Molecular Gas and Star Formation in Early Merger Arp 240

Abstract

This thesis presents $^{12}$CO $J$=1-0, $J$=2-1 and $^{13}$CO $J$=1-0 maps of an early-stage galaxy merger Arp 240 observed with ALMA. Arp 240 has a high star formation efficiency but relatively normal gas fraction, which is in contrast with typical merging pairs. We applied RADEX modeling to fit the molecular gas properties. We found that the gas concentrated regions in this system have CO-to-H$_2$ conversion factors closer to typical ultra-luminous infrared galaxy (ULRG) values. This conversion factor value is consistent with that derived from LTE analysis. Adopting the ULIRG conversion factor, we explored the relationship between molecular gas surface density $\Sigma_{mol}$ and star formation rate surface density $\Sigma_{SFR}$, which is traced by 33 GHz radio continuum from the VLA. We found that the star forming regions generally have a combined Toomre factor $Q_{tot}$ < 0.5, with the exception of the center of NGC 5257, which has $Q_{tot} \sim 1$. This suggests these star forming regions are undergoing gravitational instability. We further calculated the star formation efficiency per freefall time $\epsilon_{ff}$ on galactic scales for these regions. It turns out some regions have $\epsilon_{ff}$ exceeding 100\%. We argue that $\epsilon_{ff}$ on giant molecular cloud (GMC) scales should be about a factor of 10 lower, which might suggest the star forming activity in this system is regulated by cloud collapse on GMC scales.