Interpreting the ionization sequence in star-forming galaxy emission-line spectra

Chris T. Richardson, James T. Allen, Jack A. Baldwin, Paul C. Hewett, Gary J. Ferland, Anthony Crider, Helen Meskhidze

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


High-ionization star-forming (SF) galaxies are easily identified with strong emission-line techniques such as the BPT diagram, and form an obvious ionization sequence on such diagrams. We use a locally optimally emitting cloud model to fit emission-line ratios that constrain the excitation mechanism, spectral energy distribution, abundances and physical conditions along the star formation ionization sequence. Our analysis takes advantage of the identification of a sample of pure SF galaxies, to define the ionization sequence, via mean field independent component analysis. Previous work has suggested that the major parameter controlling the ionization level in SF galaxies is themetallicity.Herewe showthat the observed SF sequence could alternatively be interpreted primarily as a sequence in the distribution of the ionizing flux incident on gas spread throughout a galaxy. Metallicity variations remain necessary to model the SF sequence, however, our best models indicate that galaxies with the highest and lowest observed ionization levels (outside the range -0.37 < log [OIII]/Hβ < -0.09) require the variation of an additional physical parameter other than metallicity, which we determine to be the distribution of ionizing flux in the galaxy.

Original languageEnglish
Pages (from-to)988-1012
Number of pages25
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Feb 22 2016

Bibliographical note

Publisher Copyright:
© 2016 The Authors.


  • Galaxies: ISM
  • Galaxies: evolution
  • Galaxies: starburst
  • Galaxies: structure

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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