Star Formation On Sub-kpc Scale Triggered By Non-linear Processes In Nearby Spiral Galaxies

TL;DR

This study reveals a super-linear relationship between star formation rate and molecular gas density on sub-kpc scales in nearby spiral galaxies, indicating non-linear processes like gravitational collapse influence star formation efficiency.

Contribution

It provides new high-resolution CO($J$=1-0) data showing a super-linear star formation law, contrasting with previous CO($J$=2-1) results, and highlights the role of non-linear processes in star formation.

Findings

Super-linear star formation law with slope N=1.3 to 1.8.

Star formation efficiency increases with molecular gas surface density.

Diffuse emission and CO excitation effects can flatten the observed correlation.

Abstract

We report a super-linear correlation for the star formation law based on new CO($J$=1-0) data from the CARMA and NOBEYAMA Nearby-galaxies (CANON) CO survey. The sample includes 10 nearby spiral galaxies, in which structures at sub-kpc scales are spatially resolved. Combined with the star formation rate surface density traced by H$\alpha$ and 24 $\mu$m images, CO($J$=1-0) data provide a super-linear slope of $N$ = 1.3. The slope becomes even steeper ($N$ = 1.8) when the diffuse stellar and dust background emission is subtracted from the H$\alpha$ and 24 $\mu$m images. In contrast to the recent results with CO($J$=2-1) that found a constant star formation efficiency (SFE) in many spiral galaxies, these results suggest that the SFE is not independent of environment, but increases with molecular gas surface density. We suggest that the excitation of CO($J$=2-1) is likely enhanced in the regions with higher star formation and does not linearly trace the molecular gas mass. In addition, the diffuse emission contaminates the SFE measurement most in regions where star formation rate is law. These two effects can flatten the power law correlation and produce the apparent linear slope. The super linear slope from the CO($J$=1-0) analysis indicates that star formation is enhanced by non-linear processes in regions of high gas density, e.g., gravitational collapse and cloud-cloud collisions.