Dissecting the Gaseous Halos of z~2 Damped Ly$\alpha$ Systems with Close Quasar Pairs

We use spectroscopy of close pairs of quasars to study diffuse gas in the circumgalactic medium (CGM) surrounding a sample of 40 Damped Lya systems (DLAs). The primary sightline in each quasar pair probes an intervening DLA in the redshift range 1.6 10^17.2 cm^-2) to be greater than ~30% within $R_{\perp} 0.2 Ang occurs with an incidence f_C (W_1526 > 0.2 Ang) = 20(+12/-8)% within $R_{\perp} 0.2 Ang) = 57(+12/-13)% within $R_{\perp} 98% confidence, suggesting that DLAs arise close to the centers of their host halos rather than on their outskirts. Finally, the average Lya, CII and CIV equivalent widths are consistent with those measured around z~2 Lyman Break Galaxies. Assuming that DLAs trace a galaxy population with lower masses and luminosities, this finding implies that the absorption strength of cool circumgalactic material has a weak dependence on dark matter halo mass for M_h < 10^12 M_sun.