The Millennium Arecibo 21 Centimeter Absorption‐Line Survey. II. Properties of the Warm and Cold Neutral Media

Abstract

We use the Gaussian-fit results of Paper I to investigate the properties of\ninterstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM\nand CNM) are physically distinct components. The CNM spin temperature histogram\npeaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume\nfilling fraction of about 0.50 for the WNM; this value is very rough. The\nupper-limit WNM temperatures determined from line width range upward from about\n500 K; a minimum of about 48% of the WNM lies in the thermally unstable region\n500 to 5000 K. The WNM is a prominent constituent of the interstellar medium\nand its properties depend on many factors, requiring global models that include\nall relevant energy sources, of which there are many. We use Principal\nComponents Analysis, together with a form of least squares fitting that\naccounts for errors in both the independent and dependent parameters, to\ndiscuss the relationships among the four CNM Gaussian parameters. The spin\ntemperature T_s and column density N(HI) are, approximately, the two most\nimportant eigenvectors; as such, they are sufficient, convenient, and\nphysically meaningful primary parameters for describing CNM clouds. The Mach\nnumber of internal macroscopic motions for CNM clouds is typically 2.5, but\nthere are wide variations. We discuss the historical tau-T_s relationship in\nsome detail and show that it has little physical meaning. We discuss CNM\nmorphology using the CNM pressure known from UV stellar absorption lines.\nKnowing the pressure allows us to show that CNM structures cannot be isotropic\nbut instead are sheetlike, with length-to-thickness aspect ratios ranging up to\nabout 280. We present large-scale maps of two regions where CNM lies in very\nlarge ``blobby sheets''.\n