arXiv subjects

M. Girardi

Publications and source records attributed to M. Girardi.

At least 109 records · Page 6Linked to original sources

arXiv2001

Observational Mass-to-Light Ratio of Galaxy Systems: from Poor Groups to Rich Clusters

We study the mass-to-light ratio of galaxy systems from poor groups to rich clusters, and present for the first time a large database for useful comparisons with theoretical predictions. We extend a previous work, where B_j band luminosities and optical virial masses were analyzed for a sample of 89 clusters. Here we also consider a sample of 52 more clusters, 36 poor clusters, 7 rich groups, and two catalogs, of about 500 groups each, recently identified in the Nearby Optical Galaxy sample by using two different algorithms. We obtain the blue luminosity and virial mass for all systems considered. We devote a large effort to establishing the homogeneity of the resulting values, as well as to considering comparable physical regions, i.e. those included within the virial radius. By analyzing a fiducial, combined sample of 294 systems we find that the mass increases faster than the luminosity: the linear fit gives M\propto L_B^{1.34 \pm 0.03}, with a tendency for a steeper increase in the low--mass range. In agreement with the previous work, our present results are superior owing to the much higher statistical significance and the wider dynamical range covered (about 10^{12}-10^{15} M_solar). We present a comparison between our results and the theoretical predictions on the relation between M/L_B and halo mass, obtained by combining cosmological numerical simulations and semianalytic modeling of galaxy formation.

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arXiv2000

Optical/Infrared Survey of Galaxy Clusters

In this contribution the ongoing effort to build a statistical sample of clusters of galaxies over a wide range of redshifts to study the evolution of clusters and member galaxies is reviewed. The starting point for this project has been the list of candidate clusters identified from the I-band EIS-WIDE survey data. Since the completion of this survey, new optical/infrared observations have become available and have been used to confirm some of these candidates using the photometric data alone or in combination with the results of follow-up spectroscopic observations. Our preliminary results show that the yield of real physical associations from the original catalog is conservatively >60% and that a large sample of clusters in the southern hemisphere, extending to high-redshifts, is within reach.

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arXiv2000

Clustering of Galaxies and Groups in the NOG Sample

We use the two-point correlation function in redshift space, $ξ(s)$, to study the clustering of the galaxies and groups of the Nearby Optical Galaxy (NOG) Sample, which is a nearly all-sky, complete, magnitude-limited sample of \~7000 bright and nearby optical galaxies. The correlation function of galaxies is well-described by a power-law, $ξ(s)= (s/s_0)^{-γ}$, with $γ\sim1.5$ and $s_0\sim 6.4 h^{-1}$ Mpc. We find evidence of morphological segregation between early- and late-type galaxies, with a gradual decreasing of the strength of clustering from the S0 to the late-type spirals, on intermediate scales. Furthermore, luminous galaxies (with $M_B\leq -19.5 + 5 \log h$) are more clustered than dim galaxies. The groups show an excess of clustering with respect to galaxies. Groups with greater velocity dispersions, sizes, and masses are more clustered than those with lower values of these quantities.

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arXiv2000

Evolution of the Internal Dynamics of Galaxy Clusters

We consider a sample of 51 distant galaxy clusters at 0.15 about 0.3), each cluster having at least 10 galaxies with available redshift in the literature. We select member galaxies, analyze the velocity dispersion profiles, and evaluate in a homogeneous way cluster velocity dispersions and virial masses. We apply the same procedures already recently applied on a sample of nearby clusters (z<0.15, Girardi et al. 1998b) in order to properly analyze the possible dynamical evolution of galaxy clusters. We remark problems induced by the poor sampling and the small spatial extension of the sampled cluster region in the computation of velocity dispersion. We do not find any significant difference between nearby and distant clusters. In particular, we consider the galaxy spatial distribution, the shape of the velocity dispersion profile, and the relations between velocity dispersion and X-ray luminosity and temperature. Our results imply little dynamical evolution in the range of redshift spanned by our cluster sample, and suggest that the typical redshift of cluster formation is higher than that of the sample we analyze.

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arXiv2000

Dynamical Evolution of Clusters

We consider a sample of about 50 distant galaxy clusters at z>0.15 ( about 0.3), each cluster having at least 10 galaxies with available redshift in the literature. We select member galaxies, analyze the velocity dispersion profiles, and evaluate in a homogeneous way cluster velocity dispersions and virial masses. We apply the same procedures already recently applied on a sample of nearby clusters (z<0.15) in order to properly analyze the possible dynamical evolution of galaxy clusters. We do not find any significant difference between nearby and distant clusters. In particular, we consider the galaxy spatial distribution, the shape of the velocity dispersion profile, and the relations between velocity dispersion and X-ray luminosity and temperature. Our results imply little dynamical evolution in the range of redshift spanned by our cluster sample, and suggest that the typical redshift of cluster formation is higher than that of the sample we analyze.

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arXiv2000

Weak lensing mass reconstruction of MS1008.1-1224

We present an in-depth weak lensing analysis of the cluster MS1008 based on deep multicolor imaging obtained during the Science Verification of FORS1 at the VLT. The image quality (half arcsec seeing) and depth of the VLT images allow the shear signal to be mapped with high signal-to-noise and to be traced out to 1.2 h_50^-1 Mpc, near the edge of the 6'.8x6'.8 field of view. Using BVRI color information, as well as 81 redshifts in the field from the CNOC survey, background galaxies can be effectively separated from cluster and foreground objects. PSF distorsions are found to be moderate across the FORS images and thus easily removed. Due to the small statistical errors in the mass reconstruction, this dataset provides a testing ground where several systematic effects (e.g. mass-sheet degeneracy, redshift distribution of the background sources, cluster galaxy contamination) can be quantified. Several methods are used to remove the mass-sheet degeneracy which is found to dominate the systematic error budget. We measure a lower limit to the mass of 2.6 10^14 h_50^-1 M_sun within 1 h_50^-1 Mpc and a ``total'' mass of 5.3 10^14 h_50^-1 M_sun by fitting a softened isothermal sphere. The availability of the CNOC redshift data and X-ray observations on this cluster allow a comparison of different determinations of the mass radial profile. We find the lensing and X-ray measurements in excellent agreement, while the mass derived from the virial analysis is marginally (1-2 sigma) in agreement at radii where both methods are reliable.

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arXiv2000

Spectroscopic confirmation of clusters from the ESO imaging survey

We measure redshifts for 67 galaxies in the field of six cluster candidates from the ESO Imaging Survey (EIS). The cluster candidates are selected in the EIS patches C and D among those with estimated mean redshifts between 0.5 and 0.7. The observations were made with EFOSC2 at the 3.6m ESO telescope. In the six candidate cluster fields, we identify 19 possible sets of 2 to 7 galaxies in redshift space. In order to establish which of the 19 sets are likely to correspond to real dense systems we compare our counts with those expected from a uniform distribution of galaxies with given luminosity function. In order to take into account the effect of the Large Scale Structure, we modulate the probability computed from the luminosity function with random samplings of the Canada-France Redshift Survey. We find that four out of six candidate EIS clusters are likely to correspond to real systems in redshift space (> 95 % confidence level). Two of these systems have mean redshift in agreement with the redshift estimate given by the matched filter algorithm. The other two systems have significantly lower redshifts. We discuss the implications of our results in the context of our ongoing research projects aimed at defining high-redshift optically-selected cluster samples.

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arXiv2000

The Observational Mass Function of Loose Galaxy Groups

We analyze the three catalogs of nearby loose groups identified by Garcia (1993). She used a percolation and a hierarchical method, and proposed a third group catalog defined as a combination of the two. Each catalog contains almost 500 groups. In agreement with previous works on earlier catalogs, we find that groups can be described as collapsing systems. Their sampled size is much larger than their expected virialized region. We compute the virial masses and correct them by taking into account the young dynamical status. We estimate group masses, M, for two cosmological models, a flat one with Omega_0=1 and an open one with Omega_0=0.2. For each of the three catalogs we calculate the mass function, MF. The number density of groups with M>9x10^{12}\msun, which is the adopted limit of sample completeness, ranges within 1.3-1.9x10^{-3}h^3/Mpc^3 for Omega_0=1, and it is about a factor of 15% lower for Omega_0=0.2. The MFs of the hierarchical and combined catalogs have essentially the same shape, while the MF of the percolation catalog shows a flattening towards large masses. However, the difference decreases if we do not consider the most massive groups, for which reliable results come from galaxy cluster studies. After having estimated the mass contained within the central, presumably virialized, regions of groups by adopting a reduction in mass of 30-40%, we do a comparison with the results coming from the virial analysis of nearby rich clusters (Girardi et al. 1998). All three group MFs turn out to be a smooth extrapolation of the cluster MF at M<4x10^{14}\msun, which is the completeness limit of the cluster sample. The resulting optical virial MF of galaxy systems, which extends over two orders of magnitude, is fitted to a Schechter expression with a slope of about -1.5 and a characteristic mass of about 3x10^{14}\msun.

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arXiv1999

The redshift-space two-point correlation function of galaxy groups in the CfA2 and SSRS2 surveys

We measure the two-point redshift-space correlation function of loose groups of galaxies, xi_GG, for the combined CfA2 and SSRS2 surveys. Our combined group catalog constitutes the largest homogeneous sample available (885 groups). We compare xi_GG with the correlation function of galaxies, xi_gg, in the same volume. We find that groups are significantly more clustered than galaxies: =1.64+-0.16. A similar result holds when we analyze a volume-limited sample (distance limit 78 h^-1 Mpc) of 139 groups. For these groups, with median velocity dispersion sigma_v sim 200 km s^-1 and mean group separation d sim 16 h^-1 Mpc, we find that the correlation length is s_0=8+-1 h^-1 Mpc, which is significantly smaller than that found for rich clusters. We conclude that clustering properties of loose groups of galaxies are intermediate between galaxies and rich clusters. Moreover, we find evidence that group clustering depends on physical properties of groups: correlation strengthens for increasing sigma_v.

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arXiv1999

Confirming EIS Clusters. Multi-object Spectroscopy

Using EFOSC2 at the 3.6m ESO telescope, we obtained redshifts for 68 galaxies in the field of six cluster candidates from the ESO Imaging Survey (EIS). The cluster candidates were selected in the EIS patches C and D and have estimated mean redshifts between z=0.5 and z=0.7. In the six candidate cluster fields, we identify possible systems of galaxies in the redshift space. The likelihoods of these systems are established by comparison with random resamplings of the Canada-France Redshift Surveys, and using the redshift distribution expected from Postman et al.'s luminosity function, with Poggianti's K- and evolutionary-corrections. Four of the six candidate EIS clusters are found to correspond to a real system in the redshift space, with > 95 % probability. Two of them have a mean redshift in agreement with the estimate from the matched filter algorithm, while the other two have a significantly smaller redshift. The independent analysis of the V-I vs. I color-magnitude diagrams for five of our six cluster fields, supports our conclusions based on the spectroscopic data.

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arXiv1999

Velocity Dispersions of CNOC Clusters and the Evolution of the Cluster Abundance

We present the results of the analysis of the internal velocity dispersions, σ_v, for the CNOC sample of distant galaxy clusters, based on an interlopers removal algorithm, which is different from that originally applied by Carlberg et al. (1996, C96). We find that the resulting σ_v values are consistent within <10% with the original C96 estimates. This result points in favor of a substantial robustness of currently applied methods for optical studies of the internal cluster dynamics. The resulting distribution of velocity dispersions is used to trace the redshift evolution of the cluster abundance with the aim of constraining the matter density parameter, Ω_m. We find that constraints on Ω_m are very sensitive to the adopted value of σ_8, as obtainable from the local cluster abundance: as σ_8 varies from 0.5 to 0.6 (for Omega_m=1), the best fitting Omega_m varies in the range 0.3-1.0.

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arXiv1999

Optical Luminosities and Mass--to--Light Ratios of Nearby Galaxy Clusters

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well defined high degree of completeness. In particular, we consider a subsample of 89 clusters with B_j band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalogue. We compute cluster luminosities L_{B_j} within several clustercentric distances and within the virialization radius R_{vir}. In particular, we use the luminosity function and background counts estimated by Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, L_{B_j}, and galaxy velocity dispersion, sigma_v, with L_{B_j} proportional to sigma_v^{2.1--2.3}. Our estimate of typical value for the mass-to-light ratio is M/L_{B_j} about 250 (in solar units). We do not find any correlation of M/L_{B_j} with cluster morphologies, i.e. Rood--Sastry and Bautz--Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M proportional to L_{B_j}^{1.2--1.3}. We verify the robustness of this relation against a number of possible systematics.

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arXiv1998

The Observational Mass Function of Nearby Galaxy Clusters

We present a new determination of the mass function of galaxy clusters, based on optical virial mass estimates for a large sample of 152 nearby (z\le0.15) Abell-ACO clusters, as provided by Girardi et al. (1998). This sample includes both data from the literature and the new ENACS data. The resulting mass function is reliably estimated for masses larger than M_{lim}\simeq 4\times 10^{14} \msun, while it is affected by sample incompleteness at smaller masses. We find N(>M_{lim})=(6.3\pm 1.2) 10^{-6} (h^{-1} Mpc)^{-3} for cluster masses estimated within a 1.5 h^{-1} radius. Our mass function is intermediate between the two previous estimates by Bahcall & Cen (1993) and by Biviano et al. (1993). Based on the Press-Schechter approach, we use this mass function to constrain the amplitude of the fluctuation power spectrum at the cluster scale. After suitably convolving the PS predictions with observational errors on cluster masses and COBE-normalizing the fluctuation power spectrum, we find sigma_8=(0.60\pm 0.04) Omega_0^{-0.46+0.09 Omega_0} for flat low-density models and sigma_8=(0.60\pm 0.04) Omega_0^{-0.48+0.17 Omega_0} for open models (at the 90% c.l.).

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arXiv1998

Optical Mass Estimates of Galaxy Clusters

We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z< 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.

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arXiv1997

New Optical Insights into the Mass Discrepancy of Galaxy Clusters: The Cases of A1689 and A2218

We analyze the internal structures of clusters A1689 and A2218 by applying a recent development of the method of wavelet analysis, which uses the complete information obtained from optical data, i.e. galaxy positions and redshifts. We find that both clusters show the presence of structures superimposed along the line of sight with different mean redshifts and smaller velocity dispersions than that of the system as a whole, suggesting that the clusters could be cases of the on-going merging of clumps. In the case of A2218 we find an acceptable agreement between our estimate of optical virial mass and X-ray and gravitational lensing masses. On the contrary, in the case of A1689 we find that our mass estimates are smaller than X-ray and gravitational lensing ones at both small and large radii. In any case, at variance with earlier claims, there is no evidence that X-ray mass estimates are underestimated.

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arXiv1997

Cosmology Using Cluster Internal Velocity Dispersions

We compare the distribution of internal velocity dispersions of galaxy clusters for an observational sample to those obtained from a set of N-body simulations of seven COBE-normalised cosmological scenarios: the standard CDM (SCDM) and a tilted (n=0.85) CDM (TCDM) model, a CHDM model with 25% of massive neutrinos, two low-density LCDM models with Omega_0=0.3 and 0.5, two open OCDM models with Omega_0=0.4 and 0.6. Simulated clusters are observed in projection so as to reproduce the main observational biases and are analysed by applying the same algorithm for interlopers removal and velocity dispersion estimate as for the reference observational sample. Velocity dispersions for individual clusters can be largely affected by observational biases in a model-dependent way: models in which clusters had less time to virialize show larger discrepancies between 3D and projected velocity dispersions. From the comparison with real clusters we find that both SCDM and TCDM largely overproduce clusters. The CHDM model marginally overproduces clusters and requires a somewhat larger sigma_8 than a purely CDM model in order to produce the same cluster abundance. The LCDM model with Omega_0=0.3 agrees with data, while the open model with Omega_0=0.4 and 0.6 underproduces and marginally overproduces clusters, respectively.

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arXiv1996

Optical Substructures in 48 Galaxy Clusters

We analyze the presence of substructures in a set of 48 galaxy clusters, by using galaxy positions and redshifts. We use a multi-scale analysis which couples kinematical estimators with the wavelet transform. 14% of our clusters are strongly substructured (i.e. they are bimodal or complex) and 24% of the remaining unimodal clusters contain substructures at small scales. Thus, in substantial agreement with previous studies, about one third of clusters show substructures. In unimodal clusters the presence of substructures does not affect the estimates of both virial masses and velocity dispersions, which are generally in good agreement with the X-ray temperatures. Thus, unimodal clusters are not too far from a status of dynamical equilibrium. On the contrary, velocity dispersions and masses for some bimodal or complex clusters strongly depend on whether they are treated as single systems or as sums of different clumps and X-ray temperatures and velocity dispersions may be very different.

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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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