Gas and Dust in NGC 7469: Submillimeter Imaging and COJ = 3–2

Abstract

We present sensitive submillimeter imaging of the Seyfert 1 galaxy NGC 7469 at 850 and 450 μm with the Submillimeter Common User Bolometer Array on the James Clerk Maxwell Telescope and 12CO J = 3-2 line observations of its central starbursting region. The global dust spectrum, as constrained by the new set of submillimeter data and available 1.30 mm and IRAS 100 and 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 spiral galaxies, and in order to reconcile them with the significantly lower ratio of ~100 obtained for the Milky Way, a cold dust reservoir, inconspicuous at far-infrared 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 12CO J = 1-0 maps show the bright submillimeter/CO emission confined in the inner ~2.5 kpc, where a high 12CO (J = 3-2)/(J = 1-0) ratio (~0.85-1.0) is measured. This is consistent with molecular gas at Tkin 30 K, suggesting that the bulk of the interstellar medium 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 12CO 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 12CO J = 1-0 emission has an estimated total gas-to-dust ratio of ~50-160, closer to the Galactic value.