DISSECTING THE GASEOUS HALOS OF z ~ 2 DAMPED Lyα SYSTEMS WITH CLOSE QUASAR PAIRS

Abstract

We use spectroscopy of close pairs of quasars to study diffuse gas in the circumgalactic medium (CGM) surrounding a sample of 40 damped Lyα systems (DLAs). The primary sightline in each quasar pair probes an intervening DLA in the redshift range 1.6 < z[subscript DLA] < 3.6, such that the second quasar sightline then probes Lyα, C ii, Si ii, and C iv absorption in the CGM transverse to the DLA to projected distances R[subscript ⊥] < 300 kpc. Analysis of the Lyα profiles in these CGM sightlines constrains the covering fraction (fC) of optically thick H i (having column density NH i > 10[superscript 17.2] cm[superscript -2]) to be [> over ~]30% within R[subscript ⊥] < 200 kpc. Strong Si ii λ1526 absorption with equivalent width W[subscript 1526] > 0.2 Å occurs with an incidence f[subscript C](W[subscript 1526] > 0.2 Å) = 20[+12 over -8]% within R[subscript ⊥] < 100 kpc, indicating that Si ii absorption associated with DLAs probes material within a physical distance R[subscript 3D] [< over ~] 30 kpc. However, we find that strong C iv λ1548 absorption is ubiquitous in these environments (f[subscript C](W[subscript 1548] > 0.2 Å) = 57[+12 over -13]% within R[subscript ⊥] < 100 kpc), and in addition exhibits a high degree of kinematic coherence on scales up to ~175 kpc. We infer that this high-ionization material arises predominantly in large, quiescent structures extending beyond the scale of the DLA host dark matter halos rather than in ongoing galactic winds. The Lyα equivalent width in the DLA-CGM is anticorrelated with R[subscript ⊥] at >98% confidence, suggesting that DLAs arise close to the centers of their host halos rather than on their outskirts. Finally, the average Lyα, and C ii and C iv equivalent widths measured as a function of R[subscript ⊥] are consistent with those measured around z ~ 2 Lyman break galaxies. Assuming that DLAs trace a galaxy population at lower masses and luminosities, this finding implies that the absorption strength of cool circumgalactic material has a weak dependence on dark matter halo mass at M[subscript h] [< over ~] 10[superscript 12] M[subscript ʘ].