Extreme Variation in Star Formation Efficiency across a Compact, Starburst Disk Galaxy

We report on the internal distribution of star formation efficiency in IRAS 08339+6517 (hereafter IRAS08), using similar to 200 pc resolution CO(2 - 1) observations from NOEMA. The molecular gas depletion time changes by 2 ordersof-magnitude from disk-like values in the outer parts to less than 10(8) yr inside the half-light radius. This translates to a star formation efficiency per freefall time that also changes by 2 orders-of-magnitude, reaching 50%-100%, different than local spiral galaxies and the typical assumption of constant, low star formation efficiencies. Our target is a compact, massive disk galaxy that has a star formation rate 10 x above the z = 0 main sequence; Toomre Q approximate to 0.5-0.7 and high gas velocity dispersion (sigma(mol) approximate to 25 km s(-1)). We find that IRAS08 is similar to other rotating, starburst galaxies from the literature in the resolved Sigma(SFR) proportional to Sigma(N)(mol) relation. By combining resolved literature studies we find that the distance from the main sequence is a strong indicator of the Kennicutt-Schmidt power-law slope, with slopes of N approximate to 1.6 for starbursts from 100 to 10(4) M-circle dot pc(-2). Our target is consistent with a scenario in which violent disk instabilities drive rapid inflows of gas. It has low values of Toomre-Q, and also at all radii, the inflow timescale of the gas is less than the depletion time, which is consistent with the flat metallicity gradients in IRAS08. We consider these results in light of popular star formation theories; in general observations of IRAS08 find the most tension with theories in which star formation efficiency is a constant. Our results argue for the need of high-spatial-resolution CO observations for a larger number of similar targets.

Automated summary

We report on the internal distribution of star formation efficiency in IRAS 08339+6517, and find significant variations in molecular gas depletion time and star formation efficiency. These results challenge common assumptions and emphasize the importance of high-spatial-resolution CO observations for understanding star formation.