Speaker
Rahul Shetty
(ZAH/ITA, University of Heidelberg)
Description
The star formation rate is observed to be strongly correlated with the gas surface density. This correlation is well described as a power-law with index N, now known as the Kennicutt-Schmidt (KS) relationship. Previous efforts have inferred N=1.5 when considering both resolved observations of galaxies as well as unresolved starbursting systems. More recently, analysis of resolved observations have resulted in a linear relationship, which is interpreted as evidence for a constant gas depetion time of 2 Gyr, though with significant scatter. Using a hierarchical Bayesian fitting method, I show that resolved observations of nearby disk galaxies from the STING and HERACLES sample actually strongly suggest a sub-linear relationship, with significant galaxy-to-galaxy variation. The hierarchical Bayesian method has the advantage of accurately accounting for statistical and systematic uncertainties, and simultaneously estimates the parameters for all individual galaxies as well as the mean value of the ensemble. The index N of individual galaxies ranges from 0.5 to 1.0, with the mean value of all galaxies N~0.75. Therefore, there is no universal KS law that can reproduce the observed trends for all galaxies. This indicates that other physical properties besides the CO traced gas influence the star formation rates, such as stellar content, gas fraction, and/or magnetic fields. More importantly, the sub-linearity in the KS relationship indicates that CO emission is not solely originating from star forming gas (e.g. "GMCs") but rather that CO also traces a non-star forming diffuse component. Consequently, the depletion time increases with increasing gas surface density, as traced by CO emission.
Author
Rahul Shetty
(ZAH/ITA, University of Heidelberg)