Stochastic star formation in the Milky Way inferred from the unity index of KS law

Abstract

We performed a correlation analysis between the brightness temperature of the CO line and number density of HII regions in the longitude--velocity diagram (LVD) of the Milky Way in order to investigate the volumetric star-formation law. We determined the index $\alpha$ of the Kennicutt-Schmidt (KS) law for the molecular gas defined by $\rho_{\rm SFR} \propto \rho_{\rm H_2}^\alpha$, where $\rho_{\rm SFR}$ is the SFR (star-formation rate) density and $\rho_{\rm H_2}$ is the molecular-gas density. We obtained $\alpha= 1.053 \pm 0.075$ and $1.031 \pm 0.067$ for the CO-line data from the Nobeyama 45-m and Columbia 1.2-m telescope Galactic plane surveys, respectively. This result is consistent with the KS indices currently determined for the molecular gas in the Milky Way as well as in spiral and starburst galaxies. We argue that an index close to 1 is universal in favour of stochastic (spontaneous) star formation, but is inconsistent with cloud-collision model, which predicts a steeper index of $\alpha=2$. We also suggest that the efficiency of star formation in the Galactic Centre is an order of magnitude lower than that in the disc.