The nature of the interstellar medium of the starburst low-metallicity galaxy Haro 11: a multi-phase model of the infrared emission^⋆

¹ Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: diane.cormier@cea.fr
² University of Cincinnati, Clermont College, Batavia, OH, 45103, USA
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁴ Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁶ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁷ Dept. of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
⁸ Istituto di Fisica dello Spazio Interplanetario, INAF, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 14 June 2012
Accepted: 28 August 2012

Abstract

Context. The low-metallicity interstellar medium (ISM) is profoundly different from that of normal systems, being clumpy with low dust abundance and little CO-traced molecular gas. Yet many dwarf galaxies in the nearby universe are actively forming stars. As the complex ISM phases are spatially mixed with each other, detailed modeling is needed to understand the gas emission and subsequent composition and structure of the ISM.

Aims. Our goal is to describe the multi-phase ISM of the infrared bright low-metallicity galaxy Haro 11, dissecting the photoionised and photodissociated gas components.

Methods. We present observations of the mid-infrared and far-infrared fine-structure cooling lines obtained with the Spitzer/IRS and Herschel/PACS spectrometers. We use the spectral synthesis code Cloudy to methodically model the ionised and neutral gas from which these lines originate.

Results. We find that the mid- and far-infrared lines account for  ~1% of the total infrared luminosity L_TIR, acting as major coolants of the gas. Haro 11 is undergoing a phase of intense star formation, as traced by the brightest line, [O iii] 88 μm, with L _{[O III]} /L_TIR ~ 0.3%, and high ratios of [Ne iii]/[Ne ii] and [S iv]/[S iii]. Due to their different origins, the observed lines require a multi-phase modeling comprising: a compact H ii region, dense fragmented photodissociation regions (PDRs), a diffuse extended low-ionisation/neutral gas which has a volume filling factor of at least 90%, and porous warm dust in proximity to the stellar source. For a more realistic picture of the ISM of Haro 11 we would need to model the clumpy source and gas structures. We combine these 4 model components to explain the emission of 17 spectral lines, investigate the global energy balance of the galaxy through its spectral energy distribution, and establish a phase mass inventory. While the ionic emission lines of Haro 11 essentially originate from the dense H ii region component, a diffuse low-ionisation gas is needed to explain the [Ne ii], [N ii], and [C ii] line intensities. The [O iii] 88 μm line intensity is not fully reproduced by our model, hinting towards the possible presence of yet another low-density high-ionisation medium. The [O i] emission is consistent with a dense PDR of low covering factor, and we find no evidence for an X-ray dominated component. The PDR component accounts for only 10% of the [C ii] emission. Magnetic fields, known to be strong in star-forming regions, may dominate the pressure in the PDR. For example, for field strengths of the order of 100 μG, up to 50% of the [C ii] emission may come from the PDR.