A diversity of starburst-triggering mechanisms in interacting galaxies and their signatures in CO emission

Abstract

The physical origin of enhanced star formation activity in interacting galaxies remains an open question. Knowing whether starbursts are triggered by an increase of the quantity of dense gas or an increase of the star formation efficiency would improve our understanding of galaxy evolution and allow to transpose the results obtained in the local Universe to high redshift galaxies. In this paper, we analyze a parsec-resolution simulation of an Antennae-like model of interacting galaxies. We find that the interplay of physical processes has complex and important variations in time and space, through different combinations of mechanisms like tides, shear and turbulence. These can have similar imprints on observables like depletion time and CO emission. The densest gas (> 10 4 cm −3 ) only constitutes the tail of the density distribution of some clouds, but exists in large excess in others. The super-linearity of the star formation rate dependence on gas density implies that this excess translates into a reduction of depletion times, and thus leads to a deviation from the classical star formation regime, visible up to galactic scales. These clouds are found in all parts of the galaxies, but their number density varies from one region to the next, due to different cloud assembly mechanisms. Therefore, the dependence of cloud and star formation-related quantities (like CO flux and depletion time) on the scale at which they are measured also varies across the galaxies. We find that the α CO conversion factor between the CO luminosity and molecular gas mass has even stronger spatial than temporal variations in a system like the Antennae. These results raise a number of cautionary notes for the interpretation of observations of unresolved star-forming regions, but also predict that the diversity of environments for star formation will be better captured by the future generations of instruments.Key words. galaxies: ISM -galaxies: star formation amount of gas, typically at sub-cloud scales ( 1 pc) and often implemented in sub-grid recipes of star formation. For the sake of clarity, we do not use the term "efficiency" to describe the galactic-scale quantity but rather use the depletion time, which is directly its inverse. The small scale SFE and the galactic scale depletion time are not directly related (see Semenov et al. 2018).Article number, page 1 of 15

Simulations of the star-forming molecular gas in an interacting M51-like galaxy

“…Major galaxy interactions are generally associated with enhanced SFRs (Larson & Tinsley 1978;Lonsdale et al 1984;Barton et al 2000;Ellison et al 2008;Renaud et al 2019). Outliers are however possible and the details are strongly dependent on the orbital parameters of the encounter and the stability of the isolated disc (Di Matteo et al 2007).…”

“…For instance, it has been debated whether the increase of SF during the encounter reflects an increase in the available molecular gas reservoir or whether it follows from a higher SF efficiency with strong arguments favouring both sides (Cox et al 2006;Krumholz, Dekel & McKee 2012;Pan et al 2018). Some of the observational studies endorsing the higher efficiency scenario assume a different conversion factor between CO and H 2 with respect to more quiescent galaxies (Daddi et al 2010) and some numerical studies seem to hint at such a scenario (Renaud et al 2019). However, ultimately these highly interesting questions can only be fully addressed with models capable of properly resolving the molecular phase of the ISM.…”

Highly turbulent gas on GMC scales in NGC 3256, the nearest luminous infrared galaxy

“…Weiß et al 2001;Hitschfeld et al 2008;Sliwa et al 2013). It is also likely that the conversion factor varies significantly across different regions in this merger as Renaud et al (2019) measure variations up to a factor of 2.2 in their simulation of a NGC 4038/9-like merger. They also expect spatial variations are even greater since they focused their analysis on actively star-forming regions.…”

Compact Star-forming Galaxies as Old Starbursts Becoming Quiescent

“…The conclusions drawn from the ISM and SFE diagnostics can be reconciled with that of the radio emission diagnostic if the SFE and ISM properties do not dominate the entire galaxy in an old SB phase. In that case the galaxy would not display an overall (unresolved) high SFE or SB-like ISM (see Renaud et al 2019). This scenario would be also supported by the handful of resolved follow-up studies of the ISM of cSFGs, which indicate an undergoing nuclear starburst (e.g., Spilker et al Figure 7.…”