Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities

We present an analysis of the color-magnitude-velocity dispersion relation for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey. We demonstrate that the color-magnitude relation is entirely a consequence of the fact that both the luminosities and colors of these galaxies are correlated with stellar velocity dispersions. Previous studies of the color-magnitude relation over a range of redshifts suggest that the luminosity of an early-type galaxy is an indicator of its metallicity, whereas residuals in color from the relation are indicators of the luminosity-weighted age of its stars. We show that this, when combined with our finding that velocity dispersion plays a crucial role, has a number of interesting implications. First, galaxies with large velocity dispersions tend to be older (i.e., they scatter redward of the color-magnitude relation). Similarly, galaxies with large dynamical mass estimates also tend to be older. In addition, at fixed luminosity, galaxies which are smaller, or have larger velocity dispersions, or are more massive, tend to be older. Second, models in which galaxies with the largest velocity dispersions are also the most metal poor are difficult to reconcile with our data. However, at fixed velocity dispersion, galaxies have a range of ages and metallicities: the older galaxies have smaller metallicities, and vice-versa. Finally, a plot of velocity dispersion versus luminosity can be used as an age indicator: lines of constant age run parallel to the correlation between velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A

Weak Lensing Detection of Cl 1604+4304 at z = 0.90

We present a weak lensing analysis of the high-redshift cluster Cl 1604+4304. At z=0.90, this is the highest-redshift cluster yet detected with weak lensing. It is also one of a sample of high-redshift, optically-selected clusters whose X-ray temperatures are lower than expected based on their velocity dispersions. Both the gas temperature and galaxy velocity dispersion are proxies for its mass, which can be determined more directly by a lensing analysis. Modeling the cluster as a singular isothermal sphere, we find that the mass contained within projected radius R is 3.69+-1.47 * (R/500 kpc) 10^14 M_odot. This corresponds to an inferred velocity dispersion of 1004+-199 km/s, which agrees well with the measured velocity dispersion of 989+98-76 km/s (Gal & Lubin 2004). These numbers are higher than the 575+110-85 km/s inferred from Cl 1604+4304 X-ray temperature, however all three velocity dispersion estimates are consistent within ~ 1.9 sigma.Comment: Revised version accepted for publication in AJ (January 2005). 2 added figures (6 figures total

Flaring Activity of Sgr A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma

We have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A* at 43 GHz ($\lambda$=7mm) and 22 GHz ($\lambda$=13mm). A low level of flare activity has been detected with a duration of $\sim$ 2 hours at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by $\sim20$ to 40 minutes. The overall characteristics of the flare emission are interpreted in terms of the plasmon model of Van der Laan (1966) by considering the ejection and adiabatically expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index $\nu^{-\alpha}$ of $\alpha$=1.6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically-expanding self-absorbed source. We present the expected synchrotron light curves for an expanding blob as well as the peak frequency emission as a function of the energy spectral index constrained by the available flaring measurements in near-IR, sub-millimeter, millimeter and radio wavelengths. We note that the blob model is consistent with the available measurements, however, we can not rule out the jet of Sgr A*. If expanding material leaves the gravitational potential of Sgr A*, the total mass-loss rate of nonthermal and thermal particles is estimated to be $\le 2\times10^{-8}$ M$_\odot$ yr$^{-1}$. We discuss the implication of the mass-loss rate since this value matches closely with the estimated accretion rate based on polarization measurements.Comment: Revised with new Figures 1 and 2, 17 pages, 4 figures, ApJ (in press

Towards a Holistic View of the Heating and Cooling of the Intracluster Medium

(Abridged) X-ray clusters are conventionally divided into two classes: "cool core" (CC) clusters and "non-cool core" (NCC) clusters. Yet relatively little attention has been given to the origins of this dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either type of cluster. The inferred entropy profiles of NCC clusters require that material is preheated prior to cluster collapse in order to explain the absence of low entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their properties at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent AGN feedback loop in which the lowest-entropy, most rapidly cooling material is heated so that it rises buoyantly to mix with material at larger radii. The resulting model does not require fine tuning and is in excellent agreement with a wide variety of observational data. Some of the other implications of this model are briefly discussed.Comment: 27 pages, 13 figures, MNRAS accepted. Discussion of cluster heating energetics extended, results unchange

X-ray Spectra of the RIXOS source sample

We present results of an extensive study of the X-ray spectral properties of sources detected in the RIXOS survey, that is nearly complete down to a flux limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing sources with low count rate spectral slopes estimated using simple hardness ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray data using statistical techniques appropriate to the Poisson regime which removes the effects of this bias. We have then applied this technique to the RIXOS survey to study the spectral properties of the sample. For the AGN we find an average energy index of 1.05+-0.05 with no evidence for spectral evolution with redshift. Individual AGN are shown to have a range of properties including soft X-ray excesses and intrinsic absorption. Narrow Emission Line Galaxies also seem to fit to a power-law spectrum, which may indicate a non-thermal origin for their X-ray emission. We infer that most of the clusters in the sample have a bremsstrahlung temperature >3 keV, although some show evidence for a cooling flow. The stars deviate strongly from a power-law model but fit to a thermal model. Finally, we have analysed the whole RIXOS sample containing 1762 sources. We find that the mean spectral slope of the sources hardens at lower fluxes in agreement with results from other samples. However, a study of the individual sources demonstrates that the hardening of the mean is caused by the appearance of a population of very hard sources at the lowest fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in pres

Simulating the Hot X-ray Emitting Gas in Elliptical Galaxies

We study the chemo-dynamical evolution of elliptical galaxies and their hot X-ray emitting gas using high-resolution cosmological simulations. Our Tree N-body/SPH code includes a self-consistent treatment of radiative cooling, star formation, supernovae feedback, and chemical enrichment. We present a series of LCDM cosmological simulations which trace the spatial and temporal evolution of heavy element abundance patterns in both the stellar and gas components of galaxies. X-ray spectra of the hot gas are constructed via the use of the vmekal plasma model, and analysed using XSPEC with the XMM EPN response function. Simulation end-products are quantitatively compared with the observational data in both the X-ray and optical regime. We find that radiative cooling is important to interpret the observed X-ray luminosity, temperature, and metallicity of the interstellar medium of elliptical galaxies. However, this cooled gas also leads to excessive star formation at low redshift, and therefore results in underlying galactic stellar populations which are too blue with respect to observations.Comment: 6 pages, 3 figures, to appear in the proceedings of "The IGM/Galaxy Connection - The Distribution of Baryons at z=0", ed. M. Putman & J. Rosenberg; High resolution version is available at http://astronomy.swin.edu.au/staff/dkawata/research/papers.htm

The history of mass assembly of faint red galaxies in 28 galaxy clusters since z=1.3

We measure the relative evolution of the number of bright and faint (as faint as 0.05 L*) red galaxies in a sample of 28 clusters, of which 16 are at 0.50<= z<=1.27, all observed through a pair of filters bracketing the 4000 Angstrom break rest-frame. The abundance of red galaxies, relative to bright ones, is constant over all the studied redshift range, 0<z<1.3, and rules out a differential evolution between bright and faint red galaxies as large as claimed in some past works. Faint red galaxies are largely assembled and in place at z=1.3 and their deficit does not depend on cluster mass, parametrized by velocity dispersion or X-ray luminosity. Our analysis, with respect to previous one, samples a wider redshift range, minimizes systematics and put a more attention to statistical issues, keeping at the same time a large number of clusters.Comment: MNRAS, 386, 1045. Half a single sentence (in sec 4.4) change

Untwisting of a Strained Cholesteric Elastomer by Disclination Loop Nucleation

The application of a sufficiently strong strain perpendicular to the pitch axis of a monodomain cholesteric elastomer unwinds the cholesteric helix. Previous theoretical analyses of this transition ignored the effects of Frank elasticity which we include here. We find that the strain needed to unwind the helix is reduced because of the Frank penalty and the cholesteric state becomes metastable above the transition. We consider in detail a previously proposed mechanism by which the topologically stable helical texture is removed in the metastable state, namely by the nucleation of twist disclination loops in the plane perpendicular to the pitch axis. We present an approximate calculation of the barrier energy for this nucleation process which neglects possible spatial variation of the strain fields in the elastomer, as well as a more accurate calculation based on a finite element modeling of the elastomer.Comment: 12 pages, 9 figure

The Deficit of Distant Galaxy Clusters in the RIXOS X-ray Survey

Clusters of galaxies are the largest gravitationally bound systems and therefore provide an important way of studying the formation and evolution of the large scale structure of the Universe. Cluster evolution can be inferred from observations of the X-ray emission of the gas in distant clusters, but interpreting these data is not straightforward. In a simplified view, clusters grow from perturbations in the matter distribution: their intracluster gas is compressed and shock-heated by the gravitational collapse$^{1}$. The resulting X-ray emission is determined by the hydrostatic equilibrium of the gas in the changing gravitational potential. However, if processes such as radiative cooling or pre-collapse heating of the gas are important, then the X-ray evolution will be strongly influenced by the thermal history of the gas. Here we present the first results from a faint flux-limited sample of X-ray selected clusters compiled as part of the ROSAT International X-ray and Optical Survey (RIXOS). Very few distant clusters have been identified. Most importantly, their redshift distribution appears to be inconsistent with simple models based on the evolution of the gravitational potential. Our results suggest that radiative cooling or non-gravitational heating of the intracluster gas must play an important role in the evolution of clusters.Comment: uuencoded compressed postscript. The preprint is also available at http://www.ast.cam.ac.uk/preprint/PrePrint.htm