NEUTRAL HYDROGEN OPTICAL DEPTH NEAR STAR-FORMING GALAXIES AT z ≈ 2.4 IN THE KECK BARYONIC STRUCTURE SURVEY

We study the interface between galaxies and the intergalactic medium by measuring the absorption by neutral hydrogen in the vicinity of star-forming galaxies at z approximate to 2.4. Our sample consists of 679 rest-frame UV-selected galaxies with spectroscopic redshifts that have impact parameters < 2 (proper) Mpc to the line of sight of one of the 15 bright, background QSOs and that fall within the redshift range of its Ly alpha forest. We present the first two-dimensional maps of the absorption around galaxies, plotting the median Ly alpha pixel optical depth as a function of transverse and line-of-sight separation from galaxies. The Ly alpha optical depths are measured using an automatic algorithm that takes advantage of all available Lyman series lines. The median optical depth, and hence the median density of atomic hydrogen, drops by more than an order of magnitude around 100 kpc, which is similar to the virial radius of the halos thought to host the galaxies. The median remains enhanced, at the >3 sigma level, out to at least 2.8 Mpc (i.e., >9 comoving Mpc), but the scatter at a given distance is large compared with the median excess optical depth, suggesting that the gas is clumpy. Within 100 (200) kpc, and over +/- 165 km s(-1),the covering fraction of gas with Ly alpha optical depth greater than unity is 100(-32)(+0)% (66% +/- 16%). Absorbers with tau(Ly alpha) > 0.1 are typically closer to galaxies than random. The mean galaxy overdensity around absorbers increases with the optical depth and also as the length scale over which the galaxy overdensity is evaluated is decreased. Absorbers with tau(Ly alpha) similar to 1 reside in regions where the galaxy number density is close to the cosmic mean on scales >= 0.25 Mpc. We clearly detect two types of redshift space anisotropies. On scales < 200 km s(-1), or < 1 Mpc, the absorption is stronger along the line of sight than in the transverse direction. This "finger of God" effect may be due to redshift errors, but is probably dominated by gas motions within or very close to the halos. On the other hand, on scales of 1.4-2.0 Mpc the absorption is compressed along the line of sight (with >3 sigma significance), an effect that we attribute to large-scale infall (i.e., the Kaiser effect).