Shrinking of Cluster Ellipticals: a Tidal Stripping explanation and Implications for the Intra-Cluster Light

We look for evidence of tidal stripping in elliptical galaxies through the analysis of homogeneous CCD data corresponding to a sample of 228 elliptical galaxies belonging to 24 clusters of galaxies at $0.015<z<0.080$. We investigate departures from the standard magnitude-isophotal size relation, as a function of environmental (cluster-centric distance, local galaxy density) and structural (cluster velocity dispersion, Bautz-Morgan type) properties. We find that, for any particular galaxy luminosity, the ellipticals in the inner and denser regions of the clusters are about 5% smaller than those in the outer regions, which is in good agreement with the finding of Strom & Strom (1978) based on photographic photometry. The null hypothesis (ie., galaxy sizes are independent of the cluster-centric distance or density) is rejected at a significance level of better than 99.7%. Numericals models of Aguilar & White (1986) predict that tidal stripping can lead to changes in the whole structure of ellipticals producing shrinkage and brightening of the galaxy, qualitatively consistent with our measurements and also with the findings of Trujillo et al. (2002), that more centrally concentrated ellipticals populate denser regions. Our observational results can be interpreted as evidence for stripping of stars from ellipticals in the central/denser regions of clusters, contributing to the intra-cluster light observed in these structures.Comment: AJ Accepted, 15 pages, 9 figure

Weak lensing mass distributions for 24 X-Ray Abell Clusters

We use the weak gravitational lensing effect to study the mass distribution and dynamical state of a sample of 24 X-ray luminous clusters of galaxies ($0.05<z<0.31$) observed with the VLT-FORS1 under homogeneous sky conditions and subarsecond image quality. The galaxy shapes were measured after deconvolution with a locally determined point-spread-function, while the two-dimensional mass distributions of the clusters were computed using an algorithm based on the maximum entropy method. By comparing the mass and light distributions of the clusters in our sample, we find that their mass centers, for the majority of the clusters, is consistent with the positions of optical centers. We find that some clusters present significant mass substructures which generally have optical counterparts. At least in one cluster (Abell 1451), we detect a mass substructure without an obvious luminous counterpart. The radial profile of the shear of the clusters was fitted using circular and elliptical isothermal elliptical distributions, which allowed the finding of a strong correlation between the orientation of the major-axis of the matter distribution and the corresponding major-axes of the brightest cluster galaxy light-profiles. Estimates of how close to dynamical relaxation are these clusters were obtained through comparison of our weak-lensing mass measurements with the x-ray and velocity dispersion determinations available in the literature. We find that clusters with intra-cluster gas colder than 8 keV show a good agreement between the different mass determinations, but clusters with gas hotter than 8 keV present discrepant mass values.Comment: 23 pages, 8 figures. Submitted to ApJ A version of this paper with all the figures of the appendix A can be found at http://www.astro.iag.usp.br/~eduardo/shear.ps.g

Structure and dynamics of the supercluster of galaxies SC0028-0005

According to the standard cosmological scenario, superclusters are objects that have just passed the turn around point and are collapsing. The dynamics of very few superclusters have been analysed up to now. In this paper we study the supercluster SC0028-0005, at redshift 0.22, identify the most prominent groups and/or clusters that make up the supercluster, and investigate the dynamic state of this structure. For the membership identification, we have used photometric and spectroscopic data from SDSS-DR10, finding 6 main structures in a flat spatial distribution. We have also used a deep multi-band observation with MegaCam/CFHT to estimate de mass distribution through the weak-lensing effect. For the dynamical analysis, we have determined the relative distances along the line of sight within the supercluster using the Fundamental Plane of early-type galaxies. Finally, we have computed the peculiar velocities of each of the main structures. The 3D distribution suggests that SC0028-005 is indeed a collapsing supercluster, supporting the formation scenario of these structures. Using the spherical collapse model, we estimate that the mass within $r = 10$~Mpc should lie between 4 and $16 \times 10^{15} M_\odot$. The farthest detected members of the supercluster suggest that within $\sim 60$~Mpc the density contrast is $\delta \sim 3$ with respect to the critical density at $z=0.22$, implying a total mass of $\sim 4.6$--$16 \times 10^{17} M_\odot$, most of which in the form of low-mass galaxy groups or smaller substructures.Comment: 12 pages, 9 figures, Accepted for publication in MNRA

The luminosity function of the fossil group RX J1552.2+2013

We determine the first fossil group luminosity function based on spectroscopy of the member galaxies. The fossil group RX J1552.2+2013 has 36 confirmed members, it is at a mean redshift of 0.136 and has a velocity dispersion of 623 km/s (or 797 km/s if four emission lines galaxies in the outskirts of the velocity distribution are included). The luminosity function of RX J1552.2+2013, measured within the inner region of the system ~1/3 R_vir), in the range -23.5< M_i'<-17.5, is well fitted by a Schechter function with M*i'=-21.3 +/- 0.4 and alpha = -0.6 +/- 0.3 or a Gaussian function centered on M_i'= -20.0 +/- 0.4 and with sigma=1.29 +/- 0.24 i' mag. (H_0 = 70 km/s Mpc, Omega_M=0.3, Omega_Lambda=0.7. The luminosity function obtained from a photometric survey in g', r', i'-bands (and statistical background correction) confirms the spectroscopically determined results. There is a significant dip in the luminosity function at M_r'=-18 mag, as also observed for other clusters. RX~J1552.2+2013 is a rich, strongly red-galaxy dominated system, with at least 19 galaxies with magnitudes between m_3 and m_3 + 2, within a surveyed circular area of radius 625 kpc centered on the peak of the x-ray emission. Its mass, ~3.0 10^14 M_0, M/L of 507 M_sol/L_B_sol and L_X of 6.3 10^43 ergs/s (bolometric) are more representative of a fossil cluster than of a fossil group. The central object of RX J1552.2+2013 is a cD galaxy which may have accreted the more luminous ~L* former members of the group. Although dynamical friction and subsequent merging are probably the processes responsible for the lack of bright galaxies in the system, for the fainter members, there must be another mechanism in action (perhaps tidal disruption) to deplete the fossil group from intermediate-luminosity galaxies M_r' ~ -18.Comment: 14 pages, 7 Figures. accepted by A

Velocity dispersion, mass and the luminosity function of the fossil cluster RX J1416.4+2315

We study the properties of the fossil cluster RX J1416.4+2315 through g' and i'-band imaging and spectroscopy of 25 member galaxies. The system is at a mean redshift of 0.137 and has a velocity dispersion of 584 km s^-1. Superimposed onto one quadrant of the cluster field there is a group of five galaxies at a mean redshift of 0.131, which, if included as part of the cluster, increases the velocity dispersion to 846 km/s. The central object of RX J1416.4+2315 is a normal elliptical galaxy, with no cD envelope. The luminosity function of the system, estimated by the number counts, and statistical background correction, in the range -22.6< M_g'< -16.6, is well fitted by a Schechter function with M_g'^* = -21.2 +/- 0.8 and alpha = -1.2 +/- 0.2 (H_0 = 70 km s^-1 Mpc^-1, Omega_M=0.3, Omega_Lambda=0.7). The luminosity function obtained from the spectroscopically confirmed members in both g' and i' bands agrees with the photometric results. The mass of the system, M 0.9 \times 10^14 h^-1_70 M_sun, its M/L of 445 h_70 M_sun/L_B_sun and L_X of 11 10^43 h^-2_70 ergs s^-1 (bolometric) suggest that this system is the second example of known fossil cluster, after RX J1552.2+2013, confirmed in the literature.Comment: Accepted by AJ - 14 pages, 6 figure

On the mass‐to‐light ratios of fossil groups. Are they simply dark clusters?

Defined as X‐ray bright galaxy groups with large differences between the luminosities of their brightest and second brightest galaxies, ‘fossil groups’ are believed to be some of the oldest galaxy systems in the Universe. They have therefore been the subject of much recent research. In this work we present a study of 10 fossil group candidates with an average of 33 spectroscopically confirmed members per group, making this the deepest study of its type to date. We also use these data to perform an analysis of the luminosity function of our sample of fossil groups. We confirm the high masses previously reported for many of fossil systems, finding values more similar to those of clusters than of groups. We also confirm the high dynamical mass‐to‐light ratios reported in many previous studies. While our results are consistent with previous studies in many ways, our interpretation is not. This is because we show that, while the luminosities of the bright central galaxies (BCGs) in these systems are consistent with their high dynamical masses, their richnesses (total number of galaxies above some canonical value) are extremely low. This leads us to suggest a new interpretation of fossil systems in which the large differences between the luminosities of their brightest and second brightest galaxies are simply the result of the high BCG luminosities and low richnesses, while the high masses and low richnesses also explain the high mass‐to‐light ratios. Our results therefore suggest that fossil systems can be characterized as cluster‐like in their masses and BCG luminosities, but possessing the richnesses and optical luminosities of relatively poor groups. These findings are not predicted by any of the current models for the formation of fossil groups. Therefore, if this picture is confirmed, current ideas about the formation and evolution of fossil systems will need to be reformulated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/89559/1/j.1365-2966.2011.19625.x.pd

Gravitational Lensing by Nearby Clusters of Galaxies

We present an estimation of the expected number of arcs and arclets in a sample of nearby (z<0.1) clusters of galaxies, that takes into account the magnitude limit of the objects as well as seeing effects. We show that strong lensing effects are not common, but also they are not as rare as usually stated. Indeed, for a given cluster, they present a strong dependence with the magnitude limit adopted in the analysis and the seeing of the observations. We also describe the procedures and results of a search for lensing effects in a sample of 33 clusters spanning the redshift range of 0.014 to 0.076, representative of the local cluster distribution. This search produced two arc candidates. The first one is in A3408 (z=0.042), the same arc previously discovered by Campusano & Hardy (1996), with z=0.073 and associated to the brightest cluster galaxy. The second candidate is in the cluster A3266 (z=0.059) and is near a bright elliptical outside the cluster center, requiring the presence of a very massive sub-structure around this galaxy to be produced by gravitational lensing.Comment: 22 pages including 9 Figures and 2 Tables, submitted to A

The K luminosity-metallicity relation for dwarf galaxies and the tidal dwarf galaxies in the tails of HCG 31

We determine a K-band luminosity-metallicity (K-Z) relation for dwarf irregular galaxies, over a large range of magnitudes, -20.5 < M_K < -13.5, using a combination of K photometry from either the 2-micron all sky survey (2MASS) or the recent study of Vadivescu er al. (2005), and metallicities derived mainly with the T_e method, from several different studies. We then use this newly-derived relation, together with published K_s photometry and our new spectra of objects in the field of HCG 31 to discuss the nature of the possible tidal dwarf galaxies of this group. We catalogue a new member of HCG 31, namely "R", situated ~40 kpc north of the group center, composed by a ring of H alpha knots which coincides with a peak in HI. This object is a deviant point in the K-Z relation (it has too high metallicity for its luminosity) and its projected distance to the parent galaxy and large gas reservoir makes it one of the most promising tidal dwarf galaxy candidates of HCG 31, together with object F. The subsystems A1, E, F, H and R all have metallicities similar to that of the galaxies A+C and B, result that is expected in a scenario where those were formed from material expelled from the central galaxies of HCG 31. While objects A1, E and H will most probably fall back onto their progenitors, F and R may survive as tidal dwarf galaxies. We find that two galaxies of HCG 31, G and Q, have A+em spectral signatures, and are probably evolving toward a post-starburst phase.Comment: 32 pages, 4 figures - Submitted to AJ - A version of this paper with full resolution figures can be found at http://www.astro.iag.usp.br/~eduardo/HCG31-KZrelation.pd