The unusual ISM in blue and dusty gas-rich galaxies (BADGRS). Issue 1 (11th June 2018)

• Record Type: Journal Article
• Authors: Dunne, L; Zhang, Z; De Vis, P; Clark, C J R; Oteo, I; Maddox, S J; Cigan, P; de Zotti, G; Gomez, H L; Ivison, R J; Rowlands, K; Smith, M W L; van der Werf, P; Vlahakis, C; Millard, J S
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 479:Issue 1(2018)
• Page Start: 1221
• Page End: 1239
• Publication Date: 2018-06-11
• Subjects: Galaxies: Local – Infrared – Star-forming – ISM; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty1465
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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 ($f_{\rm H\, {\small I}}=M_{\rm H\, {\small I}}/(M_{\ast }+M+_{\rm H\, {\small I}})\gt 0.5$ ). 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 $\rm {CO(J_{up}=1, 2, 3)}$ 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 $R_{21}={T_\mathrm{ b}^{21}/{ T}_\mathrm{ b}^{10}=0.6-2.1}$ and $R_{31}= {T_\mathrm{ b}^{32}/T_\mathrm{ b}^{10}=0.2-1.2}$ . Using a metallicity-dependent conversion from CO luminosity to molecular gas mass, we find $\, M_{\rm{ H}_2}/\, M_{\rm d}\sim 7-27$ and $\Sigma _{\rm {H_2}} = 0.5-6\, \rm M_{\odot }\, \rm {pc^{-2}}$, around an order of magnitude lower than expected. The BADGRS have lower molecular gas depletion time-scales (τd ∼ 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.