H. Muriel

On the relationship between environment and galaxy properties in clusters of galaxies

We study the correlation between different properties of bright (L > L*) galaxies in clusters and the environment in the Sloan Digital Sky Survey (SDSS). Samples of clusters of galaxies used in this paper are those selected by Coenda & Muriel that are drawn from the Popesso et al. and Koester et al. samples. Galaxies in these clusters have been identified in the main Galaxy sample of the Fifth Data Release (DR5) of SDSS. We analyse which galaxy properties correlate best with either cluster mass or cluster-centric distance using the technique by Blanton et al. We find that galaxy properties do not clearly depend on cluster mass for clusters more massive than M ∼ 10 14 M ⊙ . On the other hand, galaxy properties correlate with cluster-centric distance. The property most affected by the cluster-centric distance is g - r colour, closely followed by the u - r colour. These results are irrespective of the cluster selection criteria. The two samples of clusters were identified based on the X-ray emission and the galaxy colours, respectively. Moreover, the parameter that best predicts environment (i.e. cluster-centric distance) is the same found by Martinez & Muriel for groups of galaxies and Blanton et al. for the local density of field galaxies.

Relating bars with the environment in the nearby Universe

We study the correlation between the fraction of barred spiral galaxies and environmental parameters of galaxies to understand in which environments the bars are more commonly found. For this purpose, we apply the Blanton et al. technique to a sample of spiral galaxies drawn from the Nair & Abraham catalogue. Our results agree with previous findings in which the fraction of barred galaxies is almost insensitive to environment.

Comparing galaxy populations in compact and loose groups of galaxies

Aims. We compare the properties of galaxies in compact groups, loose groups, and the field to deepen our understanding of the physical mechanisms acting upon galaxy evolution in different environments. Methods. We select samples of galaxies in compact groups, loose groups, and field galaxies from the Sloan Digital Sky Survey. We compare the properties of the galaxy populations in these different environments: absolute magnitude, colour, size, surface brightness, stellar mass, and concentration. We also study the fraction of red and early-type galaxies, the luminosity function, the colourluminosity, and luminosity-size relations. Results. The population of galaxies in compact groups differ from that of loose groups and the field. The fraction of red and early-type galaxies is larger in compact groups. Galaxies in compact groups are, on average, systematically smaller in size, more concentrated, and have higher surface brightnesses than galaxies in the field and in loose groups. At a fixed absolute magnitude, or fixed surface brightness, galaxies in compact groups are smaller in size. Conclusions. The physical mechanisms that transform galaxies into earlier types could be more effective within compact groups, owing to the typically high densities and small velocity dispersions of these environments, which could explain the large fraction of red and early-type galaxies we found in compact groups. Galaxies inhabiting compact groups have undergone a major transformation compared to galaxies that inhabit loose groups.

The environmental dependence of the red galaxy sequence

The dependence of the sequence of red galaxies (RS) with the environment is investigated using volume-limited samples of field, group and cluster galaxies drawn from the Sloan Digital Sky Survey. Our work focuses on studying the mean colour (μ R ) and the scatter (σ R ) of the RS as a function of 0.1 r-band absolute magnitude in different environments characterized either by the mass of the system in which the galaxies are located or by the distance to the systems centre. These results are compared with the RS of field galaxies. The same analysis is carried out using subsamples of red galaxies classified as early types according to their concentration parameter. For a given luminosity, μ R of field galaxies is bluer and σ R is larger than their group and cluster counterparts irrespective of mass and position within the systems. Among systems of galaxies, high-mass groups and clusters have the reddest μ R and the smallest σ R . These differences almost disappear when red early-type galaxies alone are considered. Galaxies in the core and in the outskirts of groups have similar μ R , whereas galaxies in clusters show a strong dependence on cluster-centric distance, the inner galaxies being the reddest objects. Red early-type galaxies in the outskirts of clusters have σ R values as large as those of field galaxies, while galaxies in the inner regions of clusters have lower values, comparable to those of group galaxies. We find that bright red early-type galaxies have reached nearly the same evolutionary stage in all environments. Our results suggest that, although effective in drifting galaxies of intermediate luminosities towards redder colours, the cluster environment is not necessary to populate the RS. We propose a scenario in which the RS in massive systems is populated by two different star formation history galaxies: red early-type galaxies that formed the bulk of their stars during the early stages of massive halo assembly and red galaxies that passed most of their lives inhabiting poor groups or the field and fell into massive systems at lower redshifts.

Comparing galaxy populations in compact and loose groups of galaxies - II. Brightest group galaxies

Aims. The properties of the brightest galaxies (BCGs) are studied in both compact and loose groups of galaxies in order to better understand the physical mechanisms influencing galaxy evol ution in different environments. Methods. Samples of BCGs are selected in the compact groups identified by McConnachie et al. (2009), and in loose groups taken from Zandivarez & Mart´ inez (2011). The following physical properties of the BCGs in compact groups and in subsamples of loose groups are compared, defined by their mass and total luminosi ty: absolute magnitude, colour, size, surface brightness, stellar mass, concentration and morphological information from the Galaxy Zoo. The fraction of BCGs classified as red and /or early-type as a function of galaxy luminosity are studied. The fraction of t he groups total luminosity contained in the BCG and the difference in luminosity between the BCG and the second-ranked galaxy, are also analysed. Results. Some properties of BCGs in compact and loose groups are comparable. However, BCGs in compact groups are systematically more concentrated and have larger surface brightness than t heir counterparts in both, high- and low-mass loose groups. The fractions of red and early-type BCGs in compact groups are consistent with those of high-mass loose groups. Comparing BCGs in subsamples of compact and loose groups selected for their similar luminosities, BCGs in compact groups are found to be, on average, brighter, more massive, larger, redder and more frequently classified as elliptical. In compact groups, the BCG contains a larger f raction of the systems total luminosity and di ffers more in absolute magnitude from the second-ranked galaxy. Using a simple model, which dry-merges the BCG in loose groups with a random choice among the 2nd, 3rd and 4th-ranked galaxies in the group, and allowing for some star loss in the process, we show that the absolute magnitude distributions of BCGs in compact and loose groups of similar luminosities can be made more alike. Conclusions. BCGs in compact and loose groups are found to be different. Some mechanisms responsible for transforming late-type galaxies into early types, such as mergers, may be more effective within compact groups due to their high densities and small velocity dispersion, which would lead their BCGs along somewhat different evolutionary paths.