The Most Massive Ultra-Compact Dwarf Galaxy in the Virgo Cluster
  We report on the properties of the most massive ultra-compact dwarf galaxy
(UCD) in the nearby Virgo Cluster of galaxies using imaging from the Next
Generation Virgo Cluster Survey (NGVS) and spectroscopy from Keck/DEIMOS. This
object (M59-UCD3) appears to be associated with the massive Virgo galaxy M59
(NGC 4621), has an integrated velocity dispersion of 78 km/s, a dynamical mass
of $3.7\times10^8 M_\odot$, and an effective radius ($R_e$) of 25 pc. With an
effective surface mass density of $9.4\times10^{10} M_\odot/kpc^2$, it is the
densest galaxy in the local Universe discovered to date, surpassing the density
of the luminous Virgo UCD, M60-UCD1. M59-UCD3 has a total luminosity of
$M_{g'}=-14.2$ mag, and a spectral energy distribution consistent with an old
(14 Gyr) stellar population with [Fe/H]=0.0 and [$\alpha$/Fe]=+0.2. We also
examine deep imaging around M59 and find a broad low surface brightness stream
pointing towards M59-UCD3, which may represent a tidal remnant of the UCD
progenitor. This UCD, along with similar objects like M60-UCD1 and M59cO,
likely represents an extreme population of tidally stripped galaxies more akin
to larger and more massive compact early-type galaxies than to nuclear star
clusters in present-day dwarf galaxies.
