arXiv subjects

M. Girardi

Publications and source records attributed to M. Girardi.

At least 91 records · Page 5Linked to original sources

arXiv2008

Cluster Abell 520: a perspective based on member galaxies. A cluster forming at the crossing of three filaments?

The connection of cluster mergers with the presence of extended, diffuse radio sources in galaxy clusters is still debated. An interesting case is the rich, merging cluster Abell 520, containing a radio halo. A recent gravitational analysis has shown in this cluster the presence of a massive dark core suggested to be a possible problem for the current cold dark matter paradigm. We aim to obtain new insights into the internal dynamics of Abell 520 analyzing velocities and positions of member galaxies. Our analysis is based on redshift data for 293 galaxies in the cluster field obtained combining new redshift data for 86 galaxies acquired at the TNG with data obtained by CNOC team and other few data from the literature. We also use new photometric data obtained at the INT telescope. We combine galaxy velocities and positions to select 167 cluster members around z~0.201. We analyze the cluster structure using the weighted gap analysis, the KMM method, the Dressler-Shectman statistics and the analysis of the velocity dispersion profiles. We compare our results with those from X-ray, radio and gravitational lensing analyses. We find that Abell 520 is definitely a very complex system. Our results suggest that we are looking at a cluster forming at the crossing of three filaments of the large scale structure. In particular, we detect a filament aligned with the LOS and projected onto the center of the forming cluster. It might explain the apparent massive dark core shown by gravitational lensing analysis.

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arXiv2008

Optical analysis of the poor clusters Abell 610, Abell 725, and Abell 796, containing diffuse radio sources

We study the dynamical status of the poor, low X-ray luminous galaxy clusters Abell 610, Abell 725, and Abell 796 (at z=0.1, 0.09, and 0.16, respectively), containing diffuse radio sources (relic, relic, and possible halo, respectively). Our analysis is based on new spectroscopic data obtained at the William Herschel Telescope for 158 galaxies, new photometry obtained at the Isaac Newton Telescope with the addition of data recovered from the Data Release 5 of the Sloan Digital Sky Survey. We use statistical tools to select 57, 36, and 26 cluster members and to analyze the kinematics of cluster galaxies, as well as to study the 2D cluster structure. The low values we compute for the global line-of-sight velocity dispersion of galaxies (420-700 km/s) confirm that these clusters are low-mass clusters. Abell 610 shows a lot of evidence of substructure. It seems to be formed by two structures separated by about 700 km/s in the cluster rest-frame, having comparable velocity dispersion (about 200 km/s) and likely causing a velocity gradient. The analysis of the 2D galaxy distribution shows a bimodal distribution in the core elongated in the SE-NW direction. Abell 725 and Abell 796, which are less sampled, show marginal evidence of substructure in the velocity space. They are elongated in the 2D galaxy distribution. For both Abell 610 and Abell 725 we shortly discuss the possible connection with the hosted diffuse radio relic. Our results show that relic radio sources are likely connected with merger events,but are not limited to massive clusters. About the possible halo source in Abell 796, there is some evidence of a merger event in this non-massive cluster, but a pointed radio observation is necessary to confirm this halo.

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arXiv2007

VLT and ACS observations of RDCS J1252.9-2927: dynamical structure and galaxy populations in a massive cluster at z=1.237

We present results from an extensive spectroscopic survey, carried out with VLT FORS, and from an extensive multiwavelength imaging data set from the HST Advanced Camera for Surveys and ground based facilities, of the cluster of galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster members in the redshift range 1.22 < z < 1.25. A cluster median redshift of z=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s are obtained. Using the 38 confirmed redshifts, we were able to resolve, for the first time at z > 1, kinematic structure. The velocity distribution, which is not Gaussian at the 95% confidence level, is consistent with two groups that are also responsible for the projected east-west elongation of the cluster. The groups are composed of 26 and 12 galaxies with velocity dispersions of 486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation is also seen in the intracluster gas and the dark matter distribution. This leads us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state. We extend the analysis of the color-magnitude diagram of spectroscopic members to more than 1 Mpc from the cluster center. The scatter and slope of non-[OII]-emitting cluster members in the near-IR red sequence is similar to that seen in clusters at lower redshift. Furthermore, most of the galaxies with luminosities greater than ~ K_s*+1.5 do not show any [OII], indicating that these more luminous, redder galaxies have stopped forming stars earlier than the fainter, bluer galaxies. Our observations provide detailed dynamical and spectrophotometric information on galaxies in this exceptional high-redshift cluster, delivering an in-depth view of structure formation at this epoch only 5 Gyr after the Big Bang.

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arXiv2007

Internal dynamics of the radio halo cluster Abell 773: a multiwavelength analysis

We conduct an intensive study of the rich, X-ray luminous galaxy cluster A773 at z=0.22 containing a diffuse radio halo to determine its dynamical status. Our analysis is based on new spectroscopic data obtained at the TNG telescope for 107 galaxies, 37 spectra recovered from the CFHT archive and new photometric data obtained at the INT telescope. We use statistical tools to select 100 cluster members (out to ~1.8 Mpc from the cluster centre), to analyse the kinematics of cluster galaxies and to determine the cluster structure. Our analysis is also performed by using X-ray data stored in the Chandra archive. Results: The 2D distribution shows two significant peaks separated by ~2 arcmin in the EW direction with the main western one closely located at the position of the two dominant galaxies and the X-ray peak. The velocity distribution of cluster galaxies shows two peaks at v~65000 and ~67500 km/s, corresponding to the velocities of the two dominant galaxies. The low velocity structure has a high velocity dispersion sigma_v=800-1100 km/s and its galaxies are centred on the western 2D peak. The high velocity structure has intermediate velocity dispersion sigma_v~500 km/s and is characterized by a complex 2D structure with a component centred on the western 2D peak and a component centred on the eastern 2D peak, these components probably being two independent groups.We estimate a cluster mass within 1 Mpc of 6--11 e^14 h_70 Msun. Our analysis of Chandra data shows the presence of two very close peaks in the core and the elongation of the X-ray emission in the NEE--SWW direction. Our results suggest we are looking at probably two groups in an advanced stage of merging. In particular, the radio halo seems to be related to the merger of the eastern group.

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arXiv2007

The dynamical status of the galaxy cluster Abell 115

We present the results of a new spectroscopic and photometric survey of the hot, binary X-ray cluster A115 at z=0.193, containing a radio relic. We combine galaxy velocity and position information to select 85 galaxies recognized as cluster members (from 115 galaxies observed at TNG telescope), determine global dynamical properties and detect substructures. We find that A115 presents velocity dispersion of sigma_v=1362_{-108}^{+126} km/s. Our analysis confirms the presence of two structures (A115N and A115S) of cluster--type well recognizable in the plane of the sky and shows that they differ of ~2000 km/s in the LOS velocity. We estimate that A115S is slightly dynamically more important than A115N having sigma_v=900-1100 km/s vs sigma_v=750-850 km/s. Moreover, we find evidence for two small groups at low velocities. We estimate a global cluster virial mass of 2.2--3.5 e15 h_70 Msun. Our results agree with a pre--merging scenario where A115N and A115S are colliding with a LOS impact velocity Delta v~1600 km/s.The most likely solution to the two--body problem suggests that the merging axis lies at $\sim 20$ degrees from the plane of the sky and that the cores will cross after ~0.1 Gyr. The radio relic with its largest dimension perpendicular to the merging axis is likely connected to this merger.

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arXiv2007

Low-Frequency study of two clusters of galaxies: A2744 and A2219

Spectral index images can be used to constraint the energy spectrum of relativistic electrons and magnetic field distribution in radio halos and relics, providing useful information to understand their formation, evolution and connection to cluster merger processes. We present low-frequency images of the two clusters of galaxies: A2744 and A2219, in which a wide diffuse emission is detected. Observations were made with the Very Large Array at the frequency of 325 MHz. For both clusters deep Very Large Array 1.4 GHz observations are available. Combining the 325 MHz and 1.4 GHz data, we obtained the spectral index images and the brightness radial profiles of the diffuse radio emission with a resolution of ~ 1'. The azimuthally averaged spectral index in A2744 is constant to a value close to alpha ~ 1 up to a distance of 1 Mpc from the cluster center. However, the spectral index image shows the presence of localized regions in which the radio spectrum is significantly different from the average. The observed spectral index variations range from a minimum of alpha ~ 0.7 +/- 0.1 to a maximum alpha ~ 1.5 +/- 0.2. From the comparison of the spectral index with the X-rays data it is found for the first time that the flat spectrum regions of the radio halo tend to have higher temperature. In the case of A2219, the radio emission in the central regions of the cluster is dominated by the blend of discrete sources. The azimuthally averaged radio spectrum is alpha ~ 0.8 in the central region of the cluster and is close to a value of alpha ~ 1 in the radio halo. The limited sensitivity of the 325 MHz image does not allowed us to detect all the radio halo structure seen at 1.4 GHz and therefore no constrains on the point-to-point variations of the spectral index have been obtained for this cluster.

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arXiv2006

On the efficiency and reliability of cluster mass estimates based on member galaxies

We study the efficiency and reliability of cluster mass estimators that are based on the projected phase-space distribution of galaxies in a cluster region. To this aim, we analyse a data-set of 62 clusters extracted from a concordance LCDM cosmological hydrodynamical simulation. Galaxies (or Dark Matter particles) are first selected in cylinders of given radius (from 0.5 to 1.5 Mpc/h) and ~200 Mpc/h length. Cluster members are then identified by applying a suitable interloper removal algorithm. Two cluster mass estimators are considered: the virial mass estimator (Mvir), and a mass estimator (Msigma) based entirely on the cluster velocity dispersion estimate. Mvir overestimates the true mass by ~10%, and Msigma underestimates the true mass by ~15%, on average, for sample sizes of > 60 cluster members. For smaller sample sizes, the bias of the virial mass estimator substantially increases, while the Msigma estimator becomes essentially unbiased. The dispersion of both mass estimates increases by a factor ~2 as the number of cluster members decreases from ~400 to ~20. The bias in the Mvir estimates is reduced in clusters without significant evidence for subclustering, and when only early-type galaxies are selected. Radially-dependent incompleteness can drastically affect Mvir estimates, but leaves the Msigma estimates almost unaffected. Other observational effects, like centering and velocity errors, and different observational apertures, have little effect on the mass estimates. (Abridged)

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arXiv2006

Internal dynamics of the massive cluster Abell 697: a multiwavelength analysis

We conduct an intensive study of the rich, X-ray luminous, and hot galaxy cluster Abell 697 (at z=0.282), likely containing a diffuse radio emission, to determine its dynamical status. Our analysis is based on new spectroscopic data obtained at the TNG telescope for 93 galaxies and on new photometric data obtained at the INT telescope. We combine galaxy velocity and position information to select 68 cluster members, determine global dynamical properties, and detect possible substructures. The investigation of the dynamical status is also performed by using X-ray data stored in the Chandra archive. We compute the line-of-sight (LOS) velocity dispersion of galaxies, sigma_v=1334 km s^-1, in agreement with the high average X-ray temperature T_X=10.2 keV recovered from Chandra data. Assuming that the cluster is in dynamical equilibrium and mass follows the galaxy distribution, we find that A697 is a very massive cluster obtaining M(<R_max=0.75 Mpc h^-1)=9.5x10^14 solar masses h^-1 and M(<R_vir=3.85 Mpc h^-1)=4.5x10^15 solar masses h^-1 for the region well sampled by the spectroscopic data and for the entire virialized region, respectively. Further investigations find that A697 is not fully relaxed, as shown by the non Gaussianity of the velocity distribution, the elongation of the X-ray emission, and the presence of small-size substructures in the central region. Our results suggest that we are looking at a cluster undergone to a complex cluster merger occurring roughly mainly along the LOS, with a transverse component in the SSE-NNW direction. Our study supports the hypothesis of a relation between extended radio emission and merging phenomena.

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arXiv2006

Internal dynamics of the radio-halo cluster A2744

We present a detailed dynamical analysis of the rich galaxy cluster A2744, containing a powerful diffuse radio halo.Our analysis is based on redshift data for 102 galaxies, part of them recovered from unexplored spectra in the ESO archive. We combine galaxy velocity and position information to select the cluster members and determine global dynamical properties of the cluster. We use a variety of statistical tests to detect possible substructures. We find that A2744 appears as a well isolated peak in the redshift space at =0.306, which includes 85 galaxies recognized as cluster members. We compute the line-of-sight (LOS) velocity dispersion of galaxies (~1750 km/sec), which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 8 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non-Gaussian nature of the velocity distribution, the presence of a velocity gradient and a significant substructure. In particular, our results suggest a merging scenario of two clumps with a mass ratio of 3:1 and a LOS impact velocity (rest frame) of ~3000 km/sec, likely observed just after the core passage. The merging is occuring roughly in the NS direction with the axis close to the LOS. This scenario agrees with that proposed on the basis of recent Chandra results in its general lines, although suggesting a somewhat more advanced merging phase.

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arXiv2005

Internal dynamics of the z sim 0.8 cluster RX J0152.7-1357

We present the results from the dynamical analysis of the cluster of galaxies RX J0152-1357, which shows a complex structure in its X-ray emission, with two major clumps in the central region, and a third clump in the Eastern region. Our analysis is based on redshift data for 187 galaxies. We compute the line-of-sight velocity dispersion of galaxies, sigma_V=1322^+74_-68 kms^-1, which is significantly larger than what is expected in the case of a relaxed cluster with an observed X-ray temperature of 5-6 keV. We find evidence that this cluster is far from dynamical equilibrium, as shown by the non Gaussianity of the velocity distribution, the presence of a velocity gradient and significant substructure. Our analysis shows that the high value of sigma_V is due to the complex structure of RX J0152-1357, i.e. to the presence of three galaxy clumps of different mean velocity. Using optical data we detect a low-velocity clump (with sigma_V=300-500 kms^-1) in the central SW region and a high--velocity clump (with sigma_V sim 700 kms^-1) in the E region, well corresponding to the SW and E peaks detected in the X-ray emission. The central NE X-ray peak is associated to the main galaxy structure with a velocity which is intermediate between those of the other two clumps and sigma_V sim 900 kms^-1. The mass of the whole system within 2 Mpc is estimated to lie in the range (1.2-2.2)X10^{15} M_sun. Analytic calculations based on the two-body model indicate that the system is most likely bound, currently undergoing merging. In particular, we suggest that the SW clump is not a small group, but rather the dense cluster-core of a massive cluster.

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arXiv2004

A VLT spectroscopic survey of RX J0152.7-1357, a forming cluster of galaxies at z=0.837

We present the results of an extensive spectroscopic survey of RX J0152.7-1357, one of the most massive distant clusters of galaxies known. Multi-object spectroscopy, carried out with FORS1 and FORS2 on the ESO Very Large Telescope (VLT), has allowed us to measure more than 200 redshifts in the cluster field and to confirm 102 galaxies as cluster members. The mean redshift of the cluster is $z=0.837 \pm 0.001$ and we estimate the velocity dispersion of the overall cluster galaxy distribution to be $\sim 1600 \mathrm{km \ s^{-1}}$. The distribution of cluster members is clearly irregular, with two main clumps that follow the X-ray cluster emission mapped by Chandra. A third clump of galaxies to the east of the central structure and at the cluster redshift has also been identified. The two main clumps have velocity dispersions of $\sim919$ and $\sim737 \mathrm{km s^{-1}}$ respectively, and the peculiar velocity of the two clumps suggests that they will merge into a single more massive cluster. A segregation in the star formation activity of the member galaxies is observed. All star forming galaxies are located outside the high-density peaks, which are populated only by passive galaxies. A population of red galaxies (belonging to the cluster red sequence) with clear post-starburst spectral features and [OII] ($λ$3727) emission lines is observed in the outskirts of the cluster. Two AGNs, which were previously confused with the diffuse X-ray emission from the intracluster medium in ROSAT and BeppoSAX observations, are found to be cluster members.

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arXiv2004

Transformations of galaxies in the environments of the cluster ABCG 209 at z~0.2

We analyse the properties of galaxy populations in the rich Abell cluster ABCG 209 at redshift z~0.21, on the basis of spectral classification of 102 member galaxies. We take advantage of available structural parameters to study separately the properties of bulge-dominated and disk-dominated galaxies. The star formation histories of the cluster galaxy populations are investigated by using line strengths and the 4000 A break, through a comparison to stellar population synthesis models. The dynamical properties of different spectral classes are examined in order to infer the past merging history of ABCG 209. The cluster is characterized by the presence of two components: an old galaxy population, formed very early (z_f >~ 3.5), and a younger (z$_f >~ $ 1.2) population of infalling galaxies. We find evidence of a merger with an infalling group of galaxies occurred 3.5-4.5 Gyr ago. The correlation between the position of the young H_delta-strong galaxies and the X-ray flux shows that the hot intracluster medium triggered a starburst in this galaxy population ~ 3 Gyr ago.

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arXiv2004

Galaxy Evolution in the Environment of ABCG 209

We examine the environmental effects on the photometric properties of galaxies for the rich galaxy cluster ABCG 209 at z=0.209. We use archive CFHT optical imaging of a 42'x28' field centred on the cluster to produce a galaxy sample complete to B=25.0 and R=24.5. Both the composite and red sequence galaxy luminosity functions are found to be dependent on the local galaxy surface density, their faint-end slopes becoming shallower with increasing density. We explain this as a combination of the morphology-density relation, and dwarf galaxies being cannibalised and/or disrupted by the cD galaxy and the ICM in the cluster core. The B-R colour of the red sequence itself appears 0.02mag redder for the highest-density regions, indicative of their stellar populations being marginally (<5%) older or (<20%) more metal-rich. This may be due to the galaxies themselves forming earliest in the rarest overdensities marked by rich clusters, or their star-formation being suppressed earliest by the ICM.

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arXiv2003

Internal dynamics of the radio--halo cluster A2219: a multi-wavelength analysis

We present the results of the dynamical analysis of the rich, hot, and X-ray very luminous galaxy cluster A2219, containing a powerful diffuse radio-halo. Our analysis is based on new redshift data for 27 galaxies in the cluster region, measured from spectra obtained at the TNG, with the addition of other 105 galaxies recovered from reduction of CFHT archive data in a cluster region of about 5 arcmin radius (about 0.8 Mpc h^-1 at the cluster distance) centered on the cD galaxy. The investigation of the dynamical status is also performed by using X-ray data stored in the Chandra archive. Further, valuable information comes from other bands - optical photometric, infrared, and radio data - which are analyzed and/or discussed, too. Our analyses support the view of the connection between extended radio emission and merging phenomena in galaxy clusters.

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arXiv2003

Galaxies in group and field environments: a comparison of optical-NIR luminosities and colors

We compare properties of galaxies in loose groups with those in field environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy systems. We consider as group galaxies, objects belonging to systems with at least five members identified by means of the "friends of friends method", and, as field galaxies, all galaxies with no companions. We analyze both a magnitude--limited sample of 959 and 2035 galaxies (groups vs. field galaxies, respectively, B<14 mag, and 2000<cz<6000 km/s) and a volume-limited sample (M_B <-19.01 mag, 2000<cz<4000 km/s 369 group and 548 field galaxies). For all these galaxies, blue corrected magnitudes and morphological types are available. The cross-correlation of NOG with the 2MASS second release allow us to assign K magnitudes and obtain B-K colors for about half of the galaxies in our samples. We analyze luminosity and color segregation-effects in relation with the morphological segregation. For both B and K bands, we find that group galaxies are, on average, more luminous than field galaxies and this effect is not entirely a consequence of the morphological segregation. After taking into account the morphological segregation, the luminosity difference between group and field galaxies is about 10%. When considering only very early-type galaxies (T<-2) the difference is larger than 30%. We also find that group galaxies are redder than field galaxies, Delta(B-K) about 0.4 mag. However, after taking into account the morphological segregation, we find a smaller B-K difference, poorly significant (only at the c.l. of about 80%).

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arXiv2003

Optical Luminosity Functions of the Abell Galaxy Cluster ABCG 209 at z=0.21

We derive the luminosity functions in three bands (BVR) for the rich galaxy cluster ABCG 209 at z=0.21. The data cover an area of ~78 arcmin^2 in the B and R bands, while a mosaic of three pointings was obtained in the V band, covering an area of approximately 160 arcmin^2. The galaxy sample is complete to B = 22.8 (N_gal = 339), V = 22.5 (N_gal = 1078) and R = 22.0 (N_gal = 679). Although the fit of a single Schechter function cannot be rejected in any band, the luminosity functions are better described by a sum of two Schechter functions for bright and faint galaxies, respectively. There is an indication for a presence of a dip in the luminosity functions in the range V = 20.5--21.5 and R = 20.0--21.0. We find a marked luminosity segregation, in the sense that the number ratio of bright--to--faint galaxies decreases by a factor 4 from the center to outer regions. Our analysis supports the idea that ABCG 209 is an evolved cluster, resulting from the merger of two or more sub--clusters.

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arXiv2003

Morphology and luminosity segregation of galaxies in nearby loose groups

We study morphology and luminosity segregation of galaxies in loose groups. We analyze the two catalogs of groups which have been identified in the Nearby Optical Galaxy (NOG) sample, by means of hierarchical and percolation ``friends-of-friends'' methods (HG and PG catalogs, respectively). In the first part of our analysis we consider 387 and 436 groups of HG and PG, respectively, and compare morphology- (luminosity-) weighted to unweighted group properties: velocity dispersion, mean pairwise distance, and mean groupcentric distance of member galaxies. The second part of our analysis is based on two ensemble systems, one for each catalog, built by suitably combining together galaxies of all groups (1584 and 1882 galaxies for HG and PG groups, respectively). We find that earlier-type (brighter) galaxies are more clustered and lie closer to the group centers, both in position and in velocity, than later-type (fainter) galaxies. Spatial segregations are stronger than kinematical segregations. These effects are generally detected at the larger than 3-sigma level. Luminosity segregation is shown to be independent of morphology segregation. Our main conclusions are strengthened by the detection of segregation in both hierarchical and percolation catalogs. Our results agree with a continuum of segregation properties of galaxies in systems, from low-mass groups to massive clusters.

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arXiv2002

Structure and Evolution of Galaxy Clusters: Internal Dynamics of ABCG 209 at z~0.21

We study the internal dynamics of the rich galaxy cluster ABGC 209 on the basis of new spectroscopic and photometric data. The distribution in redshift shows that ABCG 209 is a well isolated peak of 112 detected member galaxies at z=0.209, characterised by a high value of the line-of-sight velocity dispersion, sigma_v=1250-1400 Km/s, on the whole observed area (1 Mpc/h from the cluster center), that leads to a virial mass of M=1.6-2.2x10^15 M_sun within the virial radius, assuming the dynamical equilibrium. The presence of a velocity gradient in the velocity field, the elongation in the spatial distribution of the colour-selected likely cluster members, the elongation of the X-ray contour levels in the Chandra image, and the elongation of cD galaxy show that ABCG 209 is characterised by a preferential NW-SE direction. We also find a significant deviation of the velocity distribution from a Gaussian, and relevant evidence of substructure and dynamical segregation. All these facts show that ABCG 209 is a strongly evolving cluster, possibly in an advanced phase of merging.

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