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

We use the Gaussian-fit results of Paper I to investigate the properties ofinterstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNMand CNM) are physically distinct components. The CNM spin temperature histogrampeaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volumefilling fraction of about 0.50 for the WNM; this value is very rough. Theupper-limit WNM temperatures determined from line width range upward from about500 K; a minimum of about 48% of the WNM lies in the thermally unstable region500 to 5000 K. The WNM is a prominent constituent of the interstellar mediumand its properties depend on many factors, requiring global models that includeall relevant energy sources, of which there are many. We use PrincipalComponents Analysis, together with a form of least squares fitting thataccounts for errors in both the independent and dependent parameters, todiscuss the relationships among the four CNM Gaussian parameters. The spintemperature T_s and column density N(HI) are, approximately, the two mostimportant eigenvectors; as such, they are sufficient, convenient, andphysically meaningful primary parameters for describing CNM clouds. The Machnumber of internal macroscopic motions for CNM clouds is typically 2.5, butthere are wide variations. We discuss the historical tau-T_s relationship insome detail and show that it has little physical meaning. We discuss CNMmorphology using the CNM pressure known from UV stellar absorption lines.Knowing the pressure allows us to show that CNM structures cannot be isotropicbut instead are sheetlike, with length-to-thickness aspect ratios ranging up toabout 280. We present large-scale maps of two regions where CNM lies in verylarge ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of turbulent Mach number in equation 16 and figure 12; the new typical value is 1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the astro-ph version to conserve paper. 60 pages, 16 figure