Astronomy, Department of

The SDSS-C4 DR2 Galaxy Cluster Catalog

Thu, 01 Sep 2005 00:00:00 GMT

The SDSS-C4 DR2 Galaxy Cluster Catalog Miller, Christopher We present the C4 Cluster Catalog, a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 cluster-finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects that have plagued previous optical cluster selection. The present C4 catalog covers ~2600 deg2 of sky and ranges in redshift from z=0.02 to 0.17. The mean cluster membership is 36 galaxies (with measured redshifts) brighter than r=17.7, but the catalog includes a range of systems, from groups containing 10 members to massive clusters with over 200 cluster members with measured redshifts. The catalog provides a large number of measured cluster properties including sky location, mean redshift, galaxy membership, summed r-band optical luminosity (Lr), and velocity dispersion, as well as quantitative measures of substructure and the surrounding large-scale environment. We use new, multicolor mock SDSS galaxy catalogs, empirically constructed from the ΛCDM Hubble Volume (HV) Sky Survey output, to investigate the sensitivity of the C4 catalog to the various algorithm parameters (detection threshold, choice of passbands, and search aperture), as well as to quantify the purity and completeness of the C4 cluster catalog. These mock catalogs indicate that the C4 catalog is ~=90% complete and 95% pure above M200=1×1014 h-1 Msolar and within 0.03<=z<=0.12. Using the SDSS DR2 data, we show that the C4 algorithm finds 98% of X-ray-identified clusters and 90% of Abell clusters within 0.03<=z<=0.12. Using the mock galaxy catalogs and the full HV dark matter simulations, we show that the Lr of a cluster is a more robust estimator of the halo mass (M200) than the galaxy line-of-sight velocity dispersion or the richness of the cluster. However, if we exclude clusters embedded in complex large-scale environments, we find that the velocity dispersion of the remaining clusters is as good an estimator of M200 as Lr. The final C4 catalog will contain ~=2500 clusters using the full SDSS data set and will represent one of the largest and most homogeneous samples of local clusters.

Imprints of recoiling massive black holes on the hot gas of early-type galaxies

Wed, 01 Apr 2009 00:00:00 GMT

Imprints of recoiling massive black holes on the hot gas of early-type galaxies Devecchi, B.; Rasia, E.; Dotti, M.; Volonteri, M.; Colpi, M. Anisotropic gravitational radiation from a coalescing black hole (BH) binary is known to impart recoil velocities of up to ∼1000 km s −1 to the remnant BH. In this context, we study the motion of a recoiling BH inside a galaxy modelled as a Hernquist sphere, and the signature that the hole imprints on the hot gas, using N -body/smoothed particle hydrodynamics simulations. Ejection of the BH results in a sudden expansion of the gas ending with the formation of a gaseous core, similarly to what is seen for the stars. A cometary tail of particles bound to the BH is initially released along its trail. As the BH moves on a return orbit, a nearly spherical swarm of hot gaseous particles forms at every apocentre: this feature can live up to ≈10 8 years. If the recoil velocity exceeds the sound speed initially, the BH shocks the gas in the form of a Mach cone in density near each supersonic pericentric passage. We find that the X-ray fingerprint of a recoiling BH can be detected in Chandra X-ray maps out to a distance of Virgo. For exceptionally massive BHs, the Mach cone and the wakes can be observed out to a few hundred of milliparsec. The detection of the Mach cone is of twofold importance as it can be a probe of high-velocity recoils, and an assessment of the scatter of the M BH − M bulge relation at large BH masses.

Detection of non-radial pulsation and faint companion in the symbiotic star CH Cyg

Tue, 21 Jul 2009 00:00:00 GMT

Detection of non-radial pulsation and faint companion in the symbiotic star CH Cyg Pedretti, E.; Monnier, John D.; Lacour, S.; Traub, W. A.; Danchi, W. C.; Tuthill, P. G.; Thureau, N. D.; Millan-Gabet, R.; Berger, J. -P.; Lacasse, M. G.; Schuller, P. A.; Schloerb, F. P.; Carleton, N. P. We have detected asymmetry in the symbiotic star CH Cyg through the measurement of precision closure phase with the Integrated Optics Near-Infrared Camera (IONIC) beam combiner, at the infrared optical telescope array interferometer. The position of the asymmetry changes with time and is correlated with the phase of the 2.1-year period found in the radial velocity measurements for this star. We can model the time-dependent asymmetry either as the orbit of a low-mass companion around the M giant or as an asymmetric, 20 per cent change in brightness across the M giant. We do not detect a change in the size of the star during a 3-year monitoring period neither with respect to time nor with respect to wavelength. We find a spherical dust shell with an emission size of 2.2 ± 0.1 D * full width at half-maximum around the M giant star. The star to dust flux ratio is estimated to be 11.63 ± 0.3. While the most likely explanation for the 20 per cent change in brightness is non-radial pulsation, we argue that a low-mass companion in close orbit could be the physical cause of the pulsation. The combined effect of pulsation and low-mass companion could explain the behaviour revealed by the radial velocity curves and the time-dependent asymmetry detected in the closure-phase data. If CH Cyg is a typical long secondary period variable then these variations could be explained by the effect of an orbiting low-mass companion on the primary star.

Simulations of deep pencil-beam redshift surveys

Wed, 01 Aug 2001 00:00:00 GMT

Simulations of deep pencil-beam redshift surveys Yoshida, N.; Colberg, J.; White, S. D. M.; Evrard, August E.; MacFarland, T. J.; Couchman, H. M. P.; Jenkins, A.; Frenk, C. S.; Pearce, F. R.; Efstathiou, G.; Peacock, J. A.; Thomas, P. A.