The ALMaQUEST survey – III. Scatter in the resolved star-forming main sequence is primarily due to variations in star formation efficiency

Abstract

Using a sample of 11 478 spaxels in 34 galaxies with molecular gas, star formation, and stellar maps taken from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we investigate the parameters that correlate with variations in star formation rates on kpc scales. We use a combination of correlation statistics and an artificial neural network to quantify the parameters that drive both the absolute star formation rate surface density (ΣSFR), as well as its scatter around the resolved star-forming main sequence (ΔΣSFR). We find that ΣSFR is primarily regulated by molecular gas surface density ($\Sigma _{\rm H_2}$) with a secondary dependence on stellar mass surface density (Σ⋆), as expected from an ‘extended Kennicutt–Schmidt relation’. However, ΔΣSFR is driven primarily by changes in star formation efficiency (SFE), with variations in gas fraction playing a secondary role. Taken together, our results demonstrate that whilst the absolute rate of star formation is primarily set by the amount of molecular gas, the variation of star formation rate above and below the resolved star-forming main sequence (on kpc scales) is primarily due to changes in SFE.

Spatially resolved star formation and fuelling in galaxy interactions

“…We find that such variations are driven by changes in SFE and the colddense gas reservoir -in line with prior observational work focused on how SFE and available fuel in galaxies regulate their location within (and departure from) the global SFMS (e.g. Saintonge et al 2017;Piotrowska et al 2020;Ellison et al 2020b). In this section, we probe deeper into the following question: what drives star formation in the central kiloparsec, efficiency or available fuel?…”

“…It would be interesting to check if the three starbursts with signs of merger features in the Ellison et al (2020a) sample are secondary companions in a galaxy pair. For galaxies not classified as starbursts, Ellison et al (2020b) find that enhanced central SFR is more likely to be driven by high levels of H 2 gas in the inner regions (see also Piotrowska et al 2020;Bluck et al 2020). The large fraction of fuel-driven cases with mild SFR enhancement in the centre (lighest-blue hexagons in both panels) are in line with these observations.…”

Cool outflows in galaxies and their implications

“…Finally, reviews on more specific topics are mentioned throughout the text. formation rate Ṁ * , the depletion timescale t dep ≡ M gas / Ṁ * of the molecular gas in galaxies due to star formation is typically ∼ 1-2 Gyr on kpc scales (e.g., Bigiel et al 2008), although t dep tends to decrease with increasing specific star formation rates, both spatially integrated ( Ṁ * /M * ; Saintonge et al 2011) and spatially resolved (Σ SFR /Σ * ; Ellison et al 2020), and there is considerable scatter on scales below ∼ 100 pc (Kennicutt and Evans 2012;Kruijssen and Longmore 2014). This depletion timescale is roughly two orders of magnitude longer than the dynamical timescale of giant molecular clouds (GMCs): t ff /t dep ≡ ff 1%, where t ff = √ 3π/32Gρ is the local free-fall time and ff is the star formation efficiency per free-fall time.…”

Variations in the slope of the resolved star-forming main sequence: a tool for constraining the mass of star-forming regions

“…We adopt these specific time-scales because they are commonly used in the literature, but our conclusions are insensitive to these choices. Ellison et al 2020). In the simulations, we find that as the spatial resolution increases, the rSFMS becomes shallower for all SFR tracers.…”