ALMA resolves giant molecular clouds in a tidal dwarf galaxy

M Querejeta,D Colombo,F Lelli,J Pety,F Bigiel,A J Rigby,A Hughes,S García-Burillo,J M D Kruijssen,U Lisenfeld,E Schinnerer,C N Herrera,C G Mundell,S E Meidt,T J T Moore

doi:10.1051/0004-6361/202038955

Abstract

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″∼45 pc 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 a 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 predominantly takes place in the south of the TDG, where we observe projected offsets between GMCs and young stellar clusters ranging from ∼50 pc 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 (∼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.

Highlights

When galaxies collide, a portion of the gas is ejected by tidal forces and may eventually collapse under self-gravity, giving rise to new low-mass galaxies along the tidal debris
Star formation predominantly takes place in the south of the Tidal dwarf galaxies (TDGs), where we observe projected offsets between giant molecular clouds (GMCs) and young stellar clusters ranging from ∼50 pc to ∼200 pc; the largest offsets correspond to the oldest knots, as seen in other galaxies
While TDGs have been extensively mapped through deep optical imaging, only a few studies have looked at their molecular gas content and distribution, and these studies target spatial resolutions that are too coarse to resolve individual giant molecular clouds (GMCs; e.g. Braine et al 2000, 2001; Duc et al 2007; Lisenfeld et al 2016)

Summary

Introduction

A portion of the gas is ejected by tidal forces and may eventually collapse under self-gravity, giving rise to new low-mass galaxies along the tidal debris. Movie is available at https://www.aanda.org lar populations, but this is not always the case (Fensch et al 2016) Such a dark matter-free setting, which is subject to tidal forces from their neighbours, affords the possibility of testing theories of star formation in an extreme dynamical environment. SCIMES (Colombo et al 2015), a dendrogram-based method (Rosolowsky et al 2008) which, on top of segmenting the molecular ISM, informs on how it is hierarchically structured In both cases, we follow the same strategy to derive physical properties

Results

Discussion

Conclusion