THE GUNN-PETERSON EFFECT FROM UNDERDENSE REGIONS IN A PHOTOIONIZED INTERGALACTIC MEDIUM

We use the Zel'dovich approximation and another analytical approximation to calculate the evolution under gravitational instability of the underdense regions of a photoionized intergalactic medium (IGM). We find that over most of the spectrum of a quasar, the optical depth to \lya scattering originates from gas in underdense regions, or voids. This causes the ratio of the median Gunn-Peterson (GP) absorption to the value for a uniform medium containing all the baryons in the universe to be very small, and to decrease as gravitational collapse proceeds. We apply our calculations to the observations of the intensity distribution in a $z=4.11$ quasar by Webb and coworkers. We show that if \lya clouds arise from gravitational collapse, their observations must be interpreted as the first detection of the (fluctuating) GP effect, with a median value $\tau_{GP}\simeq 0.06$ at $z=4$. For typical low-density ($\Omega\sim 0.4$) cosmological models, this is consistent with the predicted baryon density from primordial nucleosynthesis, and the intensity of the ionizing background derived from the proximity effect. From the numerical simulations of Cen \etal, such models also predict correctly the number of \lya absorption lines observed.Comment: compressed uuencoded postscript files, 28 pages including 5 figure

Soft X-ray Absorption by High-Redshift Intergalactic Helium

The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has been measured with HST in four quasars over the last few years, over the redshift range 2.4 < z < 3.2. These observations have indicated that the HeII reionization may not have been completed until z\simeq 2.8, and that large fluctuations in the intensity of the HeII-ionizing background were present before this epoch. The detailed history of HeII reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, because the IGM can be completely optically thick to Lya photons when only a small fraction of the helium remains as HeII. In addition, finding quasars in which the HeII Lya absorption can be observed becomes increasingly difficult at higher redshift, owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that HeII in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was HeII should recover from the HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic HeII absorption can be detected as an excess over the expected Galactic absorption from the 21cm HI column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift, if the measurement is done on a large sample of high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter

Gravitational Collapse of Small-Scale Structure as the Origin of the Lyman Alpha Forest

If gravitational clustering is a hierarchical process, the present large-scale structure of the galaxy distribution implies that structures on smaller scales must have formed at high redshift. We simulate the formation of small-scale structure (average cell mass: $\Delta \bar m_b=10^{4.2}$M$_\odot$) and the evolution of photoionized gas, in the specific case of a CDM model with a cosmological constant. The photoionized gas has a natural minimal scale of collapse, the Jeans scale ($m_{b,J}\simeq 10^{9}$M$_\odot$). We find that low column density (\nhi \le 10^{14}\cm^{-2}) lines originate in regions resembling Zel'dovich pancakes, where gas with overdensities in the range $3 - 30$ is enclosed by two shocks but is typically re-expanding at approximately the Hubble velocity. However, higher column density (\nhi \ge 10^{15}\cm^{-2}) lines stem from more overdense regions where the shocked gas is cooling. We show that this model can probably account for the observed number of lines, their distribution in column density and b-parameters, as well as the cloud physical sizes as observed in gravitationally lensed quasars. We find a redshift evolution that isComment: 15p postscript file to appear in The Astrophysical Journal Letters (1994