Is this an Early Stage Merger? A Case Study on Molecular Gas and Star
  Formation Properties of Arp 240
  We present new high resolution $^{12}$CO $J$=1-0, $J$=2-1, and $^{13}$CO
$J$=1-0 maps of the early stage merger Arp 240 (NGC5257/8) obtained with the
Atacama Large Millimeter/submillimeter Array (ALMA). Simulations in the
literature suggest that the merger has just completed its first passage;
however, we find that this system has a lower global gas fraction but a higher
star formation efficiency compared to typical close galaxy pairs, which
suggests that this system may already be in an advanced merger stage. We
combine the ALMA data with $^{12}$CO $J$=3-2 observations from the
Submillimeter Array and carry out RADEX modeling on several different regions.
Both the RADEX modeling and a local thermal equilibrium (LTE) analysis show
that the regions are most likely to have a CO-to-H$_2$ conversion factor
$\alpha_{\mathrm{CO}}$ close to or perhaps even smaller than the typical value
for (ultra-)luminous infrared galaxies. Using 33 GHz data from the Very Large
Array to measure the star formation rate, we find that most star forming
regions have molecular gas depletion times of less than 100 Myr. We calculated
the star formation efficiency (SFE) per free-fall time for different regions
and find some regions appear to have values greater than 100%. We find these
regions generally show evidence for young massive clusters (YMCs). After
exploring various factors, we argue that this is mainly due to the fact that
radio continuum emission in those regions is dominated by that from YMCs, which
results in an overestimate of the SFE per free-fall time.
