Ionization of the intercloud medium and the central disk regions of spiral galaxies

view Abstract Citations (11) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ionization of the Intercloud Medium and the Central Disk Regions of Spiral Galaxies Silk, Joseph Abstract The two-phase model of the interstellar medium is reconsidered in terms of a hydrogen ionization rate that may vary significantly between cloud and intercloud medium. In view of recent observational and theoretical studies directly pertaining to the ionization of H clouds, it is proposed that the observed free-free absorption of extragalactic radio sources and nonthermal radio continuum can be produced predominantly in the intercloud medium. Various observational of this hypothesis include interpretations of diffuse Ha and Hfl emission, radio recombination lines from distributed interstellar gas, and line emission from the central disk regions of nearby Sc galaxies. All these observational data can be explained in terms of a simple model of the intercloud medium, which is consistent with recent 21-cm studies of the interstellar medium. This necessitates a somewhat higher value of the hydrogen ionization rate than previously accepted, amounting to (1-3) x 10-14 1 in the intereloud medium, but permits a lower ionization rate ( l0-' 1) in H clouds. An important difference with regard to earlier work is that the inter- cloud medium is required to possess a relatively clumpy distribution of ionization. Moreover, the intereloud rate of hydrogen ionization is required to increase by about an order of magnitude within the central regions of the Galaxy. Several ionization sources are discussed, and of these, blast-wave emission from supernova remnants and soft X-rays from ultraviolet dwarfs are found to be potentially viable candidates for the role of ionizing the intercloud medium. The role of these mechanisms is quantitatively evaluated, both in the solar vicinity and in the central disk regions of spiral galaxies. Subject headiiigs: interstellar matter - nebulae - supernova remnants - 21-cm radiation - X-rays Publication: The Astrophysical Journal Pub Date: May 1973 DOI: 10.1086/152082 Bibcode: 1973ApJ...181..707S full text sources ADS |