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Register a new userThe transverse proximity effect in the z ~ 2 Lyman-alpha forest suggests QSO episodic lifetimes of ~1 Myr
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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.
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The quasar in the Hubble Deep Field South (HDFS), J2233-606 (z=2.23) has been exhaustively observed by ground based telescopes and by the STIS spectrograph on board the Hubble Space Telescope at low, medium and high resolution in the spectral interval from 1120 A to 10000 A. This very large base-line represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDFS campaign. Here we report the main properties of the Lyman-alpha clouds in the intermediate redshift range 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63pm8 lines with log N_{HI}geq14.0 are found (including metal systems) at <z>=1.7, to be compared with ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyman-alpha clouds shows a region spanning z=1.383-1.460 (comoving size of 94 h^{-1}_{65} Mpc, Omega_o=1) with a low density of absorption lines; we detect 5 lines in this region, compared with the 16 expected from an average density along the line of sight. The two point correlation function shows a positive signal up to scales of about 3 h^{-1}_{65} Mpc and an amplitude that is larger for larger HI column densities. The average Doppler parameter is about 27 km/s, comparable to the mean value found at z > 3, thus casting doubts on the temperature evolution of the Lyman-alpha clouds.
We present the highest quality Lyman Alpha forest spectra published to date, from the QSO HS 1946+7658. The distribution of H I column densities is a power law of slope -1.5 from Log N = 12.1 - 14. This power law can extend to N = 0, because lines weaker than Log N = 12.1 do not have a large H I optical depth. Low column lines with Log N > 9 could account for all observed He II absorption, but lines with Log N > 12 alone are unlikely to do so. The b distribution between 20 and 60 km/sec is a Gaussian with a mean of 23 km/sec (less than reported in past at this z), and a sigma b of 14 km/sec. We report no evolution in the Lyman alpha forest (except the number of lines), because Lu et al. (1997) found the same N and b distributions at <z> = 3.7. We see lines with 14 < b < 20 km/sec and b > 80 km/sec that cannot be accounted for by noise or blending effects. We discover that the lower cutoff in the b distribution varies with N, from b = 14 km/sec at Log N = 12.5 to b = 22 km/sec at Log N = 14.0, but otherwise b and N are not correlated. We see no Lyman Alpha line clustering above 50 kms, in disagreement with previous results from lower signal to noise data, but we do see a 3 sigma clustering signal at 25 - 50 km/sec among lines with Log N > 13.6
We present the analysis of a sample of the Ly-$alpha$ forest spectra of 152 quasars taken with the HST FOS. The Ly-$alpha$ lines show little evolution at $0<z<1.7$. We see a difference between the evolution indices for weak and strong lines.
We use hydrodynamic simulations to predict correlations between Lya forest absorption and galaxies at redshift z~3. The probability distribution function (PDF) of Lya flux decrements shifts systematically towards higher values in the vicinity of galaxies, reflecting the overdense environments in which these galaxies reside. The predicted signal remains strong in spectra smoothed over 50-200 km/s, allowing tests with moderate resolution quasar spectra. The strong bias of high redshift galaxies towards high density regions imprints a clear signature on the flux PDF, but the predictions are not sensitive to galaxy baryon mass or star formation rate, and they are similar for galaxies and for dark matter halos. The dependence of the flux PDF on galaxy proximity is sensitive to redshift determination errors, with rms errors of 150-300 km/s substantially weakening the predicted trends. On larger scales, the mean galaxy overdensity in a cube of 5 or 10 Mpc/h (comoving) is strongly correlated with the mean Lya flux decrement on a line of sight through the cube center. The slope of the correlation is ~3 times steeper for galaxies than for dark matter as a result of galaxy bias. The predicted large scale correlation is in qualitative agreement with recently reported observational results. However, observations also show a drop in absorption in the immediate vicinity of galaxies, which our models do not predict even if we allow the galaxies or AGNs within them to be ionizing sources. This decreased absorption could be a signature of galaxy feedback on the surrounding IGM, perhaps via galactic winds. Peculiar velocities often allow gas at comoving distances ~1.5 Mpc/h to produce saturated absorption at the galaxy redshift, so any feedback mechanism must suppress neutral hydrogen out to these radii to match the data. (Abridged)
The angular positions of quasars are deflected by the gravitational lensing effect of foreground matter. The Lyman-alpha forest seen in the spectra of these quasars is therefore also lensed. We propose that the signature of weak gravitational lensing of the forest could be measured using similar techniques that have been applied to the lensed Cosmic Microwave Background, and which have also been proposed for application to spectral data from 21cm radio telescopes. As with 21cm data, the forest has the advantage of spectral information, potentially yielding many lensed slices at different redshifts. We perform an illustrative idealized test, generating a high resolution angular grid of quasars (of order arcminute separation), and lensing the Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field. We find that standard quadratic estimators can be used to reconstruct images of the foreground mass distribution at z~1. There currently exists a wealth of Lya forest data from quasar and galaxy spectral surveys, with smaller sightline separations expected in the future. Lyman-alpha forest lensing is sensitive to the foreground mass distribution at redshifts intermediate between CMB lensing and galaxy shear, and avoids the difficulties of shape measurement associated with the latter. With further refinement and application of mass reconstruction techniques, weak gravitational lensing of the high redshift Lya forest may become a useful new cosmological probe.
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