  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_HI = M_HI/(M*+M_HI)>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 ISM in a representative sample of BADGRS, using
very deep CO(J_up=1,2,3) observations across the central and outer disk
regions. We find very low CO brightnesses (Tp=15-30 mK), despite the bright
far-infrared emission and metallicities in the range 0.5<Z/Z_sun<1.0. The CO
line ratios indicate a range of conditions with R_21=0.6-2.1 and R_31=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 Sigma_H2=0.5-6 M_sun pc^-2, around an order of
magnitude lower than expected. The BADGRS have lower molecular gas depletion
timescales (tau_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 temperatures in BADGRS (13-16 K) require 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=5 galaxies and may be useful as local analogues
for high gas fraction galaxies in the early Universe.
