2018MNRAS.479.1221D

The Herschel-ATLAS unbiased survey of cold dust in the local Universe is 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 metallicities in the range 0.5 < Z/Z_☉ < 1.0. The CO line ratios indicate a range of conditions with R₂₁=T_ b_²¹/ T_ b_¹⁰=0.6-2.1 and R₃₁= T_ b32^/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_∼7-27 and Σ_ H₂ = 0.5-6 M_☉ pc^–2, around an order of magnitude lower than expected. The BADGRS have lower molecular gas depletion time-scales (τ_d ∼ 0.5 Gyr) 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-β space as z ∼ 5 galaxies and may be useful as local analogues for high gas fraction galaxies in the early Universe.