ALMA resolves giant molecular clouds in a tidal dwarf galaxy^⋆,⋆⋆

¹ Observatorio Astronómico Nacional, Alfonso XII, 3, 28014 Madrid, Spain
e-mail: m.querejeta@oan.es
² School of Physics and Astronomy, Cardiff University, 5 The Parade, CF24 3AA Cardiff, UK
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 9, 53121 Bonn, Germany
⁵ Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁶ Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias, 18071 Granada, Spain
⁷ Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK
⁸ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ IRAM, 300 rue de la Piscine, 38406 Saint Martin, d’Hères, France
¹⁰ Université de Toulouse, UPS-OMP, 31028 Toulouse, France
¹¹ CNRS, IRAP, Av. du Colonel Roche BP 44346, 31028 Toulouse Cedex 4, France
¹² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
¹³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹⁴ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁵ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 75005 Paris, France

Received: 16 July 2020
Accepted: 30 October 2020

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.