Fast and inefficient star formation due to short-lived molecular clouds and rapid feedback

Abstract

The physics of star formation and the deposition of mass, momentum, and energy into the interstellar medium by massive stars (‘feedback’) are the main uncertainties in modern cosmological simulations of galaxy formation and evolution1, 2. These processes determine the properties of galaxies3, 4, but are poorly understood on the ≲100 pc scale of individual giant molecular clouds (GMCs)5, 6 resolved in modern galaxy formation simulations7, 8. The key question is why the timescale for depleting molecular gas through star formation in galaxies (tdep ≈ 2 Gyr)9, 10 exceeds the dynamical timescale of GMCs by two orders of magnitude11. Either most of a GMC’s mass is converted into stars over many dynamical times12, or only a small fraction turns into stars before the GMC is dispersed on a dynamical timescale13, 14. Here we report our observation that molecular gas and star formation are spatially decorrelated on GMC scales in the nearby flocculent spiral galaxy NGC300, contrary to their tight correlation on galactic scales5. We demonstrate that this de-correlation implies rapid evolutionary cycling between GMCs, star formation, and feedback. We apply a novel statistical method15, 16 to quantify the evolutionary timeline and find that star formation is regulated by efficient stellar feedback, driving GMC dispersal on short timescales (~1.5 Myr) due to radiation and stellar winds, prior to supernova explosions. This feedback limits GMC lifetimes to about one dynamical timescale (~10 Myr), with integrated star formation efficiencies of only 2–3%. Our findings reveal that galaxies consist of building blocks undergoing vigorous, feedback-driven lifecycles, that vary with the galactic environment and collectively define how galaxies form stars. Systematic applications of this multi-scale analysis to large galaxy samples will provide key input for a predictive, bottom-up theory of galaxy formation and evolution.

The lifecycle of molecular clouds in nearby star-forming disc galaxies

“…When comparing the measured separation lengths to the physical resolutions listed in Table 1, we typically find larger separation lengths at coarser resolution (also see the discussion in Section 4.3.1). We have combined our results with other studies performing the same analysis for NGC300, M33, and the LMC (Hygate et al 2019a; Kruijssen et al 2019;Ward et al 2019, with the latter measuring the separation length for HI clouds and HII regions), which all have resolutions of 50 pc or better, to determine whether this constitutes a systematic trend. We find that the region separation length mirrors the behaviour of the feedback time-scale.…”

“…1 Myr, see the discussion in Sections 2.3 and 4.2, as well as e.g. Prescott et al 2007;Hollyhead et al 2015;Kruijssen et al 2019), this implies that, in these environments, early feedback mechanisms such as winds, photoionisation or radiation pressure must be the dominant processes driving the destruction of molecular clouds. The short feedback time-scales are not achieved by dynamical cloud dispersal without associated massive star formation.…”

“…Inspired by the interpretation first proposed by Schruba et al (2010), recent work has now demonstrated that the observed smallscale scatter around the global star formation relation (e.g. Bigiel et al 2008;Blanc et al 2009;Onodera et al 2010;Schruba et al 2010;Leroy et al 2013;Kreckel et al 2018;Kruijssen et al 2019) can be understood by assuming that individual regions in a galaxy independently undergo an evolutionary lifecycle during which a molecular cloud assembles, collapses, forms stars, and is disrupted by feedback, with molecular gas and SFR tracers probing different evolutionary phases (e.g. Feldmann et al 2011;Kruijssen & Longmore 2014).…”

“…Previous studies of nearby galaxies have shown a high spatial correlation between 24µm emission and Hα emission (e.g. Pérez-González et al 2006;Prescott et al 2007;Kruijssen et al 2019). Most importantly, we aim to derive visibility time-scales rather than absolute flux levels.…”

“…In practice, we perform our measurement by centring circular apertures of a certain size on molecular clouds or young HII regions, and measuring the relative change of the gas-to-SFR flux ratio within these apertures with respect to the galactic average as the aperture size is varied (see e.g. Supplementary Video 1 of Kruijssen et al 2019). At large aperture sizes (centred on either emission peak), the galactic average gas-to-SFR flux ratio is recovered.…”