Evidence for a Hard Ionizing Spectrum from a
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Mainali-2017-Evidence for a Hard Ionizing Spec.pdf
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Author(s)
Mainali, Ramesh
Kollmeier, Juna A.
Stark, Daniel P.
Walth, Gregory
Newman, Andrew B.
Miller, Daniel Robert
Simcoe, Robert A.
Date Issued
February 2017
Journal
Astrophysical Journal. Letters
Publisher
IOP Publishing
Citation
Mainali, Ramesh; Kollmeier, Juna A.; Stark, Daniel P.; Simcoe, Robert A.; Walth, Gregory; Newman, Andrew B. and Miller, Daniel R. “Evidence for a Hard Ionizing Spectrum from a z = 6.11 Stellar Population.” The Astrophysical Journal 836, no. 1 (February 2017): L14 © 2017 The American Astronomical Society
Version
Final published version
Abstract
We present the Magellan/FIRE detection of highly ionized C iv λ1550 and O iii]λ1666 in a deep infrared spectrum of the z = 6.11 gravitationally lensed low-mass galaxy RXC J2248.7-4431-ID3, which has previously known Lyα. No corresponding emission is detected at the expected location of He ii λ1640. The upper limit on He ii, paired with detection of O iii] and C iv, constrains possible ionization scenarios. Production of C iv and O iii] requires ionizing photons of 2.5–3.5 Ryd, but once in that state their multiplet emission is powered by collisional excitation at lower energies (~0.5 Ryd). As a pure recombination line, He ii emission is powered by 4 Ryd ionizing photons. The data therefore require a spectrum with significant power at 3.5 Ryd but a rapid drop toward 4.0 Ryd. This hard spectrum with a steep drop is characteristic of low-metallicity stellar populations, and less consistent with soft AGN excitation, which features more 4 Ryd photons and hence higher He ii flux. The conclusions based on ratios of metal line detections to helium non-detection are strengthened if the gas metallicity is low. RXJ2248-ID3 adds to the growing handful of reionization-era galaxies with UV emission line ratios distinct from the general z=2-3 population in a way that suggests hard ionizing spectra that do not necessarily originate in AGNs.
MIT Department
MIT Kavli Institute for Astrophysics and Space Research
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DOI of Published Version
https://doi.org/10.3847/2041-8213/836/1/L14
