Cool Atomic Clouds in Spiral Galaxies

这项研究将包括对附近螺旋星系圆盘中星际介质的冷原子阶段的研究。这项研究的目的是测量温暖和寒冷星际原子氢（HI）的相对比例。这个分数是加热过程和星际压力的敏感函数。第二个目标是测量100到几千太阳质量范围内的云大小光谱。观测结果将表明，这些原子云的丰度是否会随着星系半径的减小而增加，就像大多数螺旋星系中的分子云一样，或者原子云是否会在分子云最丰富的地方减少。第三个目标是测量螺旋星系21cm波长吸收的横截面。这将有助于更好地理解高红移处HI吸收谱线的起源。观测技术将是通过50个附近螺旋星系的圆盘测量21厘米的吸收光谱。所有Shapley- Ames星系表（蓝等小于12）中有合适背景源的星系都将被观测到，而非常近的星系（被命名为M31、M33和M81）将沿着几条视线被研究。最近超大阵列（VLA）在线数据约简系统的扩展首次使这成为可能。这项研究的基本目的是了解螺旋星系中冷星际云的丰度和分布，并确定分子和原子云是单一种群还是两个不同的物种。

The research will involve a study of the cool atomic phase of the interstellar medium in the disks of nearby spiral galaxies. The purpose of the research is to measure the relative fractions of warm and cool interstellar atomic hydrogen (HI). This fraction is a sensitive function of the heating process and the interstellar pressure. The second objective is to measure the cloud size spectrum in the mass range one hundred to several thousand solar masses. The observations will show whether the abundance of these of these atomic clouds increases with decreasing galactic radius, as do the molecular clouds in most spirals, or whether the atomic clouds are depopulated where the molecular clouds are most abundant. The third objective is to measure the cross-sections of spiral galaxies for 21 cm wavelength absorption. This will provide a better understanding of the origin of HI absorption lines at high redshifts. The observing technique will be to measure 21 cm absorption spectra through the disks of 50 nearby spiral galaxies. All Shapley- Ames catalog of galaxies (blue magnitude less than 12) with suitable background sources will be observed, and very nearby galaxies (designated M31, M33, and M81) will be studied along several lines of sight. The recent expansion of the Very large Array (VLA) on-line data reduction system makes this feasible for the first time. The basic purpose of this research will be to understand the abundance and distribution of cool interstellar clouds in spiral galaxies and to determine whether molecular and atomic clouds are a single population or two distinct species.