Constraining the Dense Gas Fraction and Star Formation Efficiency in Extreme Starbursts
Abstract
We explore the prediction that the fraction of star-forming gas decreases with increasing mach number, which is predicted in gravoturbulent models of star formation (Burkhart et al. 2019). A proxy for the fraction of star-forming gas is the dense gas fraction, as observed by the HCN/CO luminosity ratio. We model the HCN/CO ratio using the non-LTE radiative transfer code RADEX, and we weight these emissivities by physically-motivated gas density distributions that span a range of environments (cf. Leroy et al. 2017). We compare the results of these models to archival ALMA data of the HCN/CO and HCN/HCO+ line ratios in a sample of ten starbursts. Using analytical star-formation models (e.g. Padoan & Nordlund 2011, Burkhart et al. 2019) we predict the SFR for a given HCN/CO ratio, and compare this with SFRs derived from the radio continuum and IR. We find that the modeled PDF-weighted emissivities of the HCN/CO ratio agree well with observations for a range of mach numbers. However, we find an inverse relationship between the modeled HCN/CO ratio and the predicted fraction of star-forming gas, which suggests the HCN/CO ratio is a poor tracer of this quantity. Instead, we find the HCN/CO ratio is a better tracer of the mean gas density.
- Publication:
-
American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23540701B