Is this an Early Stage Merger? A Case Study on Molecular Gas and Star Formation Properties of Arp 240

TL;DR

This study uses high-resolution ALMA observations to analyze molecular gas and star formation in the early stage merger Arp 240, revealing that it may be more advanced in merging than previously thought, with efficient star formation and young clusters.

Contribution

It provides detailed molecular gas and star formation analysis of Arp 240, challenging previous merger stage classifications and exploring star formation efficiencies in this system.

Findings

Arp 240 has a lower gas fraction but higher star formation efficiency than typical close pairs.

Most regions have molecular gas depletion times under 100 Myr.

Some regions show star formation efficiency per free-fall time exceeding 100%, likely due to YMCs.

Abstract

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.