The properties of the heterogeneous Shakhbazyan groups of galaxies in the SDSS

Abstract

We present a systematic study of the subsample of Shakhbazyan (SHK) groups covered by the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). SHK groups probe an environment with characteristics which are intermediate between those of loose and very compact groups. Surprisingly, we found that several groups identifying algorithms, e. g. Berlind et al. and Tago et al., miss this type of structures. Using the SDSS DR5 spectroscopic data and the photometric redshifts derived in D'Abrusco et al., we identified possible group members in photometric redshift space and derived, for each group, several individual properties (richness, size, mean photometric redshift, fraction of red galaxies, etc.). We also combined pointed and stacked Rosat All Sky Survey (RASS) data to investigate the X-ray luminosities of these systems. Our study confirms that the majority of groups are physical entities with richness in the range 3-13 galaxies, and properties ranging between those of loose and compact groups. We confirm that SHK groups are richer in early-type galaxies than the surrounding environment and the field, as expected from the morphology - density relation and from the selection of groups of red galaxies. Furthermore, our work supports the existence of two subclasses of structures, the first one being formed by compact and isolated groups and the second formed by extended structures. We suggest that while the first class of objects dwells in less dense regions like the outer parts of clusters or the field, possibly sharing the properties of Hickson Compact Groups, the more extended structures represent a mixture of [core + halo] configurations and cores of rich clusters. X-ray luminosities for SHK groups are generally consistent with these results and with the expectations for the L(X)-sigma(v) relation, but also suggest the velocity dispersions reported in literature are underestimated for some of the richest systems

Fossil group origins

“…The SDSS photometry of point-like sources is nominally 95% complete down to a model magnitude r = 22 (Stoughton et al 2002) and the star/galaxy classification is still reliable down to r ∼ 21.5 (Lupton et al 2001; see also Capozzi et al 2009). Accordingly, we adopt here a limiting magnitude of r ∼ 21.5 for the entire SDSS catalog.…”

METAPHOR: a machine-learning-based method for the probability density estimation of photometric redshifts

“…Redshifts, by being directly correlated to the distance of the sources, lay at the very heart of almost all studies of the extragalactic universe and are used for a wide variety of tasks: to constrain the dark matter and dark energy contents of the Universe through weak gravitational lensing (Serjeant 2014), to understand the cosmic large scale structure (Aragon Calvo et al 2015) by identifying galaxies clusters and groups (Capozzi et al 2009;Annunziatella et al 2016), to map the galaxy color-redshift relationships (Masters et al 2015), to classify astronomical sources Tortora et al 2016), to quote just a few. Due to the multitude of ongoing multi-band photometric galaxy surveys such as KiDS (Kilo-Degree Survey;de Jong et al 2015), DES (Dark Energy Survey, Annis 2013), Pan-STARRS (Kaiser 2004), and future E-mail: cavuoti@na.astro.it facilities like the Large Synoptic Survey Telescope (LSST, Ivezic 2009) and Euclid (Laureijs et al 2014), we have entered an era requiring redshift estimates for billions of galaxies.…”

Photometric redshifts with the quasi Newton algorithm (MLPQNA) Results in the PHAT1 contest

“…For instance, they are essential in constraining dark matter and dark energy studies by means of weak gravitational lensing for the identification of galaxy clusters and groups (e.g. Capozzi et al 2009), for type Ia supernovae, and for the study of the mass function of galaxy clusters (Albrecht et al 2006;Peacock et al 2006;Keiichi et al 2012).…”