The Twin Paradoxes of Compact Groups: Discordant Excess Muted But the Dynamical Puzzle Persists

We consider the twin paradoxes posed by compact groups that involve: (1) gravitational instability of physically dense accordant groups and (2) a plethora of discordant redshift components. We reconsider the statistics of Hickson compact groups and discordant redshift components and find that: (a) only 82 of the 100 groups actually satisfy the stated selection criteria, (b) 61 of the 82 are bona fide compact groups defined as accordant systems of at least four galaxies, (c) the 39 "false" groups are found in regions with twice as high an average galaxy surface density as the true groups, and (d) of the 17 discordant components in 13 true groups, only six fall inside the smallest circle enclosing the accordant system. Thus the verifiable, and hence testable, discordant excess is reduced to six compared to an expectation of 1.5 galaxies.

Much more effort has been devoted to finding a solution to the second paradox. We discuss the challenge to current models posed by observations that imply little or no present, or past, merging in compact groups.