Rahul Shetty (ITA Heidelberg)

Rahul Shetty Colloquium: Interpreting the sub-linear Kennicutt-Schmidt Relationship: The case for diffuse mole

Marsfield Lecture Theatre

Abstract

Title: Interpreting the sub-linear Kennicutt-Schmidt Relationship: The case for diffuse molecular gas

Abstract: On galactic scales, the star formation rate is observed to be strongly correlated with the gas surface density. Accurately measuring the parameters of this power-law correlation, now known as the
Kennicutt-Schmidt (KS) relationship, is important for building realistic models of star formation and the ISM. Recent hierarchical statistical analysis has shown that there is no "universal" KS relationship, as the parameters varies between galaxies. Moreover, many individual galaxies, as well as the mean trend for two observational surveys, clearly evince a sub-linear KS relationship. This suggests that a number of other physical properties besides the CO traced gas influence the star formation rates. Further, 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 diffuse non-star forming diffuse component. The presence of such diffuse molecular gas is supported by a number of other observational results, such as extended CO emission, the similarity between CO and HI linewidths, and high 12CO/13CO intensity ratios. I will discuss how such a diffuse component is a natural outcome of a hierarchical, turbulent ISM. Consequently, the inferred molecular gas depletion time is not constant, but rather increases with increasing molecular gas surface density, as traced by CO emission.