The Galaxy Proximity Effect in the Lyman-alpha Forest

Abstract

Hydrodynamic cosmological simulations predict that the average opacity of the Ly-alpha forest should increase in the neighborhood of galaxies because galaxies form in dense environments. Recent observations (Adelberger et al. 2002) confirm this expectation at large scales, but they show a decrease of absorption at comoving separations Delta_r <~ 1 Mpc/h. We show that this discrepancy is statistically significant, especially for the innermost data point at Delta_r <= 0.5 Mpc/h, even though this data point rests on three galaxy-quasar pairs. Galaxy redshift errors of the expected magnitude are insufficient to resolve the conflict. Peculiar velocities allow gas at comoving distances >~ 1 Mpc/h to produce saturated absorption at the galaxy redshift, putting stringent requirements on any ``feedback'' solution. Local photoionization is insufficient, even if we allow for recurrent AGN activity that keeps the neutral hydrogen fraction below its equilibrium value. A simple ``wind'' model that eliminates all neutral hydrogen in spheres around the observed galaxies can marginally explain the data, but only if the winds extend to comoving radii ~1.5 Mpc/h.

4 pages, 1 figure; To appear in proceedings of the 13th Annual Astrophysics Conference in College Park, Maryland, The Emergence of Cosmic Structure, eds. S.Holt and C. Reynolds, (AIP)