arXiv subjects

A. Biviano

Publications and source records attributed to A. Biviano.

323 records · Page 18Linked to original sources

arXiv1998

ENACSVII, Galaxy density profiles of rich clusters of galaxies

We have analyzed the projected galaxy distributions in a subset of the ENACS/ COSMOS cluster sample. We made Maximum-Likelihood fits to the distribution of COSMOS galaxies for 4 theoretical profiles, with `cores' (generalized King- and Hubble-profiles) and with `cusps' (generalized Navarro et al., or NFW, and de Vaucouleurs profiles). We use the Likelihood ratio to investigate whether the observations are better described by profiles with cusps or with cores. Taking the King and NFW profiles as model of either class, we find that about 75% of the clusters are better fitted by the King profile than by the NFW profile. However, for the individual clusters the preference for the King profile is rarely significant. When we limit ourselves to the central regions it appears that the significance increases drastically, with 65% of the clusters showing a strong preference for a King over an NFW profile. We constructed composite clusters from the COSMOS and ENACS data, taking special care to avoid the creation or the destruction of cusps. We scale by different ways (projected distances, core radii, r_{200}).In all three cases we find that the King profile is clearly preferred over the NFW profile. However, this preference is not shared by the brightest galaxies. We conclude that these galaxies are represented almost equally well by King and NFWprofiles, but that the distribution of the fainter galaxies clearly shows a core rather than a cusp. Finally, we compared the outer slope of the galaxy distributions in ourclusters with results for model calculations with different cosmological parameters. We conclude that the observed profile slope indicates a low value for Omega_0. This is consistent with the direct estimate of Omega_0 based on the M/L ratios of the individual clusters.

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arXiv1998

ISO Mid-Infrared Observations of Abell 370

We report on the mid-IR imaging at 7 and 10 microns of the galaxy cluster Abell 370, obtained with the ISOCAM instrument onboard ESA's Infrared Space Observatory (ISO), as part of an ongoing program to image gravitational arcs and arclets in distant clusters. We have recorded initial detections of the A0 giant arc in both bandpasses, the two central dominant galaxies at 7 microns, and several other cluster members and field galaxies. These preliminary results are indicative of the potential output of our program when more extensive observational data become available.

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arXiv1998

Deep ISOCAM Observations of Abell 2218

We report on the mid-infrared imaging at 5, 7, 10 and 15 microns of the galaxy cluster Abell 2218 obtained with the ISOCAM instrument onboard ESA's Infrared Space Observatory (ISO), as part of an on-going program to image gravitational arcs and arclets in distant clusters. Several cluster galaxies as well as field galaxies are detected. We discuss their mid-IR flux properties.

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arXiv1997

Segregations in clusters of galaxies

We analyse a sample of about 2000 galaxies in 40 regular clusters, to look for evidence of segregation with respect to galaxy luminosities and morphological types. We find evidence of luminosity segregation for galaxies brighter than $M_R < -21.5$, i.e. typically the four brightest members of each cluster. We also find evidence of morphological segregation: both the core-radius and the velocity dispersion increase along the Hubble sequence (ellipticals - S0 - early spirals - late spirals). Galaxies of different types have different velocity dispersion profiles, being steeper for later type galaxies. Simple modelling allows us to show that elliptical (and, to a lesser extent, S0) orbits are mostly tangential in the cluster core, and nearly isotropic outside, while spiral (in particular late-spiral) orbits are predominantly radial. A viable interpretation of our results is that (1) late spirals, at variance with other type galaxies, are a non-virialized cluster population, still on partially radial infalling orbits, (2) the elliptical phase-space distribution is evolving towards energy equipartition through the process of dynamical friction, (3) S0 and early-spirals have intermediate distributions between these two extremes.

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arXiv1997

The Fundamental Plane of clusters of galaxies

We have used the ESO Nearby Abell Cluster Survey (ENACS) in combination with the Cosmos Galaxy Catalogue, to investigate the existence of a Fundamental Plane (FP) for rich clusters of galaxies. The 20 clusters with the most regular projected galaxy distributions appear to define a quite narrow FP, which is similar to the FP found by Schaeffer et al., who used other clusters. Our cluster FP appears to be different from that of ellipticals, as well as from the virial prediction. The latter fact may have several physical explanations, or a combination thereof. If M/L varies with L this will change the FP slope away from the virial slope. Differences in dynamical structure between clusters will also produce deviations from the virial FP. In view of the long virialization time-scales in all but the very central parts of galaxy clusters, the deviation of the cluster FP from the virial expectation may also result from clusters not being totally virialized. The scatter of the observations around the cluster FP is fairly small. An important part of the observed scatter is likely to be intrinsic. If this intrinsic spread were due exclusively to deviations from the Hubble flow it would imply cluster peculiar velocities of at most about 1000 km.s-1.

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arXiv1996

The ESO Nearby Abell Cluster Survey. III. Distribution and Kinematics of Emission-Line Galaxies

We have studied the frequency of occurrence, the kinematics and the spatial distribution of Emission-Line Galaxies (ELG) in clusters in the ESO Nearby Abell Cluster Survey (ENACS). More than 90% of the ELG are spirals, but the ELG represent only 30% of all spirals. The true ELG fraction in clusters is 0.10; this is consistent with that in the field (0.21) as a result of differences in morphological mix. The average ELG fraction decreases with increasing velocity dispersion of the cluster. Only 1/5 of the clusters shows a significant difference in the average velocities of the ELG and the other (non-ELG) galaxies; in a similar fraction of the clusters significant differences in velocity dispersion are seen. Yet, the velocity dispersion of the ELG is, on average, 20% larger than that of the non-ELG. This larger velocity dispersion is largely intrinsic, i.e. it is not likely to be the result of velocity offsets of the ELG. The surface density of the ELG is significantly less peaked towards the centre than that of the non-ELG. The ELG are distributed fairly smoothly within the cluster with at most 25% in compact substructures. The distribution and kinematics of the ELG yield virial mass estimates that, on average, are 50% higher than those based on the non-ELG. This probably means that the ELG are on more radial orbits than the non-ELG. All observed properties of the ELG are consistent with most of them being on their first approach of the high-density cluster core.

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arXiv1996

A photometric catalogue of the Coma cluster core

We have obtained a mosaic of CCD images of the Coma cluster in the V-band covering a region of approximately 0.4 degrees$^2$ around both central cluster galaxies NGC 4889 and NGC 4874. An additional frame of $\sim$ 90 arcmin$^2$ was taken of the south-west region around NGC 4839. We derived a catalogue of 7023 galaxies and 4096 stars containing positions, central surface brightnesses and isophotal V$_{26.5}$ magnitudes. We estimate that data is complete up to V $_{26.5}\sim$ 22.5 and the surface brightness limiting detection value is $μ$ $\sim$ 24 mag/arcsec$^2$. In this paper we present the catalogue (available in electronic form alone), along with a detailed description of the steps concerning the data reduction and quality of the computed parameters.

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arXiv1996

Environmental effects on the Coma cluster luminosity function

Using our catalogue of V$_{26.5}$ isophotal magnitudes for 6756 galaxies in a region covering 60~$\times$~25~arcmin$^2$ in the center of the Coma cluster, plus 267 galaxies in a region of 9.7~$\times$~9.4~arcmin$^2$ around NGC~4839, we derive the luminosity function in the magnitude range 13.5$\leq V_{26.5} <$ 21.0 (corresponding to the absolute magnitude range $-22.24 < M_{V26.5} \leq -14.74$). The luminosity function for this region is well fitted by the combination of a gaussian in its bright part and of a steep Schechter function (of index $α=-1.8$) in its faint part. Luminosity functions derived for individual regions surrounding the brightest galaxies show less steep slopes, strongly suggesting the existence of environmental effects. The implications of such effects and galaxy formation scenarios are discussed.

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arXiv1995

Unveiling hidden structures in the Coma cluster

We have assembled a large data-set of 613 galaxy redshifts in the Coma cluster, the largest presently available for a cluster of galaxies. We have defined a sample of cluster members complete to b$_{26.5}=20.0$, using a membership criterion based on the galaxy velocity, when available, or on the galaxy magnitude and colour, otherwise. Such a data set allows us to define nearly complete samples within a region of 1~\Mpc\ radius, with a sufficient number of galaxies per sample to make statistical analyses possible. Using this sample and the {\em ROSAT} PSPC X--ray image of the cluster, we have re-analyzed the structure and kinematics of Coma, by applying the wavelet and adaptive kernel techniques. A striking coincidence of features is found in the distributions of galaxies and hot intracluster gas. The two central dominant galaxies, NGC4874 and NGC4889, are surrounded by two galaxy groups, mostly populated with galaxies brighter than b$_{26.5}=17$ and well separated in velocity space. On the contrary, the fainter galaxies tend to form a single smooth structure with a central peak coinciding in position with a secondary peak detected in X--rays, and located between the two dominant galaxies; we suggest to identify this structure with the main body of the Coma cluster. A continuous velocity gradient is found in the central distribution of these faint galaxies, a probable signature of tidal interactions rather than rotation. There is evidence for a bound population of bright galaxies around other brightest cluster members. Altogether, the Coma cluster structure seems to be better traced by the faint galaxy population, the bright galaxies being located in subclusters. We discuss this evidence in terms of an ongoing accretion of groups onto the cluster.

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arXiv1995

The Kinematics of Emission-Line Galaxies in Clusters from ENACS

Summary - By using the data from the ESO Nearby Abell Cluster Survey (ENACS), we have studied the properties of emission-line galaxies (ELG) in clusters. The fraction of ELG in clusters is 16~\%; this fraction is slightly dependent on the cluster velocity dispersion, being larger for ``colder'' systems. ELG tend to occur more frequently than other cluster galaxies in substructures. However, only a minor cluster fraction has an ELG population with a mean velocity significantly different from that of the overall cluster population. The velocity distribution of cluster ELG is broader than that of the other cluster galaxies, which translates in a larger observed velocity dispersion. The velocity distribution for ELG in the inner cluster regions suggests that ELG are on mostly radial orbits.

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arXiv1995

The Kinematics of Emission-Line Galaxies in Clusters from ENACS

By using the ENACS data, we have studied the properties of emission-line galaxies (ELG) in clusters. The fraction of ELG in clusters is 16~%; this fraction is slightly dependent on the cluster velocity dispersion, being larger for ``colder'' systems. ELG tend to occur more frequently than other cluster galaxies in substructures. However, only a minor cluster fraction has an ELG population with a mean velocity significantly different from that of the overall cluster population. The velocity distribution of cluster ELG is broader than that of the other cluster galaxies, which translates in a larger observed velocity dispersion. The velocity distribution for ELG in the inner cluster regions suggests that ELG are on mostly radial orbits.

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arXiv1995

New Results on the Coma cluster: Revealing the Primary Component

Recent observations of the Coma cluster of galaxies in its central region have provided approximately 250 new redshifts - allowing a good membership criterion to be established for brighter galaxies - and magnitudes for 8000 objects in the same region derived from photometric data complete up to V$_{26.5}$=22.5. A thorough structural study of the galaxy distribution of the cluster galaxies, jointly with an X-ray wavelet analysis, and with a kinematical analysis of the velocity distribution allows us to uncover a primary body of the cluster, with evidence for a velocity gradient.

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arXiv1995

Velocity Dispersions and X-Ray Temperatures of Galaxy Clusters

Using a large and well-controlled sample of clusters of galaxies, we investigate the relation between cluster velocity dispersions and X-ray temperatures of intra-cluster gas. In order to obtain a reliable estimate of the total velocity dispersion of a cluster, independent of the level of anisotropies in galaxy orbits, we analyze the integrated velocity dispersion profiles over increasing distances from the cluster centers. Distortions in the velocity fields, the effect of close clusters, the presence of substructures, and the presence of a population of (spiral) galaxies not in virial equilibrium with the cluster potential are taken into account. Using our final sample of 37 clusters, for which a reliable estimate of the velocity dispersion could be obtained, we derive a relation between the velocity dispersions and the X-ray temperatures, with a scatter reduced by more than 30 % with respect to previous works. A chi square fit to the temperature-velocity dispersion relation does not exclude the hypothesis that the ratio between galaxy and gas energy density (the so-called spectral beta) is a constant for all clusters. In particular, the value of beta=1, corresponding to energy equipartition, is acceptable.

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arXiv1994

On the galaxy luminosity function in the central regions of the Coma cluster

We have obtained new redshifts for 265 objects in the central 48~$\times$~25~arcmin$^2$ region of the Coma cluster. When supplemented with literature data, our redshift sample is 95~\% complete up to a magnitude b$_{26.5}$=18.0 (the magnitudes are taken from the photometric sample of Godwin et al. 1983). Using redshift-confirmed membership for 205 galaxies, and the location in the colour-magnitude diagram for another 91 galaxies, we have built a sample of cluster members which is complete up to b$_{26.5}$=20.0. We show that the Coma cluster luminosity function cannot be adequately fitted by a single Schechter (1976) function, because of a dip in the magnitude distribution at b$_{26.5}$$\sim$17. The superposition of an Erlang (or a Gauss) and a Schechter function provides a significantly better fit. We compare the luminosity function of Coma to those of other clusters, and of the field. Luminosity functions for rich clusters look similar, with a maximum at $M_{b} \simeq -19.5 + 5 \times \log h_{50}$, while the Virgo and the field luminosity functions show a nearly monotonic behaviour. These differences may be produced by physical processes related to the environment which affect the luminosities of a certain class of cluster galaxies.

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arXiv1994

Optical Radii of Galaxy Clusters

We analyze the density profiles and virial radii for a sample of 90 nearby clusters, using galaxies with available redshifts and positions. Each cluster has at least 20 redshifts measured within an Abell radius, and all the results come from galaxy sets of at least 20 members. Most of the density profiles of our clusters are well fitted by hydrostatic-isothermal-like profiles. The slopes we find for many cluster density profiles are consistent with the hypothesis that the galaxies are in equilibrium with the binding cluster potential. The virial radii correlate with the core radii at a very high significance level. The observed relationship between the two size estimates is in agreement with the theoretical one computed by using the median values of the density profile parameters fitted on our clusters. After correcting for incompleteness in our cluster sample, we provide the universal distributions functions of core and virial radii (obtained within half an Abell radius).

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arXiv1993

Structures in Galaxy Clusters

The analysis of the presence of substructures in 16 well-sampled clusters of galaxies suggests a stimulating hypothesis: Clusters could be classified as unimodal or bimodal, on the basis of to the sub-clump distribution in the {\em 3-D} space of positions and velocities. The dynamic study of these clusters shows that their fundamental characteristics, in particular the virial masses, are not severely biased by the presence of subclustering if the system considered is bound.

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arXiv1993

The Mass Function of Nearby Galaxy Clusters

We present the distribution of virial masses for nearby galaxy clusters, as obtained from a data-set of 75 clusters, each having at least 20 galaxy members with measured redshifts within 1 Abell radius. After having accounted for problems of incompleteness of the data-set, we fitted a power-law to the cluster mass distribution.

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