论文标题
这是早期合并吗？ ARP 240的分子气和恒星形成特性的案例研究
Is this an Early Stage Merger? A Case Study on Molecular Gas and Star Formation Properties of Arp 240
论文作者
论文摘要
我们提出了新的高分辨率$^{12} $ co $ j $ = 1-0，$ j $ = 2-1，以及$^{13} $ co $ j $ j $ = 1-0映射的早期阶段合并ARP ARP 240（NGC5257/8）（ngc5257/8）与Atacama大型米计/超米计阵列（Alimeter/Allay（Alma）一起获得。文献中的模拟表明合并刚刚完成了第一段。但是，我们发现该系统的全球气体分数较低，但与典型的近距离银河系对相比，恒星形成效率更高，这表明该系统可能已经处于高级合并阶段。我们将ALMA数据与$^{12} $ co $ j $ = 3-2观测到亚毫米阵列的观测值，并在几个不同区域进行Radex建模。 RADEX建模和局部热平衡（LTE）分析都表明，这些区域最有可能具有Co-to-H $ _2 $转换因子$α_ {\ Mathrm {Co}} $接近或甚至小于（Ultra-）灯泡的典型值。使用来自非常大阵列的33 GHz数据来测量恒星的形成速率，我们发现大多数恒星形成区的分子气体耗竭时间小于100 MYR。我们在不同区域计算了每个自由下落时间的恒星形成效率（SFE），发现某些区域的值似乎大于100％。我们发现这些区域通常显示出年轻的大型集群（YMC）的证据。在探索了各种因素之后，我们认为这主要是由于这些区域中的无线电连续体发射由YMC所主导的，这导致每个自由下降时间的SFE高估。
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 $α_{\mathrm{CO}}$ 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.
