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