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

TL;DR

This paper introduces an improved algorithm for identifying compact galaxy groups directly in redshift space, significantly increasing the detection rate and capturing lower surface brightness groups missed by previous methods.

Contribution

The authors develop and test a new Hickson-like algorithm that identifies compact groups in redshift space, resulting in nearly double the number of detected groups and a more complete sample from SDSS data.

Findings

Nearly twice as many compact groups identified compared to traditional methods.

The new sample includes lower surface brightness, looser groups with fainter brightest galaxies.

Largest sample of Hickson-like groups with spectroscopic confirmation from SDSS.

Abstract

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. 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. 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.