ALMA Resolves Giant Molecular Clouds in a Tidal Dwarf Galaxy
  Tidal dwarf galaxies (TDGs) are gravitationally bound condensations of gas
and stars formed during galaxy interactions. Here we present
multi-configuration ALMA observations of J1023+1952, a TDG in the interacting
system Arp 94, where we resolve CO(2-1) emission down to giant molecular clouds
(GMCs) at 0.64" ~ 45pc resolution. We find a remarkably high fraction of
extended molecular emission (~80-90%), which is filtered out by the
interferometer and likely traces diffuse gas. We detect 111 GMCs that give a
similar mass spectrum as those in the Milky Way and other nearby galaxies (a
truncated power law with slope of -1.76+/-0.13). We also study Larson's laws
over the available dynamic range of GMC properties (~2 dex in mass and ~1 dex
in size): GMCs follow the size-mass relation of the Milky Way, but their
velocity dispersion is higher such that the size-linewidth and virial relations
appear super-linear, deviating from the canonical values. The global
molecular-to-atomic gas ratio is very high (~1) while the CO(2-1)/CO(1-0) ratio
is quite low (~0.5), and both quantities vary from north to south. Star
formation is predominantly taking place in the south of the TDG, where we
observe projected offsets between GMCs and young stellar clusters ranging from
~50pc to ~200pc; the largest offsets correspond to the oldest knots, as seen in
other galaxies. In the quiescent north, we find more molecular clouds and a
higher molecular-to-atomic gas ratio (~1.5); atomic and diffuse molecular gas
also have a higher velocity dispersion there. Overall, the organisation of the
molecular ISM in this TDG is quite different from other types of galaxies on
large scales, but the properties of GMCs seem fairly similar, pointing to near
universality of the star-formation process on small scales.
