Revisiting the fossil group candidates UGC 842 and NGC 6034
  We present a new insight on NGC 6034 and UGC 842, two groups of galaxies
previously reported in the literature as being fossil groups. The study is
based on optical photometry and spectroscopy obtained with the CTIO Blanco
telescope and Sloan Digital Sky Survey archival data. We find that NGC 6034 is
embedded in a large structure, dominated by three rich clusters and other small
groups. Its first and next four ranked galaxies have magnitude differences in
the r band and projected distances which violate the optical criteria to
classify it as a fossil group. We confirm that the UGC 842 group is a fossil
group, but with about half the velocity dispersion that is reported in previous
works. The velocity distribution of its galaxies reveals the existence of two
structures in its line of sight, one with sigmaV ~ 223 km/s and another with
sigmaV ~ 235 km/s, with a difference in velocity of ~820 km/s. The main
structure is dominated by passive galaxies, while these represent ~60% of the
second structure. The X-ray temperature for the intragroup medium of a group
with such a velocity dispersion is expected to be kT ~0.5-1 keV, against the
observed value of kT ~1.9 keV reported in the literature. This result makes UGC
842 a special case among fossil groups because (1) it represents more likely
the interaction between two small groups, which warms the intragroup medium
and/or (2) it could constitute evidence that member galaxies lost energy in the
process of spiraling toward the group center, and decreased the velocity
dispersion of the system. As far as we know, UGC 842 is the first low-mass
fossil group studied in detail.
