The X-Ray Zurich Environmental Study (X-ZENS). II. X-ray Observations of the Diffuse Intragroup Medium in Galaxy Groups

TL;DR

This study uses X-ray observations to explore the faint diffuse intragroup medium in nearby galaxy groups, revealing their under-luminosity and providing insights into their mass and dynamical state, which are crucial for understanding galaxy evolution.

Contribution

It presents the first X-ray analysis of optically-selected galaxy groups at this mass scale, highlighting their under-luminosity and the importance of combined optical and X-ray data for dynamical insights.

Findings

X-ray emission detected in the 0.5--2 keV band, fainter than typical groups.

Many groups are under-luminous compared to X-ray scaling relations.

X-ray mass estimates align best with optical bulge luminosity.

Abstract

We present the results of a pilot XMM-$Newton$ and $Chandra$ program aimed at studying the diffuse intragroup medium (DIM) of optically-selected nearby groups from the Zurinch ENvironmental Study (ZENS) catalog. The groups are in a narrow mass range about $10^{13}M_\odot$, a mass scale at which the interplay between the DIM and the group member galaxies is still largely unprobed. X-ray emission from the DIM is detected in the energy band 0.5--2 keV with flux $\le 10^{-14}$ erg cm$^{-1}$ s$^{-1}$, which is one order of magnitude fainter than for typical ROSAT groups (RASS). For many groups we set upper limits to the X-ray luminosity, indicating that the detections are likely probing the upper envelope of the X-ray emitting groups. We find evidence for our optically selected groups to be under-luminous with respect to predictions from X-ray scaling relations. X-ray mass determinations are in best agreement with those based on the member galaxies bulge luminosity, followed by their total optical luminosity and velocity dispersion. We measure a stellar mass fraction with a median value of about 1$\%$, with a contribution from the most massive galaxies between 30 to 50 \%. Optical and X-ray data give often complementary answers concerning the dynamical state of the groups, and are essential for a complete picture of the system. Extending this pilot program to a larger sample of groups is mandatory to unveil any imprint of interaction between member galaxies and DIM in halo potentials of key importance for environmentally-driven galactic evolution.