Extended dust emission and atomic hydrogen, a reservoir of diffuse H_2 in NGC 1068

Abstract

We report on sensitive sub-mm imaging observations of the prototype Seyfert~2/starburst galaxy NGC 1068 at 850 $\mu$m and 450 $\mu$m using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). We find clear evidence of dust emission associated with the extended HI component which together with the very faint ${}^{12}$CO J=1--0 emission give a gas-to-dust ratio of ${\mathrm{M}}_{\mathrm{gas}}/{\mathrm{M}}_{\mathrm{dust}}\sim 70-150$. This contrasts with the larger ratio ${\mathrm{M}}_{\mathrm{gas}}/{\mathrm{M}}_{\mathrm{dust}}\sim 330$ estimated within a galactocentric radius of $r\le 1.36$ kpc, where the gas is mostly molecular and starburst activity occurs. The large gas-to-dust ratio found for the starburst region is attributed to a systematic overestimate of the molecular gas mass in starburst environments when the luminosity of the ${}^{12}$CO J=1--0 line and a standard galactic conversion factor is used. On the other hand sub-mm imaging proves to be a more powerful tool than conventional CO imaging for revealing the properties of the diffuse ${\mathrm{H}}_2$ that coexists with HI. This molecular gas phase is characterized by low densities ($\mathrm{n}({\mathrm{H}}_2)<10^3$ cm${}^{-3}$), very faint emission from sub-thermally excited CO, and contains more mass than HI, namely $\mathrm{M}({\mathrm{H}}_2)/M(\mathrm{HI})\sim 5$.

Accepted for publication in the Astrophysical Journal Letters

References in corpus (1)

• Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies

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• Herschel-PACS Observations of Far-IR CO Line Emission in NGC 1068: Highly Excited Molecular Gas in the Circumnuclear Disk
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• Millimeter observations of obscured Spitzer 24 micron sources
• Herschel Far-Infrared Photometry of the Swift Burst Alert Telescope Active Galactic Nuclei Sample of the Local Universe. II. SPIRE Observations
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