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)
Artikel i vetenskaplig tidskrift, 2014

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 ATNE Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [CII] 157 mu m and [OI] 63 mu m 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-H-2 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 similar to 0.4 Z(circle dot)), Mrk 930 (0.2 Z(circle dot)), and UM 311 (0.5 Z(circle dot)), but CO remains undetected in NGC 4861 (0.2 Z(circle dot)). The CO luminosities are low, while [CII] is bright in these galaxies, resulting in [CII]/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 HI 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.

GLITSCH A

V454

BLUE COMPACT GALAXIES

V322

ISM: molecules

C-II LINE

ASTROPHYSICAL JOURNAL

STAR-FORMING GALAXIES

P423

1991

ACEY GJ

INTERSTELLAR-MEDIUM

V373

infrared: galaxies

SPITZER-SPACE-TELESCOPE

P293

ALPHA VELOCITY-FIELDS

LMC-N 11

1987

radio lines: galaxies

LARGE-MAGELLANIC-CLOUD

ASTROPHYSICAL JOURNAL

NEARBY

1995

P681

ASTROPHYSICAL JOURNAL

CCONI LJ

GALAXIES

CO-TO-H-2 CONVERSION FACTOR

galaxies: ISM

galaxies: dwarf

Författare

D. Cormier

Universität Heidelberg

S. C. Madden

CNRS Centre National de la Recherche Scientifique

V. Lebouteiller

CNRS Centre National de la Recherche Scientifique

S. Hony

Max Planck-institutet

Susanne Aalto

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Francesco Costagliola

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

A. Hughes

Max Planck-institutet

A. Remy-Ruyer

CNRS Centre National de la Recherche Scientifique

N. Abel

University of Cincinnati

E. Bayet

University of Oxford

F. Bigiell

Universität Heidelberg

J. M. Cannon

Macalester College

Robert Cumming

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

M. Galametz

University of Cambridge Institute of Astronomy

F. Galliano

CNRS Centre National de la Recherche Scientifique

S. Viti

University College London (UCL)

R. Wu

CNRS Centre National de la Recherche Scientifique

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 564 A121

Ämneskategorier

Astronomi, astrofysik och kosmologi

DOI

10.1051/0004-6361/201322096