The Herschel Dwarf Galaxy Survey

I. Properties of the low-metallicity ISM from PACS spectroscopy^{⋆,⋆⋆,⋆⋆⋆}

¹ Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
e-mail: diane.cormier@zah.uni-heidelberg.de
² Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
³ University of Cincinnati, Clermont College, Batavia, OH 45103, USA
⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁵ Laboratoire d’Astrophysique de Marseille – LAM, Université d’Aix-Marseille & CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁸ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁹ ESO, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹⁰ Department of Physics and Astronomy, University of Sussex, Brighton, BN1 9QH, UK
¹¹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹² Istituto di Fisica dello Spazio Interplanetario, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 23 October 2014
Accepted: 5 February 2015

Abstract

Context. The far-infrared (FIR) lines are important tracers of the cooling and physical conditions of the interstellar medium (ISM) and are rapidly becoming workhorse diagnostics for galaxies throughout the universe. There are clear indications of a different behavior of these lines at low metallicity that needs to be explored.

Aims. Our goal is to explain the main differences and trends observed in the FIR line emission of dwarf galaxies compared to more metal-rich galaxies, and how this translates in ISM properties.

Methods. We present Herschel/PACS spectroscopic observations of the [C ii] 157 μm, [O i] 63 and 145 μm, [O iii] 88 μm, [N ii] 122 and 205 μm, and [N iii] 57 μm fine-structure cooling lines in a sample of 48 low-metallicity star-forming galaxies of the guaranteed time key program Dwarf Galaxy Survey. We correlate PACS line ratios and line-to-L_TIR ratios with L_TIR, L_TIR/L_B, metallicity, and FIR color, and interpret the observed trends in terms of ISM conditions and phase filling factors with Cloudy radiative transfer models.

Results. We find that the FIR lines together account for up to 3 percent of L_TIR and that star-forming regions dominate the overall emission in dwarf galaxies. Compared to metal-rich galaxies, the ratios of [O iii]₈₈/[N ii]₁₂₂ and [N iii]₅₇/[N ii]₁₂₂ are high, indicative of hard radiation fields. In the photodissociation region (PDR), the [C ii]₁₅₇/[O i]₆₃ ratio is slightly higher than in metal-rich galaxies, with a small increase with metallicity, and the [O i]₁₄₅/[O i]₆₃ ratio is generally lower than 0.1, demonstrating that optical depth effects should be small on the scales probed. The [O iii]₈₈/[O i]₆₃ ratio can be used as an indicator of the ionized gas/PDR filling factor, and is found to be ~4 times higher in the dwarfs than in metal-rich galaxies. The high [C ii]/L_TIR, [O i]/L_TIR, and [O iii]/L_TIR ratios, which decrease with increasing L_TIR and L_TIR/L_B, are interpreted as a combination of moderate far-UV fields and a low PDR covering factor. Harboring compact phases of a low filling factor and a large volume filling factor of diffuse gas, the ISM of low-metallicity dwarf galaxies has a more porous structure than that of metal-rich galaxies.