Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-01-25
Astronomy and Astrophysics
Astrophysics
Accepted for publication in the Astrophysical Journal Letters
Scientific paper
10.1086/311953
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 $\rm M_{\rm gas}/M_{\rm dust} \sim 70-150$. This contrasts with the larger ratio $\rm M_{\rm gas}/M_{\rm dust}\sim 330$ estimated within a galactocentric radius of $r\leq 1.36$ 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 $\rm H_2$ that coexists with HI. This molecular gas phase is characterized by low densities ($\rm n(H_2)<10^3$ cm$ ^{-3}$), very faint emission from sub-thermally excited CO, and contains more mass than HI, namely $\rm M(H_2)/M(HI)\sim 5$.
Papadopoulos Padeli P.
Seaquist Ernest R.
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