Gas and dust in NGC 7469: sub-mm imaging and CO J=3-2

Abstract

We present sensitive sub-mm imaging of the Seyfert 1 galaxy NGC 7469 at 850 $\mu $m and 450 $\mu $m with the Submillimetre Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) and ^{12}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 $\mu $m, 60 $\mu $m data reveals a dominant warm dust component with a temperature of $\rm T_{\rm d} \sim 35$ K and a global molecular gas-to-dust ratio $\rm M(H_2)/M_{\rm d}\sim 600$. 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. We present evidence that, rather than unaccounted cold dust mass, this high ratio suggests an overestimate of $\rm M(H_2)$ from its associated ^{12}CO J=1-0 line luminosity by a factor of ~ 5 when a Milky Way value for this conversion is used in starburst nuclei environments.