Where are compact groups in the local Universe?

TL;DR

This study investigates the spatial distribution of compact galaxy groups in the local universe, revealing most are isolated and providing insights into their physical density and environmental context using observational and simulated data.

Contribution

It introduces a new method to identify embedded compact groups in large-scale structures and compares observational data with semi-analytical models to understand their distribution.

Findings

Only 27% of compact groups are embedded in larger overdense systems.

Embedded groups tend to be smaller and have brighter surface brightness.

Approximately 70% of embedded groups are predicted to be physically dense in 3D.

Abstract

The purpose of this work is to perform a statistical analysis of the location of compact groups in the Universe from observational and semi-analytical points of view. We used the velocity-filtered compact group sample extracted from the Two Micron All Sky Survey for our analysis. We also used a new sample of galaxy groups identified in the 2M++ galaxy redshift catalogue as tracers of the large-scale structure. We defined a procedure to search in redshift space for compact groups that can be considered embedded in other overdense systems and applied this criterion to several possible combinations of different compact and galaxy group subsamples. We also performed similar analyses for simulated compact and galaxy groups identified in a 2M++ mock galaxy catalogue constructed from the Millennium Run Simulation I plus a semi-analytical model of galaxy formation. We observed that only $\sim27\%$ of the compact groups can be considered to be embedded in larger overdense systems, that is, most of the compact groups are more likely to be isolated systems. The embedded compact groups show statistically smaller sizes and brighter surface brightnesses than non-embedded systems. No evidence was found that embedded compact groups are more likely to inhabit galaxy groups with a given virial mass or with a particular dynamical state. We found very similar results when the analysis was performed using mock compact and galaxy groups. Based on the semi-analytical studies, we predict that $70\%$ of the embedded compact groups probably are 3D physically dense systems. Finally, real space information allowed us to reveal the bimodal behaviour of the distribution of 3D minimum distances between compact and galaxy groups. The location of compact groups should be carefully taken into account when comparing properties of galaxies in environments that are a priori different.