The transverse proximity effect in the z ~ 2 Lyman-alpha forest suggests QSO episodic lifetimes of ~1 Myr

We look for signs of the H~I transverse proximity effect in the spectra of 130 QSO pairs, most with transverse separations in the plane of the sky of 0.1 -- 3 Mpc at z ~ 2.2. We expected to see a decrease in Lyman-alpha forest HI absorption in the spectrum of background QSOs near the position of foreground QSOs. Instead we see no change in the absorption in front of the foreground QSOs, and we see evidence for a 50% increase in the absorption out to 6 Mpc behind the foreground QSOs. Further, we see no change in the H I absorption along the line-of-sight to the foreground QSOs, the normal line-of-sight proximity effect. We may account for the lack of change in the H I absorption if the effect of extra UV photons is canceled by higher gas density around QSOs. If so, the increase in absorption behind the QSOs then suggests that the higher gas density there is not canceled by the UV radiation from the QSOs. We can explain our observations if QSOs have had their current UV luminosities for less than approximately a million years, a time scale that has been suggested for accretion disk instabilities and gas depletion.

The Lyman-alpha forest at redshifts 0.1 -- 1.6: good agreement between a large hydrodynamic simulation and HST spectra

We give a comprehensive statistical description of the Lyman-alpha absorption from the intergalactic medium in a hydrodynamic simulation at redshifts 0.1-1.6, the range of redshifts covered by HST spectra of QSOs. We use the ENZO code to make a 76 comoving Mpc cube simulation using 75 kpc cells, for a Hubble constant of 71 km/s/Mpc. The best prior work, by citet{dave99},used an SPH simulation in a 15.6 Mpc box with an effective resolution of 245 kpc and slightly different cosmological parameters. At redshifts z=2 this simulation is different from data. citet{tytler07b} found that the simulated spectra at z=2 have too little power on large scales, Lyman-alpha lines are too wide, there is a lack high column density lines, and there is a lack of pixels with low flux. Here we present statistics at z<1.6, including the flux distribution, the mean flux, the effective opacity, and the power and correlation of the flux. We also give statistics of the lyman alpha lines including the line width distribution, the column density distribution, the number of lines per unit equivalent width and redshift, and the correlation between the line width and column density. We find that the mean amount of absorption in the simulated spectra changes smoothly with redshift with DA(z)=0.01(1+z)^{2.25}. Both the trend and absolute values are close to measurements of HST spectra by citet{kirkman07a}. The column density and line width distributions are also close to those measured from HST spectra by citet{janknecht06a}, except for the mode of the line width distribution which is smaller in the HST spectra. Although some differences that we saw at z=2 are too subtle to be seen in existing HST spectra, overall, the simulation gives an good description of HST spectra at 0.1<z<1.6.

Metal Absorption Systems in Spectra of Pairs of QSOs

We present the first large sample of absorption systems in paired QSOs consisting of 691 absorption systems in the spectra of 310 QSOs including 170 pairings. All these absorption systems have metal lines, usually C IV or Mg II. We see 17 cases of absorption in one line-of-sight within 200 km/s (1 Mpc) of absorption in the paired line-of-sight with the probability at least approx 50% at 100kpc, declining rapidly to 23% at 100 - 200 kpc. We detect clustering on 0.5Mpc scales and see a hint of the fingers of God redshift-space distortion. The distribution matches absorbers arising in galaxies at z=2 with a normal correlation function and systematic infall velocities but unusually low random pair-wise velocity differences. Absorption in gas flowing out from galaxies at a mean velocity of 250 km/s would produce vastly more elongation than we see. The UV absorption from fast winds that Adelberger et al. 2005 see in spectra of LBGs is not representative of the absorption that we see. Either the winds are confined to LBGs, or they can not extend to 40 kpc with large velocities, while continuing to make UV absorption we see, implying most metals were in place in the IGM long before z=2. Separately, when we examine the absorption seen when a sight line passes a second QSO, we see 19 absorbers within 400 km/s of the partner QSO. The probability of seeing absorption is approximately constant for impact parameters 0.1 - 1.5 Mpc. Perhaps we do not see a rapid rise in the probability at small impact parameters because the UV from QSOs destroys some absorbers near to the QSOs. The 3D distribution of 64 absorbers around 313 QSOs is to first order isotropic, with just a hint of the anisotropy expected if the QSO UV emission is beamed, or alternatively QSOs might emit UV isotropically but for a surprisingly short time of only 0.3Myr.