On the Metallicities and Kinematics of the Circumgalactic Media of Damped Ly$\alpha$ Systems at $z \sim 2.5$

Abstract

We use medium- and high-resolution spectroscopy of close pairs of quasars to analyze the circumgalactic medium (CGM) surrounding 32 damped Ly$\alpha$ absorption systems (DLAs). The primary quasar sightline in each pair probes an intervening DLA in the redshift range $1.6<z_\text{abs}<3.5$, such that the secondary sightline probes absorption from Ly$\alpha$ and a large suite of metal-line transitions (including $~\rm OI$, $~\rm CII$, $~\rm CIV$, $~\rm SiII$, and $~\rm SiIV$) in the DLA host galaxy's CGM at transverse distances $24\ \text{kpc}\le R_\bot\le284~\rm kpc$. Analysis of Ly$\alpha$ in the CGM sightlines shows an anti-correlation between $R_\bot$ and $~\rm HI$ column density ($N_\text{HI}$) with 99.8$\%$ confidence, similar to that observed around luminous galaxies. The incidences of $~\rm CII$ and $~\rm SiII$ with $N>10^{13}~\rm cm^{-2}$ within 100 kpc of DLAs are larger by $2\sigma$ than those measured in the CGM of Lyman break galaxies (C$_f(N_\text{CII})>0.89$ and C$_f(N_\text{SiII})=0.75_{-0.17}^{+0.12}$). Metallicity constraints derived from ionic ratios for nine CGM systems with negligible ionization corrections and $N_\text{HI}>10^{18.5}~\rm cm^{-2}$ show a significant degree of scatter (with metallicities/limits across the range $-2.06\lesssim\log Z/Z_{\odot}\lesssim-0.75$), suggesting inhomogeneity in the metal distribution in these environments. Velocity widths of $\text{CIV}\lambda1548$ and low-ionization metal species in the DLA vs. CGM sightlines are strongly ($>2\sigma$) correlated, suggesting they trace the potential well of the host halo over $R_\bot\lesssim300$ kpc scales. At the same time, velocity centroids for $\text{CIV}\lambda1548$ differ in DLA vs. CGM sightlines by $>100~\rm km\ s^{-1}$ for $\sim50\%$ of velocity components, but few components have velocities that would exceed the escape velocity assuming dark matter host halos of $\ge10^{12}M_\odot$.