Abstract
(abridged) Our goal is to describe the multi-phase ISM of the IR bright low-metallicity galaxy Haro 11, dissecting the photoionised and photodissociated gas components. We present observations of the mid- and far-IR 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. We find that the mid- and far-IR lines account for ~1% of the total IR luminosity L_TIR. Haro 11 is undergoing a phase of intense star formation, as traced by the brightest line [OIII] 88um, with L_[OIII]/L_TIR ~0.3%, and high ratios of [NeIII]/[NeII] and [SIV]/[SIII]. Due to their different origins, the observed lines require a multi-phase modeling comprising: a compact HII 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 HII region component, a diffuse low-ionisation gas is needed to explain the [NeII], [NII], and [CII] line intensities. The [OIII] 88um line intensity is not fully reproduced by our model, hinting towards the possible presence of yet another low-density high-ionisation medium. The [OI] 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 [CII] emission.
Highlights
Star formation in the most pristine environments of the early universe is poorly understood
We find that the mid- and far-infrared lines account for ∼1% of the total infrared luminosity LTIR, acting as major coolants of the gas
Local as well as global, within the dwarf galaxies are we witnessing with these observational signatures? How do we turn these signatures into a realistic view of star formation under early universe conditions? How do basic local parameters, such as radiation field, density, compactness, filling factors, metallicity, etc. figure in the picture we have of star-forming dwarf galaxies? Much of the ambiguity surrounding these questions is due to the lack of understanding the precise role of critical diagnostics and of spatial resolution in the most nearby objects, especially in the FIR/submm, resulting in a mixture of different interstellar medium (ISM) phases in the integrated view of galaxies
Summary
Star formation in the most pristine environments of the early universe is poorly understood. The closest analogs to chemically unevolved systems are the low-metallicity dwarf galaxies of our local universe From an observational point of view, at all wavelengths, local universe low-metallicity dwarf galaxies show dramatic differences compared to their more metal-rich counterparts (Kunth & Östlin 2000, for a review), in the midinfrared (MIR), far-infrared (FIR), and submillimeter (submm) wavelength regimes. These include the dearth of polycyclic aromatic hydrocarbons (PAHs), prominent hot MIR-emitting dust, Article published by EDP Sciences. Local as well as global, within the dwarf galaxies are we witnessing with these observational signatures? How do we turn these signatures into a realistic view of star formation under early universe conditions? How do basic local parameters, such as radiation field, density, compactness, filling factors, metallicity, etc. figure in the picture we have of star-forming dwarf galaxies? Much of the ambiguity surrounding these questions is due to the lack of understanding the precise role of critical diagnostics and of spatial resolution in the most nearby objects, especially in the FIR/submm, resulting in a mixture of different interstellar medium (ISM) phases in the integrated view of galaxies
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