Interpreting the Ionization Sequence in Star-Forming Galaxy Emission-Line Spectra

Abstract

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 star-forming 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 the metallicity. Here we show that 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 [O III]/H\b{eta} < -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.

41 pages, 17 figures, 9 tables, accepted to MNRAS

References in corpus (14)

• The Host Galaxies and Classification of Active Galactic Nuclei
• Galaxy Zoo 1 : Data Release of Morphological Classifications for nearly 900,000 galaxies
• Gas-to-Dust mass ratios in local galaxies over a 2 dex metallicity range
• The 2013 Release of Cloudy
• Theoretical Evolution of Optical Strong Lines across Cosmic Time
• The Effects of Stellar Rotation. II. A Comprehensive Set of Starburst99 Models
• Grids of stellar models with rotation - III. Models from 0.8 to 120 Msun at a metallicity Z = 0.002
• Emission-line Diagnostics of Low Metallicity AGN
• [NeIII]/[OII] as an Ionization Parameter Diagnostic in Star-Forming Galaxies
• [Ne V] Emission in Optically Classified Starbursts
• Interpreting high [O III]/Hbeta ratios with maturing starbursts
• Interpreting the Ionization Sequence in AGN Emission-Line Spectra
• Mid-infrared diagnostics of starburst galaxies: clumpy, dense structures in star-forming regions in the Antennae (NGC 4038/4039)
• Theoretical Modeling of Emission-Line galaxies: New Classification Parameters for Mid-Infrared and Optical Spectroscopy

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