Nebular emission from star-forming galaxies

Abstract

We present a new model for computing consistently the line and continuum\nemission from galaxies, based on a combination of recent population synthesis\nand photoionization codes. We use effective parameters to describe the HII\nregions and the diffuse gas ionized by single stellar generations in a galaxy\n[...]. We calibrate the nebular properties of our model using the observed\n[OIII]/Hbeta, [OII]/[OIII], [SII]/Halpha, and [NII]/[SII] ratios of a\nrepresentative sample of nearby spiral and irregular, starburst, and HII\ngalaxies. To compute whole (line plus continuum) spectral energy distributions,\nwe include the absorption by dust in the neutral interstellar medium (ISM)\nusing a recent simple prescription, which is consistent with observations of\nnearby starburst galaxies. Our model enables us to interpret quantitatively the\nobserved optical spectra of galaxies in terms of stars, gas, and dust\nparameters. We find that the range of ionized-gas properties spanned by nearby\ngalaxies implies factors of 3.5 and 14 variations in the Halpha and [OII]\nluminosities produced per unit star formation rate (SFR). When accounting for\nstellar Halpha absorption and absorption by dust in the neutral ISM, the actual\nuncertainties in SFR estimates based on the emergent Halpha and [OII]\nluminosities are as high as several decades. We derive new estimators of the\nSFR, the gas-phase oxygen abundance, and the effective absorption optical depth\nof the dust in galaxies. We show that, with the help of other lines such as\n[OII], Hbeta, [OIII], [NII], or [SII], the uncertainties in SFR estimates based\non Halpha can be reduced to a factor of only 2-3, even if the Halpha line is\nblended with the adjacent [NII] lines. Without Halpha, however, the SFR is\ndifficult to estimate from the [OII], Hbeta, and [OIII] lines. (abridged)\n

Keywords

Affiliated Institutions

• Centre National de la Recherche Scientifique FR
• Mineral Products Association GB
• Institut d'Astrophysique de Paris FR

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