The detection of large amounts of cool, x ray absorbing gas in distant clusters of galaxies. What does this mean?

We present an x-ray spectral study of 12 distant (z = 0.17-0.54) rich clusters of galaxies observed with the Einstein Observatory Imaging Proportional Counter. These x-ray spectral data show evidence for substantial excess absorptions beyond those expected in the galaxy, indicating the presence of large amounts of x-ray absorbing cool gas in these distant clusters. The mean value of the excess absorptions corresponds to an absorbing gas column density approximately greater than 10(exp 21)/sq cm. We calculate the x-ray luminosities of the clusters with observed fluxes only in the 0.8-3.5 keV band where the fluxes are less effected by the absorptions, and use the temperature-to-luminosity correlation (known only for nearby clusters) to estimate the temperatures of the hot intracluster medium (ICM) in the distant clusters. These temperature estimates, together with the spectral fits, provide further constraints on the column densities in the individual clusters. For the cluster CL 0016+16, the lower limit on the column density is found to be 8 x 10(exp 20)/sq cm at the 99 percent confidence limit. We also show that the ratio of the temperature obtained from the spectral fit to the temperature expected from the correlation tends to decrease with increasing look-back time, indicating possible temperature evolution of the hot ICM in the recent past. The inclusion of this evolutionary effect further increases the absorptions required in fitting the spectra

BL Lac evolution revisited

BL Lac objects are an elusive and rare class of active galactic nuclei. For years their evolutionary behavior has appeared inconsistent with the trend observed in the population of AGN at large. The so-called ``negative'' evolution implies that BL Lacs were either less or fainter in the past. This effect is stronger for BL Lacs selected in X-ray surveys. We have investigated if one of the selection criteria, namely the flat-radio spectrum (imposed on the Radio-selected but not on the X-ray-selected samples), might explain the different evolutionary trend.Comment: Proceedings of "Multiwavelength AGN Surveys", Cozumel, Dec 200

Radio spectra of a sample of X-ray selected BL Lacs

We present simultaneous multifrequency radio observations for a complete subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey, obtained with the Very Large Array (VLA). Spectra are computed using fluxes at 20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose any criterion on the radio spectrum to identify BL Lac objects. It is therefore possible to investigate the intrinsic radio spectral slope distribution and to determine the effect produced by this selection criterion. We find that 15% of the observed objects do not meet the flat-spectrum criterion imposed on some other BL Lac samples. A dataset that includes non-simultaneous data (that are also taken with different VLA configurations) shows an even higher percentage of steep spectrum sources. This effect can be ascribed to a larger fraction of extended flux detected with the more compact VLA configuration.Possible biases introduced by the flat--radio-spectrum criterion in the radio-selected BL Lac samples cannot explain the discrepancies observed in the evolutionary properties of Radio and X-ray selected samples of BL Lacs.Comment: 9 pages, 5 figures, to be published in Astronomy and Astrophysic

The Low-Redshift Intergalactic Medium

The low-redshift Ly-alpha forest of absorption lines provides a probe of large-scale baryonic structures in the intergalactic medium, some of which may be remnants of physical conditions set up during the epoch of galaxy formation. We discuss our recent Hubble Space Telescope (HST) observations and interpretation of low-z Ly-alpha clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to Omega-baryon. Our HST/GHRS detections of 70 Ly-alpha absorbers with N_HI > 10^12.6 cm-2 along 11 sightlines covering pathlength Delta(cz) = 114,000 km/s show f(>N_HI) ~ N_HI^{-0.63 +- 0.04} and a line frequency dN/dz = 200 +- 40 for N_HI > 10^12.6 cm-2 (one every 1500 km/s of redshift). A group of strong absorbers toward PKS 2155-304 may be associated with gas (400-800) h_75^-1 kpc from 4 large galaxies, with low metallicity (< 0.003 solar) and D/H < 2 x 10^-4. At low-z, we derive a metagalactic ionizing radiation field from AGN of J_0 = 1.3^{+0.8 -0.5} x 10^-23 ergs/cm2/s/Hz/sr and a Ly-alpha-forest baryon density Omega-baryon = (0.008 +- 0.004) h_75^-1 [J_-23 N_14 b_100]^{1/2} For clouds of characteristic size b = (100 kpc)b_100.Comment: 5 figure