Galactic Neutral Hydrogen Emission Profile Structure

Abstract

Analysis of Galactic neutral hydrogen emission proÐles that have been corrected for sidelobe radiation conÐrm the existence of three distinct component line width regimes identiÐed by Verschuur & Magnani in 1994. In addition, a fourth becomes recognizable in the data in directions of low total column density. The line width regimes are around 50 km s~1 (component 1a), 31 km s~1 (component 1b), 13 km s~1 (component 2), and 5.2 km s~1 for the narrow lines arising from cool H I (component 3). In this paper, the new data are presented and compared with previously published results. The possible origin of the distinct line width regimes is brieÑy examined, and it is concluded that a new interpretation is needed, one that involves a plasma phenomenon known as the critical ionization velocity, which will be fully discussed in a subsequent paper.

Gaussian decomposition of $\ion{H}{i}$ surveys

“…Earlier, similar results have been reported by Verschuur and coauthors, who have published several papers (1989( , 1999( hereafter VP, 2002 hereafter V4) where they argue that neutral hydrogen emission profiles produced by gas in the local interstellar medium are characterised by three, or probably four, line-width regimes where dominant and pervasive features have widths of the order 5.2 ± 0.4 (VP regime 3), 12.8 ± 0.4 (VP regime 2), 31.0 ± 0.5 (VP regime 1b), and 50.1 ± 0.6 km s −1 (VP regime 1a). They also note that these line-width regimes show a striking resemblance to a set of velocity regimes described by a plasma physical mechanism called the critical ionization phenomenon.…”

The spin temperature of warm interstellar H I

“…Of course, if Ly-α excitation is actually operating at a Doppler temperature above T K , the measured spin temperatures are already somewhat inflated, and the turbulent velocities Recent studies of stray-radiation-corrected Galactic emission profiles find evidence for three kinematic components having linewidths of 5, 13, and 31 km s −1 (Verschuur & Peratt 1999). These correspond respectively to individual clouds (or to T D = 550 K; see Takakubo 1967, and references therein), to the 6 km s −1 cloud-cloud velocity dispersion (or T D = 3700 K), and to an anonymous component with T D = 21 000 K or a combination of T K = 8000 K, v turb = 10.4 km s −1 .…”

On the Critical Ionization Velocity Effect in Interstellar Space and Possible Detection of Related Continuum Emission

“…The 34 km/s wide family of HI components is found in high-latitude, low-velocity HI [1], [2], in Very High-Velocity Clouds [12], the Magellanic Stream [13], [14] and…”

“…Verschuur & Peratt [1] have described the Gaussian decomposition of interstellar HI emission profile shapes and find that at least three families of linewidths of order 34, 13 and 6 km/s characterize the profiles. If the linewidths are taken to indicate kinetic temperature these would imply that the HI along a given line of sight manifests 3 temperatures of order 24,000, 3,500 & 750 K. However, at the former value the hydrogen atoms should be ionized and hence not observable at 21-cm wavelength.…”

“…It has been suggested [1], [4] that the linewidth families are related to a plasma phenomenon known as the critical ionization velocity (CIV) effect based on a numerical similarity between families of CIV values and HI linewidths. The CIV is defined as that velocity at which a neutral particle traveling into plasma permeated by a magnetic field becomes ionized when its kinetic energy is equal to its ionization potential [5].…”