Nebular properties and the ionizing radiation field in the Galactic center

Joseph C. Shields, Gary J. Ferland

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


Nebulosity in the central parsec of the Milky Way exhibits a low-ionization spectrum that has led previous analyses of this region to conclude that this material is photoionized by a relatively soft continuum. We have reanalyzed the infrared emission-line spectrum of the Galactic center in order to consider whether the data could actually be explained with photoionization by a relatively hard, yet dilute continuum, and additionally to constrain the properties of the radiating plasma. We conclude that the composite infrared spectrum does not place strong restrictions on the nature of the ionizing continuum and that much of the ionized gas in the Galactic center is probably quite dense (≳105 cm-3). If the ionizing spectral energy distribution is, in fact, relatively hard, this material additionally must be highly clumped. Indications that the ionizing continuum is probably fairly soft are provided by radio recombination-line studies, however. Shocks are unlikely to be important for generating the observed nebulosity in light of the large far-infrared continuum luminosity of the central parsec. The infrared fine-structure spectrum provides mixed evidence for enhanced heavy-element abundances, and enrichment is probably limited to ≲2 times solar levels. We predict the optical spectrum of the Galactic center and conclude that it probably appears as an H II nucleus to external observers. Comparisons between the nucleus of the Milky Way and nuclei of external galaxies remain uncertain, however, since the parsec-scale metric apertures typically employed for studies of Galactic-center nebular emission are 1-2 orders of magnitude smaller than those used in extragalactic measurements.

Original languageEnglish
Pages (from-to)236-251
Number of pages16
JournalAstrophysical Journal
Issue number1
StatePublished - Jul 20 1994


  • Galaxies: nuclei
  • Galaxy: center
  • H II regions
  • Infrared: ISM: lines and bands

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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