THE STAR FORMATION LAW IN NEARBY GALAXIES ON SUB-KPC SCALES

Abstract

(Abridged) We present a comprehensive analysis of the relationship between\nstar formation rate surface density (SFR SD) and gas surface density (gas SD)\nat sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution\nHI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from\nthe Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral\ngalaxies and 11 late-type/dwarf galaxies and investigate how the star formation\nlaw differs between the H2-dominated centers of spiral galaxies, their\nHI-dominated outskirts and the HI-rich late-type/dwarf galaxies.\n We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR\nSD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms\nstars at a constant efficiency in spirals. The average molecular gas depletion\ntime is ~2*10^9 yrs. We interpret the linear relation and constant depletion\ntime as evidence that stars are forming in GMCs with approximately uniform\nproperties and that the H2 SD may be more a measure of the filling fraction of\ngiant molecular clouds than changing conditions in the molecular gas.\n The relationship between total gas SD and SFR SD varies dramatically among\nand within spiral galaxies. Most galaxies show little or no correlation between\nthe HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR\nSD / gas SD) varies strongly across our sample and within individual galaxies.\nWe show that in spirals the SFE is a clear function of radius, while the dwarf\ngalaxies in our sample display SFEs similar to those found in the outer optical\ndisks of the spirals. Another general feature of our sample is a sharp\nsaturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.\n