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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)

We aim to quantify the molecular gas reservoir in a subset of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data, and link this reservoir to the observed star formation activity. 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 [CII] 157um and [OI] 63um observations obtained with the Herschel/PACS spectrometer in the 6 galaxies: Haro11, Mrk1089, Mrk930, NGC4861, NGC625, and UM311. We derive molecular gas mass from several methods including the use of the CO-to-H2 conversion factor Xco (both Galactic and metallicity-scaled values) and of dust measurements. The molecular and atomic gas reservoirs are compared to the star formation activity. We also constrain the physical conditions of the molecular clouds using the non-LTE code RADEX and the spectral synthesis code Cloudy. We detect CO in 5 of the 6 galaxies, including first detections in Haro11 (Z~0.4 Zsun), Mrk930 (0.2 Zsun), and UM311 (0.5 Zsun), but CO remains undetected in NGC4861 (0.2 Zsun). The CO luminosities are low while [CII] is bright in these galaxies, resulting in [CII]/CO(1-0)>10000. Our dwarf galaxies are in relatively good agreement with the Schmidt-Kennicutt relation for total gas. They show short molecular depletion time scales, even when considering metallicity-scaled Xco factors. Those galaxies are dominated by their HI gas, except Haro11 which has high star formation efficiency and is dominated by ionized and molecular gas. We determine the mass of each ISM phase in Haro11 using Cloudy and estimate an equivalent Xco factor which 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.