We analyze the association of galaxies to Lya and OVI absorption, the most commonly detected transitions in the low-z intergalactic medium (IGM), in the fields of 14 quasars with z_em = 0.06-0.57. Confirming previous studies, we observe a high coveri
ng fraction for Lya absorption to impact parameter rho = 300kpc: 33/37 of our L>0.01L* galaxies show Lya equivalent width W_Lya>50mA. Galaxies of all luminosity L>0.01L* and spectral type are surrounded by a diffuse and ionized circumgalactic medium (CGM), whose baryonic mass is estimated at ~10^(10.5 +/- 0.3) Msun for a constant N_H. The virialized halos and extended CGM of present-day galaxies are responsible for most strong Lya absorbers (W_Lya > 300mA) but cannot reproduce the majority of observed lines in the Lya forest. We conclude that the majority of Lya absorption with W_Lya=30-300mA occurs in the cosmic web predicted by cosmological simulations and estimate a characteristic width for these filaments of ~400kpc. Regarding OVI, we observe a near unity covering fraction to rho=200kpc for L>0.1L* galaxies and to rho = 300kpc for sub-L* (0.1 L*<L<L*) galaxies. Similar to our Lya results, stronger OVI systems (W_OVI > 70mA) arise in the virialized halos of L>0.1L* galaxies. Unlike Lya, the weaker OVI systems (W_OVI~30mA) arise in the extended CGM of sub-L* galaxies. The majority of OVI gas observed in the low-z IGM is associated with a diffuse medium surrounding individual galaxies with L~0.3L*, and rarely originates in the so-called warm-hot IGM (WHIM) predicted by cosmological simulations.
We publish the survey for galaxies in 20 fields containing ultraviolet bright quasars (with z_em 0.1 to 0.5) that can be used to study the association between galaxies and absorption systems from the low-z intergalactic medium (IGM). The survey is ma
gnitude limited (R~19.5 mag) and highly complete out to 10 from the quasar in each field. It was designed to detect dwarf galaxies (L ~ 0.1 L*) at an impact parameter rho 1Mpc (z=0.1) from a quasar. The complete sample (all 20 fields) includes R-band photometry for 84718 sources and confirmed redshifts for 2800 sources. This includes 1198 galaxies with 0.005 < z < (z_em - 0.01) at a median redshift of 0.18, which may associated with IGM absorption lines. All of the imaging was acquired with cameras on the Swope 40 telescope and the spectra were obtained via slitmask observations using the WFCCD spectrograph on the Dupont 100 telescope at Las Campanas Observatory (LCO). This paper describes the data reduction, imaging analysis, photometry, and spectral analysis of the survey. We tabulate the principal measurements for all sources in each field and provide the spectroscopic dataset online.
We present results from an imaging and spectroscopic study of four strong MgII absorbers of W(2796) >~ 1 Ang revealed by the afterglow of GRB060418 at z_GRB=1.491. These absorbers, at z=0.603,0.656,1.107 and z_GRB, exhibit large ion abundances that s
uggest neutral gas column densities characteristic of damped Lya systems. The imaging data include optical images obtained using LRIS on the Keck I telescope and using ACS on board HST, and near-infrared H-band images obtained using PANIC on the Magellan Baade Telescope and K-band images obtained using NIRC2 with LGSAO on the Keck II telescope. These images reveal six distinct objects at <~ 3.5 of the afterglows position, two of which exhibit well-resolved mature disk morphology, one shows red colors, and three are blue compact sources. Follow-up spectroscopic observations using LRIS confirm that one of the disk galaxies coincides with the MgII absorber at z=0.656. The observed broad-band spectral energy distributions of the second disk galaxy and the red source indicate that they are associated with the absorbers at z=0.603 and z=1.107, respectively. These results show that strong MgII absorbers identified in GRB afterglow spectra are associated with typical galaxies of luminosity ~ (0.1-1) L* at impact parameter <~ 10 h^-1 kpc. The close angular separation would preclude easy detections toward a bright quasar. Finally, we associate the remaining three blue compact sources with the GRB host galaxy, noting that they are likely star-forming knots located at projected distances 2-12 h^-1 kpc from the afterglow. At the afterglows position, we derive a 2-sigma upper limit to the underlying SFR intensity of 0.0074 M_sun yr^-1 kpc^-2.
