Galaxy and Mass Assembly (GAMA): Accurate number densities &
  environments of massive ultracompact galaxies at 0.02 < z < 0.3
  Massive Ultracompact Galaxies (MUGs) are common at z=2-3, but very rare in
the nearby Universe. Simulations predict that the few surviving MUGs should
reside in galaxy clusters, whose large relative velocities prevent them from
merging, thus maintaining their original properties (namely stellar
populations, masses, sizes and dynamical state). We take advantage of the
high-completeness, large-area spectroscopic GAMA survey, complementing it with
deeper imaging from the KiDS and VIKING surveys. We find a set of 22 bona-fide
MUGs, defined as having high stellar mass (>8x10^10 M_Sun) and compact size
(R_e<2 Kpc) at 0.02 < z < 0.3. An additional set of 7 lower-mass objects
(6x10^10 < M_star/M_Sun < 8x10^10) are also potential candidates according to
typical mass uncertainties. The comoving number density of MUGs at low redshift
(z < 0.3) is constrained at $(1.0\pm 0.4)x 10^-6 Mpc^-3, consistent with galaxy
evolution models. However, we find a mixed distribution of old and young
galaxies, with a quarter of the sample representing (old) relics. MUGs have a
predominantly early/swollen disk morphology (Sersic index 1<n<2.5) with high
stellar surface densities (<Sigma_e> ~ 10^10 M_Sun Kpc^-2). Interestingly, a
large fraction feature close companions -- at least in projection -- suggesting
that many (but not all) reside in the central regions of groups. Halo masses
show these galaxies inhabit average-mass groups. As MUGs are found to be almost
equally distributed among environments of different masses, their relative
fraction is higher in more massive overdensities, matching the expectations
that some of these galaxies fell in these regions at early times. However,
there must be another channel leading some of these galaxies to an abnormally
low merger history because our sample shows a number of objects that do not
inhabit particularly dense environments. (abridged)
