Héctor J. Martínez

On the luminosity function of galaxies in groups in the Sloan Digital Sky Survey

Using galaxy groups identified in the fourth data release of the Sloan Digital Sky Survey (SDSS), we compute the luminosity function for several subsamples of galaxies in groups. In all cases, the luminosity functions are well described by Schechter functions, down to the faintest magnitudes we probe, M0.1r - 5 log(h) ~ -16. For the general luminosity function of galaxies in groups in the five SDSS bands, we observe that the characteristic magnitude is brighter at ~0.5 mag than those obtained for field galaxies by Blanton et al. Even when the observed faint-end slope is steeper in galaxy groups, it is statistically comparable with the field value. We analyze the dependence of the galaxy luminosity function with system masses, finding two clear trends: a continuous brightening of the characteristic magnitude and a steepening of the faint-end slope as mass increases. The results in 0.1g, 0.1r, 0.1i, and 0.1z bands show the same behavior. Using the u - r color to split the galaxy sample into red and blue galaxies, we show that the changes observed as a function of the system mass are mainly seen in the red, passively evolving galaxy population, while the luminosities of blue galaxies remain almost unchanged with mass. Finally, we observe that groups having an important luminosity difference between the two brightest galaxies of a system show a steeper faint-end slope than the other groups. Our results can be interpreted in terms of galaxy mergers as the main driving force behind galaxy evolution in groups.

Groups of galaxies: relationship between environment and galaxy properties

We analyse how the properties of galaxies in groups identified in the Sloan Digital Sky Survey depend on the environment. In particular, we study the relationship between galaxy properties and group mass and group-centric distance. Among the galaxy properties we have considered here, we find that the g - r colour is the most predictive parameter for group mass, while the most predictive pair of properties are g - r colour and r-band absolute magnitude. Regarding the position inside the systems, the g - r colour is the best tracer of group-centric distance and the most predictive pair of properties are g - r colour and spectral type taken together. These results remain unchanged when a subsample of high-mass groups is analysed. The same happens if the brightest group galaxies are excluded.

The galaxy density environment of gamma-ray burst host galaxies

We analyze cross-correlation functions between Gamma-Ray Burst (GRB) hosts and surrounding galaxies. We have used data obtained with the Very Large Telescope at Cerro Paranal (Chile), as well as public Hubble Space Telescope data. Our results indicate that Gamma-Ray Burst host galaxies do not reside in high galaxy density environments. Moreover, the host-galaxy cross-correlations show a relatively low amplitude. Our results are in agreement with the cross-correlation function between star-forming galaxies and surrounding objects in the HDF-N.

The evolution of the bimodal colour distribution of galaxies in Sloan Digital Sky Survey groups

We analyse u - r colour distributions for several samples of galaxies in groups drawn from the fourth data release of the Sloan Digital Sky Survey. For all luminosity ranges and environments considered, the colour distributions are well described by the sum of two Gaussian functions. We find that the fraction of galaxies in the red sequence is an increasing function of group virial mass. We also study the evolution of the galaxy colour distributions at low redshift, z ≤ 0.18 in the field and in groups for galaxies brighter than M r - 5 log (h) = -20, finding significant evidence of recent evolution in the population of galaxies in groups. The fraction of red galaxies monotonically increases with decreasing redshift, this effect implies a much stronger evolution of galaxies in groups than in the field.

Galaxies infalling into groups: filaments versus isotropic infall

We perform a comparative analysis of the properties of galaxies infalling into groups classifying them accordingly to whether they are: falling along filamentary structures; or they are falling isotropically. For this purpose, we identify filamentary structures connecting massive groups of galaxies in the SDSS. We perform a comparative analysis of some properties of galaxies in filaments, in the isotropic infall region, in the field, and in groups. We study the luminosity functions (LF) and the dependence of the specific star formation rate (SSFR) on stellar mass, galaxy type, and projected distance to the groups that define the filaments. We find that the LF of galaxies in filaments and in the isotropic infalling region are basically indistinguishable between them, with the possible exception of late-type galaxies. On the other hard, regardless of galaxy type, their LFs are clearly different from that of field or group galaxies. Both of them have characteristic absolute magnitudes and faint end slopes in between the field and group values. More significant differences between galaxies in filaments and in the isotropic infall region are observed when we analyse the SSFR. We find that galaxies in filaments have a systematically higher fraction of galaxies with low SSFR as a function of both, stellar mass and distance to the groups, indicating a stronger quenching of the star formation in the filaments compared to both, the isotropic infalling region, and the field. Our results suggest that some physical mechanisms that determine the differences observed between field galaxies and galaxies in systems, affect galaxies even when they are not yet within the systems.

The environment of active objects in the nearby Universe

We study the galaxy environment of active galaxies, radio-loud and radio-quiet quasars in the redshift range 0.1 ≤ z ≤ 0.25. We use galaxies from the APM survey in order to explore the local galaxy overdensity and the b J - R colour distribution of neighbouring galaxies of these target samples. For comparison, we perform similar analysis on samples of Abell clusters with X-ray emission, and samples of Abell clusters with richness R = 1 and 0. The projected cross-correlations show that the samples of quasars and active galaxies reside in regions of galaxy density enhancements lower than those typical of R = 0 clusters. We also find that in the nearby Universe the local galaxy overdensities of radio-loud and radio-quiet quasars are comparable. The analysis of the distribution of b J - R galaxy colour indices suggests that the environment of quasars is not strongly dominated by a population of red galaxies, characteristic of rich Abell clusters, an effect that is more clearly appreciated for our sample of radio-loud quasars.

Comparing galaxy populations in compact and loose groups of galaxies - III. Effects of environment on star formation

This paper is part of a series in which we perform a systematic comparison of the galaxy properties inhabiting compact groups, loose groups and the field. In this paper we focus our study to the age and the star formation in galaxies. For galaxies in selected samples of compact groups, loose groups and field, we compare the distributions of the following parameters: D

Green valley galaxies as a transition population in different environments

_n(4000)

Luminosity function of galaxies in groups in the Sloan Digital Sky Survey Data Release 7: the dependence on mass, environment and galaxy type: Luminosity function of galaxies in groups

as an age indicator, and the specific star formation rate as indicator of ongoing star formation. We analyse the dependence of these parameters on galaxy type, stellar mass and, for group galaxies, their dependence on the dynamic state of the system. We also analyse the fraction of old, and of high star forming galaxies as a function of galaxy stellar mass in the environments we probe. Galaxies in compact groups have, on average, older stellar populations than their loose group or field counterparts. Early-type galaxies in compact groups formed their stars and depleted their gas content more rapidly than in the other environments. We have found evidence of two populations of late-type galaxies in dynamically old compact groups: one with normal specific star formation rates and another with markedly reduced star formation. Processes that transform galaxies from star forming to quiescent act upon galaxies faster and more effectively in compact groups. The unique characteristics of compact groups make them an extreme environment for galaxies, where the transition to quiescence occurs rapidly.

The dynamical state of galaxy groups and their luminosity content

We present a comparative analysis of the properties of passive, star-forming and transition (green valley) galaxies, in four discrete environments: field, groups, the outskirts and the core of X-ray clusters. We construct samples of galaxies from the SDSS in these environments so that they are bound to have similar redshift distributions. The classification of galaxies into the three sequences is based on the UV-optical colour