2008AJ....136.2846B

We present a comprehensive analysis of the relationship between star formation rate surface density, Σ_SFR, and gas surface density, Σ_gas, at sub-kpc resolution in a sample of 18 nearby galaxies. We use high-resolution H I data from The H I Nearby Galaxy Survey, CO data from HERACLES and the BIMA Survey of Nearby Galaxies, 24 µm data from the Spitzer Space Telescope, and UV data from the Galaxy Evolution Explorer. We target seven spiral galaxies and 11 late-type/dwarf galaxies and investigate how the star formation law differs between the H₂ dominated centers of spiral galaxies, their H I dominated outskirts and the H I rich late-type/dwarf galaxies. We find that a Schmidt-type power law with index N = 1.0±0.2 relates Σ_SFRand Σ_H2 across our sample of spiral galaxies, i.e., that H₂forms stars at a constant efficiency in spirals. The average molecular gas depletion time is ∼2x10⁹ years. The range of Σ_H2 over which we measure this relation is ∼3-50 M_☉/pc2, significantly lower than in starburst environments. We find the same results when performing a pixel-by-pixel analysis, averaging in radial bins, or when varying the star formation tracer used. We interpret the linear relation and constant depletion time as evidence that stars are forming in giant molecular clouds with approximately uniform properties and that Σ_H2 may be more a measure of the filling fraction of giant molecular clouds than changing conditions in the molecular gas. The relationship between total gas surface density (Σ_gas) and Σ_SFR varies dramatically among and within spiral galaxies. Most galaxies show little or no correlation between Σ_HI and Σ_SFR. As a result, the star formation efficiency (SFE), Σ_SFR/Σ_gas, varies strongly across our sample and within individual galaxies. We show that this variation is systematic and consistent with the SFE being set by local environmental factors: in spirals the SFE is a clear function of radius, while the dwarf galaxies in our sample display SFEs similar to those found in the outer optical disks of the spirals. We attribute the similarity to common environments (low density, low metallicity, H I dominated) and argue that shear (which is typically absent in dwarfs) cannot drive the SFE. In addition to a molecular Schmidt law, the other general feature of our sample is a sharp saturation of H I surface densities at Σ_HI~ 9 M_☉/pc2 in both the spiral and dwarf galaxies. In the case of the spirals, we observe gas in excess of this limit to be molecular.