Small-scale systems of galaxies. IV. Searching for the faint galaxy population associated with X-ray detected isolated E+S pairs

TL;DR

This study investigates faint galaxy populations around isolated E+S galaxy pairs using optical and X-ray data, revealing differences in evolutionary stages and luminosity functions among X-ray bright and faint systems.

Contribution

It provides new insights into the evolutionary status of E+S galaxy pairs and their surrounding faint galaxy populations through combined optical and X-ray analysis.

Findings

X-ray bright systems are more dynamically evolved.

OLF varies between X-ray bright and faint systems.

X-ray faint pairs may be transitional phases in group evolution.

Abstract

In hierarchical evolutionary scenarios, isolated, physical pairs may represent an intermediate phase, or "way station", between collapsing groups and isolated elliptical (E) galaxies (or fossil groups). We started a comprehensive study of a sample of galaxy pairs composed of a giant E and a spiral (S) with the aim of investigating their formation/evolutionary history from observed optical and X-ray properties. Here we present VLT-VIMOS observations designed to identify faint galaxies associated with the E+S systems from candidate lists generated using photometric criteria on WFI images covering an area of ~ 0.2 h^{-1} Mpc radius around the pairs. The results are discussed in the context of the evolution of poor galaxy group associations. A comparison between the Optical Luminosity Functions (OLFs) of our E+S systems and a sample of X-ray bright poor groups suggest that the OLF of X-ray detected poor galaxy systems is not universal. The OLF of our X-ray bright systems suggests that they are more dynamically evolved than our X-ray faint sample and some X-ray bright groups in the literature. However, we suggest that the X-ray faint E+S pairs represent a phase in the dynamical evolution of some X-ray bright poor galaxy groups. The recent or ongoing interaction in which the E member of the X-ray faint pairs is involved could have decreased the luminosity of any surrounding X-ray emitting gas.