Gas and Dust in Ngc 7469: Sub-mm Imaging and Co J=3–2

We present sensitive sub-mm imaging of the Seyfert 1 galaxy NGC 7469 at 850 μm and 450 μm with the Submillimetre Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) and CO J=3–2 line observations of its central starbursting region. The global dust spectrum, as constrained by the new set of sub-mm data and available 1.30 mm and IRAS 100 μm, 60 μm data reveals a dominant warm dust component with a temperature of Td ∼ 35 K and a global molecular gas-to-dust ratio M(H2)/Md ∼ 600. Including the atomic gas component yields a total gas-to-dust ratio of ∼ 830. Such high values are typical for IR-bright spirals and in order to reconcile them with the significantly lower ratio of ∼ 100 obtained for the Milky Way a cold dust reservoir, inconspicuous at FIR wavelengths, is usually postulated. However, while there is good evidence for the presence of cold gas/dust in NGC 7469 beyond its central region, our 450 μm map and available interferometric CO J=1–0 maps show the bright sub-mm/CO emission confined in the inner ∼ 2.5 kpc, where a high CO (J=3–2)/(J=1–0) ratio (∼ 0.85− 1.0) is measured. This is consistent with molecular gas at Tkin ∼> 30K, suggesting that the bulk of the ISM in the starburst center of NGC 7469 is warm. Nevertheless the corresponding total gas-to-dust ratio there remains high, of the order of ∼ 500. We argue that, rather than unaccounted cold dust mass, this high ratio suggests an overestimate of M(H2) from its associated CO J=1–0 line luminosity by a factor of ∼ 5 when a Milky Way value for this conversion is used. Finally the diffuse cold gas and dust that is the likely source of the observed faint extended 450 μm and CO J=1–0 emission has an estimated total gas-to-dust ratio of ∼ 50− 160, closer to the Galactic value.