Improving Hickson-like compact group finders in redshift surveys: an implementation in the SDSS

Abstract

Aims. In this work we present an algorithm to identify compact groups (CGs) that closely follows Hickson's original aim and that improves the completeness of the samples of compact groups obtained from redshift surveys. Methods. Instead of identifying CGs in projection first and then checking a velocity concordance criterion, we identify them directly in redshift space using Hickson-like criteria. The methodology was tested on a mock lightcone of galaxies built from the outputs of a recent semi-analytic model of galaxy formation run on top of the Millennium Simulation I after scaling to represent the first-year Planck cosmology. Results. The new algorithm identifies nearly twice as many CGs, no longer missing CGs that failed the isolation criterion because of velocity outliers lying in the isolation annulus. The new CG sample picks up lower surface brightness groups, which are both looser and with fainter brightest galaxies, missed by the classic method. A new catalogue of compact groups from the Sloan Digital Sky Survey is the natural corollary of this study. The publicly available sample comprises 462 observational groups with four or more galaxy members, of which 406 clearly fulfil all the compact group requirements: compactness, isolation, and velocity concordance of all of their members. The remaining 56 groups need further redshift information of potentially contaminating sources. This constitutes the largest sample of groups that strictly satisfy all the Hickson's criteria in a survey with available spectroscopic information.

Compact Groups of Galaxies in Sloan Digital Sky Survey and LAMOST Spectral Survey. I. The Catalogs

“…Recently, Díaz-Giménez et al (2018) combined the advantages of above two selection algorithms and applied their new algorithm on the galaxy catalogue of SDSS-DR12 (Alam et al 2015). In specific, they first apply a redshift filter to remove the background galaxies.…”

“…They also applied the similar richness and compactness criteria as used byMcConnachie et al (2008), but the isolation criterion was not taken into account.3. HMCG: As we already mentioned,Díaz-Giménez et al (2018) applied a redshift filter on galaxy sample before starting the Hickson's criteria to search CGs, where a sample of 462 CGs are finally extracted from the SDSS-DR12.…”

Compact groups from semi-analytical models of galaxy formation – I. A comparative study of frequency and nature

“…Moreover, DGM10 found that the frequency (space density) of CGs was roughly 10 times higher in the simulations than observed, suggesting that the HCG sample was 90 per cent incomplete, mainly in groups dominated by a single galaxy (as previously noted by Prandoni et al 1994). In subsequent studies by our team (Díaz-Giménez et al 2012;Díaz-Giménez & Zandivarez 2015;Taverna et al 2016;Díaz-Giménez et al 2018), CGs were extracted from the public outputs of different SAMs based on different physics or from those of older SAMs run on the dark matter haloes extracted from cosmological simulations run with different cosmological parameters. These studies led to different fractions of physically dense groups.…”

“…Absolute magnitudes were interpolated between different snapshots. Since the SAMs provide the galaxy absolute magnitudes, to compute the observer-frame apparent magnitudes it is necessary to include a k-decorrection procedure, which was done following the recipes described in Díaz-Giménez et al (2018).…”

“…Several subsequent attempts were de-eugeniadiazz@gmail.com voted to the construction of catalogues via automatic search algorithms (e.g. Mamon 1989 for a search within the Virgo cluster yielding a single group; Prandoni et al 1994;Focardi & Kelm 2002;Lee et al 2004;McConnachie et al 2009;Díaz-Giménez et al 2012;Hernández-Fernández & Mendes de Oliveira 2015;Sohn et al 2015Sohn et al , 2016Díaz-Giménez et al 2018).…”