Cosmic Pathways for Compact Groups in the Milli-Millennium Simulation
  We detected 10 compact galaxy groups (CGs) at $z=0$ in the semi-analytic
galaxy catalog of Guo et al. (2011) for the milli-Millennium Cosmological
Simulation (sCGs in mGuo2010a). We aimed to identify potential canonical
pathways for compact group evolution and thus illuminate the history of
observed nearby compact groups. By constructing merger trees for $z=0$ sCG
galaxies, we studied the cosmological evolution of key properties, and compared
them with $z=0$ Hickson CGs (HCGs). We found that, once sCG galaxies come
within 1 (0.5) Mpc of their most massive galaxy, they remain within that
distance until $z=0$, suggesting sCG "birth redshifts". At $z=0$ stellar masses
of sCG most-massive galaxies are within $10^{10} \lesssim M_{\ast}/M_{\odot}
\lesssim 10^{11}$. In several cases, especially in the two 4- and 5-member
systems, the amount of cold gas mass anti-correlates with stellar mass, which
in turn correlates with hot gas mass. We define the angular difference between
group members' 3D velocity vectors, $\Delta\theta_{\rm vel}$, and note that
many of the groups are long-lived because their small values of
$\Delta\theta_{\rm vel}$ indicate a significant parallel component. For
triplets in particular, $\Delta\theta_{\rm vel}$ values range between
$20^{\circ}$ and $40^{\circ}$ so that galaxies are coming together along
roughly parallel paths, and pairwise separations do not show large pronounced
changes after close encounters. The best agreement between sCG and HCG physical
properties is for $M_{\ast}$ galaxy values, but HCG values are higher overall,
including for SFRs. Unlike HCGs, due to a tail at low SFR and $M_{\ast}$, and a
lack of $M_{\ast}\gtrsim 10^{11}M_{\odot}$ galaxies, only a few sCG galaxies
are on the star-forming main sequence.
