Physical properties of diffuse H° gas in the Galaxy

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

Abstract

The diffuse (non self-gravitating) interstellar medium of the Galaxy is almost impossibly complex and diverse. Temperatures and densities range over many orders of magnitude. The magnetic field links all regimes of the gas, including cosmic rays. Shocks from supernovae and other sources buffet the medium. Into this maelstrom, theorists have ventured. Some models emphasize time-independent thermal equilibrium between hot and cold phases of the gas. Other models stress frequent dynamical events that throw much of the medium out of thermal equilibrium. Some models include magnetic field effects, others do not. Here we report on the nature of diffuse atomic gas, emphasizing observational results from a recent, extensive survey of HI emission and absorption along random lines of sight in the local diffuse medium. We find much of the warmer diffuse atomic gas is out of thermal equilibrium, yet the medium retains a clear dichotomy between warmer and cooler phases. The warmer phase comprises about half of the total diffuse atomic hydrogen gas. Magnetic fields have a median value of about 6 μG in the cold gas, insuring that their dynamical effects cannot be ignored. The conundrum of similar magnetic field strengths in diffuse gas at widely disparate densities remains as an observational fact and a challenge to explain theoretically.

Original languageEnglish
Title of host publicationEAS - Sky Polarisation at Far-Infrared to Radio Wavelengths
Subtitle of host publicationThe Galactic Screen before the Cosmic Microwave Background
Pages55-72
Number of pages18
DOIs
StatePublished - 2007
EventSky Polarisation at Far-Infrared to Radio Wavelengths: The Galactic Screen before the Cosmic Microwave Background - Orsay, France
Duration: Sep 12 2005Sep 15 2005

Publication series

NameEAS Publications Series
Volume23
ISSN (Print)1633-4760
ISSN (Electronic)1638-1963

Conference

ConferenceSky Polarisation at Far-Infrared to Radio Wavelengths: The Galactic Screen before the Cosmic Microwave Background
Country/TerritoryFrance
CityOrsay
Period9/12/059/15/05

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • General Engineering
  • Space and Planetary Science

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