ALMA resolves giant molecular clouds in a tidal dwarf galaxy

Abstract

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 ∼ 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 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 ∼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 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.

Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

“…CO observations with the Atacama Large Millimeter/ submillimeter Array (ALMA) have successfully made detections of individual clumps of molecular gas in the wakes of ram-pressure-stripped galaxies (e.g., Jáchym et al 2019) as well as individual giant molecular clouds in TDGs (Querejeta et al 2021). If BCs still contain significant quantities of molecular gas (as would be expected based on their recent SF), then they should be readily detectable with ALMA, especially as their high metallicity measurements imply a favorable CO-to-H 2 conversion factor in comparison to other low-mass objects (Bolatto et al 2013).…”

WISDOM Project – XIII. Feeding molecular gas to the supermassive black hole in the starburst AGN-host galaxy Fairall 49

“…In addition, there could be plausible variations in the line ratio 𝑅 21 . Multi-transition CO observations have shown that 𝑅 21 can vary from ∼0.3 to ∼1.5 in typical spiral galaxies (Leroy et al 2009;Querejeta et al 2021;Den Brok et al 2021). Interestingly, in the Seyfert prototype NGC 1068, the value of 𝑅 21 ranges from 1.6 to 2.5 (Viti et al 2014).…”

Detection of CO(1−0) Emission at the Tips of the Tidal Tail in the Antennae Galaxies

“…As TDGs are recycled objects, they inherit their parent galaxies' metallicity, making CO a suitable molecular gas tracer for comparison with nearby disk galaxies (e.g., Duc et al 2000). Previous CO observations toward TDGs have shown that molecular gas is abundant in TDGs (e.g., Braine et al 2000Braine et al , 2001Lisenfeld et al 2002Lisenfeld et al , 2004Lisenfeld et al , 2008Lisenfeld et al , 2016Duc et al 2007;Boquien et al 2011;Querejeta et al 2021;Kovakkuni et al 2023). Whether the molecular K-S relation (i.e., H 2 S versus Σ SFR ) of the TDGs is consistent with that in the disk galaxies is under debate.…”

“…Braine et al (2001) = S S ) in the TDGs is comparable to that in spiral galaxies. On the other hand, TDGs that do not follow the molecular K-S relation of normal disk galaxies have been found (Lisenfeld et al 2008(Lisenfeld et al , 2016Querejeta et al 2021;Kovakkuni et al 2023), suggesting a diversity of SFE H 2 in TDGs. In particular, Querejeta et al (2021) reported that there are regions in tails where star formation is severely suppressed despite the presence of large amounts of molecular gas.…”