TY - GEN
T1 - Physical properties of diffuse H° gas in the Galaxy
AU - Troland, T. H.
PY - 2007
Y1 - 2007
N2 - 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.
AB - 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.
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U2 - 10.1051/eas:2007005
DO - 10.1051/eas:2007005
M3 - Conference contribution
AN - SCOPUS:55849121883
SN - 9782868839800
T3 - EAS Publications Series
SP - 55
EP - 72
BT - EAS - Sky Polarisation at Far-Infrared to Radio Wavelengths
T2 - Sky Polarisation at Far-Infrared to Radio Wavelengths: The Galactic Screen before the Cosmic Microwave Background
Y2 - 12 September 2005 through 15 September 2005
ER -