The unusual ISM in blue and dusty gas-rich galaxies (BADGRS)

The Herschel-ATLAS unbiased survey of cold dust in the local Universeis dominated by a surprising population of very blue (FUV-K < 3.5), dust-rich galaxies with high gas fractions (f(H i) = M-H i/(M-* + M-H i) > 0.5). Dubbed 'Blue and Dusty Gas-Rich Sources' (BADGRS) they have cold diffuse dust temperatures, and the highest dust-to-stellar mass ratios of any galaxies in the local Universe. Here, we explore the molecular interstellar medium in a representative sample of BADGRS, using very deep CO(J(up) = 1, 2, 3) observations across the central and outer disc regions. We find very low CO brightnesses (T-p = 5-30 mK), despite the bright far-infrared emission and metallici ties in the range 0.5 < Z/Z(circle dot) < 1.0. The CO line ratios indicate a range of conditions with R-21 = T-b(21)/T-b(10)) = 0.6 - 2.1 and R-31 = T-b(32)/T-b(10) = 0.2 - 1.2. Using a metallicity-dependent conversion from CO luminosity to molecular gas mass, we find M-H2 / M-d similar to 7 - 27 and Sigma(H2) = 0.5-6 M-circle dot pc(-2), around an order of magnitude lower than expected. The BADGRS have lower molecular gas depletion time-scales (tau(d) similar to 0.5 Ciyr) than other local spirals, lying offset from the Kennicutt Schmidt relation by a similar factor to Blue Compact Dwarf galaxies. The cold diffuse dust temperature in BADGRS (13-16 K) requires an interstellar radiation field 10-20 times lower than that inferred from their observed surface brightness. We speculate that the dust in these sources has either a very clumpy geometry or a very different opacity in order to explain the cold temperatures and lack of CO emission. BADGRS also have low UV attenuation for their UV colour suggestive of an SMC-type dust attenuation curve, different star formation histories or different dust/star geometry. They lie in a similar part of the IRX-beta space as z similar to 5 galaxies and may be useful as local analogues for high gas fraction galaxies in the early Universe.