The fraction of BL Lac objects in groups of galaxies

Context: BL Lac objects are a rare class of active galactic nuclei that typically show featureless optical spectra that make it difficult to estimate the redshift. A novel method for estimating the redshift of BL Lac objects has recently been proposed which assumes that these objects typically reside in groups of galaxies. Aims: The aim of this work is to estimate the fraction of BL Lac objects that reside in groups of galaxies. Methods: We use a sample of groups (M12 catalogue) selected by applying a friends-of-friends algorithm in the SDSS DR12. Galaxies in the M12 sample were cross-correlated with the sample of blazars in the BZCAT. Results: We found that 121 galaxies in the M12 catalogue are blazars in the BZCAT sample, all but one are BL Lac objects, and a large fraction are classified as BL Lac-galaxy dominated. Analysing the fraction of galaxies in groups as a function of redshift we have estimated a correction factor that takes into account the typical incompleteness of the catalogues based on the friends-of-friends algorithm. Once this factor was applied to the sample of BL Lac objects with a counterpart in the M12 catalogue, we found that the percentage of BL Lac objects in groups is $\gtrsim 67 \pm 8\%$. Conclusions: The high rate of BL Lac objects in groups found in this work strongly supports a recent method that has successfully estimated the redshift of BL Lac objects with featureless spectra.Comment: Accepted for publication in Astronomy and Astrophysics (Letter

Comparing galaxy populations in compact and loose groups of galaxies II: brightest group galaxies

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 evolution in different environments. 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 luminosity. The fraction of BCGs classified as red and/or early-type as a function of galaxy luminosity are studied. The fraction of the group's total luminosity contained in the BCG and the difference in luminosity between the BCG and the second-ranked galaxy, are also analysed. 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 their 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 fraction of the system's total luminosity and differs more in absolute magnitude from the second-ranked galaxy. 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.Comment: Accepted for publication in Astronomy & Astrophysics (A&A

Tidal stripping of globular clusters in a simulated galaxy cluster

Using a cosmological N-body numerical simulation of the formation of a galaxy cluster- sized halo, we analyze the temporal evolution of its globular cluster population. We follow the dynamical evolution of 38 galactic dark matter halos orbiting in a galaxy cluster that at redshift z=0 has a virial mass of 1.71 * 10 ^14 Msol h^-1. In order to mimic both "blue" and "red" populations of globular clusters, for each galactic halo we select two different sets of particles at high redshift (z ~ 1), constrained by the condition that, at redshift z=0, their average radial density profiles are similar to the observed profiles. As expected, the general galaxy cluster tidal field removes a significant fraction of the globular cluster populations to feed the intracluster population. On average, halos lost approximately 16% and 29% of their initial red and blue globular cluster populations, respectively. Our results suggest that these fractions strongly depend on the orbital trajectory of the galactic halo, specifically on the number of orbits and on the minimum pericentric distance to the galaxy cluster center that the halo has had. At a given time, these fractions also depend on the current clustercentric distance, just as observations show that the specific frequencyof globular clusters S_N depends on their clustercentric distance.Comment: 11 pages, 8 figures. Accepted for publication in Ap

An improved method for the identification of galaxy systems: Measuring the gravitational redshift by Dark Matter Haloes

We introduce a new method for the identification of galaxy systems in redshift surveys based on the halo model. This method is a modified version of the K-means identification algorithm developed by Yang et al (2005). We have calibrated and tested our algorithms using mock catalogs generated using the Millennium simulations (Springel et al. 2005) and applied them to the NYU-DR7 galaxy catalog (based on the SDSS datasets). Using this local sample of groups and clusters of galaxies we have measured the effect of gravitational redshift produced by their host dark matter haloes. Our results shows radial velocity decrements consistent with general relativity predictions and previous measurements by Wojtak et al (2011) in clusters of galaxies.Comment: Accepted for Publication in MNRAS Letters, 2 figures. arXiv admin note: text overlap with arXiv:astro-ph/0405234, arXiv:astro-ph/0406593 by other author

Galaxy Morphological Segregation in Clusters: Local vs. Global Conditions

We study the relative fraction of galaxy morphological types in clusters, as a function of the projected local galaxy density and different global parameters: cluster projected gas density, cluster projected total mass density , and reduced clustercentric distance. Since local and global densities are correlated, we have considered different tests to search for the parameters to which segregation show the strongest dependence. Also, we have explored the results of our analysis applied to the central regions of the clusters and their outskirts. We consider a sample of clusters of galaxies with temperature estimates to derive the projected mass density profile and the 500 density contrast radius ($r_{500}$) using the NFW model and the scaling relation respectively. The X-ray surface brightness profiles are used to obtain the projected gas density assuming the hydrostatic equilibrium model. Our results suggest that the morphological segregation in clusters is controlled by the local galaxy density in the outskirts. On the other hand, the global projected mass density, shows the strongest correlation with the fraction of morphological types in the central high density region, with a marginal dependence on the local galaxy density.Comment: 10 pages, 8 figures, Accepted AJ (February 2001 issue

Systems of Galaxies in the SDSS: the fundamental plane

We analyse a subsample of the galaxy groups obtained by Merch\'an & Zandivarez (2005) from the SDSS DR3 to study the fundamental plane and the mass to light ratio of galaxy groups. We find a fundamental plane given by $L_R \propto R^{1.3} \sigma^{0.7}$. We do not find differences when different dynamical sates or redshift ranges are analysed. We find that the mass to light ratio increases with group mass as $M/L_R \propto M^{0.36}$.Comment: 5 pages, 5 figures, 1 table. Accepted to be published in MNRA

Agricultural dynamics associated with forest recovery. A case study in Chorotega, Costa Rica

A forest transition is observed in many countries around the world (Barbier et al., 2010) and in particular in Central America (Reno et al., 2012). Many explanations have been given to the deforestation phase and to the reforestation that follows (Meyfroidt el al., 2010). Our research question is to investigate types of agricultural dynamics associated with forest recovery. Our theoretical framework is based on a typology of drivers of land use changes (Geist&Lambin, 2002). Our case study is in Costa Rica, characterized by a high reforestation rate, especially in the Región Chorotega (Calvo-Alvaro et al., 2009). Our methodology combines bibliographic review, satellite image analysis (Landsat in 2000; RapidEye in 2012), mapping of census data, and interviews with key actors. We identified three types of agricultural dynamics associated with reforestation: 1) expansion of the sugar cane agroindustry in irrigated zone at the expense of food production (rice) driven by the dynamic of private agroindustry and privileged access to public investment in irrigation infrastructure and a transition from pasture to forest; 2) the integration of a diversified agriculture within an integrated rural development in hinterland zone, with commercial forest plantations supported by rural development programs and local community organizations, and 3) abandonment of agriculture in coastal areas in favor of tourism development boosted by foreign investment. Our empirical evidences go beyond the duality intensification/abandonment and highlight the spatial diversity of agricultural dynamics to explain forest recovery emphasizing the role of stakeholders within the drivers of these dynamics (agro-industrials, rural community, foreign investors). (Résumé d'auteu