Cold Atomic Gas in the Milky Way

Steven J. Gibson
University of Calgary

Proceedings Abstract

In this review, I consider a number of basic questions about cold Galactic H I: What are the physical properties of the cold atomic phase, and how are they maintained? How much mass is there in the cold H I and in the other phases in the disk? Are the true populations of H₂, cold H I, and warm H I distinctly separated from each other in temperature-density space, or do some phase properties overlap? What is the geometric structure of cold interstellar clouds? What is the phase structure? What brings these structures about? What is the role of cold H I in the phase dynamics of the interstellar medium, in star formation, and in the evolution of the Galaxy as a whole? Some of these questions have more complete answers than others at the present time. I conclude with a few comments on the directions of current research and desirable future observations.

Conference Abstract

The Milky Way Galaxy, like other disk galaxies, is a complex system in which the interplay of stars and interstellar material reveals the ongoing evolution of matter in the Universe. The interstellar gas from which stars form, and to which they return mass and energy, exists in several apparently discrete phases. These phases are labeled in reference to the dominant species of hydrogen (H II, H I, or H₂) as hot (~10⁶ K) and warm (~8000 K) ionized gas, warm (<~8000 K) and cold (~80 K) neutral atomic gas, and very cold (~10 K) molecular gas.

The cold atomic phase is important for several reasons: (1) it contains a large fraction of the total gas mass in the Galactic disk; (2) it is often found in quiescent regions along with the molecular gas that star formation requires; and (3) considerable fine-scale structure is seen in cold H I that may offer a window into the workings of interstellar turbulence and magnetic fields. Unfortunately, the cold atomic phase is not easy to observe, because cold and warm H I are difficult to separate in 21cm emission maps, and absorption studies are limited by the spatial sampling of continuum backgrounds and by the uncertainties in estimated line emission backgrounds. Overcoming these difficulties is essential for understanding how the interstellar medium works.

In this review I will consider a number of basic questions about cold Galactic H I: What are the physical properties of the cold atomic phase, and how are they maintained? How much mass is there in the cold H I and in the other phases in the disk? Are the true populations of H₂, cold H, and warm H I distinctly separated from each other in temperature-density space, or do some phase properties overlap? What is the geometric structure of cold interstellar clouds? What is the phase structure? What brings these structures about? What is the role of cold H I in the phase dynamics of the interstellar medium, in star formation, and in the evolution of the Galaxy as a whole? Some of these questions have more complete answers than others at the present time. I will conclude with a few comments on the directions of current research and desirable future observations.