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Accepted to ApJ
Preprint typeset using LATEX style emulateapj v. 5/2/11
THE EFFECTS OF VARYING COSMOLOGICAL PARAMETERS ON HALO SUBSTRUCTURE
Gregory A. Dooley1, Brendan F. Griffen1, Phillip Zukin2,3, Alexander P. Ji1, Mark Vogelsberger1, Lars E.
Hernquist4, Anna Frebel1
Accepted to ApJ
ABSTRACT
We investigate how different cosmological parameters, such as those delivered by the WMAP and
Planck missions, affect the nature and evolution of dark matter halo substructure. We use a series of
flat Λ cold dark matter (ΛCDM) cosmological N -body simulations of structure formation, each with
a different power spectrum but the same initial white noise field. Our fiducial simulation is based on
parameters from the WMAP 7th year cosmology. We then systematically vary the spectral index, ns,
matter density, ΩM , and normalization of the power spectrum, σ8, for 7 unique simulations. Across
these, we study variations in the subhalo mass function, mass fraction, maximum circular velocity
function, spatial distribution, concentration, formation times, accretion times, and peak mass. We
eliminate dependence of subhalo properties on host halo mass and average over many hosts to reduce
variance. While the “same” subhalos from identical initial overdensity peaks in higher σ8, ns, and Ωm
simulations accrete earlier and end up less massive and closer to the halo center at z = 0, the process
of continuous subhalo accretion and destruction leads to a steady state distribution of these properties
across all subhalos in a given host. This steady state mechanism eliminates cosmological dependence
on all properties listed above except subhalo concentration and Vmax, which remain greater for higher
σ8, ns and Ωm simulations, and subhalo formation time, which remains earlier. We also find that the
numerical technique for computing scale radius and the halo finder used can significantly affect the
concentration-mass relationship computed for a simulation.
Subject headings: galaxy: formation — cosmology: theory
1. INTRODUCTION
The cold dark matter (ΛCDM) model of our
universe has been well constrained to be flat,
dark energy dominated, and filled predominantly
with cold, collisionless dark matter (Bennett et al.
2012; Planck Collaboration et al. 2013). It is partly
parametrized by four quantities: the matter fraction of
the universe at present day, Ωm, the primordial power
spectrum scalar spectral index ns, the Hubble constant
at present day H0 = 100h km s
−1 Mpc−1 , and the am-
plitude of the linear power spectrum at the scale of 8 h−1
Mpc, σ8. In a flat universe the dark energy content ΩΛ, is
constrained by ΩΛ+ΩM = 1. While adherence to ΛCDM
has not changed, the best estimates of these parameters
have varied between recent Planck and WMAP measure-
ments fromWMAP values of Ωm = 0.27, ns = 0.96, σ8 =
0.80, and h = 0.71 to Planck values of Ωm = 0.32, ns =
0.96, σ8 = 0.83, and h = 0.67 (Spergel et al. 2003, 2007;
Dunkley et al. 2009; Larson et al. 2011; Bennett et al.
2012; Planck Collaboration et al. 2013).
For over a decade now, numerical simulations adopt-
ing the ΛCDM paradigm have shown that large dark
matter halos contain substructures or subhalos which
survive to the present day (e.g., Moore et al. 1999;
1 Kavli Institute for Astrophysics and Space Research and
Department of Physics, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, MA 02139, USA; gdoo-
ley@mit.edu
2 Astronomy Department, University of California, Hearst
Field Annex, Berkeley, CA 94720
3 Academia Sinica Institute of Astronomy and Astrophysics,
P.O. Box 23-141, Taipei, 10617, Taiwan
4 Department of Astronomy, Harvard University, 60 Garden
St., Cambridge, MA 02138, USA
Tormen et al. 1998; Diemand et al. 2007; Springel et al.
2008; Giocoli et al. 2010). Various studies which at-
tempt to connect the properties of dark matter halos
to present day observables have yielded some conflict-
ing results. There remain two key problems: the so-
called “missing satellite problem” where there is a dearth
of observed low Vmax subhalos when compared to sim-
ulations (Moore et al. 1999), and the “too big to fail”
problem where there is a lack of dark (LV < 10
5L⊙),
dense, high Vmax subhalos when compared to simula-
tions (Boylan-Kolchin et al. 2011). While these issues
depend critically on the influence of baryonic and radia-
tive processes (Brooks et al. 2013), as well as the possi-
ble warm (Lovell et al. 2013) or self-interacting nature
of dark matter (Vogelsberger et al. 2012; Zavala et al.
2013; Vogelsberger & Zavala 2013), a more complete un-
derstanding of the characteristics of the dominant dark
matter structures is also required, including the effect of
changes in cosmological parameters on halo properties.
Recently, Angulo & White (2010) developed a tech-
nique, whose accuracy was tested by Ruiz et al. (2011),
to transform large simulation results into those of a
slightly modified cosmology by adjusting length, mass,
and velocity units as well as changing time time scale
and amplitudes of large scale fluctuations to successfully
reproduce the mass power spectrum of a given target
cosmology to better than 0.5 per cent on large scales (k
< 0.1 h−1 Mpc). Guo et al. (2013) recently used this
technique to compare WMAP1 and WMAP7 cosmolo-
gies on the Millennium and Millenium-II simulations
(Springel et al. 2005, Boylan-Kolchin et al. 2009). As-
suming a halo mass-luminosity relationship, they deter-
mined that the differences were not significant enough to
2 Dooley et al.
be found observationally. Wang et al. (2008) conducted
a similar study between WMAP1 and WMAP3 cosmolo-
gies, but used two distinct simulations instead of trans-
forming one into another cosmology. While the cosmolo-
gies produced minimal observable differences at low red-
shifts, they had to use significantly different star forma-
tion and feedback efficiencies in their models to match re-
sults to observational data. Furthermore, they concluded
observational differences should be noticeable at high
redshifts (z > 3). A number of studies have also been
made of the concentration, spin and shape of dark mat-
ter halos across various WMAP cosmologies (Duffy et al.
2008, Maccio` et al. 2008) and within the larger hierarchi-
cal framework (e.g., Zhao et al. 2009). These studies all
focused on host halos and have largely ignored subhalo
properties across cosmologies. Guo et al. (2013) did ex-
amine subhalos, but only their global abundances, not as
a function of their host halos.
The dependence of host halo abundance on cosmo-
logical parameters is well understood within the Press-
Schechter (Press & Schechter 1974) and improved Ex-
tended Press-Schechter (see Sheth & Tormen (2002) for
example) formalism of halo mass functions. Sub-
halo mass functions (SHMFs), however, are not as
well predicted analytically and have not been stud-
ied extensively with respect to changing cosmologies.
Zentner & Bullock (2003) did partially investigate the
effect of input cosmology on substructure using their
merger history, destruction rate and survival probability
as a function of spectral index, ns, as well as a running
spectral index model. They did not, however, investi-
gate how these properties depend on other cosmological
parameters (e.g., σ8 or Ωm).
Several studies have investigated more general sub-
halo properties. Oguri & Lee (2004) developed an an-
alytic model for the subhalo mass function based on the
Extended Press-Schechter formalism and took account
of the effects of tidal disruption and dynamical friction
to estimate that the subhalo mass function is virtually
independent of host halo mass. But since they used
the host mass at the present day to calculate the im-
pact of dynamical friction they inaccurately predicted
the SHMF. In turn, van den Bosch et al. (2005) exam-
ined the SHMF, mass fraction, and accretion history and
found that the SHMF may not be universal, arguing that
the slope and normalization depend on the ratio of the
parent mass to that of the characteristic non-linear mass,
M∗. M∗ indicates the typical mass scale of halos that
are collapsing as a function of redshift and is defined by
σ(M∗, z) = δc(z) where σ(M, z) is the rms density fluc-
tuation for a spherical volume of mass M at redshift z,
and δc(z) = 0.15(12pi)
2/3[Ωm(z)]
0.0055 ≈ 1.68 is the criti-
cal threshold for spherical collapse (Navarro et al. 1997).
The value of M∗ depends on cosmology and is larger for
cosmologies where objects collapse sooner, i.e., higher
σ8, ns and Ωm. van den Bosch et al. (2005) along with
Giocoli et al. (2010) and Gao et al. (2004) however, fur-
ther found that the SMHF per unit host halo mass at z
= 0 is universal.
Whilst these studies have determined many of the fun-
damental properties of substructures, they ultimately do
not systematically investigate the effect a varied cosmol-
ogy has on their properties. Those that attempt to, only
focus on the variance of their properties using one cosmo-
logical parameter (e.g., Zentner & Bullock 2003). In this
work, we vary three key cosmological parameters (σ8, ns,
Ωm) systematically and quantify the effect it has on the
substructure population using simulations. In this man-
ner, we help quantify what effect these variations have
on the subhalo mass function, mass fraction, maximum
circular velocity function, spatial distribution, concen-
tration, formation times, accretion times, and peak mass
in a self-consistent manner.
This paper is organized as follows. In Section 2, the
simulations and halo finders used in this work are pre-
sented. Section 3.1 presents the known major effects of
cosmology on host halos. In Sections 3.2 and 3.3, we dis-
cuss the averaged statistical properties of substructure
as a function of cosmology. In Section 3.4 we discuss the
differences in substructure that is directly matched be-
tween each of our cosmological simulations. A summary
of the effects of cosmology on subhalo properties is given
in Section 4 and conclusions are given in Section 5.
2. NUMERICAL METHODS
2.1. Simulations
For our fiducial simulation we adopt an approximately
WMAP7 cosmology characterized by the present-day
matter density parameter: Ωm = 0.27; a cosmological
constant contribution, ΩΛ = 0.73; and Hubble parame-
ter: h = 0.7 (H0 = 100 h km s
−1 Mpc−1). The mass
perturbation spectrum has a spectral index, ns = 0.95,
and is normalized by the linear rms fluctuation on 8 Mpc
h−1 radius spheres, σ8 = 0.8. Six of our seven simulations
adopt cosmologies which are identical to our fiducial run
except for individual variations in ns, Ωm and σ8.
Cosmological initial conditions were generated at
redshift z = 127 using the public code graphic
(Bertschinger 2001) with an Eisenstein-Hu transfer func-
tion (Eisenstein & Hu 1998). All seven simulations em-
ployed N = 5123 dark matter particles, a Plummer-
equivalent comoving softening length of 1.22 h−1 kpc,
the same comoving box size, Lbox = 25 h
−1 Mpc, and
were evolved using an unreleased version of gadget3
(Springel et al. 2005). Our particle masses across the
seven runs range from 6.14 to 11.3 × 106 h−1 M⊙, and
all simulations were written out at 64 snapshots. Our
entire suite with all pertinent parameters are listed in
Table 1.
Rather than performing convergence test runs, we only
investigate halos above an appropriate minimum num-
ber of particles as determined by previous studies for
each halo property. We also note that our small simula-
tion volume could lead to different absolute results than
those obtained from larger simulation volumes that con-
tain larger wavelengths of the power spectrum, particu-
larly for halo mass functions (Sirko 2005; Power & Knebe
2006; Bagla et al. 2009). However, we are only concerned
with the relative differences in results between cosmolo-
gies for halo substructure as a function of its host halo.
In light of the conclusion by Power & Knebe (2006) that
the internal properties of CDM halos are relatively un-
affected by a finite box size, it is reasonable to assume
that our box’s lack of larger wavelength modes does not
affect any of our conclusions.
Varied Cosmologies & Halo Substructure 3
13.0 13.5 14.0
14.5
15.0
15.5
16.0 16.5 17.0
x [h
−1
 Mpc]
19.5
20.0
20.5
21.0
21.5
22.0
22.5
23.0
y
 
[
h
−
1
 
M
p
c
]
13.0 13.5 14.0
14.5
15.0
15.5
16.0 16.5 17.0
x [h
−1
 Mpc]
19.5
20.0
20.5
21.0
21.5
22.0
22.5
23.0
y
 
[
h
−
1
 
M
p
c
]
Fig. 1.— Projected dark matter column density (log ρ) of corresponding halo objects in the WMAP7 cosmology simulation with σ8 = 0.8
at z = 0 (left panel) and the σ8 = 0.9 simulation (right panel). The largest four halos in each visualization are matched pairs, as explained
in Section 2.4. The cosmology with higher σ8 results in the “same” halos forming earlier and merging earlier. This is visually exemplified
by the pair of halos in the right side of each panel that are in the process of merging in the σ8 = 0.9 simulation, but not in the WMAP7,
σ8 = 0.8 simulation.
TABLE 1
Summary of the cosmological simulations
Run Ωm σ8 ns h mp (h−1 M⊙)
WMAP7 0.27 0.8 0.95 0.7 8.72× 106
Ωm = 0.35 0.35 0.8 0.95 0.7 11.3× 106
Ωm = 0.19 0.19 0.8 0.95 0.7 6.14× 106
σ8 = 0.9 0.27 0.9 0.95 0.7 8.72× 106
σ8 = 0.7 0.27 0.7 0.95 0.7 8.72× 106
ns = 1.0 0.27 0.8 1.0 0.7 8.72× 106
ns = 0.9 0.27 0.8 0.9 0.7 8.72× 106
Note. — Ωb = 0.045 in all runs. ΩΛ + Ωm = 1. The
cosmological parameters used in the seven simulations
are shown above. Each uses WMAP7 values with one
parameter (highlighted in boldface) varied at a time.
2.2. Halo Finders
Two different halo finders are used throughout our
analysis so as to not bias our results by using a particular
algorithm: rockstar (Behroozi et al. 2013b) and sub-
find (Springel et al. 2001). As each halo finder produced
the same conclusions for the relative effect of cosmology,
all figures present data from rockstar only. Any im-
portant systematic differences due to the halo finder are
discussed in the text of the results.
• rockstar (Robust Overdensity Calculation using
K-Space Topologically Adaptive Refinement) is a phase-
space halo finder which considers the position and ve-
locity of all particles to determine the location of halos
in an N -body simulation. The algorithm initially se-
lects groups of particles based on a 3D Friends-of-Friends
(FOF) algorithm with a large linking length (b = 0.28).
Within each FOF group, Rockstar builds more FOF
groups in a hierarchical fashion by adapting the linking
length such that a user-specified percentage of the parti-
cles contained within a subgroup is also contained within
the parent group. Once complete, these FOF groups are
used to generate seed halos from the innermost level of
the tree. This process is repeated until all particles at
every level of the FOF group have been assigned to a
halo. A further unbinding procedure is carried out to
determine the final list of particles contained within each
halo. For more details, see Behroozi et al. (2013b).
• subfind defines halos as locally over-dense, gravi-
tationally bound groups of particles and begins by con-
ducting a FOF search of the simulation volume. The
local density around each particle is calculated using a
smoothing Kernel over its nearest neighbors. Whenever
a saddle point in a density contour which bridges two
regions is reached, the smallest of the two is considered a
subhalo candidate. As with the other halo finders, these
candidates then undergo an unbinding procedure to pro-
duce a list of halos and subhalos. For more details, see
Springel et al. (2001).
It is now well established that different halo finders
have various strengths and weaknesses depending on the
environment in which they are used (Knebe et al. 2011,
Onions et al. 2012, Knebe et al. 2013). For example,
since subfind is based upon an over-density criterion,
its ability to identify substructure is strongly dependent
on the radial position of the structure from the host
(Muldrew et al. 2011). rockstar is based on a phase
space algorithm and thus it does not suffer from the same
problem. Onions et al. (2012) found that for properties
which rely on particles near the outer edge of the sub-
halo (e.g., total halo mass), the majority of available halo
finders agree to within 10 per cent. Basic properties such
as mass or the maximum circular velocity can also be re-
liably recovered if the subhalo contains more than 100
particles. To ensure we are not resolution limited, we
only include halos and subhalos which have 300 particles
or more, unless otherwise specified.
4 Dooley et al.
The mass and radius of host halos used throughout
this work are r200 and M200. r200 is defined such that the
average density of a halo within r200 is ρ¯ = ∆hρcrit where
∆h = 200, independent of cosmology as in Jenkins et al.
(2001), and ρcrit = 3H0
2/8piG. The mass M200 is then
simply found as M200 =
4
3pir
3
200∆hρcrit.
For subhalos, which experience tidal stripping, m200
is a less meaningful quantity. We therefore define sub-
halo mass, Msub, as the total bound mass. This is com-
puted as an extra parameter within rockstar’s source
code. Defining the radius of a subhalo is even more trou-
blesome. The distance to the furthest bound particle
depends on the random kinematics of a few particles,
and the tidal radius is not a spherically symmetric value.
Since defining a useful measure of the outer edge of a
subhalo is difficult, we avoid using it. In the one analysis
where it could be used, in finding subhalo concentration,
we instead use a concentration definition that is indepen-
dent of outer radius.
2.3. Merger Trees
For the merger tree component of this work (Section
3.3), we use only the merger tree generated by rock-
star consistent trees on the 64 snapshots per simula-
tion. Behroozi et al. (2013a) have shown that the merger
tree catalogues produced by rockstar are consistent in
determining halo masses, positions, and velocities when
compared to merger trees constructed via different meth-
ods, e.g., bdm (Klypin et al. 1999, 2011) and subfind.
2.4. Halo Matching Across Simulations
Since the same white noise field was used in each simu-
lation, particles tagged with the same ID in each simula-
tion have a low mean displacement from each other, ∼ 4
h−1 kpc at z = 0. The aggregate of these similarly dis-
tributed particles are halos of similar size, position, and
formation history between simulations. These “same”
halos can be matched to each other, enabling a direct
assessment of how cosmological parameters affect prop-
erties of individual halos and subhalos. This analysis
is carried out in Section 3.4. Figure 1 shows a visual
example of matched halos in two different simulations.
Halos are matched by their particle content. In a given
set of two simulations, one halo from each− a halo pair −
is compared and given a matching strength value based
on the number of particles they have in common (same
ID), weighted by the gravitational boundedness of each
particle. For a pair of (sub)halos A and B, there are two
match values, one where particles are weighted by bound-
edness within A, and one where particles are weighted by
boundedness within B. In order for a pair to qualify as
a match, (sub)halo B must be the best possible match
out of all (sub)halos in its simulation to (sub)halo A, and
(sub)halo A must be the best possible match to (sub)halo
B. This dual requirement eliminates cases when a small
fragment of a halo matches to a larger encompassing halo.
The particle weighting of the ith most bound particle
(starting from 0) in a (sub)halo is
Wi ∝
n− i
n
(1)
where n is the number of particles in the halo. The total
match value is given as
∑
Wi for all particles in common
to the (sub)halo pair. In order to sum to unity for a
perfect match, the normalization constant is chosen as
2n
n+1 . A perfect match is when all particles in (sub)halo
A are found in (sub)halo B or vice-versa. Additional
constraints of
∑
Wi > 0.2 and n > 40 are imposed to
eliminate uncertain matches. Host halos are matched
first, then subhalos are matched within matched host
halo pairs.
3. RESULTS
This section presents results on the differences and lack
of differences induced by varying cosmology on halo and
subhalo properties. Effects on host halos are presented in
Section 3.1. Section 3.2 shows results for subhalo charac-
teristics at z = 0, Section 3.3 presents subhalo character-
istics frommerger tree analysis, and Section 3.4 compares
subhalo characteristics for matched subhalos.
3.1. Averaged Properties of Host Halos
While the focus of this paper is on how cosmological
parameters affect substructure, we first verify and sum-
marize for reference how cosmological parameters affect
the host halo mass function and concentration.
3.1.1. Mass Function
In accordance with Jenkins et al. (2001), we compute
dn/d logM where dn(M) is the number of host halos on
an infinitesimal mass interval centered at M, and d logM
is the logarithm of the width of the mass interval. This
function shows a characteristic monotonic trend of higher
abundances of low mass objects. Rather than presenting
the mass function directly, we compute the ratio of the
mass function at redshift z = 0 in each cosmology to
our fiducial simulation mass function in order to accen-
tuate any differences. This is shown in Figure 2. We
also use the Extended Press-Schechter formalism devel-
oped by Sheth & Tormen (2002) with the Eisenstein-Hu
transfer function (Eisenstein & Hu 1998) to compute an-
alytic estimates of these mass function ratios. We com-
pute error bars using a jackknife method (Tinker et al.
2008). We create five sub-volumes, each with 45 of the
original volume, by removing a different fifth of the orig-
inal volume for each sub-volume. Error bars are then
the 1 σ standard deviation of the mass function ratio as
computed in each sub-volume. Our data agree with an-
alytic estimates except on the high mass end above 1012
h−1M⊙, where there are fewer halos and Poisson noise
becomes important.
In the top panel of Figure 2, we show that higher
σ8, which means higher amplitude for primordial density
fluctuations, produces more halos above the characteris-
tic mass scale of collapse at z = 0, M∗ ≈ 1013 h−1M⊙,
and fewer halos below. Intuitively, a higher initial den-
sity amplitude causes more mass to end up in large halos
after hierarchical merging, leaving less mass available for
low mass halos. This agrees with a study by Guo et al.
(2013). The reverse is true for lower σ8.
In the middle panel, we plot the scalar spectral index,
ns, which controls the slope of the initial power spectrum,
P0(k) ∝ k
ns . Larger values of ns correspond to more
initial power on small scales (and less on large scales),
and thus more collapsed objects early on. The earlier
collapsing small objects then hierarchically merge into
Varied Cosmologies & Halo Substructure 5
0.7
0.8
0.9
1.0
1.1
1.2
1.3
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
0.7
0.8
0.9
1.0
1.1
1.2
1.3
d
n
/
d
l
o
g
1
0
(
M
h
o
s
t
)
 
/
 
d
n
f
i
d
u
c
i
a
l
/
d
l
o
g
1
0
(
M
h
o
s
t
)
n
s
= 1.00
n
s
= 0.95
n
s
= 0.90
10
9
10
10
10
11
10
12
10
13
10
14
M
host
[h
−1
M
⊙
]
0.7
0.8
0.9
1.0
1.1
1.2
1.3
Ω
m
= 0.35
Ω
m
= 0.27
Ω
m
= 0.19
Fig. 2.— Ratio of host halo mass functions to our fiducial sim-
ulation for different cosmological parameters. Dots correspond to
data points computed from rockstar halo finder data. Lines corre-
spond to the Sheth & Tormen analytical prediction. Error bars are
computed using a jackknife method. As anticipated, in cosmologies
where structure forms earlier there are more halos at z = 0
larger objects. As evidenced by Figure 2 larger ns con-
tinues to correspond to more collapsed objects by z = 0
for the mass range considered.
We also show the effect of more exaggerated variations
in Ωm. More matter content in the universe translates
into objects of higher mass, and more objects, as demon-
strated in the bottom panel.
Since z = 0 subhalos form independently of their hosts
at high redshifts, these qualitative mass function differ-
ences apply just as well to subhalos before accretion. Any
changes in mass and abundance are due to the dynam-
ics of subhalo-host interactions, including tidal stripping,
dynamical friction, and collisions with other subhalos.
3.1.2. Host Concentration
The density profile of dark matter halos are well ap-
proximated by the spherically symmetric Navarro, Frenk,
& White (NFW) Profile (Navarro et al. 1996). The pro-
file is defined by
ρ(r) = ρcrit
δc
(r/rs)(1 + r/rs)2
, (2)
where the dimensionless δc is the characteristic over-
density, rs is the scale radius, and ρcrit is the criti-
cal density of the universe. Concentration, defined by
c200 ≡ r200/rs, gives a measure of how centrally con-
centrated the particles in the halo are. Concentration
could also be defined in terms of the spherically symmet-
ric Einasto density profile, given as
ρ(r) = ρ(r−2) exp
[
−
2
α
((
r
r−2
)α
− 1
)]
, (3)
where r−2 is the radius at which the logarithmic slope
of the profile is −2, the shape parameter α is an extra
free parameter to fit to, and ρ(r−2) is the density at r−2
(Einasto 1965). In this case the concentration would be
c200 ≡ r200/r−2. Furthermore, many studies use outer
radii other than r200. Since each definition quantifies the
same qualitative idea, we consider only the first definition
in terms of the NFW profile. We use the default r200 as
determined by the halo finder, but compute rs directly
from the particle data of the halo. To facilitate a direct
comparison to past studies, we find rs by fitting an NFW
profile to the density profile of each halo, determined by
binning particles in 16 bins equally spaced in log space
from log10(
r
r200
) = −1.25 to 0, as in Duffy et al. (2008).
Host halo concentrations have been studied extensively
(e.g., Zhao et al. 2009). It is now well established that
halos which collapse earlier have higher concentrations
(Neto et al. 2007, Duffy et al. 2008, Maccio` et al. 2008,
Giocoli et al. 2010). This is simply because the universe
was of higher density at early times. One would then
expect cosmologies with a higher M∗ to have higher con-
centrations. Similarly, halos with a higher mass should
have smaller concentrations since they formed later. In
particular, for increasing values of σ8 we expect the con-
centration to increase since it reflects the background
density of the universe at the time when the halo formed.
To test this, we consider all “relaxed” halos whose
hosts satisfy the following criteria similar to Neto et al.
(2007):
• The host must contain at least 600 particles.
• The substructure mass fraction of a given host must
be, fs < 0.1.
• The offset parameter or center of mass displace-
ment must be s < 0.07 where s = |rc − rcm|/r200.
rc and rcom are the center of halo peak density and
mass respectively.
• The virial ratio, 2T/|U |, must be less than a pre-set
value of 1.35, where T is the total kinetic energy
and U is the gravitational potential self-energy of
the host halo.
This combination of mass and kinematic information
encoded in the substructure mass fraction, offset param-
eter, and virial ratio ensures that the hosts in our relaxed
sample are well fitted by NFW profiles (Navarro et al.
1996). Unrelaxed halos are more difficult to accurately fit
a scale radius to. Figure 3 shows the concentration-mass
(c −M) relation for each of our simulations. Using the
mean and rms deviation of log10(c200), we fit the binned
c−M relation using,
log10(c200) = A log10(Mvir) +B, (4)
where log10(c200) and Mvir are the mean values in each
bin. For the error of the mean in each bin, σc, we use the
rms deviation of log10(c200) divided by the square root
of the number of objects in each bin. For each fit, we
define
χ2 =
Nb∑
j=1
(
log10(c200)− log10(c200fitj )
σc¯j
)2
, (5)
where Nb is the number of bins over which the fit is
performed and c200fitj is obtained from the best fit of
6 Dooley et al.
Equation (4), and find the values of A and B such that
the reduced chi-squared, χ¯2 (χ2 divided by the number
of degrees of freedom), is minimized. All error bars and
best fit functions in future figures are found using this
same method.
Our data and best fit functions for the relaxed sam-
ple are shown in Figure 3. We also add for compari-
son the relation for c200 derived by Duffy et al. (2008)
who used WMAP5 data (σ8 = 0.796, ns = 0.963,
Ωm = 0.258). The data confirms that higher M
∗ cos-
mologies and smaller host halos do in fact have higher
concentrations. The slopes of each c−M relation range
from −0.060 to −0.097 with a typical value of −0.083,
but do not vary systematically with cosmological pa-
rameters. Furthermore, the slope is sensitive to within
±0.008 to how the host halos are binned and the halo
mass range considered. Therefore we can only defini-
tively confirm how the amplitude of the c−M relation is
affected by cosmological parameters, and not the slope.
The slopes obtained do agree, within error, with the slope
of −0.092 obtained by Duffy et al. (2008) for relaxed ha-
los.
We further note that the slope and magnitude of the
c −M relation is very sensitive to the method for find-
ing rs and the halo finder. By default, rockstar com-
putes rs by fitting an NFW profile to a density profile
found by dividing particles into 50 radial bins such that
each bin contains the same mass (Behroozi et al. 2013b).
This results in a much steeper slope for all cosmologies,
with a typical slope of −0.15. Furthermore, the concen-
trations computed using rockstar’s r200 and particle
assignments are systematically higher than those found
using subfind’s parameters. Using subfind, our fidu-
cial simulation’s c − M relation is very similar to the
Duffy et al. (2008) relationship in magnitude and slope,
which was also computed using subfind. The c−M re-
lationships found using rockstar, however, are on aver-
age 12% greater in overall magnitude. Both halo finders
do, however, agree on the relative differences between
cosmologies. The large dependencies of concentration on
its method of computation are examined in greater detail
by Meneghetti & Rasia (2013).
3.2. Averaged Properties of Subhalos
Subhalo abundances and properties depend dramati-
cally on the size of their host halo. Any useful compar-
ison between simulated and observed subhalo distribu-
tions thus requires comparing distributions within hosts
of the same size. In this section, we explore the effects of
cosmology on averaged subhalo properties as a function
of their host halo mass.
3.2.1. Subhalo Mass Function
Similar to the host halo mass function, the sub-
halo mass function, SHMF, counts subhalo abundance
within a chosen host halo as a function of mass.
Whereas the host halo mass function has been stud-
ied extensively by numerical simulations and agrees
well with fully analytic predictions, see Figure 2, only
semi-analytic models exist for the SHMF (Oguri & Lee
2004; van den Bosch et al. 2005; Springel et al. 2008;
Zentner & Bullock 2003; Gao et al. 2004). This is due
to the more complex effects of collisions, dynamical fric-
6
8
10
12
14
16
A: -0.078, B: 1.89
A: -0.078, B: 1.84
A: -0.095, B: 1.97
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
6
8
10
12
14
16
C
2
0
0
A: -0.096, B: 2.08
A: -0.078, B: 1.84
A: -0.080, B: 1.82
n
s
= 1.00
n
s
= 0.95
n
s
= 0.90
10
10
10
11
10
12
10
13
M
host
[h
−1
M
⊙
]
6
8
10
12
14
16
A: -0.086, B: 2.01
A: -0.078, B: 1.84
A: -0.060, B: 1.53
Ω
m
= 0.35
Ω
m
= 0.27
Ω
m
= 0.19
Fig. 3.— The averaged host halo concentration as a function of
host halo mass and cosmology. Best fit lines and best fit parameters
as described in Eq. 4, and error of the mean bars are shown. The
c − M relation derived by Duffy et al. (2008) (solid black line)
is consistent in its slope to our findings. Its overall magnitude,
however, is lower primarily due to its computation using subfind
and our data using rockstar. Cosmologies with higher M∗ lead
to more concentrated halos, but do not affect the slope of the c−M
relation.
tion, and tidal stripping changing subhalo masses over
time.
We investigate the SHMF, dn/dMsub vs Msub, where
dn is the number of subhalos in a mass interval dMsub.
Within each particular cosmology, we find that the
SHMF per unit host halo mass (i.e., dividing the dif-
ferential abundance by the mass of the host halo), yields
a universal function. This confirms the same result found
in Gao et al. (2004) and van den Bosch et al. (2005). We
use this trait to average the normalized subhalo mass
functions for all hosts halos with Mhost > 10
12.5 h−1M⊙
to account for halo-to-halo variance of the SHMF. Lower-
ing the Mhost mass threshold simply increases variance,
due to resolution effects, without changing the value of
the average mass function. The top panel of Figure 4
shows the SHMF per unit host halo mass scaled for a
Milky Way sized host of Mhost = 0.84 × 10
12 h−1M⊙
(1.2× 1012 M⊙). We also indicate the magnitude of 1 σ
halo-to-halo variation with bars on each point to com-
pare changes due to cosmology with inherent variance be-
tween halos. This strategy of handling variance by find-
ing characterizations that are independent of host halo
mass, then averaging the characterizations over all host
halos, is used throughout the remainder of Section 3.2.
Varying the parameters σ8 and Ωm has no noticeable
effect on the subhalo abundance at z = 0. Our simula-
tions with higher values of ns have a greater abundance
of subhalos in each bin. However, the error on this over-
all trend is equal to the magnitude of the trend so it may
have arisen due to chance. Additionally, variations from
halo-to-halo in the SHMF exceed any possible small cos-
mological effect. The best fit to the data produced from
Varied Cosmologies & Halo Substructure 7
10
-10
10
-9
10
-8
d
n
/
d
M
s
u
b
[
h
M
−
1
⊙
]
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
0.5
1.0
1.5
2.0
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
0.5
1.0
1.5
2.0
(
d
n
/
d
M
s
u
b
)
/
(
d
n
f
i
d
u
c
i
a
l
/
d
M
s
u
b
)
n
s
= 1.00 n
s
= 0.95 n
s
= 0.90
10
9
10
10
M
sub
[h
−1
M
⊙
]
0.5
1.0
1.5
2.0
Ω
m
= 0.35 Ω
m
= 0.27 Ω
m
= 0.19
Fig. 4.— Top Panel: Subhalo mass function normalized to host
halo mass and scaled to a Milky Way sized host of 0.84 × 1012
h−1M⊙ (1.2×1012 M⊙), averaged over all host halos above 1012.5
h−1M⊙ for cosmologies with different σ8. Resolution constraints
on low masses and large Poisson errors for high masses constrain
the subhalo mass range considered. Best fit lines and 1 σ halo-
to-halo variation bars, not error bars, are shown. Bottom Panels:
Ratio of subhalo mass function to the fiducial WMAP7 subhalo
mass function. Data points for the non WMAP7 cosmologies are
shifted slightly left and right of their true values to help distin-
guish them. The effects of varying σ8 and Ωm are consistent with
no change in the SHMF. A small trend of higher abundance with
higher ns exists but is also consistent with no change in the SHMF
within error. Variation on a halo-to-halo basis dominates any ef-
fects of cosmology on the SHMF as seen in the 1 σ variation bars.
rockstar data is given by
dn
dMsub
= K ×
(
Msub
h−1M⊙
)−α
Mhost
h−1M⊙
(6)
Where K = 4.5± 0.3× 10−5h M−1⊙ and α = 1.78± 0.04.
The same results and fit values were reproduced within
error using subfind, the halo finder used in most pre-
vious studies. The power law exponent of −1.78 falls
between the values of −1.73 reported by Helmi et al.
(2002) and −1.9 reported by Springel et al. (2008),
van den Bosch et al. (2005) and Gao et al. (2004). In-
dividual halos in this study have SHMFs with α ranging
from 1.65 to 1.95, encompassing the range of values re-
ported in the literature.
Following from an unchanging SHMF per host halo
mass, the subhalo mass fraction is also not changed by
10
-2
10
-1
10
0
10
1
d
n
/
d
V
m
a
x
,
s
u
b
[
k
m
−
1
s
]
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
(
d
n
/
d
V
m
a
x
,
s
u
b
)
/
(
d
n
f
i
d
u
c
i
a
l
/
d
V
m
a
x
,
s
u
b
)
n
s
= 1.00 n
s
= 0.95 n
s
= 0.90
10 20 30 40 50 60 70 80
V
max, sub
[km s
−1
]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ω
m
= 0.35 Ω
m
= 0.27 Ω
m
= 0.19
Fig. 5.— Top Panel: Subhalo Vmax function normalized to host
halo Vmax and scaled to a Milky Way sized host of 220 km s−1,
averaged over all host halos above 1012.5 h−1M⊙ for cosmologies
with different σ8. Best fit lines and 1 σ halo-to-halo variation bars
are shown. Bottom Panels: Ratio of subhalo Vmax function to
the fiducial WMAP7 subhalo Vmax function. Data points for the
non WMAP7 cosmologies are shifted slightly left and right of their
true values to help distinguish them. Higher values of each of the
cosmological parameter (higher M∗) result in higher abundances
of Vmax subhalos over the mass range considered. The cosmologies
chosen correspond to ∼ 18% greater abundance for the higher M∗
cosmologies and ∼ 10% less abundance for the lower M∗ cosmolo-
gies. Variation on a halo-to-halo basis dominates any effects of
cosmology on the Vmax function as seen in the 1 σ variation bars.
variations in cosmological parameters. This was directly
confirmed by simulation data.
3.2.2. Maximum Circular Velocity
While mass may be the most intuitive description of a
subhalo’s size, the maximum circular velocity, Vmax, is
a related measure that is easier to ascertain observation-
ally and more robust to measure from simulations. It is
defined as the maximum velocity of an orbiting body in
the potential of a halo:
Vmax = max
(
GM(< r)
r
) 1
2
. (7)
Unlike mass, Vmax is not sensitive to the poorly de-
fined boundary between the halo and the background
(Kravtsov 2010). Additionally, it avoids the arbitrari-
ness that plagues any definition of mass. While closely
8 Dooley et al.
related, Vmax does not just trace mass, but also has a
weak dependence on concentration: higher concentration
leads to higher Vmax (Bullock et al. 2001). Thus, if there
is no cosmology dependence on the SHMF, there should
in principle still be a weak dependence on Vmax due to
the dependence of subhalo concentration on cosmology
(see Figure 9). For these reasons, we repeat the subhalo
abundance analysis done for subhalo mass with subhalo
Vmax instead. The subhalo Vmax function is computed
for each host halo, normalized to the host’s Vmax, av-
eraged over all host halos with Mhost > 10
12.5 h−1M⊙,
then scaled to a host with Vmax = 220 km s
−1. The re-
sulting function and the ratio of it to our fiducial simula-
tion’s function for each cosmology are shown in Figure 5.
We indeed find that this Vmax function depends weakly
on the cosmological parameters considered. The greater
value of each cosmological parameter increases the abun-
dance of subhalos as a function of Vmax by ∼ 18% and
the lower values of each parameter decreases abundance
by ∼ 10%. Still, the error on this trend is substantial
at ∼ 10%, and variation from halo to halo exceeds this
effect.
Just as for the SHMF, we fit an exponential function
to the data. This equation is
dn
dVmax, sub
= Kv ×
(
Vmax, sub
km s−1
)−αv Vmax, host
km s−1
(8)
and the best fit parameters to the fiducial simulation are
Kv = 12.5± 0.9 km
−1 s and αv = −2.3± 0.2.
Vmax is of particular interest in relation to the Missing
Satellite Problem. Wang et al. (2012) discuss the dispar-
ity between subhalo abundance above Vmax = 30 km s
−1
observed in the Milky Way and the abundance predicted
in simulations of a 1012 h−1M⊙ halo. The Aquarius sim-
ulations predict eight subhalos, whereas the Milky Way
has only three with Vmax above 30 km s
−1 (the Large
Magellanic Cloud, the Small Magellanic cloud, and the
Sagittarius Dwarf). They discover that this number is
very sensitive to the host halo mass and very insensitive
to the cosmology. With the data to test this more explic-
itly, we show in Figure 6 the average number of subhalos
with Vmax > 30 km s
−1 as a function of host halo mass.
We find that this Vmax function is consistent with being
independent of the variation in cosmological parameters
considered, confirming the result of Wang et al. (2012).
3.2.3. Spatial Distribution
The subhalo spatial distribution in terms of its number
density as a function of radius is also studied. Follow-
ing Springel et al. (2008), we first investigate the num-
ber density profile, n(r)/<n>, where n(r) is the local
number density of subhalos, and <n> is the average
subhalo number density within the virial radius. With
the radial distance normalized to r200, we discover that
this distribution takes on a characteristic form for all
host halos, regardless of mass, for r/r200 > 0.4. We
tested for this independence of mass by computing the
average distribution over several host halo mass inter-
vals in the range 1010 h−1M⊙ < Mhost < 10
13.6 h−1M⊙.
No mass dependent difference was found in the profiles
above r/r200 > 0.4. Below this threshold, corresponding
to only 6.4% of the halo volume, there is a clear mass
dependent trend with smaller host halos having higher
10
-1
10
0
10
1
10
2
N
s
u
b
(
V
m
a
x
>
3
0
k
m
/
s
)
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
(
N
s
u
b
(
V
m
a
x
>
3
0
k
m
/
s
)
)
/
(
N
f
i
d
u
c
i
a
l
s
u
b
(
V
m
a
x
>
3
0
k
m
/
s
)
)
n
s
= 1.00 n
s
= 0.95 n
s
= 0.90
10
11
10
12
10
13
M
host
[h
−1
M
⊙
]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ω
m
= 0.35 Ω
m
= 0.27 Ω
m
= 0.19
Fig. 6.— Top Panel: Average number of subhalos with Vmax >
30 km/s as a function of host halo mass and cosmology for vari-
ations in σ8. The number of subhalos above this threshold is not
affected by cosmology, but is substantially affected by the mass of
the host halo. Halo-to-halo variation is shown in the 1 σ vari-
ation bars. Bottom Panels: Ratio of the number of subhalos
with Vmax > 30 km/s in a given cosmology to that of the fiducial
WMAP7 cosmology. Data points for the non WMAP7 cosmolo-
gies are shifted slightly left and right of their true values to help
distinguish them. All changes in cosmology tested are consistent
with having no effect on this function. Variation on a halo-to-halo
basis dominates any effects of cosmology as seen in the 1σ variation
bars.
number densities than larger host halos. However, this
region is also one where halo finders have difficulty iden-
tifying subhalos, so it is unclear whether the effect is
real, or a halo finding artifact. We resolve to exclude
this range from the cosmology comparison analysis.
Figure 7 shows the characteristic subhalo radial distri-
bution averaged over all host halos with Mhost > 10
12.5
h−1M⊙ for each cosmology. Once again, this distribution
is found to be independent of the cosmological parame-
ters studied.
3.2.4. Subhalo Concentration
Using similar methodology presented in Section 3.1.2,
we examine the subhalo c − M relation for subhalos
within relaxed hosts at redshift z = 0. Due to difficulty
in defining subhalo mass, we also explored the subhalo c-
− Vmax relation and found all of the same results. Since
Varied Cosmologies & Halo Substructure 9
0.5
1.0
1.5
2.0
2.5
n
(
r
)
/
<
n
(
r
)
>
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
0.4
0.6
0.8
1.0
1.2
1.4
1.6
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
0.4
0.6
0.8
1.0
1.2
1.4
1.6
(
n
(
r
)
/
<
n
(
r
)
>
)
/
(
n
f
i
d
u
c
i
a
l
(
r
)
/
<
n
f
i
d
u
c
i
a
l
(
r
)
>
)
n
s
= 1.00 n
s
= 0.95 n
s
= 0.90
0.4 0.5 0.6 0.7 0.8 0.9 1.0
r/r
200
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Ω
m
= 0.35 Ω
m
= 0.27 Ω
m
= 0.19
Fig. 7.— Top Panel: The averaged subhalo number density as a
function of r/rvir normalized to the mean subhalo number density
for cosmologies with varied σ8. This function is found to not vary
with the mass of the host halo nor the cosmological parameters
considered for r/rvir > 0.4. At smaller radii, halo finding becomes
too uncertain to make any claim. Variation bars show 1σ variations
in the density profiles of individual halos. Bottom Panels: Ratio
of the subhalo spatial distribution as in the top panel to that of
the to the fiducial WMAP7 cosmology for all cosmologies. Data
points for the non WMAP7 cosmologies are shifted slightly left
and right of their true values to help distinguish them. All changes
in cosmology tested are consistent with having no effect on this
function. Variation on a halo-to-halo basis dominates any effects
of cosmology on subhalo spatial distribution as seen in the 1 σ
variation bars.
both options lead to the same conclusions, we choose
to present only the c −M relation in order to compare
to previous literature and the host halo c −M relation.
The concentration, c, is once again defined by the ratio
c ≡ r200/rs. However, directly computing r200 and rs for
subhalos is troublesome. As noted in Section 3.1.2, the
c −M relation is very sensitive to how rs in computed.
Whereas for host halos the method used by Duffy et al.
(2008) of dividing particles into 16 logarithmic bins to
make a density profile, then fitting an NFW profile to find
rs works consistently well, this method does not work re-
liably for subhalos which typically have too few particles
to populate all 16 bins. The default method from rock-
star overcomes this issue by choosing 50 bins such that
each contains an equal number of particles. However,
we find that this method results in erratically varying
subhalo concentrations with standard deviations of con-
centration per subhalo mass interval exceeding 80.
We therefore avoid computing rs directly and instead
use other parameters and properties of an NFW profile
to infer c. (Klypin et al. 2011) uses Vmax and M200 to
numerically solve for c assuming an NFW profile. While
we did test out this method, M200 is nonsensical for very
tidally stripped subhalos where r200 exceeds the furthest
bound particle. This occurs in ≈ 5% of subhalos above
109 h−1M⊙. We thus finally turn to the method used by
Springel et al. (2008) which also assumes an NFW pro-
file, but uses Vmax and the radius where Vmax occurs,
rvmax, to compute c. The concentration is found numer-
ically from the equation
c3
ln(1 + c)− c/(1 + c)
= .21639
(
Vmax
H0rvmax
)2
. (9)
The c −M relation from this method for all subhalos
with greater than 500 particles is shown in Figure 8. As
a function of cosmology, we discover that the differences
in concentration demonstrated in Figure 3 still last in the
subhalo population. Cosmologies with higherM∗ (higher
σ8, ns, and Ωm) lead to subhalos with higher concentra-
tion. We further fit the form of Eq. 4 to the data and find
concentrations similar to the host halos but with more
shallow negative slopes. Due to a more limited sample
of subhalos than host halos after the selection criteria,
∼ 900 vs. ∼ 6000 in the 109.64 − 1012 h−1M⊙ interval,
and density profiles that deviate further from NFW, the
strength of each fit is worse and the variation in slopes
between simulations is much greater for subhalos. Fur-
thermore, using the Klypin method of computing subhalo
concentration yields steeper negative slopes than the host
halo c−M relation (it yields a median slope of −0.122, in
agreement with the slope of −0.12 found by Klypin et al.
(2011)), and a 70 − 95% increase in concentration rela-
tive to hosts for the mass range considered. We therefore
cannot definitively claim a characteristic slope value or
magnitude for subhalo concentrations and caution that
results are very sensitive to the method used to compute
halo concentration. The Klypin method does nonetheless
agree with the relative differences between cosmologies,
and we conclude that varying cosmology does have a real
effect on subhalo concentrations.
With a consistent computation of concentration for
host halos and subhalos according to Eq. 9, we find that
the median slope of the c−M relation for hosts is −0.06,
which is consistent with the slopes found for the sub-
halos. The concentration of subhalos in the same mass
intervals is typically≈ 15% greater. Merger tree histories
of subhalos demonstrate that their concentrations do in
fact continually rise after accretion, and at a faster rate
than host halos. This can be explained by a change in
subhalo density profiles due to tidal stripping and tidal
heating. Hayashi et al. (2003) models the density profiles
as a modified NFW function that changes over time as a
function of the ratio of current subhalo mass to its mass
at infall. The modified NFW profile and supporting fit-
ting functions are given in Hayashi et al. (2003) as equa-
tions 8, 9, and 10. Following this model, both Vmax and
rvmax decrease as a function of mass loss, and thus time
since accretion. However, since rvmax decreases faster
than Vmax, subhalo concentration as computed in Eq 9
increases monotonically. These trends are confirmed in
the merger tree histories of the subhalos, but with sig-
nificant variance. Since the subhalo profile is no longer
NFW in this model, Eq 9 only approximates the original
definition of concentration, c = r200/rs. We therefore
10 Dooley et al.
also study the poorly defined rs and r200. This similarly
shows rs decreasing at a faster rate than r200. Below the
limit of 500 particles, the concentration of subhalos and
host halos systematically and unrealistically decreases as
a function of mass, but the relative differences between
cosmologies remains.
We further investigate the subhalo concentration as a
function of host mass in Figure 9. We find that the av-
erage subhalo concentration is weakly dependent on the
host mass, with larger hosts having more concentrated
subhalos. We account for the bias of larger hosts having a
different distribution of subhalos by only considering sub-
halos in d log10(Msub) = 0.5 mass intervals from 10
8 to
1011 h−1M⊙. In every case we observe the same positive
slope. We also rule out the possibility that larger hosts
tend to have halos which were accreted earlier through
studies of the merger tree history, as presented in Sec-
tions 3.3.1 and 3.3.2. Without a strong theoretical moti-
vation for any functional form, we abstain from making
a best fit. We speculate that the trend may be due to
stronger tidal forces in larger hosts. This would indi-
cate that the subhalo profile models from Hayashi et al.
(2003) should depend weakly on the host halo’s size in ad-
dition to the fraction of remaining bound subhalo mass.
A full explanation is beyond the scope of this paper.
4
6
8
10
12
14
16
20
A: -0.059, B: 1.71
A: -0.075, B: 1.81
A: -0.047, B: 1.48
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
4
6
8
10
12
14
16
20
C
s
u
b
A: -0.024, B: 1.34
A: -0.075, B: 1.81
A: -0.084, B: 1.88
n
s
= 1.00
n
s
= 0.95
n
s
= 0.90
10
9
10
10
10
11
M
sub
[h
−1
M
⊙
]
4
6
8
10
12
14
16
20
A: -0.051, B: 1.64
A: -0.075, B: 1.81
A: -0.050, B: 1.48
Ω
m
= 0.35
Ω
m
= 0.27
Ω
m
= 0.19
Fig. 8.— The averaged subhalo concentration as a function of
subhalo mass and cosmology. Concentration is computed using
rvmax and Vmax according to Eq. 9 which assumes an NFW profile.
The differences in concentration due to cosmology seen in the host
halos remain in the subhalos. Cosmologies with higher M∗ lead to
subhalos with higher concentration. The same trend holds for the
c − Vmax relationship. The magnitude of the subhalo concentra-
tions cannot be reliably compared to the host halo concentrations
of Figure 3 since they were computed differently. Variation bars
show large 1 σ variations in the concentration of individual subha-
los. Best fit lines and best fit parameters as described in Eq. 4 are
also shown. Difficulty in computing concentration reliably below
500 particles per subhalo sets the low mass cut-off in the plot.
6
8
10
12
14
18
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
6
8
10
12
14
18
C
s
u
b
n
s
= 1.00
n
s
= 0.95
n
s
= 0.90
10
11
10
12
10
13
10
14
M
host
[h
−1
M
⊙
]
6
8
10
12
14
18
Ω
m
= 0.35
Ω
m
= 0.27
Ω
m
= 0.19
Fig. 9.— The averaged subhalo concentration as a function of
host halo mass and cosmology for subhalos between 109 and 1010
h−1M⊙ in relaxed hosts. The slight positive trend suggests that
larger host halos cause a greater increase in subhalo concentra-
tion than smaller host halos, possibly due to stronger tidal forces.
Without a theory to predict a form of this trend, no function is
fit to the data. Variation bars show large 1 σ variations in the
concentration of individual subhalos.
3.3. Merger Tree Analysis
While the previous subsection focused on static dis-
tributions at z = 0, we now investigate the evolution
of subhalos as a function of cosmology through merger
tree analysis. Halos that become subhalos go through a
trajectory of forming, gaining mass, reaching a maximal
mass, losing mass due to tidal stripping from a nearby
halo, entering r200 of their eventual host (accretion), and
ultimately merging with their host through stripping and
disruption. Subhalos detected at z = 0 are ones that have
not yet been fully torn apart by their host. The following
sections explore these steps with the approach of finding
average distributions for subhalos within a host. Rather
than presenting results for all cosmologies, we highlight
only simulations with varied σ8, which has historically
been the least well constrained parameter of the three
studied in this paper.
3.3.1. Formation Time of Subhalos
Numerous definitions exist for the formation time of
dark matter halos. As discussed in Li et al. (2008), these
definitions generally fall into two classes:
1. When a halo reaches a fraction of its final mass or
Vmax.
2. When a halo first reaches a threshold mass or Vmax.
While both types of definitions can be applied to host
halos, the first class does not apply well to subhalos.
Subhalos do not monotonically grow in mass, but rather
reach a peak before the effects of tidal stripping from
Varied Cosmologies & Halo Substructure 11
a nearby host removes mass. Thus, the time when a
subhalo reaches a fraction of its final or maximal mass
intertwines its formation and post-accretion history. We
therefore use the definition that a halo forms once it first
surpasses a threshold mass of M200 = 3 × 10
8 h−1M⊙,
corresponding to ∼ 35 particles. Changing the mass
threshold shifts when halos have “formed”, but does not
influence how cosmology affects formation times. This
mass was chosen to minimize the number of halos which
were first detected above the threshold mass, and those
which form but never reach the threshold mass. It suc-
ceeds in characterizing ∼ 55% of subhalos at z = 0.
The remaining 45% of subhalos typically have incom-
plete mass histories, so their formation time would be
hard to deduce under any definition of formation time.
The threshold mass definition has the useful property
that the formation time of subhalos is a tracer of for-
mation time of the oldest stars in each subhalo (Li et al.
2008).
We use the merger trees to study the probability den-
sity distribution of subhalo formation times for all sur-
viving subhalos at z = 0 within a host. This distribution
is similar for all host halos in a simulation, and thus av-
eraged over all z = 0 host halos within the mass range
1012 < M < 1013.7 h−1M⊙ to characterize a particular
cosmology. A comparison of the averaged distribution is
seen in the top panel of Figure 10. Note that the distri-
bution is computed as a function of look-back time for
the given cosmology, not redshift. A small effect is seen
demonstrating that in cosmologies with higher σ8, z = 0
subhalos tend to be older. The mean age of subhalos
in the σ8 = 0.9, 0.8, and 0.7 simulations are 11.1, 11.0,
and 10.6 billion years respectively. Higher σ8 indicates
that halos of the same mass should collapse sooner, so
this result is expected for host halos. For subhalos, the
age distribution can change due to their destruction over
time, but evidently this effect is not enough to erase the
difference in ages between lasting z = 0 subhalo popula-
tions in different cosmologies.
3.3.2. Accretion Time of z = 0 Subhalos
In addition to formation time, the same analysis is ap-
plied to subhalo accretion time. The probability density
distribution of subhalo accretion times is similar for all
host halos and thus averaged to reduce scatter. The ac-
cretion time is defined by when the center of an eventual
z = 0 subhalo crosses its host’s r200, defined at the time
of crossing, and remains inside. This characterization is
relevant in that it roughly indicates the time when ram
pressure stripping and tidal stripping become important
mechanisms of gas loss, which suppresses star formation
(Mayer et al. 2006; Nickerson et al. 2011). The accre-
tion time probability density distribution is shown in the
bottom panel of Figure 10. Unlike formation time, no
difference inconsistent with scatter is observed between
cosmologies. This indicates that subhalos are destroyed
as a function of when they were accreted in a process
that is independent of small variations in cosmology.
3.3.3. Maximal Mass
Finally, we study the averaged mass function per unit
z = 0 host halo mass for the maximal masses of eventual
z = 0 subhalos. As in Figure 4, this mass function is
scaled to a host of mass 0.84 × 1012 h−1M⊙. The total
0.0
0.1
0.2
0.3
0.4
0.5
P
f
o
r
m
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
02468101214
Lookback Time (Gya)
0.0
0.1
0.2
0.3
0.4
0.5
P
a
c
c
r
e
t
e
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
Fig. 10.— Top Panel : Average formation time probability den-
sity distribution of z = 0 subhalos within a host for cosmologies
with varied σ8. Formation time defined by when halos first reach
a mass of M200 = 3 × 108 h−1M⊙. Subhalos lasting to z = 0
typically formed earlier in higher σ8 cosmologies by .1 to .4 billion
years for a 0.1 increment in σ8. Bottom Panel : Average accretion
time probability density distribution of subhalos within a host for
cosmologies with varied σ8. No significant differences are found
between the cosmologies. Error bars show 1 σ uncertainty of each
data point.
stellar mass/luminosity is related to the maximum mass
obtained by the subhalo. Since the more concentrated
luminous matter is less easily stripped than dark matter,
the maximal mass is a much better indicator of stellar
mass than the z = 0 mass of a subhalo (Springel et al.
2001; Gao et al. 2004; Guo et al. 2013).
Figure 11 compares the Mmax mass function for two
different cosmologies, showing no dependence on σ8.
This suggests varying σ8 has no effect on the stellar
mass/luminosity functions.
3.4. Properties of Matched Subhalos
In order to ascertain the difference between specific
subhalos due to cosmology, host halos are matched to
each other by a procedure described in Section 2.4.
About 90% of hosts above 1010 h−1M⊙ are success-
fully matched. Subhalos within matched hosts are then
matched to each other, and their history is compared.
The fraction of subhalos from one host successfully
matched to subhalos from the other matched host are
shown in Fig. 12. The remaining unmatched subhalos
either are the result of an imperfect matching system,
or are ones that correspond to subhalos whose counter-
part in the other simulation have already merged with
the host, or have not yet been accreted into the host,
and are thus not identified as subhalos. The following
subsections investigate the same averaged distributions
as done in Section 3.3, except with only the subset of
subhalos that have been matched to each other at z = 0.
Additionally, all figures show only the data matched be-
tween σ8 = 0.8 and σ8 = 0.9 boxes for conciseness. All
of the same trends are prevalent in the σ8 = 0.7 and
12 Dooley et al.
10
-12
10
-11
10
-10
10
-9
10
-8
d
n
/
d
M
s
u
b
[
h
M
−
1
⊙
]
σ
8
= 0.9
σ
8
= 0.8
σ
8
= 0.7
10
9
10
10
10
11
M
max
[h
−1
M
⊙
]
0.0
0.5
1.0
1.5
2.0
2.5
d
n
/
d
M
s
u
b
R
a
t
i
o
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
Fig. 11.— Top Panel: Average mass function of Mmax for sub-
halos that survive until z = 0, normalized by z = 0 host halo mass,
and scaled to a host of 0.84 × 1012 h−1M⊙ for cosmologies with
different σ8. 1 σ halo-to-halo variation bars are shown. Bottom
Panel: Ratio of the subhalo Mmax function for each cosmology to
the fiducial WMAP7 cosmology. Data points for the non WMAP7
cosmologies are shifted slightly left and right of their true values
to help distinguish them. The subhalo Mmax function is found to
not vary with σ8. Large variation on a halo-to-halo basis is seen in
the 1 σ variation bars.
σ8 = 0.8 matches.
10
10
10
11
10
12
10
13
M
host
[h
−1
M
⊙
]
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
%
o
f
S
u
b
h
a
l
o
s
M
a
t
c
h
e
d
Fig. 12.— Fraction of subhalos in a matched pair of hosts between
simulations of different cosmology that are successfully matched to
each other as a function of their halo mass. Stronger dynamical
processes in the larger halos inhibit subhalo matches more than in
smaller host halos. 1σ error bars also shown.
3.4.1. Formation and Accretion Time
Figure 13 shows the probability density distributions
of matched subhalos for subhalo formation time and
accretion time for two different cosmologies. As ex-
pected, matched subhalos from the higher M∗ simula-
tion, σ8 = 0.9, form earlier on average as seen in the top
panel of the figure. The difference in the mean ages of
the matched subhalos between σ8 = 0.9 and σ8 = 0.8 is
0.2 billion years, slightly greater than the 0.1 billion year
difference found in the general sample.
In addition to forming earlier, the matched subhalos
in the higher σ8 simulation are accreted earlier. This
is seen in the bottom panel of Figure 13. Comparison
with Figure 10 shows that the matching process prefer-
entially selects subhalos in the σ8 = 0.9 simulation that
were accreted ∼ 2 billion years earlier than the typical
subhalo.
Thus, in a direct subhalo to subhalo comparison, vary-
ing cosmological parameters do have significant effects
on the life of a subhalo, even though there are small
or no differences found when averaging over all subha-
los. Reconciliation of Figure 13 and Figure 10 indicates
that the unmatched subhalos in the σ8 = 0.9 simulation
must be ones that were very recently accreted. Following
the trend of earlier formation and earlier accretion with
higher σ8, the counterparts of these unmatched subhalos
must be ones that have not yet accreted in the σ8 = 0.8
simulation. Similarly, the unmatched subhalos in the
σ8 = 0.8 simulation are ones that are close to the end of
their life. Their counterparts in the σ8 = 0.9 simulation
are ones that were already tidally destroyed. Therefore,
the majority of unmatched subhalos are not stragglers
from a faulty matching scheme, but rather the necessary
result of different cosmologies leading to a shift in the
formation, accretion, and thus destruction of subhalos.
Evidently, when studying the accretion times of all sub-
halos in a host, this accretion and destruction process
erases hints of the cosmology the subhalos formed in.
0.0
0.2
0.4
0.6
0.8
1.0
P
f
o
r
m
σ
8
= 0.9
σ
8
= 0.8
02468101214
Lookback Time (Gya)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
P
a
c
c
r
e
t
e
σ
8
= 0.9
σ
8
= 0.8
Fig. 13.— Top Panel : Average formation time probability
density distribution of subhalos matched between σ8 = 0.9 and
σ8 = 0.8 simulations. Matched subhalos in the higher σ8 simula-
tion form earlier on average, just as in the general case. Bottom
Panel : Average accretion time probability density distribution of
subhalos matched between σ8 = 0.9 and σ8 = 0.8 simulations. Un-
like the general case, matched subhalos in the higher σ8 simulation
are accreted significantly earlier. Error bars show 1 σ uncertainty
of each data point.
3.4.2. Maximal Mass and z = 0 Mass
Comparison of the mass function of the maximal mass
of matched subhalos, as seen in the top panel of Fig-
ure 14, shows a weak dependence on cosmology. Subha-
los in the σ8 = 0.9 simulation have their life cycle shifted
earlier, forming and accreting earlier. The earlier accre-
Varied Cosmologies & Halo Substructure 13
tion leads to less mass at its peak. Likewise subhalos
in the σ8 = 0.7 simulation peak at a higher mass than
in the σ8 = 0.8 simulation. Knowing that such subha-
los in the higher σ8 simulations accrete earlier, one can
expect them to reach z = 0 at an even smaller mass
due to more time spent being tidally stripped. This is
confirmed in the mass function displayed in the bottom
panel of Figure 14. Matched subhalos are on average 25%
less massive in the σ8 = 0.9 simulation than the σ8 = 0.8
simulation, and 50% more massive in the σ8 = 0.7 simu-
lation than the σ8 = 0.8 simulation.
M
max
[h
−1
M
⊙
]
0.6
0.8
1.0
1.2
1.4
1.6
d
n
/
d
M
s
u
b
R
a
t
i
o
σ
8
= 0.9 σ
8
= 0.8 σ
8
= 0.7
10
9
10
10
10
11
M
sub
[h
−1
M
⊙
]
0.5
1.0
1.5
2.0
2.5
d
n
/
d
M
s
u
b
R
a
t
i
o
σ
8
= 0.9 σ
8
= 0.8 σ
8
= 0.7
Fig. 14.— Top Panel : Ratio of the average mass function of
Mmax normalized by the z = 0 host halo mass for matched sub-
halos that survive until z = 0 to the Mmax function of the fiducial
WMAP7 simulation for cosmologies with varied σ8. Higher σ8 re-
sults in lower peak masses in each pair of cosmologies tested. Error
bars show 1 σ uncertainty of each data point. See Fig. 11 for the
true Mmax function. Bottom Panel : Ratio of the average SHMF
normalized to host halo mass for matched subhalos to the SHMF
of the fiducial WMAP7 simulation for cosmologies with varied σ8.
Unlike the general SHMF, the matched SHMF is significantly af-
fected by cosmology. Subhalos in higher σ8 simulations end up less
massive. Error bars show 1 σ uncertainty of each data point. See
Fig. 4 for the true SHMF.
3.4.3. Spatial Distribution
The spatial distribution of matched subhalos, charac-
terized by number density as a function of r/r200 as in
Figure 7, is shown in Figure 15. The local density is nor-
malized by <n>, the average number density of matched
subhalos within r200. Relative to the σ8 = 0.8 simulation,
the matched subhalos within the σ8 = 0.9 simulation are
closer to the center of their host: the number density is
higher below r/r200 = 0.5 and lower above r/r200 = 0.8.
The σ8 = 0.9 subhalos which accreted earlier on aver-
age have more time for dynamical friction effects to slow
down their orbits and thus drag them closer to the halo
center. The net average change of position is 3.5% closer.
This result helps further confirm the conclusions made
at the end of Section 3.4.1 that matched subhalos in the
σ8 = 0.9 simulation are closer to the end of their life and
may be destroyed earlier, leaving their counterparts in
the σ8 = 0.8 simulation matchless.
0.4 0.5 0.6 0.7 0.8 0.9 1.0
r/r
200
0.5
1.0
1.5
2.0
2.5
n
(
r
)
/
<
n
(
r
)
>
R
a
t
i
o
σ
8
= 0.9 σ
8
= 0.8
σ
8
= 0.7
Fig. 15.— The averaged subhalo number density for matched
subhalos as a function of r/rvir normalized to the mean matched
subhalo number density for cosmologies with varied σ8 divided
by the same function for the fiducial WMAP7 simulation. This
demonstrates the trend that matched subhalos in higher σ8 cos-
mologies are closer to the halo center on average. Subhalos are
3.5% closer to the halo center in the σ8 = 0.9 simulation than
in the σ8 = 0.8 simulation on average. Variation bars show 1 σ
halo-to-halo variation for each data point.
4. SUMMARY OF SUBHALO PROPERTY CHANGES
In this investigation, we found that the way in which
σ8, ns and Ωm affect subhalo properties can be more con-
cisely summarized in terms of changes in M∗, which is
positively correlated to all three parameters. The fol-
lowing list details how subhalo properties change with
higher M∗ cosmologies for all properties that vary with
cosmological parameters:
• Subhalo Vmax function as a function of host halo
mass is greater.
• Subhalo concentration is greater.
• Subhalo formation time is earlier on average and
on a matched subhalo to subhalo basis.
• Subhalo accretion time is earlier on a matched sub-
halo to subhalo basis.
• Subhalo peak mass is smaller on a matched subhalo
to subhalo basis.
• Subhalo mass at z = 0 is smaller on a matched
subhalo to subhalo basis.
• Subhalo radial distribution (number density as a
function of r/r200) is shifted toward the host’s cen-
ter on a matched subhalo to subhalo basis.
Individual subhalos in cosmologies with higher M∗ are
shown to have formed earlier, accreted into their eventual
host earlier, both in agreement with Zentner & Bullock
(2003); van den Bosch et al. (2005), and as a result of
spending more time within their host, lost more mass
due to tidal stripping and moved closer to the center of
the host due to dynamical friction. This leads to subhalos
in higher M∗ cosmologies having, on average, less mass
at z = 0. This is opposite of the trend for host halo
masses, where large hosts have more mass in higher M∗
cosmologies.
14 Dooley et al.
Including subhalo abundances, the averaged subhalo
properties that remain unchanged under variations in
cosmological parameters are:
• subhalo mass function as a function of host halo
mass,
• subhalo abundance above Vmax = 30 km s
−1 as a
function of host halo mass,
• subhalo mass fraction as a function of host halo
mass,
• radial distribution of subhalos (number density as
a function of r/r200),
• subhalo accretion time probability distribution
function, and
• distribution of maximal mass attained by subhalos
before they enter their host’s tidal field.
5. DISCUSSION AND CONCLUSION
Changes in the cosmological parameters σ8, ns, and
Ωm affect the abundance of dark matter halos in the
universe. Larger values of ns and Ωm lead to a larger
abundance of halos. Larger values of σ8 lead to a higher
abundance of halos above the current characteristic mass
scale of collapse, M∗, and a lower abundance below that
threshold. These differences, however, as tested by a
suite of cosmological dark matter only simulations, are
erased when considering local subhalo populations (all
subhalos within a given host halo) as a function of their
host halo mass. For any given host halo mass, the aver-
age subhalo mass function is independent of small vari-
ations in cosmological parameters on the order of the
changes in the best estimate values ofWMAP and Planck
(Planck Collaboration et al. 2013). Thus, the expected
abundance of dark matter subhalos by mass in the Milky
Way, for example, should not change with the recent re-
vision of parameters by Planck.
Subhalo concentrations, Vmax functions, and forma-
tion times, on the other hand, retain the same trends
in cosmological dependence as the host halos. In cos-
mologies where objects tend to collapse sooner (higher
values of σ8, ns, and Ωm), both host halos and subhalos
have systematically more concentrated cores for all mass
ranges considered and form earlier. More concentrated
cores lead to subhalos with higher Vmax relative to less
concentrated halos of the same mass. Other cosmology
induced differences arise on a subhalo to subhalo basis.
Since all simulations used the same initial random white
noise field, it was possible to identify halos and subhalos
which arose from the same density fluctuation. A com-
parison of these “matched” subhalos indicates that cos-
mological parameters do in fact have a significant effect
on individual halos, even ones that become subhalos. In
the higher σ8 simulation, matched subhalos formed ear-
lier, accreted earlier, are located closer to the host halo
center, are more concentrated, and are smaller in mass.
Clearly, there is a disparity of results between aver-
aging properties of all subhalos within a host and av-
eraging only the subset of matched subhalos. This in-
dicates that the mechanisms of subhalo accretion, mass
loss, and ultimately destruction lead to a cosmology in-
dependent steady state distribution of subhalos within
a host. While subhalos in the higher M∗ simulations
are accreted earlier on average, there is a steady rate of
subhalo accretion that is the same in all cosmologies at
low redshift for hosts of the same size. In addition, even
though the overall mass function of halos changes, the
mass function of subhalos at accretion for a given host
halo does not change. Since subhalo mass and angular
momentum loss is governed by cosmology independent
physics, it follows that the resulting size, position, num-
ber, and accretion time of subhalos will be unaffected
by cosmology. In contrast, the formation time and con-
centration, and thus Vmax, of subhalos at accretion do
vary with cosmology. These differences are not erased
subhalo-host interactions.
The results of this study indicate that in simulations
with pre-Planck cosmological parameters, characteriza-
tions of subhalo mass function, mass fraction, spatial dis-
tribution, accretion time, and peak mass at infall are all
still correct. This indicates that expensive high resolu-
tion simulations remain valid when used to study subha-
los as long as the properties are appropriately normalized
to the mass of the host. There are a few exceptions for
when subhalo concentration matters, such as in comput-
ing a dark matter annihilation rate estimate from sub-
structure, or when subhalo formation time (defined by
when halos first reach a threshold mass) is important, or
to a lesser extent when the subhalo Vmax function mat-
ters. In such cases a simple way to estimate how these
properties change is to compute M∗ for two cosmologies
and know that in higher M∗ cosmologies subhalos will be
more concentrated and have formed earlier. For exam-
ple, between WMAP7 and Planck, M∗ increased by an
amount similar to the effect of an increase of σ8 of 0.07.
The results also indicate that variations in cosmological
parameters should have negligible effect on the magni-
tude of the missing satellite problem, and some affect
on the too big to fail problem. Decreases in predicted
subhalo Vmax and concentration, which could come from
lower values of ns, σ8, and Ωm, both serve to alleviate
the too big to fail problem. Polisensky & Ricotti (2013)
confirms this for the cases of ns, σ8. Still, based on the
study of subhalo properties over many host halos, the
mass of the host halo and variations from halo to halo
are much more important in controlling the distribution
of subhalo properties within a galaxy than cosmological
parameters.
We thank Peter Behroozi for help with using rock-
star and Volker Springel for giving access to gadget3.
G.A.D. acknowledges support from an NSF Graduate
Research Fellowship under Grant No. 1122374.
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