A Search for Optical Variability of Type 2 Quasars in SDSS Stripe 82

Hundreds of Type 2 quasars have been identified in Sloan Digital Sky Survey (SDSS) data, and there is substantial evidence that they are generally galaxies with highly obscured central engines, in accord with unified models for active galactic nuclei (AGNs). A straightforward expectation of unified models is that highly obscured Type 2 AGNs should show little or no optical variability on timescales of days to years. As a test of this prediction, we have carried out a search for variability in Type 2 quasars in SDSS Stripe 82 using difference-imaging photometry. Starting with the Type 2 AGN catalogs of Zakamska et al. (2003) and Reyes et al. (2008), we find evidence of significant g-band variability in 17 out of 173 objects for which light curves could be measured from the Stripe 82 data. To determine the nature of this variability, we obtained new Keck spectropolarimetry observations for seven of these variable AGNs. The Keck data show that these objects have low continuum polarizations (p<~1% in most cases) and all seven have broad H-alpha and/or MgII emission lines in their total (unpolarized) spectra, indicating that they should actually be classified as Type 1 AGNs. We conclude that the primary reason variability is found in the SDSS-selected Type 2 AGN samples is that these samples contain a small fraction of Type 1 AGNs as contaminants, and it is not necessary to invoke more exotic possible explanations such as a population of "naked" or unobscured Type 2 quasars. Aside from misclassified Type 1 objects, the Type 2 quasars do not generally show detectable optical variability over the duration of the Stripe 82 survey.Comment: 14 pages, 8 figures. Accepted for publication in A

X-ray Observations of Optically Selected Giant Elliptical-Dominated Galaxy Groups

We present a combined optical and X-ray analysis of three optically selected X-ray bright groups with giant elliptical galaxies in the center. These massive ellipticals were targeted for XMM-Newton X-ray observations based on their large velocity dispersions and their proximity to a nearby ROSAT X-ray source. Additionally, these targets are significantly brighter in the optical than their nearest neighbors. We show that one of these systems meets the standard criteria for a fossil group. While the other two systems have a prominent magnitude gap in the E/S0 ridgeline, they do not appear to have reached the fossil-like final stage of group evolution.Comment: 8 pages, 6 figures, Accepted for publication in Ap

Chandra Cluster Cosmology Project II: Samples and X-ray Data Reduction

We discuss the measurements of the galaxy cluster mass functions at z=~0.05 and z=~0.5 using high-quality Chandra observations of samples derived from the ROSAT PSPC All-Sky and 400deg^2 surveys. We provide a full reference for the data analysis procedures, present updated calibration of relations between the total cluster mass and its X-ray indicators (T_X, Mgas, and Y_X) based on a subsample of low-z relaxed clusters, and present a first measurement of the evolving L_X-Mtot relation (with Mtot estimated from Y_X) obtained from a well-defined statistically complete cluster sample and with appropriate corrections for the Malmquist bias applied. Finally, we present the derived cluster mass functions, estimate the systematic uncertainties in this measurement, and discuss the calculation of the likelihood function. We confidently measure the evolution in the cluster comoving number density at a fixed mass threshold, e.g., by a factor of 5.0 +- 1.2 at M_500=2.5e14 h^-1 Msun between z=0 and 0.5. This evolution reflects the growth of density perturbations and can be used for the cosmological constraints complementing those from the distance-redshift relation.Comment: ApJ in press (Feb 10, 2009 issue); replacement to match accepted version, includes revisions in response to referee's and community comment

Soft X-Ray Sources at the Centers of the Elliptical Galaxies NGC 4472 and NGC 4649

Analysis of recent Chandra observations of the elliptical galaxies NGC 4472 and NGC 4649 has revealed faint soft X-ray sources at their centers. The sources are located to within 1'' of the optical centers of the galaxies. They are most likely associated with the central supermassive black holes. Interest in these and several other similar objects stems from the unusually low luminosity of the supermassive black holes embedded in dense interstellar medium. Our Chandra sources have very soft spectra. They are detectable only below ~0.6 keV and have luminosities in the 0.2-2.5 keV energy band of ~ 6 * 10^{37} erg/s and ~1.7 * 10^{38} erg/s in NGC 4649 and NGC 4472, respectively.Comment: Shortened version of the paper published in Astronomy Letter

Cosmological constraints from evolution of cluster baryon mass function at z~0.5

We present a new method for deriving cosmological constraints based on the evolution of the baryon mass function of galaxy clusters, and implement it using 17 distant clusters from our 160deg2 ROSAT survey. The method uses the cluster baryon mass as a proxy for the total mass, thereby avoiding the large uncertainties of the M_tot-T or M_tot-L_X relations used in all previous studies. Instead, we rely on a well-founded assumption that the M_b/M_tot ratio is a universal quantity, which should result in a much smaller systematic uncertainty. 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 Omega_m-Lambda plane, Omega_m + 0.23Lambda = 0.41+-0.10 at 68% confidence, which intersects with constraints from the Cosmic Microwave Background and supernovae Ia near Omega_m=0.3 and Lambda=0.7.Comment: ApJ in press, 11 pages; new emulateapj.cls. Better treatment of the mass measurement scatter increased the final Omega,Lambda uncertainties by 20

The XMM Cluster Survey: The Stellar Mass Assembly of Fossil Galaxies

This paper presents both the result of a search for fossil systems (FSs) within the XMM Cluster Survey and the Sloan Digital Sky Survey and the results of a study of the stellar mass assembly and stellar populations of their fossil galaxies. In total, 17 groups and clusters are identified at z < 0.25 with large magnitude gaps between the first and fourth brightest galaxies. All the information necessary to classify these systems as fossils is provided. For both groups and clusters, the total and fractional luminosity of the brightest galaxy is positively correlated with the magnitude gap. The brightest galaxies in FSs (called fossil galaxies) have stellar populations and star formation histories which are similar to normal brightest cluster galaxies (BCGs). However, at fixed group/cluster mass, the stellar masses of the fossil galaxies are larger compared to normal BCGs, a fact that holds true over a wide range of group/cluster masses. Moreover, the fossil galaxies are found to contain a significant fraction of the total optical luminosity of the group/cluster within 0.5R200, as much as 85%, compared to the non-fossils, which can have as little as 10%. Our results suggest that FSs formed early and in the highest density regions of the universe and that fossil galaxies represent the end products of galaxy mergers in groups and clusters. The online FS catalog can be found at http://www.astro.ljmu.ac.uk/~xcs/Harrison2012/XCSFSCat.html.Comment: 30 pages, 50 figures. ApJ published version, online FS catalog added: http://www.astro.ljmu.ac.uk/~xcs/Harrison2012/XCSFSCat.htm

Evidence for new physics from clusters ?

The abundance of local clusters is a traditional way to derive the amplitude of matter fluctuations. In the present work, by assuming that the observed baryon content of clusters is representative of the universe, we show that the mass temperature relation (M-T) can be specified for any cosmological model. This approach allows one to remove most of the uncertainty coming from M-T relation, and to provide an estimation of sigma\_8 whose uncertainty is essentially statistical. The values we obtain are fortuitously almost independent of the matter density of the Universe (sigma\_8 ~ 0.6-0.63) with an accuracy better than 5%. Quite remarkably, the amplitude of matter fluctuations can be also tightly constrained to similar accuracy from existing CMB measurements alone. However, the amplitude inferred in this way in a concordance model (Lambda-CDM) is significantly larger than the value derived from the above method based on X-ray clusters. Such a discrepancy would almost disappear if the actual optical thickness of the Universe was 0 but could also be alleviated from more exotic solutions: the existence of a new dark component in the Universe as massive neutrinos. However, recent other indications of sigma\_8 favor a high normalization. In this case, the assumption that the baryonic content observed in clusters actually reflects the primordial value has to be relaxed : either there exists a large baryonic dark component in the Universe or baryons in clusters have undergone a large depletion during the formation of these structures. We concluded that the baryon fraction in clusters is not representative and therefore that an essential piece of the physics of baryons in clusters is missing in standard structure formation scenario.Comment: New version including changes and typos corrected, accepted for publication in A&

The mass assembly of galaxy groups and the evolution of the magnitude gap

We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated gas simulations and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z=1 with dark matter halo mass M(R200) > 1E13/h Msun, and trace their properties until the present time (z=0). We consider only the systems with X-ray luminosity L_X> 0.25E42/h^2 erg/s at z=0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90% of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0<z<1) cease to be fossils within 4 Gyr, mostly because other massive galaxies are assembled within their cores, even though most of the mass in their haloes might have been assembled at early times. We show that, compared to the conventional definition of fossil galaxy groups based on the magnitude gap Delta m(12)> 2 (in the R-band, within 0.5R200 of the centre of the group), an alternative criterion Delta m(14)>2.5 (within the same radius) finds 50% more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.Comment: 16 pages, 11 figures, 2 tables. Accepted for publication in MNRA