The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey

A ground-based follow-up survey of CO(1–0), CO(2–1), and CO(3–2)

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 Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁵ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁶ University of Cincinnati, Clermont College, Batavia OH 45103, USA
⁷ Sub-Dept. of Astrophysics, Dept. of Physics at University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁸ Department of Physics & Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul MN 55105, USA
⁹ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

Received: 17 June 2013
Accepted: 17 November 2013

Abstract

Context. Observations of nearby starburst and spiral galaxies have revealed that molecular gas is the driver of star formation. However, some nearby low-metallicity dwarf galaxies are actively forming stars, but CO, the most common tracer of this reservoir, is faint, leaving us with a puzzle about how star formation proceeds in these environments.

Aims. We aim to quantify the molecular gas reservoir in a subset of 6 galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data and to link this reservoir to the observed star formation activity.

Methods. We present CO(1–0), CO(2–1), and CO(3–2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [C ii] 157μm and [O i] 63μm observations obtained with the Herschel/PACS spectrometer in the 6 low-metallicity dwarf galaxies: Haro 11, Mrk 1089, Mrk 930, NGC 4861, NGC 625, and UM 311. We derived their molecular gas masses from several methods, including using the CO-to-H₂ conversion factor X_CO (both Galactic and metallicity-scaled values) and dust measurements. The molecular and atomic gas reservoirs were compared to the star formation activity. We also constrained the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy.

Results. We detect CO in 5 of the 6 galaxies, including first detections in Haro 11 (Z ~ 0.4 Z_⊙), Mrk 930 (0.2 Z_⊙), and UM 311 (0.5 Z_⊙), but CO remains undetected in NGC 4861 (0.2 Z_⊙). The CO luminosities are low, while [C ii] is bright in these galaxies, resulting in [C ii]/CO(1–0) ≥ 10 000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion timescales, even when considering metallicity-scaled X_CO factors. Those galaxies are dominated by their H i gas, except Haro 11, which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro 11 using Cloudy and estimate an equivalent X_CO factor that is 10 times higher than the Galactic value. Overall, our results confirm the emerging picture that CO suffers from significant selective photodissociation in low-metallicity dwarf galaxies.