Dense Gas and Star Formation in Nearby Galaxies

Abstract

Star formation occurs within the densest regions of molecular clouds. This is confirmed by the tight scaling relations between star formation and dense gas content in galaxies. One of the most common tracers of dense molecular gas is emission from HCN. Recent studies using HCN have found evidence for variations in the star formation efficiency of dense gas at sub-kiloparsec (kpc) scales in nearby galaxies. This may indicate that environment plays a role in regulating the connection between dense gas and star formation in galaxies. Alternatively, HCN emissivity may also depend on environment within galaxies which could also explain the observed trends. In this thesis, I study the relationship between dense gas and star formation in nearby galaxies. Specifically, I explore the origin of these apparent variations in the star formation efficiency of dense gas. I begin with a case study of the Antennae Galaxies, the nearest major merger. I explore the star formation efficiency of dense gas and dense gas fraction across different regions of the Antennae system, at sub-kpc scales. I find lower star formation efficiencies of dense gas in the two nuclei relative to their dense gas fractions. I conclude that these low efficiencies are either due to temporal variations in star formation or variations in the emissivity of HCN. I extend this analysis to a sample of nine other galaxies and incorporate analytical models of star formation to provide context to the observational data. Models which include a varying density threshold for star formation are able to best reproduce the observational trends. This is consistent with an environmental dependence of the star formation efficiency of dense gas within galaxies. I validate these findings with numerical modelling of the HCN and CO emissivities using the radiative transfer code RADEX, which can then be compared directly to analytic models of star formation. I specifically consider whether the HCN/CO ratio universally tracks the fraction of gravitationally bound gas within molecular clouds. The modelled emissivities are consistent with observations and suggest that the HCN/CO ratio is not universally tracking the fraction of gravitationally bound gas, in the case that there are varying threshold densities for star formation. However, the HCN/CO ratio is still a useful tracer of the fraction of gas above 10^4.5 cm^−3. The results of this thesis imply that there are clear variations of star formation efficiency of dense gas across galaxies. It is important to consider these variations when interpreting observed trends. Furthermore, when estimating dense gas fractions it is important to incorporate a varying HCN conversion factor, as has been done for CO.