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The Herschel Dwarf Galaxy Survey. I. Properties of the low-metallicity ISM from PACS spectroscopy

Cormier, D. and Madden, S. C. and Lebouteiller, V. and Abel, N. and Hony, S. and Galliano, F. and Rémy-Ruyer, A. and Bigiel, F. and Baes, M. and Boselli, A. and Chevance, M. and Cooray, A. and De Looze, I. and Doublier, V. and Galametz, M. and Hughes, T. M. and Karczewski, O. Ł. and Lee, M.-Y. and Lu, N. and Spinoglio, L. (2015) The Herschel Dwarf Galaxy Survey. I. Properties of the low-metallicity ISM from PACS spectroscopy. Astronomy and Astrophysics, 578 . A53. ISSN 0004-6361.

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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.

Item Type:Article
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URLURL TypeDescription Paper
Cooray, A.0000-0002-3892-0190
Galametz, M.0000-0002-0283-8689
Lu, N.0000-0002-8948-1044
Additional Information:© 2015 ESO. Article published by EDP Sciences. Received 23 October 2014; Accepted 5 February 2015; Published online 3 June 2015. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We are grateful to A. Poglitsch, A. Contursi, J. Graciá Carpio, B. Vandenbussche, and P. Royer for their help with the PACS data. We thank D. Dale for sharing the B-band data of the normal galaxies with us and the referee for careful reading of the manuscript. We acknowledge support from the Agence Nationale de la Recherche (ANR) through the programme SYMPATICO (Program Blanc Projet ANR-11-BS56-0023). IDL is a postdoctoral researcher of the FWO-Vlaanderen (Belgium). PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESAPRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain). This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Agence Nationale pour la Recherche (ANR)ANR-11-BS56-0023
Fonds Wetenschappelijk Onderzoek - Vlaanderen (FWO)UNSPECIFIED
Subject Keywords:galaxies: dwarf; infrared: ISM; photon-dominated region (PDR); techniques: spectroscopic; radiative transfer; HII regions
Record Number:CaltechAUTHORS:20150811-091726523
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Official Citation:D. Cormier, S. C. Madden, V. Lebouteiller, N. Abel, S. Hony, F. Galliano, A. Rémy-Ruyer, F. Bigiel, M. Baes, A. Boselli, M. Chevance, A. Cooray, I. De Looze, V. Doublier, M. Galametz, T. Hughes, O. Ł. Karczewski, M.-Y. Lee, N. Lu and L. Spinoglio A&A 578 A53 (2015) DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:59380
Deposited By: Tony Diaz
Deposited On:11 Aug 2015 16:32
Last Modified:03 Oct 2019 08:46

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