Physical properties of neutral gas in M31 and the Galaxy

Abstract

view Abstract Citations (87) References (44) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Physical Properties of Neutral Gas in M31 and the Galaxy Braun, Robert ; Walterbos, Rene A. M. Abstract Neutral hydrogen (H I) absorption and deduced emission detected along seven lines of sight through the disk of M31 are analyzed in parallel with published data for the Galaxy. It is shown that the brightness temperature of H I emission is coupled to the opacity of the gas. The Galactic relationship shows asymptotic trends at both large and small opacities. A simple yet effective physical model which accounts for this behavior consists of only two independent components: a high-opacity, cool component of fixed mean temperature, and a low-opacity, warm component of fixed mean brightness. The fitted asymptotic values not only provide a good representation of the data but also are consistent with the much more general constraints imposed by the maximum observed brightness of H I emission and the "threshold" effect of finding detectable absorption whenever the emission brightness exceeds about 5 K. The warm component of H I in the Galaxy is responsible for a mean brightness of about 4 K. The limited data at low opacity in M31 are consistent with this value. The cool Galactic component (in the extended solar neighborhood) has a mean temperature of 105 K, while a radial dependence of gas temperature is observed in M31. Cool-component temperatures in M31 appear to vary from about 70 K at radii between 5 and 10 kpc to about 175 K in the outer disk (10-20 kpc). Intrinsic temperature variations by about a factor of 2 around these mean values are implied by the data. The gas-to-dust ratio is derived from opacity-corrected integrals of H I to the midplane of M31 versus Balmer-decrement-derived extinctions toward H II regions and supernova remnants. A possible gradient of the gas-to-dust ratio is observed with radius. The slope, -0.039 +/- 0.008 dex kpc^-1^, is consistent with the abundance gradient. Gas-to-dust ratios comparable to solar neighborhood values are found at radii between about 10 and 15 kpc in M31. The most plausible mechanism for accounting for a higher cool- component H I temperature in M3 1 appears to be a lower mean gas pressure by a factor of about 2. By adopting this gas pressure and the component temperatures of the model fit the physical properties of the neutral gas are estimated. Deduced volume filling factors of the Galactic H I are about 1% and 15%, respectively, for the cool and warm components while for M31 they are 8% and 30%. The higher filling factors in M31 are coupled with lower gas densities especially in the cool phase. The large ratio of surface to volume filling factors for both cool and warm H I suggests that these components are distributed predominantly as large sheet- or shell-like structures. The warm H I component is likely to be physically distinct from the cool component and may instead be associated with diffuse ionized gas at large scale heights in the spiral arms. Publication: The Astrophysical Journal Pub Date: February 1992 DOI: 10.1086/170998 Bibcode: 1992ApJ...386..120B Keywords: Andromeda Galaxy; Emission Spectra; H Lines; Interstellar Gas; Milky Way Galaxy; Neutral Gases; Absorption Spectra; Computational Astrophysics; Cosmic Dust; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M31; ISM: DUST; EXTINCTION; ISM: GENERAL; RADIO LINES: ATOMIC full text sources ADS | data products SIMBAD (8) NED (1)