Isabella M. Gioia

The Einstein Observatory Extended Medium-Sensitivity Survey. II - The optical identifications

The optical identifications are presented of the Einstein Extended Medium-Sensitivity Survey (EMSS), including the methodology used to optically identify the EMSS sources and the uncertainties involved with that process. The optical properties of the classes of X-ray, optical, and radio data for each of the identified and, as yet, unidentified sources of the survey are described. A new class of X-ray emitters, cooling flow galaxies, is proposed. The criteria used to determine whether the proposed optical counterpart to the X-ray source is a plausible identification are described. Plausibility is based on the optical classification of the counterpart, e.g., AGN, cluster, G star, and the X-ray-to-optical flux ratios previously observed for these classes of X-ray emitters. Two independent schemes of optical classification of the counterparts are used to check the plausibility of these identifications; one is based on moderate-resolution optical spectroscopy, and the other, on inferred X-ray luminosity and the overall energy distribution. 110 refs.

The Einstein Observatory extended medium-sensitivity survey. I: X-ray data and analysis

This paper presents the results of the analysis of the X-ray data and the optical identification for the Einstein Observatory Extended Medium-Sensitivity Survey (EMSS). The survey consists of 835 serendipitous sources detected at or above 4 times the rms level in 1435 imaging proportional counter fields with centers located away from the Galactic plane. Their limiting sensitivities are about (5-300) x 10 to the -14th ergs/sq cm sec in the 0.3-3.5-keV energy band. A total area of 778 square deg of the high-Galactic-latitude sky has been covered. The data have been analyzed using the REV1 processing system, which takes into account the nonuniformities of the detector. The resulting EMSS catalog of X-ray sources is a flux-limited and homogeneous sample of astronomical objects that can be used for statistical studies. 82 refs.

The X-ray spectra of the extragalactic sources in the Einstein extended medium sensitivity survey

The X-ray energy distribution of a large sample of faint extragalactic X-ray sources is studied. Two independent methods are used to determine that the average spectral index for the sources is about 0.95. The intrinsic dispersion of the spectral index distribution is about 0.36. The sources are characterized by a range of power law slopes in the soft X-ray band with a mean of 1.03 + 0.05 or - 0.06 and an intrinsic dispersion of about 0.36. The present analysis extends knowledge of the AGN spectral properties to sources which are about three times more distant and more than 10 times fainter than those previously studied. There is no evidence in the data for any change in the mean spectral index of either extragalactic sources as a whole or AGN in particular, as a function of the X-ray flux in the flux range 10 to the -11th to 10 to the -13th ergs/sq cm/s. 22 references.

The Extended Medium Sensitivity Survey distant cluster sample : X-ray cosmological evolution

The X-ray luminosity function of clusters of galaxies is determined at different cosmic epochs using data from the Einstein Observatory Extended Medium Sensitivity Survey. The sample consists of 67 X-ray-selected clusters that have been grouped in three redshift shells. Evolution is detected in the X-ray properties of clusters. The present volume density of high-luminosity clusters is found to be greater than it was in the past. Given the still limited data set, this result should be regarded as preliminary. It can be interpreted as the consequence of either luminosity evolution or modest density evolution.

The 160 Square Degree ROSAT Survey: The revised catalog of 201 clusters with spectroscopic redshifts

We present the revised catalog of galaxy clusters detected as extended X-ray sources in the 160 Square Degree ROSAT Survey, including spectroscopic redshifts and X-ray luminosities for 200 of the 201 members. The median redshift is zmedian = 0.25, and the median X-ray luminosity is LX,median = 4.2 × 1043 h ergs s-1 (0.5-2.0 keV). This is the largest high-redshift sample of X-ray-selected clusters published to date. There are 73 objects at z > 0.3 and 22 objects at z > 0.5 drawn from a statistically complete flux-limited survey with a median object flux of 1.4 × 10-13 ergs cm-2 s-1. We describe the optical follow-up of these clusters with an emphasis on our spectroscopy, which has yielded 155 cluster redshifts, 110 of which are presented here for the first time. These measurements, combined with 45 from the literature and other sources, provide near-complete spectroscopic coverage for our survey. We discuss the final optical identifications for the extended X-ray sources in the survey region and compare our results to similar X-ray cluster searches.

Weak Lensing by High-Redshift Clusters of Galaxies. I. Cluster Mass Reconstruction

We present the results of a weak lensing survey of six high-redshift (z > 0.5), X-ray-selected clusters of galaxies. We have obtained ultra-deep R-band images of each cluster with the Keck Telescope and have measured a weak lensing signal from each cluster. From the background galaxy ellipticities we create two-dimensional maps of the surface mass density of each cluster. We find that the substructure seen in the mass reconstructions typically agree well with substructure in both the cluster galaxy distributions and X-ray images of the clusters. We also measure the one-dimensional radial profiles of the lensing signals and fit these with both isothermal spheres and universal CDM profiles. We find that the more massive clusters are less compact and not as well fitted by isothermal spheres as the less massive clusters, possibly indicating that they are still in the process of collapse.

A search for gravitational lensing in 38 X-ray selected clusters of galaxies

We present the results of a CCD imaging survey for gravitational lensing in a sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the most X-ray luminous (Lx 21 0 44 erg s 1 )c lus- ters selected from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are observable from Mauna Kea ( > 40 o ). The sample spans a red- shift range of 0:15 z 0:823 and includes 5 clusters with z> 0.5. CCD images of the clusters were obtained in excellent seeing. There is evidence of strong gravita- tional lensing in the form of giant arcs (length l 8 00 , axis ratio l=w 10) in 8 of the 38 clusters. Two addi- tional clusters contain shorter arclets, and 6 more clusters contain candidate arcs that require follow-up observations to conrm their lensing origin. Since the survey does not have a uniform surface brightness limit we do not draw any conclusion based on the statistics of the arcs found. We note, however, that 60% (3 of 5) of the clusters with Lx > 10 45 erg s 1 , and none of the 15 clusters with Lx 10), and are aligned or- thogonal to the optical major axes of the clusters, indicate the cluster core mass density proles must be compact (steeper than isothermal). In several cases, however, there is also some evidence, in the form of possible radial arcs, for density proles with nite core radii.

Weak Lensing by Two z ~ 0.8 Clusters of Galaxies

We report the detection of weak gravitational lensing of distant background galaxies by two rich, X-ray-luminous clusters of galaxies, MS 1137+66 at z=0.783 and RXJ 1716+67 at z = 0.813. We detect a shear signal in each cluster over the radial range 100≤r ≤ 700 h−1 kpc. Assuming that the background galaxies lie in a sheet at z = 2, we measure a mass of 2.45 × 1014 h−1 M☉ and M/LV = 270 at 500 h−1 kpc radius for MS 1137+66 and a mass of 2.6 × 1014 h−1 M☉ and M/LV = 190 for RXJ 1716+67 at the same radius. Both the optical light maps and the weak lensing mass maps of RXJ 1716+67 show two spatially distinct subclusters, as well as a long filamentary structure extending out of the cluster to the northeast. In contrast, MS 1137+66 is an ultracompact massive cluster in both mass maps and optical light maps and contains a strongly lensed arc system in the cluster core. These data add to the growing number of massive clusters at z 0.8.

Cosmological Constraints from the Evolution of the Cluster Baryon Mass Function

We determine the evolution of the number density of the galaxy clusters between z=0 and z=0.5. Our method uses the cluster baryon mass as a proxy for the total mass, thereby avoiding any uncertainties of the M tot−T or M tot−L X relations. Instead, we rely on a well-founded assumption that the M b /M tot ratio is a universal quantity. Taking advantage of direct and accurate Chandra measurements of the gas masses for distant clusters, we find strong evolution of the baryon mass function between z>0.4 and the present. The observed evolution defines a narrow band in the Ωm−Λ plane which intersects with constraints from the Cosmic Microwave Background and supernovae Ia near Ωm=0.3 and Λ=0.7. Assuming the flat Universe, cluster evolution favors the values of the dark energy equation of state parameter, w≈−1.

Constraints on cold dark matter theories from observations of massive x-ray-luminous clusters of galaxies at high redshift

During the course of a gravitational lensing survey of distant, X-ray selected Einstein Observatory Extended Medium Sensitivity Survey (EMSS) clusters of galaxies, we have studied six X-ray-luminous (L(sub x) greater than 5 x 10(exp 44)(h(sub 50)(exp -2))ergs/sec) clusters at redshifts exceeding z = 0.5. All of these clusters are apparently massive. In addition to their high X-ray luminosity, two of the clusters at z approximately 0.6 exhibit gravitationally lensed arcs. Furthermore, the highest redshift cluster in our sample, MS 1054-0321 at z = 0.826, is both extremely X-ray luminous (L(sub 0.3-3.5keV)=9.3 x 10(exp 44)(h(sub 50)(exp -2))ergs/sec) and exceedingly rich with an optical richness comparable to an Abell Richness Class 4 cluster. In this Letter, we discuss the cosmological implications of the very existence of these clusters for hierarchical structure formation theories such as standard Omega = 1 CDM (cold dark matter), hybrid Omega = 1 C + HDM (hot dark matter), and flat, low-density Lambda + CDM models.