A systematic characterisation of the evolutionary cycling between molecular clouds, star formation, and feedback in nearby galaxies
Mélanie Chevance
Thursday December 4th, 09:35
Star formation is one of the main drivers of galaxy evolution, but an understanding of this process remains elusive. This is caused by a lack of systematic observational constraints on cloud scales - previous attempts to observationally constrain the physics of star formation and feedback have necessarily been limited to few anecdotal cases in the Local Group. However, star formation in galaxies is expected to be highly dependent on the galactic structure and environment, as it results from a competition between mechanisms such as gravitational collapse, shear, spiral arm passages, cloud-cloud collisions, and feedback. A statistically representative sample of galaxies is therefore needed to probe the wide range of conditions under which stars form. For the first time, we can now carry out a systematic characterisation of the evolutionary timeline of molecular clouds and star-forming regions across the nearby Universe, enabling us to determine which of these different processes drive the cloud lifecycle as a function of the environment. I will present the results from systematically applying the statistical method of Kruijssen & Longmore (2014) and Kruijssen et al. (2018) to homogeneous ALMA + optical observations at 50 pc resolution of a large sample of star-forming disc galaxies out to 17 Mpc, obtained in the context of the PHANGS collaboration. This method exploits the spatial scale dependence of the star formation law to constrain the timeline and efficiencies for star formation and feedback on the cloud scale, across a wide variety of galactic environments. I will show that molecular clouds undergo universally fast and inefficient star formation, due to short molecular cloud lifetimes (10-30 Myr) and rapid cloud destruction by stellar feedback (1-6 Myr), but will also demonstrate that the details of the star formation and feedback processes vary considerably with the galactic environment. I will conclude by discussing the physical implications of these results, as well as some of the major open questions. These observations settle a long-standing question on the multi-scale lifecycle of gas and stars in galaxies, and open up the exciting prospect of studying cloud-scale star formation and feedback in galaxies across cosmic time.