M51星系盘上HCN J=1-0发射线的成图观测

Mapping observations of the nearby spiral galaxy M51 in HCN J=1-0

为了解致密分子气体在星系盘上的分布,我们使用IRAM 30 m望远镜对M51的HCN J=1-0分子发射线进行了成图观测（角分辨率为28＂）.成图区域达4＇×5＇（1＇~2.9 kpc）,是目前为止对M51 HCN J=1-0发射线进行的最为延展的一次成图观测.将之与NRO 45 m望远镜观测得到的CO J=1-0发射线数据进行比较,我们对M51星系内致密分子气体与总分子气体分布的差异与相关性等特性进行了讨论.HCN分布呈现出向星系中心集中的趋势,中心区域的HCN积分强度是星系盘上的3–4倍.在HCN积分强度图中也观测到了由CO发射所示踪的气体旋臂.M51总HCN光度是中心主波束区域（kpc尺度）的5倍,这说明了依然存在很多的致密分子气体分布在星系中心核区之外.在距星系光学中心42＇＇内的区域（包含核球区的星系内盘）,距离星系中心越远,HCN/CO的积分强度比越小;而在远离星系中心的星系盘上（大于42＂）该比值基本保持恒定.对于星系盘上的大部分区域,IHCN和ICO具有非常紧密的线性相关性,最小二乘法拟合的斜率为1.2.而对于核球区它们的拟合斜率则要高得多（2.3）.所有这些都说明了致密分子气体的性质（例如含量、发射条件、HCN的丰度）在核球区和星系盘上是不一样的.

We have mapped the Whirlpool galaxy M51 in HCN J=1-0 transition with the IRAM 30m telescope in order to investigate the distribution of dense molecular gas in M51. The map covers an extent of 4＇x5＇ with a spatial resolution of 28＂, which is the largest HCN map of M51. Comparing with the CO map observed by the NRO 45m telescope, convolved to the same resolution as our HCN map, we discuss the different distribution （and correlation） between the molecular gas and dense molecular gas traced by CO and HCN respectively. HCN shows a concentration in the center, the integrated intensity of the central regions is 3--4 times of the outer disk. The inner spiral arms traced nicely in CO map are all detected in HCN. The total HCN integrated intensity of M51 is 5 times of the central beam （kpc scales）, illustrating that there is an extended dense molecular gas distribution outside of the -1 kpc nuclear region, The ratio of HCN/CO decreases along the radii in the central 42＂ （roughly corresponding to the stellar bulge component） and stays fiat at larger radii in the spiral disk. For most of the regions in the disk, HCN correlates tightly with CO, following a nearly linear relation with a slope of 1.2, Whereas in the bulge the correlation slope is much higher （2.3）. All these suggest a dramatic difference in the dense molecular gas properties （e.g., content, excitation or HCN abundances） between the bulge and spiral disk regions.