High-resolution imaging of compact high-velocity clouds
High-resolution imaging of compact high-velocity clouds
复制标题
致密高速云的高分辨率成像
DOI：
10.1051/0004-6361:20020640
复制
发表时间：
2002
影响因子：
6.5
通讯作者：
Nrao
中科院分区：
文献类型：
--
作者：
V. D. Heij;R. Braun;W. Univ.;Nfra;Nrao
我们用韦斯特堡综合射电望远镜拍摄了5个高空致密的高速云团，其角分辨率和KM S 1谱分辨率都达到了极小值。这些CHVC具有独特的形态，由一个或多个静止的、低分散的致密核心嵌入到一个双色温晕中。这些致密的核可以毫不含糊地与凝聚原子氢的冷中性介质相一致，因为它们的线宽比热中性介质的热线宽要窄得多。由于干涉测量数据对脉冲发射的灵敏度有限，在WSRT观测中不能直接探测到热介质。补充的总功率数据对Hi的冷和暖分量都非常敏感，可用于所有来源的比较，尽管角度分辨率从3 0到36 0不等。在我们的样本中，致密CNM组件中的分数Hi通量从4%到16%不等。所有物体在CNM柱密度中至少有一个局部峰，当用arcmin分辨率观察时，该峰超过约1019cm2。1-2×10~(19)cm~2的峰柱密度是这些污染源长期存活的先决条件。在我们的样本中，有一个天体，CHVC12020443(戴维斯的云团)，位于M31的圆盘附近。该天体的特点是其CNM浓度的线宽非常宽，比迄今以可比分辨率研究的13种CHVC的中位数高出5倍以上。这些CNM浓度位于震源边缘的弧形中，面向M31盘。从NRAO 140英尺望远镜的总功率数据中可以看到，这个震源的Diuse Hi分量在M31盘的方向上有一个位置Oset。所有这些特征表明，CHVC120、20443处于与已研究的大多数其他CHVc不同的进化状态。同样，仅在另一颗星云CHVC199107.0466(Wakker&Schwarz1991)中探测到了较宽的CNM线宽，该星云也位于局地群重心方向，具有已知的最极端的径向速度。戴维斯的云团的一种明显的可能性似乎是与M31发生某种类型的物理相互作用。这种相互作用的最可能形式可能是M31‘S可见矮星伴星M32或NGC205的冲压或潮汐剥离，或者是伴随着Hi凝聚的暗伴星。位于局部群重心方向的致密天体在约束局部群假设部署CHVC方面起着重要作用。
We have imaged five compact high-velocity clouds in Hi with arcmin angular resolution and km s 1 spectral reso- lution using the Westerbork Synthesis Radio Telescope. These CHVCs have a characteristic morphology, consisting of one or more quiescent, low-dispersion compact cores embedded in a diuse warm halo. The compact cores can be unambiguously identified with the cool neutral medium of condensed atomic hydrogen, since their linewidths are significantly narrower than the thermal linewidth of the warm neutral medium. Because of the limited sensitivity to diuse emission inherent to interfer- ometric data, the warm medium is not directly detected in the WSRT observations. Supplementary total-power data, which is fully sensitive to both the cool and warm components of Hi, is available for comparison for all the sources, albeit with angular resolutions that vary from 3 0 to 36 0 . The fractional Hi flux in compact CNM components varies from 4% to 16% in our sample. All objects have at least one local peak in the CNM column density which exceeds about 10 19 cm 2 when observed with arcmin resolution. It is plausible that a peak column density of 1-2 10 19 cm 2 is a prerequisite for the long-term survival of these sources. One object in our sample, CHVC 120 20 443 (Davies' cloud), lies in close projected proximity to the disk of M 31. This object is characterized by exceptionally broad linewidths in its CNM concentrations, more than 5 times greater than the median value found in the 13 CHVCs studied to date at comparable resolution. These CNM concentrations lie in an arc on the edge of the source facing the M 31 disk. The diuse Hi component of this source, seen in total-power data from the NRAO 140-foot telescope, has a positional oset in the direction of the M 31 disk. All of these attributes suggest that CHVC 120 20 443 is in a dierent evolutionary state than most of the other CHVCs which have been studied. Similarly broad CNM linewidths have only been detected in one other cloud, CHVC 110.6 07.0 466 (Wakker & Schwarz 1991) which also lies in the Local Group barycenter direction and has the most extreme radial velocity known. A distinct possibility for Davies' cloud seems to be physical interaction of some type with M 31. The most likely form of this interaction might be the ram-pressure or tidal-stripping by either one of M 31's visible dwarf companions, M 32 or NGC 205, or else by a dark com- panion with an associated Hi condensation. The compact objects located in the direction of the Local Group barycenter have an important role to play in constraining the Local Group hypothesis for the deployment of CHVCs.