Pre-supernova feedback mechanisms drive the destruction of molecular clouds in nearby star-forming disc galaxies
Melanie Chevance
Thursday December 3rd, 15:30
The cycling of matter in galaxies between molecular clouds, stars and feedback is a major driver of galaxy evolution. However, the detailed characteristics of this cycle are unknown due to a lack of systematic observational constrains, and it is still a key open question which physical processes drive this cycle in galaxies, and in particular which mechanisms stop the accretion of gas onto giant molecular clouds (GMCs) and limit the efficiency at which gas is converted into stars within these GMCs. While feedback from supernova explosions has been the popular feedback mechanism included in simulations of galaxy formation and evolution, 'early' feedback mechanisms such as stellar winds, photoionisation and radiation pressure are expected to play an important role in dispersing the gas after the onset of star formation. The unknowns about which feedback mechanism dominates at each step of the star formation process and what is the environment in which supernovae explode constitute major limitations of modern simulations of galaxy formation and evolution For the first time, I have used ~1'' resolution maps of CO and H_alpha emission across a sample of nine nearby disc galaxies, to measure the lifecyle of GMCs, and specifically the time over which GMCs are dispersed by feedback from young, high-mass stars, as a function of the galactic environment. 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 (1-5 Myr), and that the details of the star formation and feedback processes vary considerably with the galactic environment. Comparison of this rapid destruction time-scale with analytical predictions demonstrates that, independently of the environment, early feedback mechanisms (particularly photoionisation and stellar winds) play a crucial role in dispersing GMCs and limiting their star formation efficiency in nearby galaxies. Finally, I will show that the efficiency at which these early feedback mechanisms couples with the parent GMC is relatively low (a few tens of per cent), such that the vast majority of momentum and energy emitted by the young stellar populations escapes the parent GMC. 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.