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Register a new userThe WARPS survey for faint Clusters of Galaxies
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The WARPS cluster survey is based on the ROSAT PSPC archive of pointed observations. It includes extended X-ray sources, detected with the Voronoi Tessellation and Percolation algorithm (VTP), and point-like X-ray sources with non-stellar optical counterparts. It is designed to minimize selection effects whilst covering a large area of sky. The aims of the survey are to (a) measure the low luminosity (<10^44 ergs^-1), high redshift (z>0.2) X-ray luminosity function of clusters and groups, and (b) investigate cluster morphologies and unusual systems (e.g. merging clusters). In an initial 13 sq deg (66 fields) we have found 22 extended X-ray sources with detected flux >7x10^-14 erg cm^-2s^-1 (0.5-2.0keV) and sizes of 1 to 5 arcmin. Optically they range from a single bright nearby galaxy which has been resolved, an Abell cluster which is revealed to have two (probably merging) components and groups and clusters of galaxies at estimated redshifts beyond z=0.4.
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We have embarked on a survey of ROSAT PSPC archival data with the aim of detecting all significant surface brightness enhancements due to sources in the innermost 15 arcmin of the PSPC field of view (0.5-2.0 keV). This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. The approach we have chosen for source detection [Voronoi Tessellation and Percolation (VTP)] represents a significant advance over conventional methods and is particularly suited for the detection and accurate quantification of extended and/or low surface brightness emission. In an extensive optical follow-up programme we are identifying galaxies, groups and clusters at redshifts ranging from z~0.1 to z~0.7. We present first results for an initial 17.2 deg^2 at detected fluxes > 3.5 x 10^-14 erg s^-1 cm^-2. We find the sky density of extended objects to be in the range 2.8 to 4.0 (+- 0.4) deg^-2. A comparison with a point source detection algorithm demonstrates that our VTP approach typically finds 1-2 more objects deg^-2 to this detected flux limit. The surface brightness limit of the WARPS cluster survey is ~1x10^-15 erg sec^-1 cm^-2 arcmin^-2, approximately 6 times lower than the EMSS. The WARPS LogN-LogS shows a significant excess over previous measurements for S > 8 x 10^-14 erg sec^-1 cm^-2.
Besides giant elliptical galaxies, a number of low-mass stellar systems inhabit the cores of galaxy clusters, such as dwarf elliptical galaxies (dEs/dSphs), ultra-compact dwarf galaxies (UCDs), and globular clusters. The detailed morphological examination of faint dwarf galaxies has, until recently, been limited to the Local Group (LG) and the two very nearby galaxy clusters Virgo and Fornax. Here, we compare the structural parameters of a large number of dEs/dSphs in the more distant clusters Hydra I and Centaurus to other dynamically hot stellar systems.
We improve the accuracy of photometric redshifts by including low-resolution spectral data from the G102 grism on the Hubble Space Telescope, which assists in redshift determination by further constraining the shape of the broadband Spectral Energy Disribution (SED) and identifying spectral features. The photometry used in the redshift fits includes near-IR photometry from FIGS+CANDELS, as well as optical data from ground-based surveys and HST ACS, and mid-IR data from Spitzer. We calculated the redshifts through the comparison of measured photometry with template galaxy models, using the EAZY photometric redshift code. For objects with F105W $< 26.5$ AB mag with a redshift range of $0 < z < 6$, we find a typical error of $Delta z = 0.03 * (1+z)$ for the purely photometric redshifts; with the addition of FIGS spectra, these become $Delta z = 0.02 * (1+z)$, an improvement of 50%. Addition of grism data also reduces the outlier rate from 8% to 7% across all fields. With the more-accurate spectrophotometric redshifts (SPZs), we searched the FIGS fields for galaxy overdensities. We identified 24 overdensities across the 4 fields. The strongest overdensity, matching a spectroscopically identified cluster at $z=0.85$, has 28 potential member galaxies, of which 8 have previous spectroscopic confirmation, and features a corresponding X-ray signal. Another corresponding to a cluster at $z=1.84$ has 22 members, 18 of which are spectroscopically confirmed. Additionally, we find 4 overdensities that are detected at an equal or higher significance in at least one metric to the two confirmed clusters.
We present results from an on-going follow-up campaign of far-infrared sources detected as part of our ISOPHOT Cosmic IR Background project. Fields have been imaged in the optical and near-infrared, and we find at least a third of the FIR targets areas to contain a bright and nearby star-forming galaxy. We also explore the largely neglected possibility that instead of individual galaxies some of the fainter FIR sources are confused sums of several sources - or even whole cores of galaxy clusters at redshifts of z sim 0.4-0.8. We look for correlations in the FIR positions with extremely red objects (EROs) and significant peaks in the galaxy surface density and peaks in cluster red sequence signal. Several matches are found and we have set out to study cluster candidates spectroscopically. The campaign is producing an interesting base to study IR-luminous, strongly star-forming galaxies in potential cluster environments.
In this paper, we present results of a photometric and kinematic study for a sample of 13 edge-on spiral galaxies with pronounced integral-shape warps of their stellar discs. The global structure of the galaxies is analyzed on the basis of the Sloan Digital Sky Survey (SDSS) imaging, in the g, r and i passbands. Spectroscopic observations are obtained with the 6-m Special Astrophysical Observatory telescope. In general, galaxies of the sample are typical bright spiral galaxies satisfying the Tully-Fisher relation. Most of the galaxies reside in dense spatial environments and, therefore, tidal encounters are the most probable mechanism for generating their stellar warps. We carried out a detailed analysis of the galaxies and their warps and obtained the following main results: (i) maximum angles of stellar warps in our sample are about 20{deg}; (ii) warps start, on average, between 2 and 3 exponential scale lengths of a disc; (iii) stronger warps start closer to the center, weak warps start farther; (iv) warps are asymmetric, with the typical degree of asymmetry of about several degrees (warp angle); (v) massive dark halo is likely to preclude the formation of strong and asymmetric warps.
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