ALMA resolves giant molecular clouds in a tidal dwarf galaxy

Tidal dwarf galaxies (TDGs) are gravitationally bound condensations of gas and stars that formed during galaxy interactions. Here we present multi-configuration ALMA observations of J1023+1952, a TDG in the interacting system Arp 94, where we resolved CO(2-1) emission down to giant molecular clouds (GMCs) at 0.64 '' similar to 45 pc resolution. We find a remarkably high fraction of extended molecular emission (similar to 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 a slope of -1.76 +/- 0.13). We also study Larson's laws over the available dynamic range of GMC properties (similar to 2 dex in mass and similar to 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 (similar to 1) while the CO(2-1)/CO(1-0) ratio is quite low (similar to 0.5), and both quantities vary from north to south. Star formation predominantly takes place in the south of the TDG, where we observe projected offsets between GMCs and young stellar clusters ranging from similar to 50 pc to similar to 200 pc; 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 (similar to 1.5); atomic and diffuse molecular gas also have a higher velocity dispersion there. Overall, the organisation of the molecular interstellar medium 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.

Automated summary

Tidal dwarf galaxies (TDGs) are condensations of gas and stars formed during galaxy interactions. The ALMA observations of J1023+1952 in the interacting system Arp 94 showed a high fraction of extended molecular emission and GMC properties similar to the Milky Way. Star formation occurs predominantly in the south of the TDG, with more molecular clouds and higher molecular-to-atomic gas ratio in the quiescent north.