Evolution of Lyman-Alpha Clouds at Low Redshifts

Abstract

We examine the evolution of an intergalactic gas cloud confined by the gravity of cold dark matter in the minihalo model. Assuming that the evolution of the diffuse ultraviolet (UV) flux follows a power law function of the form (1 + Z)^alphai^ with a change in exponent at redshift z_c_ and a constant comoving number density of gas clouds, we can reproduce the evolution of the number of clouds per unit redshift at both high and low redshifts with α_L_ = 2 for z <= z_c_ and α_H_ = -4 (or -3) for z > z_c_ with z_c_ = 1.9 (or 1.8). Such a small value of α-implies much more UV flux at low redshifts than is expected from the luminosity function of observed quasars. Moreover, we find that the required intensity of this UV flux is consistent with observations of the Gunn-Peterson effect and of the continuum depression.