Showing papers on "Brightest cluster galaxy published in 1997"

The Average Mass and Light Profiles of Galaxy Clusters

Abstract: The systematic errors in the virial mass-to-light ratio, Mv/L, of galaxy clusters as an estimator of the field M/L value are assessed. We overlay 14 clusters in redshift space to create an ensemble cluster that averages over substructure and asymmetries. The combined sample, including background, contains about 1150 galaxies, extending to a projected radius of about twice r200. The radius r200, defined as where the mean interior density is 200 times the critical density, is expected to contain the bulk of the virialized cluster mass. The dynamically derived M(r200)/L(r200) of the ensemble is (0.82 ? 0.14)Mv/L. The Mv/L overestimate is attributed to not taking into account the surface pressure term in the virial equation. Under the assumption that the velocity anisotropy parameter is in the range 0 ? ? ? 2/3, the galaxy distribution accurately traces the mass profile beyond about the central 0.3r200. There are no color or luminosity gradients in the galaxy population beyond 2r200, but there is 0.11 ? 0.05 mag fading in the r-band luminosities between the field and cluster galaxies. We correct the cluster virial mass-to-light ratio, Mv/L = 289 ? 50 h M?/L? (calculated assuming q0 = 0.1), for the biases in Mv and mean luminosity to estimate the field M/L = 213 ? 59 h M?/L?. With our self-consistently derived field luminosity density, j/?c = 1136 ? 138 h M?/L? (at z 1/3), the corrected M/L indicates ?0 = 0.19 ? 0.06 ? 0.04 (formal 1 ? random error and estimated potential systematic errors) for those components of the mass field in rich clusters.

The Homogeneity of Spheroidal Populations in Distant Clusters

Abstract: The small scatter observed for the (U-V) colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0 < z < 1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high-precision rest-frame UV-optical photometry of a large sample of morphologically selected spheroidal galaxies in three z ~ 0.54 clusters that have been observed with the Hubble Space Telescope (HST). We demonstrate the robustness of using the HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (<0.1 mag rms) for galaxies classed as E's and E/S0's, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L~L*V+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z 0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z 3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z 3.

On the origin of globular clusters in elliptical and cD galaxies

Abstract: Perhaps the most noteworthy of recent findings in extragalactic globular cluster (GC) research are the multimodal GC metallicity distributions seen in massive early–type galaxies. We explore the origin of these distinct GC populations, the implications for galaxy formation and evolution, and identify several new properties of GC systems. First, when we separate the metal–rich and metal–poor subpopulations, in galaxies with bimodal GC metallicity distributions, we find that the mean metallicity of the metal–rich GCs correlates well with parent galaxy luminosity but the mean metallicity of the metal–poor ones does not. This indicates that the metal–rich GCs are closely coupled to the galaxy and

Detection of Weak Lensing by a Cluster of Galaxies at z = 0.83

Abstract: We report the detection of weak gravitational lensing of faint, distant background galaxies by the rich, X-ray luminous cluster of galaxies MS 1054-03 at z = 0.83. This is the first measurement of weak lensing by a bona fide cluster at such a high redshift. We detect tangential shear at the 5%-10% level over a range of radii 50'' r 250'' centered on the optical position of the cluster. Two-dimensional mass reconstruction using galaxies with 21.5 1).

The Luminosity-Temperature Relation at z = 0.4 for Clusters of Galaxies

Abstract: We have obtained the first large sample of accurate temperatures for clusters at z > 0.14 from ASCA. We compare the luminosity-temperature (L-T) distribution for these clusters with the low-redshift sample of David and coworkers and find that there is no evidence for evolution. We also find that the intrinsic variance in this relation is roughly constant with redshift. Additionally, there is no detectable change in the relationship of optical velocity dispersion to X-ray temperature with redshift. Most cosmological simulations driven primarily by gravity predict substantial changes in the L-T relation due to the recent rapid growth of clusters. Our results are consistent either with models in which the cluster core entropy is dominated by preheating, or with low-Ω models in which cluster structure does not evolve strongly with time. The intrinsic variance in the L-T relation at a fixed redshift can be due to variety of possible causes, e.g., a change in the baryonic fraction from cluster to cluster, variation in the fraction of the total energy in the system arising from shocks or supernova heating, or variations in the emission measure distributions in multiphase gas.

Star Formation in Cluster Galaxies at 0.2<z<0.55

Abstract: The rest-frame equivalent width of the [OII] λ3727 emission line, W0(O II), has been measured for cluster and field galaxies in the Canadian Network for Observational Cosmology redshift survey of rich clusters at 0.2 10 A, as expected in a model of cluster formation in which star formation is truncated on infall. Evidence of suppressed star formation relative to the field is present in the whole cluster sample, out to 2R200, so the mechanism responsible for the differential evolution must be acting at a large distance from the cluster center and not just in the core. The mean star formation rate in the cluster galaxies with the strongest emission corresponds to an increase in the total stellar mass of less than about 4% if the star formation is due to a secondary burst lasting 0.1 Gyr.

Low-Surface-Brightness Galaxies: Hidden Galaxies Revealed

Abstract: In twenty years, low surface brightness (LSB) galaxies have evolved from being an idiosyncratic notion to being one of the major baryonic repositories in the Universe The story of their discovery and the characterization of their properties is told here Their recovery from the noise of the night sky background is a strong testament to the severity of surface brightness selection effects LSB galaxies have a number of remarkable properties which distinguish them from the more familiar Hubble Sequence of spirals The two most important are 1) they evolve at a significantly slower rate and may well experience star formation outside of the molecular cloud environment, 2) they are embedded in dark matter halos which are of lower density and more extended than the halos around high surface brightness (HSB) disk galaxies Compared to HSB disks, LSB disks are strongly dark matter dominated at all radii and show a systematic increase in M/L with decreasing central surface brightness In addition, the recognition that large numbers of LSB galaxies actually exist has changed the form of the galaxy luminosity function and has clearly increased the space density of galaxies at z =0 Recent CCD surveys have uncovered a population of red LSB disks that may be related to the excess of faint blue galaxies detected at moderate redshifts LSB galaxies offer us a new window into galaxy evolution and formation which is every bit as important as those processes which have produced easy to detect galaxies Indeed, the apparent youth of some LSB galaxies suggest that galaxy formation is a greatly extended process While the discovery of LSB galaxies have lead to new insights, it remains unwise to presume that we now have a representative sample which encompasses all galaxy types and forms

Merger Rate of Equal-Mass Spherical Galaxies

Abstract: We present cross sections and reaction rates for merging to occur during encounters of equal-mass spherical galaxies. As an application, we determine the rate of galaxy merging in clusters of galaxies. We present results for two types of Plummer models (a full and a truncated one), two King models, and the Hernquist model. Cross sections are determined on the basis of a large number (~500) of simulations of galaxy encounters, using the 10 Gigaflops GRAPE-3A special purpose computer. We characterize the overall merger rate of galaxies in a galaxy cluster by a single number, derived from our cross sections by an integration over galaxy encounter velocities in the limit of a constant density in velocity space. For small clusters, where the cluster velocity dispersion may not significantly exceed the internal velocity dispersion of the individual galaxies, this constant-density approximation may not be valid. For those cases, we present separate results, based on integrations of our cross sections over Maxwellian velocity distributions. Finally, tidal effects from the cluster potential, as well as from neighboring galaxies, may prevent a barely bound galaxy pair from spiraling in after their first encounter. We give a quantitative estimate of the resulting reduction in the actual merger rate that is due to these tidal interactions.

The Hawaii K-Band Galaxy Survey. II. Bright K-Band Imaging

Abstract: We present the results of a wide-field K-selected galaxy survey with complementary optical I- and B-band imaging in six fields with a total coverage of 9.8 deg2. The observations were carried out on the University of Hawaii 0.6 m and 2.2 m telescopes. The purpose of this survey is to study the properties of the local galaxies and explore the evolution of K-selected galaxies at low redshifts. Star-galaxy discrimination is performed using both galaxy color properties and object morphologies, and 6264 galaxies are found. This survey establishes the bright-end K-band galaxy number counts in the magnitude range 13 < K < 16 with high precision. We find that our bright-end counts have a significantly steeper slope than the prediction of a no-evolution model, which cannot be accounted for by known observational or theoretical error. We argue also against the likelihood of sufficient evolution at such low redshifts to account for this effect; we describe an alternative picture in which there is a local deficiency of galaxies by a factor of 2 on scale sizes of around 300 h-1 Mpc. Taken at face value, this would imply that local measurements of Ω0 underestimate the true value of the cosmological mass density by this factor and that local measurements of H0 could be high by as much as 33%.

The Morphology of Distant Cluster Galaxies. II. HST Observations of Four Rich Clusters at z ≃ 0.4*

Abstract: We present Hubble Space Telescope (HST) WF/PC and WFPC2 observations of four rich clusters of galaxies at redshifts z ~ 0.4: CL 1447+2619, CL 0939+4713, CL 0024+1654, and Abell 370. Extensive ground-based observations have shown that all have substantial populations of blue, star-forming, and (in some cases) starbursting galaxies. We find the galaxy populations in these clusters to be approximately normal, in that most galaxies can be accommodated in the z = 0 Hubble sequence and have the luminosities, colors, and distributions within the clusters that are expected for their Hubble types. However, in every cluster environment the galaxy populations contain a uniformly higher proportion of late-type galaxies than is found in similar environments today. Confirming previous ground-based and HST observations, we find a remarkably large fraction of disturbed systems. The appearance of many of these is suggestive of merging and tidally interacting objects, but several difficulties with this suggest that the newly described process of "galaxy harassment" may be a more likely explanation. These objects are clearly responsible for the majority of the starburst spectra detected in ground-based observations of the clusters. They cannot, however, account for most of the large population of blue galaxies. The fraction of such disturbed galaxies varies from cluster to cluster in concert with the number of blue and of spiral galaxies. All these variations are well correlated with the structure and density of the clusters, but not with cluster richness. Close examination of the population of "normal" spirals shows that a significant fraction have very abnormal patterns of star formation: Rings of star formation are much more common than two-armed spirals. Galaxies of this type are very rare in nearby populations but also have been found in HST observations of high-redshift field populations. We speculate that these forms might be products of the cluster environment or may simply represent the norm for the earlier stage of disk evolution observed at this redshift.

The APM Galaxy Survey — V. Catalogues of galaxy clusters

Abstract: We describe the construction of catalogues of galaxy clusters from the APM Galaxy survey using an automated algorithm based on Abell-like selection criteria. We investigate the effects of varying several parameters in our selection algorithm, including the magnitude range and radius from the cluster centre used to estimate the cluster richnesses. We quantify the accuracy of the photometric distance estimates by comparing them with measured redshifts, and we investigate the stability and completeness of the resulting catalogues. We find that the angular correlation functions for different cluster catalogues are in good agreement with one another, and are also consistent with the observed amplitude of the spatial correlation function of rich clusters.

X-ray emission from the Fornax cluster

Abstract: We have analyzed the ROSAT PSPC observations of the central region of the Fornax cluster, a relatively poor group of galaxies at a distance of about 24 Mpc. The brightest X-ray and optical galaxy in the group is NGC 1399, an E1 galaxy located near the center of the Fornax cluster. We characterize the hot gas around the galaxy, derived from a 2' to 18' annulus around NGC 1399, as having a mean temperature of 1.30 ± 0.05 keV and a heavy element abundance of 0.6 ± 0.1 with respect to solar abundance (Fe/H = 4.68 × 10-5 by number). Spatially resolved spectral data provide both gas temperature and gas abundance profiles extending to 125 kpc (18') from the galaxy. The temperature distribution, combined with the X-ray surface brightness profile, yields an accurate determination of the gravitating mass within 125 kpc, which falls in the range (4.3-8.1) × 1012 M☉ (95% confidence range, including systematic uncertainties). If we include the extended optical halo around NGC 1399, the mass-to-light ratio increases with radius from 33 ± 8 M☉/L☉ at 18 kpc to 70 ± 22 M☉/L☉ at 110 kpc. We compare the heavy element abundance distribution measured around NGC 1399 with that measured around the Virgo galaxy NGC 4472, as well as to models for hot coronae. We find that the abundance distribution is in good agreement with that previously measured for NGC 4472 by Forman et al. in 1993. For both galaxies, the observed abundance profiles require both a weak evolution of the type Ia supernova rate with time and a present epoch rate which agrees with that of Cappellaro et al. We compare mass measurements in NGC 1399 to those for M87. The similarity of the optical masses in these systems and their differences in gas masses and gravitating masses lead us to suggest that the optical galaxies formed at an early stage when the central potentials of these two systems were similar. Subsequent infall of gas and dark matter into the larger, deeper Virgo potential resulted in the greater mass of the Virgo cluster compared to Fornax. We also report on X-ray properties of thirteen other Fornax galaxies. Eight of these were detected in ROSAT images with luminosities in the 0.2 to 2 keV energy band from 1 × 1039 to 1.6 × 1041 ergs s-1. Five galaxies were sufficiently bright to permit spectral analyses and all but one (NGC 1380) had spectra consistent with thermal emission. Two (NGC 1404 and NGC 1387) of the four galaxies with well-constrained spectral parameters have hot coronae with characteristic gas temperatures of about 0.5 keV and iron abundances less than that found around NGC 1399 and other bright ellipticals. To maintain these hot coronae, the absolute magnitudes of these galaxies must be brighter than -19. Thus the distance to Fornax must be at least 18 Mpc, and, if there are no large peculiar velocities, the Hubble constant should be less than 75 km s-1 Mpc-1. Since these galaxies are all members of Fornax, distance uncertainties do not affect the relationship between their optical magnitude and X-ray luminosity. Analysis of the Fornax galaxies supports the contention that the scatter in the X-ray and optical relationship is intrinsic and does not arise solely from distance uncertainties. For the elliptical galaxy NGC 1404, the X-ray images show that the hot corona is distorted and likely is being stripped, indicating infall of the galaxy toward NGC 1399 and the cluster center.

The Dynamical Equilibrium of Galaxy Clusters

Abstract: If a galaxy cluster is effectively in dynamical equilibrium, then all galaxy populations within the cluster must have distributions in velocity and position that individually reflect the same underlying mass distribution, although the derived virial masses can be quite different. Specifically, within the Canadian Network for Observational Cosmology cluster sample, the virial radius of the red galaxy population is, on the average, a factor of 2.05 ± 0.34 smaller than that of the blue population. The red galaxies also have a smaller rms velocity dispersion, a factor of 1.31 ± 0.13 within our sample. Consequently, the virial mass calculated from the blue galaxies is 3.5 ± 1.3 times larger than from the red galaxies. However, applying the Jeans equation of stellar hydrodynamic equilibrium to the red and blue subsamples separately gives statistically identical cluster mass profiles. This is strong evidence that these clusters are effectively equilibrium systems and therefore demonstrates empirically that the masses in the virialized region are reliably estimated using dynamical techniques.

Projection effects in cluster catalogues

Abstract: We investigate the importance of projection effects in the identification of galaxy clusters in 2D galaxy maps and their effect on the estimation of cluster velocity dispersions. From large N-body simulations of a standard cold dark matter universe, we construct volume-limited galaxy catalogues that have similar low-order clustering properties to those of the observed galaxy distribution. We then select clusters using criteria tailored to match those employed in the construction of real cluster catalogues such as Abell’s. We find that our mock Abell cluster catalogues are heavily contaminated and incomplete. Over one third (34±6 per cent) of clusters of richness class R�1 are miclassifications arising from the projection of one or more sub-clumps onto an intrinsically poor cluster. Conversely, 32±5 per cent of intrinsically rich clusters are missed altogether from the R�1 catalogues, mostly because of statistical fluctuations in the background count. Selection by X-ray luminosity rather than optical richness reduces, but does not completely eliminate, these problems. Contamination by unvirialised sub-clumps near a cluster leads to an overestimation of the cluster velocity dispersion which can be very substantial even if the analysis is restricted only to galaxies close to the cluster centre. Thus, the distribution of cluster masses – often used to test cosmological models – is a highly unreliable statistic. The median value of the distribution, however, is considerably more robust because the main effect of contamination is to create an artificial tail of high velocity dispersion clusters. Improved estimates of the cluster velocity dispersion distribution require constructing new cluster catalogues in which clusters are defined according to the number of galaxies within a radius about three times smaller than the Abell radius.

Lensing by galaxy haloes in clusters of galaxies

Abstract: Weak shear maps of the outer regions of clusters have been successfully used to map the distribution of mass at large radii from the cluster center. The typical smoothing lengths employed thus far preclude the systematic study of the effects of galactic-scale substructure on the measured weak lensing signal. In this paper, we present two methods to infer the possible existence and extent of dark halos around bright cluster galaxies by quantifying the ‘local’ weak lensing induced by them. The proposed methods are: direct radial averaging of the shear field in the vicinity of bright cluster members and a maximum-likelihood method to extract fiducial parameters characterizing galaxy halos. The correlations observed for early-type galaxies on the Fundamental Plane are used to derive the scaling laws with luminosity in the modelling of cluster galaxies. We demonstrate using simulations that these observed local weak-shear effects on galaxy scales within the cluster can be used to statistically constrain the mean mass-to-light ratio, and fiducial parameters like the halo size, velocity dispersion and hence mass of cluster galaxies. We compare the two methods and investigate their relative drawbacks and merits in the context of feasibility of application to HST cluster data, whereby we find that the prospects are promising for detection on stacking a minimum of 20 WFPC2 deep cluster fields.

The VRI colours of H ii galaxies

Abstract: We present a high spatial resolution CCD surface photometry study in the optical V, R and I broadband filters of a sample of 15 HII galaxies. The colours of the starburst continuum and of the underlying galaxy are measured. The distribution of colours of the underlying galaxy in HII galaxies is similar to the colours of other late type low surface brightness galaxies which suggests a close kinship of these with the quiescent phases of HII galaxies. However, comparison with recent evolutionary population synthesis models shows that the observational errors and the uncertainties in the models are still too large to put strict constraints on their past star formation history.

The morphology of H II galaxies

Abstract: We present CCD images of a sample of 39 HII galaxies taken at the Danish 154m telescope on La Silla The images are used to analyse the morphology of these emission line dwarfs, and the structural properties of the knots of star formation and of the underlying galaxy The sizes of the starbursts are measured We propose a morphological classification based on the presence or absence of signs of tails, extensions, or distorted outer isophotes This criterion segregates the objects into two broad morphological types with different physical properties: the more disturbed and extended (type I) HII galaxies having larger luminosities and velocity dispersions than the more compact and regular (type II) objects The relative position of HII galaxies and of a sample of dwarf elliptical galaxies in the [R -- sigma] diagram support the hypothesis of a possible evolutionary link between the two types of galaxy

Dynamical Correlations for Globular Clusters in M31

Abstract: We present internal velocity dispersion measurements for a set of 21 globular clusters in the Andromeda galaxy (M31). We combine them with structural and photometric cluster parameters measured earlier with the Hubble Space Telescope and from the ground to explore correlations of cluster properties and to compare them with the equivalent correlations for the Galactic globular clusters. We find that the M31 globulars follow the same correlations between velocity dispersion and luminosity, central, and average surface brightness, as do their Galactic counterparts. This suggests a common physical origin for these correlations. They may be produced by the same astrophysical conditions and processes operating at the epoch of globular cluster formation in both galaxies. The very existence of these excellent correlations, and their quantitative form as scaling laws, represent challenges and constraints for theories of globular cluster formation. Preliminary estimates of the cluster M/L ratios show correlations with the cluster metallicity, in the sense of more metal-rich clusters having lower M/L, particularly in the near-infrared. At a given metallicity, there is no detectable systematic difference between the M31 globulars and their Galactic counterparts, which suggests a great similarity of their stellar populations. The observed scatter around these trends is comparable to the expected errors, which implies a small intrinsic scatter and thus which leaves little room for possible variations in the cluster age or stellar IMF at a given metallicity.

Another collision for the Coma cluster

Abstract: We describe a wavelet transform analysis of the ROSAT Position Sensitive Proportional Counter images of the Coma Cluster. On small scales, ≤1', the wavelet analysis shows substructure dominated by two extended sources surrounding the two brightest cluster galaxies, NGC 4874 and NGC 4889. On slightly larger scales, ~2', the wavelet analysis reveals a filament of X-ray emission originating near the cluster's center, curving to the south and east for ~25' in the direction of the galaxy NGC 4911, and ending near the galaxy NGC 4921. These results extend earlier ROSAT observations and further indicate the complex nature of the cluster's core. We consider two possible explanations for the production of the filamentary feature from interactions of the main cluster with a merging group. The feature could arise from either ram pressure stripped gas or a dark matter perturbation of tidally stripped material.

Globular Clusters in 19 Northern Abell Clusters

Abstract: We study the globular cluster (GC) populations of 21 giant elliptical galaxies in 19 Abell clusters. This method, applied here primarily in the R band, is based on the surface brightness fluctuations technique of extragalactic distance measurement. The sample galaxies range in redshift from 5000 to 10,000 \kms, and were selected primarily from the Lauer & Postman (1994) survey of brightest cluster galaxies (BCGs). We find a tight correlation between the GC specific frequency S_N of the central bright galaxy in the cluster and the cluster velocity dispersion. S_N also correlates well with the cluster X-ray temperature and with the number of bright neighboring galaxies, less well with the galaxy profile, and only marginally with galaxy luminosity and overall cluster richness. It does not correlate with cluster morphology class. Thus, unlike galaxy luminosity, S_N is determined by the cluster mass, or density. To account for this situation, we propose that the GCs formed early and in proportion to the available mass, while the luminosity growth of the galaxy was later halted, yielding the observed correlations of S_N with density. We introduce a quantity called $\eta_{GC}$, the number of GCs per unit local cluster mass. For a simple cluster mass model, $\eta_{GC}$ is found to be constant, indicating a uniform GC production rate per unit available mass. A measurement of the Gaussian width $\sigma$ of the GC luminosity function (GCLF) is one of the byproducts of our analysis. In the cosmic microwave background frame, the mean width for this sample is $ = 1.43$ mag, virtually identical to the HST value for M87, the galaxy used to calibrate the mean of the GCLF in this analysis.

The Luminosity Function of Galaxies in Clusters

Abstract: By means of statistical analyses, we compute the luminosity function of galaxies in clusters and explore its dependence on cluster global properties. The data consist of two different samples of southern clusters with published redshifts, taken from the Abell cluster catalog and the APM cluster catalog. Galaxy assignment to the clusters and their corresponding apparent magnitudes were derived from the Edinburgh-Durham Southern Galaxy Catalogue. We have considered two limiting absolute magnitudes, M < -16 and M < -17, comprising 16 and 55 clusters, respectively. The galaxy luminosity function in clusters derived in this work shows a large excess of faint galaxies compared with previous field determinations. In spite of their different identification procedures, we obtain similar galaxy luminosity functions in Abell and APM clusters. A Schechter function with parameters M* -20.0 ± 0.1 and α -1.4 ± 0.1 provides a satisfactory fit to the galaxy luminosity function of the sample of 55 clusters. We also find a flatter galaxy luminosity function in poor clusters (α -1.2). We have computed errors through a Monte Carlo procedure that considers different possible background corrections and errors in the photometry. Besides providing reliable values of the uncertainties in the luminosity function fitting parameters, this method poses confident limits on the effects of possible systematics. Our results strongly argue against a universal galaxy luminosity function. The high relative frequency of faint galaxies in rich clusters reported in this work may provide constraints on models for the origin and evolution of galaxies.

The Mgb-σ relation of elliptical galaxies at z ≈ 0.37

Abstract: We derive absorption line indices of elliptical galaxies in clusters at intermediate redshift (z ~ 0.37) from medium-resolution spectroscopy together with kinematical parameters. These galaxies exhibit a relationship between the line strength of Mg b p.o ~ 5170 A) and their internal velocity dispersion (1, similar to local dynamically hot galaxies. For any given (1, however, the Mg b line strength ofthe distant ellipticals is significantly lower than the mean value of the nearby samRle. The difference in Mg b between the two samples is small «.:1Mgb ) ~ - 0.4 A) and can be fully attributed to the lower age of the distant stellar populations in accordance with the passive evolution model for elliptical galaxies. The low reduction of Mgb at a look­ back time of about 5 Gyr requires that the bulk of the stars in cluster ellipticals have formed at very high redshifts of zf> 2. For the most massive galaxies, where the reduction is even lower, zf probably exceeds 4. Unlike most methods used to measure the evolution of elliptical galaxies using luminosities, surface brightnesses or colours, the Mgb -(1 test does not depend on corrections for extinction and cosmic expansion (k-correction) and depends only slightly on the slope of the initial mass function (IMF). The combination of a kinematical parameter with a stellar population indicator allows us to study the evolution of very similar objects. In addition, the good mass estimate provided by (1 means that the selection criteria for the galaxy sample as a whole are wellcontrolled. In quantitative agreement with the reduction of the Mgb absorption, we find an increase of the B magnitude of (.:1MB ) ~ -0.5 mag at fixed (1 from the Faber­ Jackson relation. The brightening of the ellipticals at Z = 0.37 arises solely from the evolution of their stellar populations and is of the same order as the change in magnitudes observed when varying the deceleration parameter qo from - 0.5 to + 0.5 at this redshift. Studying the evolution of the Mgb-(1 relation in combination with that of the Faber-Jackson relation allows us to constrain both the slope of the IMF and the value of the deceleration parameter. Our current data (including measurement errors) are compatible with the standard Salpeter IMF and qo=0.5 ± 0.5.

The Southern SHARC Survey: the z = 0.3-0.7 Cluster X-Ray Luminosity Function*

Abstract: We present the z = 0.3-0.7 cluster X-ray luminosity function (XLF) determined from the Southern Serendipitous High-Redshift Archival ROSAT Cluster (SHARC) survey. Over the luminosity range L ~ (0.3-3) × 1044 ergs s-1 (0.5-2.0 keV), the XLF is in close agreement with that of the low-redshift X-ray cluster population. This result greatly strengthens our previous claim of no evolution of the cluster population, at these luminosities, at a median redshift of z=0.44.

Spectroscopic Properties and Luminosity Distribution of the Universidad Complutense de Madrid Survey Galaxies

Abstract: A spectroscopic analysis of the entire sample of Ha emission-line galaxies (ELGs) contained in lists 1 and 2 of the Universidad Complutense de Madrid (UCM) objective-prism survey is presented. A significant fraction (59%) of star-forming galaxies with low-ionization or high-extinction properties has been found. This kind of ELG is only incompletely detected in the blue or in other ELG surveys. We have found evidence for evolution among some of the different ELG classes. A comparison between the populations detected by the Case, Kiso, University of Michigan, and UCM surveys is presented. We conclude that a deep H_α survey is better able to sample all the ages, evolutionary stages, and luminosities of currently star-forming galaxies than other surveys using blue emission lines or colors. Finally, the luminosity and spatial distributions of the UCM galaxies are determined. The contribution of the newly found, currently star-forming ELGs provides new clues to galaxy evolution and has to be taken into account when trying to consider the density of ELGs and total star formation rate in the universe.

Low Surface Brightness Galaxies in the Local Universe. III. Implications for the Field Galaxy Luminosity Function

Abstract: We present a luminosity function for low surface brightness (LSB) galaxies identi-ed in the Automatic Plate Measuring Facility survey of Impey et al. These galaxies have central surface brightnesses (k(0)) in B in the range 22.0 " k(0) " 25.0. Using standard maximum likelihood estimators, we determine that the best--t Schechter function parameters for this luminosity function (LF) are a \( 1.42, M* \( 18.34, and /* \ 0.0036, assuming km s~1 Mpc~1. We compare the luminosity and number den- H 0 \ 100 h 100 sities derived from this luminosity function with those obtained from other recent -eld galaxy studies and -nd that surveys that do not take account of the observational selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conser- vative estimates, our derivation of the LF for LSB galaxies suggests that the CfA Redshift Survey has missed at least one-third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help to close the gap between local and moderate-redshift galaxy counts. Subject headings: galaxies: fundamental parameters E galaxies: luminosity function, mass function E galaxies: statistics

Discovery of a Large-scale Abundance Gradient in the Cluster of Galaxies AWM 7 with ASCA

Abstract: A large-scale gradient in the metal abundance has been detected with ASCA from an X-ray bright cluster of galaxies, AWM 7. The metal abundance shows a peak of 0.5 solar at the center and smoothly declines to 0.2 solar at a radius of 500 kpc. The gas temperature is found to be constant at 3.8 keV. The radial distribution of iron can be fitted with a β model with β ~ 0.8 assuming the same core radius (115 kpc) as that of the intracluster medium. The metal distribution in AWM 7 suggests that the gas injected from galaxies is not efficiently mixed in the cluster space and traces the distribution of galaxies.