Star Formation in NGC 5194 (M51a). II. The Spatially-Resolved Star Formation Law

TL;DR

This study investigates the spatially-resolved star formation law in galaxy M51a, revealing a strong correlation between star formation rate and molecular gas density, and proposing a new SFR index for dust attenuation correction.

Contribution

Introduces a new extinction-corrected SFR index combining H-alpha and 24 micron data, and analyzes the star formation law on 0.5-2 kpc scales in M51a.

Findings

Star formation law follows a Schmidt power law with slope 1.37-1.56.

Strong correlation between SFR surface density and molecular gas, but not atomic gas.

Variations in the law's parameters are linked to spatial sampling scales.

Abstract

We have studied the relationship between the star formation rate (SFR) surface density and gas surface density in the spiral galaxy M51a (NGC 5194), using multi-wavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). We introduce a new SFR index based on a linear combination of H-alpha emission-line and 24 micron continuum luminosities, that provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis HI and CO maps has allowed us to probe the form of the spatially-resolved star formation law on scales of 0.5 to 2 kpc. We find that the resolved SFR vs gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N = 1.37 to 1.56, with zeropoint and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parametrization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.