  A galaxy system must have a minimum velocity dispersion for its mass to be
greater than the sum of the masses of its galaxies. Nearly half of the nearby
Hickson compact groups (HCGs) have too low a velocity dispersion in comparison
with the rotational velocities of their spiral galaxies and internal velocity
dispersions of their early types.
  A detailed study of the low velocity dispersion group, HCG 16 -- the only
known group of late-type galaxies with diffuse intergalactic X-ray emitting hot
gas -- reveals that half of the diffuse X rays are associated with
foreground/background sources and the remaining gas is clumpy and mostly
associated with the bright galaxies of the group. The large-scale environment
of the group suggests that HCG 16 lies where a cosmological filament falls
perpendicularly onto a large-scale sheet.
  The observed frequency of compact groups is lower than predicted from the
extended Press-Schechter formalism, which also predicts that most 10^13 Msun
objects in the Universe must be fairly old and hence have already coalesced
into single objects, reminiscent of elliptical galaxies over-luminous in X-rays
that are now being discovered.
  Thus, the low survival time of dense groups against the merging instability
is no longer a worry for compact groups, as they form in large enough numbers.
I show why other arguments against the reality of HCGs no longer hold, partly
because of the biases of Hickson's sample.
