Nearby Galaxy Clusters Research Articles

CAIRNS: The Cluster and Infall Region Nearby Survey. III. Environmental Dependence of Hα Properties of Galaxies

We investigate the environmental dependence of star formation in cluster virial regions and infall regions as part of the Cluster and Infall Region Nearby Survey (CAIRNS), a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. We use complete, homogeneous spectroscopic surveys of Ks-limited samples in eight of the CAIRNS clusters. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to the general large-scale structure. Galaxies in infall regions probe whether processes affecting star formation are effective over scales larger than cluster virial regions. The fraction of galaxies with current star formation in their inner disks as traced by Hα emission increases with distance from the cluster and converges to the "field" value only at 2–3 virial radii, in agreement with other investigations. However, among galaxies with significant current star formation (EW[Hα] ≥ 2 Å), there is no difference in the distribution of EW[Hα] inside and outside the virial radius. This surprising result, first seen by Carter and coworkers, suggests that (1) star formation is truncated either on very short timescales or only at moderate and high redshifts or (2) that projection effects contaminate the measurement. We quantify the possible impact of mechanisms that only affect the outer parts of galaxies and thus might not be detected in this survey or any fiber-based survey. The number density profiles of star-forming and non-star-forming galaxies indicate that, among galaxies projected inside the virial radius, at least half the former and 20% of the latter are "infall interlopers," galaxies in the infall region but outside the virial region. We show that the kinematics of star-forming galaxies in the infall region closely match those of absorption-dominated galaxies. This result shows that the star-forming galaxies in the infall regions are not interlopers from the field and excludes one model of the backsplash scenario of galaxy transformation. Finally, we quantify systematic uncertainties in estimating the global star formation in galaxies from their inner disks.

A PHOTOMETRIC STUDY ON THE FORMATION OF THE EARLY TYPE GALAXIES IN NEARBY GALAXY CLUSTERS

We present a photometric study of galaxies in the central regions of six nearby galaxy clusters at redshift z=0.0231<TEX>${\~}$</TEX>0.0951. We have derived BVI photometry of the galaxies from the CCD images obtained at the Bohyunsan Optical Astronomical Observatory (BOAO) in Korea, and JHKs photometry of the bright galaxies from the 2MASS extended source catalog. Comparing the galaxy photometry results with the simple stellar population model of Bruzual & Charlot (2003) in the optical & NIR color-color diagrams, we have estimated the ages and metallicities of early type galaxies. We have found that the observed galaxies had recent star-formation mostly 5 <TEX>${\~}$</TEX> 7 Gyrs ago but the spread in age estimation is rather large. The average metallicities are [Fe/H]=0.l<TEX>${\~}$</TEX>0.5 dex. These results support the hypothesis that large early type galaxies in clusters are formed via hierarchical merging of smaller galaxies.

ChandraObservations of the Gravitational Lenses B1600+434 and B1608+656

We observed B1600+434 and B1608+656 with Chandra ACIS, detecting both quasar images in B1600+434 and three of four images in B1608+656. We did not detect significant X-ray emission from nearby galaxy groups or clusters associated with each lens galaxy. The upper limits on the X-ray luminosity of any cluster within 4' of each lens and at each lens redshift are ~2 × 1042 and ~6 × 1042 ergs s-1 for B1600+434 and B1608+656, respectively. The radio-loud source quasars have power-law photon indices of Γ = 1.9 ± 0.2 and Γ = 1.4 ± 0.3 and X-ray luminosities of 1.4 × 1045 and 2.9 × 1044 ergs s-1 for B1600+434 and B1608+656, respectively, before correcting for the magnification. We detected a differential absorption column density of ΔNH ~ 3 × 1021 cm-2 between the two images of B1600+434, roughly consistent with expectations from differential extinction estimates of ΔE(B - V) = 0.1 mag and a standard dust-to-gas ratio. The differential absorption observed in gravitational lenses may serve as an important probe to study the gas content in high-redshift galaxies, since it can separate the absorbing column originating from the lens galaxy and those intrinsic to quasars. We also detected 157 serendipitous X-ray sources in the two Chandra fields and identified the brighter optical counterparts using the Sloan Digital Sky Survey and Palomar Digital Sky Survey.

The structural and scaling properties of nearby galaxy clusters

We present the integrated mass profiles for a sample of ten nearby (z < 0.15), relaxed galaxy clusters, covering a temperature range of (2−9) keV, observed with XMM-Newton. The mass profiles were derived from the observed gas density and temperature profiles under the hypothesis of spherical symmetry and hydrostatic equilibrium. All ten mass profiles are well described by an NFW-type profile over the radial range from 0.01 to 0.5R200 ,w hereR200 is the radius corresponding to a density contrast of 200 with respect to the critical density at the cluster redshift. A King model is inconsistent with these data. The derived concentration parameters and total masses are in the range c200 = 4− 6a ndM200 = 1.2×10 14 −1.2×10 15 M� , respectively. Our qualitative and quantitative study of the mass profile shape shows, for the first time, direct and clear observational evidence for the universality of the total mass distribution in clusters. The mass profiles scaled in units of R200 and M200 nearly coincide, with a dispersion of less than 15% at 0.1 R200 .T hec200−M200 relation is consistent with the predictions of numerical simulations for a ΛCDM cosmology, taking into account the measurement errors and expected intrinsic scatter. Our results provide further strong evidence in favour of the Cold Dark Matter cosmological scenario and show that dark matter collapse is well understood, at least down to the cluster scale.

Intergalactic Globular Clusters

AbstractWe confirm and extend our previous detection of a population of intergalactic globular clusters in Abell 1185, and report the first discovery of an intergalactic globular cluster in the nearby Virgo cluster of galaxies. The numbers, colors and luminosities of these objects can place constraints on their origin, which in turn may yield new insights to the evolution of galaxies in dense environments.

Supernovae in Galaxy Clusters

SummaryWe present the results of several surveys for supernovae (SNe) in galaxy clusters. SNe discovered in deep, archival HST images were used to measure the cluster SN Ia rate to z = 1. A search for SNe in nearby (0.06 ≤ z ≤ 0.2) Abell galaxy clusters yielded 15 SNe, 12 of which were spectroscopically confirmed. Of these, 7 are cluster SNe Ia, which we will use to measure the SN Ia rate in nearby clusters. This search has also discovered the first convincing examples of intergalactic SNe. We conclude with a brief description of ongoing and future cluster SN surveys.

Infrared luminosities of local-volume galaxies

Based on data from the Two-Micrometer All-Sky Survey (2MASS), we analyzed the infrared properties of 451 Local-Volume galaxies at distances D ≤ 10 Mpc. We determined the K-band luminosity function of the galaxies in the range of absolute magnitudes from −25m to −11m. The local luminosity density within 8 Mpc is 6.8 × 108L⊙ Mpc−3, a factor of 1.5 ± 0.1 higher than the global mean K-band luminosity density. We determined the ratios of the virial mass to the K-band luminosity for nearby groups and clusters of galaxies. In the luminosity range from 5 × 1010 to 2 × 1013L⊙, the dependence log(M/LK) ∝ (0.27 ± 0.03) log LK with a dispersion of ∼0.1 comparable to the measurement errors of the masses and luminosities of the systems of galaxies holds for the groups and clusters of galaxies. The ensemble-averaged ratio, 〈M/LK〉 ≃ (20–25) M⊙/L⊙, was found to be much smaller than the expected global ratio, (80–90)M⊙/L⊙, in the standard model with Ωm = 0.27. This discrepancy can be eliminated if the bulk of the dark matter in the Universe is not associated with galaxies and their systems.

Very Small Array observations of the Sunyaev-Zel'dovich effect in nearby galaxy clusters

We present Very Small Array (VSA) observations (centred on � 34GHz) on scales � 20 arcmin towards a complete, X-ray–flux–limited sample of seven clusters at redshift z < 0.1. Four of the clusters have significant Sunyaev-Zel’dovich (SZ) detections in the presence of CMB primordial anisotropy. For all seven, we use a Bayesian MarkovChain-Monte-Carlo (MCMC) method for inference from the VSA data, with X-ray priors on cluster positions and temperatures, and radio priors on sources. In this context, the CMB primordial fluctuations are an additional source of Gaussian noise, and are included in the model as a non–diagonal covariance matrix derived from the known angular power spectrum. In addition, we make assumptions of �–model gas distributions and of hydrostatic equilibrium, to evaluate probability densities for the gas mass (Mgas) and total mass (Mr) out to r200, the radius at which the average density enclosed is 200 times the critical density at the redshift of the cluster. This is further than has been done before and close to the classical value for a collapsed cluster. Our combined estimate of the gas fraction (fgas = Mgas/Mr) is 0.08 +0.06 0.04 h 1 . The random errors are poor (note however that the errors are higher than would have been obtained with the usual chi-squared method on the same data) but the control of bias is good. We have described the MCMC analysis method specifically in terms of SZ but hope the description will be of more general use. We find that the effects of primordial CMB contamination tend to be similar in the estimates of both Mgas and Mr over the narrow range of angular scales we are dealing with, so that there is little effect of primordials on fgas determination. Using our Mr estimates we find a normalisation of the mass – temperature relation based on the profiles from the VSA cluster pressure maps that is in good agreement with recent M T determinations from X-ray cluster measurements.

Star formation, massive stars, and super star clusters in nearby galaxies

The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z ∼ 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1 M ⊙ yr −1 in galaxies out to a redshift of z ∼ 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction.

Cosmic Filaments in Superclusters

Large-scale structure calculations show that modest overdensity filaments will connect clusters of galaxies and these filaments are reservoirs of baryons, mainly in gaseous form. To determine whether such filaments exist, we have examined the UV absorption line properties of three AGNs projected behind possible filaments in superclusters of galaxies; the AGNs lie within 3 Mpc of the centerlines of loci connecting clusters. All three lines of sight show absorption in Ly\alpha, Ly\beta, or/and OVI at redshifts within about 1300 km/s of the nearby galaxy clusters that would define the closest filaments. For one AGN, the absorption line redshifts are close to the emission line redshift of the AGN, so we cannot rule out self-absorption for this object. These absorption line associations with superclusters are unlikely to have occurred by chance, a result consistent with the presence of cosmic filaments within superclusters.

TheU‐Band Galaxy Luminosity Function of Nearby Clusters

Despite the great potential of the U-band galaxy luminosity function (GLF) to constrain the history of star formation in clusters, to clarify the question of variations of the GLF across filter bands, to provide a baseline for comparisons to high-redshift studies of the cluster GLF, and to estimate the contribution of bound systems of galaxies to the extragalactic near-UV background, determinations have so far been hampered by the generally low efficiency of detectors in the U band and by the difficulty of constructing both deep and wide surveys. In this paper we present U-band GLFs of three nearby, rich clusters to a limit of MU ≈ -17.5 (M + 2). Our analysis is based on a combination of separate spectroscopic and R- and U-band photometric surveys. For this purpose, we have developed a new maximum likelihood algorithm for calculating the LF that is particularly useful for reconstructing the galaxy distribution function in multidimensional spaces (e.g., the number of galaxies as a simultaneous function of luminosity in different filter bands, surface brightness, star formation rate, morphology, etc.) because it requires no prior assumptions as to the shape of the distribution function. The composite LF can be described by a Schechter function with characteristic magnitude M = -19.82 ± 0.27 and faint-end slope αU = -1.09 ± 0.18. The total U-band GLF is slightly steeper than the R-band GLF, indicating that cluster galaxies are bluer at fainter magnitudes. Quiescent galaxies dominate the cumulative U-band flux for MU < -14. The contribution of galaxies in nearby clusters to the U-band extragalactic background is less than 1% Gyr-1 for clusters of masses ~3 × 1014-2 × 1015 M☉.

Photometric and Morphological Properties of Nearby Cluster Galaxies

Photometric observations are presented for 8747 galaxies in the fields of 28 clusters with redshifts in the range 0.05 < z < 0.11 and spanning a wide range in Abell richness class. The primary motivation for this study is to provide a nearby sample of clusters whose galaxy populations can be used to provide an anchor point for evolutionary studies of cluster galaxy populations.

CAIRNS: The Cluster and Infall Region Nearby Survey. II. Environmental Dependence of Infrared Mass-to-Light Ratios

CAIRNS (Cluster And Infall Region Nearby Survey) is a spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. In Paper I, we used redshifts within $\sim 10\Mpc$ of the centers of the clusters to determine the mass profiles of the clusters based on the phase space distribution of the galaxies. Here, we use 2MASS photometry and an additional \ncziinew redshifts to investigate the environmental dependence of near-infrared mass-to-light ratios. In the virial regions, the halo occupation function is non-linear; the number of bright galaxies per halo increases more slowly than the mass of the halo. On larger scales, the light contained in galaxies is less clustered than the mass in rich clusters. Specifically, the mass-to-light ratio inside the virial radius is a factor of $1.8\pm0.3$ larger than that outside the virial radius. This difference could result from changing fractions of baryonic to total matter or from variations in the efficiency of galaxy formation or disruption with environment. The average mass-to-light ratio $M/L_K = 53\pm 5 h$ implies $\Omega_m = 0.18\pm 0.03$ (statistical) using the luminosity density based on 2dFGRS data. These results are difficult to reconcile with independent methods which suggest higher $\Omega_m$. Reconciling these values by invoking bias requires that the typical value of $M/L_K$ changes significantly at densities of $\lesssim3\rho_c$.

E+A Galaxies and the Formation of Early‐Type Galaxies atz ∼ 0

E+A galaxies, whose spectra have deep Balmer absorption lines but no significant [O II] emission, are the best candidates for an evolutionary link between star-forming, gas-rich galaxies and quiescent, gas-poor galaxies. However, their current morphologies are not well known. We present Hubble Space Telescope WFPC2 observations of the five bluest E+A galaxies (z ~ 0.1) in the sample of Zabludoff and coworkers to study whether their detailed morphologies are consistent with late- to early-type evolution and to determine what drives that evolution. The morphologies of four galaxies are disturbed, indicating that a galaxy-galaxy merger is at least one mechanism that leads to the E+A phase. Two-dimensional image fitting shows that the E+As are generally bulge-dominated systems, even though at least two E+As may have underlying disks. In the fundamental plane, E+As stand apart from the E/S0 galaxies mainly because of their high effective surface brightness. Fading of the young stellar population and the corresponding increase in their effective radii will cause these galaxies to migrate toward the locus of E/S0 galaxies. E+As have profiles qualitatively like those of normal power-law early-type galaxies but have higher surface brightnesses. This result provides the first direct evidence supporting the hypothesis that power-law elliptical galaxies form via gas-rich mergers. In total, at least four E+As are morphologically consistent with early-type galaxies. We detect compact sources, possibly young star clusters, associated with the galaxies. These sources are much brighter (MR ~ -13) than Galactic globular clusters, have luminosities consistent with the brightest clusters in nearby starburst galaxies, and have blue colors consistent with the ages estimated from the E+A galaxy spectra (several 108 yr). Further study of such young star cluster candidates might provide the elusive chronometer needed to break the age/burst-strength degeneracy for these postmerger galaxies.

A Redshift Survey of Nearby Galaxy Groups: The Shape of the Mass Density Profile

We constrain the mass profile and orbital structure of nearby groups and clusters of galaxies. Our method yields the joint probability distribution of the density slope n, the velocity anisotropy β, and the turnover radius r0 for these systems. The measurement technique does not use results from N-body simulations as priors. We incorporate 2419 new redshifts (included here) in the fields of 41 systems of galaxies with z < 0.04. The new groups have median velocity dispersion σ = 360 km s-1. We also use 979 archived redshifts in the fields of eight nearly relaxed clusters with z < 0.1. Within R ≲ 2r200, the data are consistent with a single power-law matter density distribution with slope n = 1.8-2.2 for systems with σ < 470 km s-1 and n = 1.6-2.0 for those with σ > 470 (95% confidence). We show that a simple, scale-free phase-space distribution function (DF) f(E,L2) ∝ (-E)α-1/2L-2β is consistent with the data as long as the matter density has a cusp. Using this DF, matter density profiles with constant-density cores (n = 0) are ruled out with better than 99.7% confidence.

The Properties of Satellite Galaxies in External Systems. II. Photometry and Colors

In this second paper dedicated to the study of satellite galaxies we present broadband photometry in the B, V, R, and I filters of 49 satellite galaxies orbiting giant isolated spiral galaxies. First analysis of the properties of these objects are presented by means of color-color and color-magnitude diagrams for early- and late-type satellites. Although we find differences in the slope of the V - I versus Mv color magnitude diagram, as a whole, the relations are in agreement with the trends known to date for galaxies of similar magnitudes in nearby clusters of galaxies. Comparison with the relations found for satellites in the Local Group allows us to sample better the bright end of the luminosity function of satellite galaxies and extends for brighter objects the validity of the color-magnitude relation found for dwarf galaxies in the Local Group. Most of the E/S0 galaxies in our sample show a negative color gradient with values similar to those known for early-type galaxies in other environments.

On the iron content in rich nearby clusters of galaxies

In this paper we study the iron content of a sample of 22 nearby hot clusters observed with BeppoSAX .W e fi nd that the global iron mass of clusters is tightly related to the cluster luminosity and that the relatively loose correlation between the iron mass and the cluster temperature follows from the combination of the iron mass vs. luminosity and luminosity vs. temperature correlations. The iron mass is found to scale linearly with the intracluster gas mass, implying that the global iron abundance in clusters is roughly constant. This result suggests that enrichment mechanisms operate at a similar rate in all clusters. By employing population synthesis and chemical enrichment models, we show that the iron mass associated with the abundance excess which is always found in the centre of cool core clusters can be entirely produced by the brightest cluster galaxy (BCG), which is always found at the centre of cool core clusters. The iron mass associated with the excess, the optical magnitude of the BCG and the temperature of the cluster are found to correlate with one another suggesting a link between the properties of the BCG and the hosting cluster. These observational facts lends strength to current formation theories which envisage a strong connection between the formation of the giant BCG and its hosting cluster.

AChandraStudy of the Effects of a Major Merger on the Structure of A2319

We present an analysis of a Chandra observation of the massive, nearby galaxy cluster A2319. A sharp surface brightness discontinuity—suggested by previous, lower angular resolution X-ray imaging—is clearly visible in the ACIS image. This ~300 kpc feature suggests that a major merger is taking place with a significant velocity component perpendicular to the line of sight. The cluster emission-weighted mean temperature is 11.8 ± 0.6 keV, somewhat higher than previous temperature measurements. The Chandra temperature map of A2319 reveals substructure resembling that anticipated based on hydrodynamic simulations of cluster mergers and shows an associated cool core not previously known. The map shows a separation between the intracluster medium (ICM) and galaxies of one subcluster, indicating a transient state in which the ICM has been stripped from the subcluster galaxies (and presumably the dark matter). Detailed analysis of the merger feature shows a pressure change across the surface brightness discontinuity by a factor of 2.5. The higher density side of the front has a lower temperature, suggesting the presence of a cold front similar to those in many other merging clusters. The velocity of the front is roughly sonic. We compare bulk properties of the ICM and galaxies in A2319 to the same properties in a large sample of clusters as a way of gauging the effects of the major merger. Interestingly, by comparing A2319 to a sample of 44 clusters studied with the ROSAT PSPC, we find that the X-ray luminosity, isophotal size, and ICM mass are consistent with the expected values for a cluster of its temperature; in addition, the K-band galaxy light is consistent with the light-temperature scaling relation derived from a sample of ~100 clusters studied with the Two Micron All Sky Survey. Together, these results indicate either that the merger in A2319 has not been effective at altering the bulk properties of the cluster or that there are large but correlated displacements in luminosity, isophotal size, ICM mass, galaxy light, and emission-weighted mean temperature in this cluster.

The distribution of mass and light in cluster infall regions

The CAIRNS (Cluster And Infall Region Nearby Survey) project is a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. I describe the survey and use the kinematics of galaxies in the infall regions to estimate the cluster mass profiles. At small radii, these mass profiles are consistent with independent mass estimates from X-ray observations and Jeans analysis. I demonstrate the dependence of mass-to-light ratios on environment by combining these mass profiles with Two-Micron All-Sky Survey (2MASS) photometry. Near-infrared light is more extended than mass in these clusters, suggesting that dense cluster cores are less efficient at forming galaxies and/or more efficient at disrupting them. At large radii, galaxy populations in cluster infall regions closely resemble those in the field. The mass-to-light ratio at these radii should therefore be a good probe of the global mass-to-light ratio. The mass-to-light ratio in the infall region yields a surprisingly low estimate of .To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Powerful, obscured active galactic nuclei among X-ray hard, optically dim serendipitousChandrasources

We present a small sample of Chandra X-ray sources selected from the fields of ACIS observations which probe fluxes around the break in the hard band source counts. The targets of these fields include nine nearby galaxy clusters, one distant cluster and two powerful radio galaxy fields. The follow-up of this serendipitous sample was biased towards X-ray hard and optically dim sources mostly not seen on the Digitized Sky Survey; for these, we present X-ray fluxes, optical and near-infrared photometry leading to 51 photometric redshifts in all and 18 independently measured spectroscopic redshifts. Few sources are associated with the target fields themselves. 56 of 58 sources imaged in the K-band are detected at K 20 with K median = 18, and of these, 38 have hard X-ray count ratios and 24 of these are significantly hard with most of the counts emerging about 2 keV. We find that almost all are active galactic nuclei (AGN) hosted in massive early-type host galaxies with a photometric redshift distribution peaking at z ∼ 1. Two type 2 quasars with intrinsic X-ray luminosity L 10 45 erg s −1 ,F e Kα emission lines and absorbing column density N H > 10 23 cm −2 ‐ and N H 10 24 cm −2 in one case ‐ are discussed in detail; the sample contains at least 12 potential type 2 quasars in all. We discuss various detection strategies for type 2 quasars and calculate their inferred space density. This combines and extends a number of results from subsamples already published by us.