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We aim to quantify the molecular gas reservoir in a subset of 6low-metallicity galaxies from the Herschel Dwarf Galaxy Survey with newlyacquired CO data, and link this reservoir to the observed star formationactivity. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at theATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [CII] 157um and[OI] 63um observations obtained with the Herschel/PACS spectrometer in the 6galaxies: Haro11, Mrk1089, Mrk930, NGC4861, NGC625, and UM311. We derivemolecular gas mass from several methods including the use of the CO-to-H2conversion factor Xco (both Galactic and metallicity-scaled values) and of dustmeasurements. The molecular and atomic gas reservoirs are compared to the starformation activity. We also constrain the physical conditions of the molecularclouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. Wedetect CO in 5 of the 6 galaxies, including first detections in Haro11 (Z~0.4Zsun), Mrk930 (0.2 Zsun), and UM311 (0.5 Zsun), but CO remains undetected inNGC4861 (0.2 Zsun). The CO luminosities are low while [CII] is bright in thesegalaxies, resulting in [CII]/CO(1-0)>10000. Our dwarf galaxies are inrelatively good agreement with the Schmidt-Kennicutt relation for total gas.They show short molecular depletion time scales, even when consideringmetallicity-scaled Xco factors. Those galaxies are dominated by their HI gas,except Haro11 which has high star formation efficiency and is dominated byionized and molecular gas. We determine the mass of each ISM phase in Haro11using Cloudy and estimate an equivalent Xco factor which is 10 times higherthan the Galactic value. Overall, our results confirm the emerging picture thatCO suffers from significant selective photodissociation in low-metallicitydwarf galaxies.

astronomy and astrophysics

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)