Program: GS-2012A-Q-12

We have mined the SDSS and discovered a sample of rare close QSO pairs at $z=2-4$ with small transverse separations $30-300\,{\rm kpc}$ ($3-30\arcsec$). These unique sightlines probe the %allow us to study the physical state of the circumgalactic medium (CGM) around galaxies hosted by low-mass ($\lesssim10^{11}\msol$) and very high-mass ($\gtrsim10^{13}\msol$) dark matter halos for the first time. Low-mass halos are traced by intervening damped Ly$\alpha$ absorption (DLA) in one of the sightlines, such that the neighboring sightline probes the CGM of the DLA-galaxy. Similarly, with projected QSO pairs, absorption in the \emph{background} QSO spectra encodes information about the CGM of the \emph{foreground} QSOs, which are signposts for massive galaxies. We propose to obtain a statistical sample ($\sim50$) of spectra with Gemini/GMOS-S and MMT/Blue Channel to measure cool gas kinematics, %abundances, and cold gas masses, spatial distribution, and covering fraction to distances $< 300$ kpc. These observations will provide %a direct test of the cosmological `cold accretion' picture new constraints on the cool gas supply and the physical effects of feedback around both low-mass galaxies and massive QSO hosts. %{\bf[Some more space here]}