J. M. Jackson

The optical depth of the 158 micron forbidden C-12 II line - Detection of the F = 1 - 0 forbidden C-13 II hyperfine-structure component

The detection of the F = 1 - 0 hyperfine component of the 158-micron forbidden C-13 II fine-structure line in the interstellar medium is reported. A 12-point intensity map was obtained of the forbidden C-13 distribution over the inner 190-arcsec (R.A.) X 190-arcsec (decl.) regions of the Orion Nebula using an imaging Fabry-Perot interferometer. The forbidden C-12 II/C-13 II line intensity ratio varies significantly over the region mapped. It is highest (86 +/-0) in the core of the Orion H II region, and significantly lower (62 +/-7) in the outer regions of the map, reflecting higher optical depth in the forbidden C-12 II line here. It is suggested that this enhanced optical depth is the result of limb brightening of the optically thin forbidden C-13 II line at the edges of the bowl-shaped H II region blister.

SPIFI Imaging of the Galactic Center

The center of the Galaxy provides a unique opportunity to study many of the fascinating phenomena assocated with galactic nuclei at relatively high spatial resolution. Of particular interest is the circumnuclear disk (CND) that circulates about Sgr A* with a sharp inner edge at 1.5 pc. Discovered in its far-IR continuum emission, the ring has since been investigated in a wide variety long wavelength spectral line and continuum probes. This work has shown that the ring is warm, dense, and turbulent. The total mass of the ring out to 4 pc radius is about 104 M⊙. Within the ring lies the ”mini-spiral”, a group of gas and dust streamers. The ring/mini-spiral morphology is well illustrated by the high resolution KAO images at 31 and 38 µm that clearly illustrate all of these features [1]. The warm dust traced by these images is heated by the far-UV radiation from the central star cluster.

CO (J = 7 → 6) Observations of NGC 253: Exited Molecular Gas in the Nucleus

We report observations of the CO J = 7 → 6 transition toward the starburst nucleus of NGC 253. This is the highest-excitation CO measurement in this source to date, and allows an estimate of the molecular gas excitation conditions. We find that the bulk of the 2-5×107 M⊙ of molecular gas in the central 180 pc is highly excited. The inferred mass of warm, dense molecular gas is 10-30 times the atomic gas mass as traced through its [C II] and [O I] line emission. This large mass ratio is inconsistent with photodissociation region models where the gas is heated by far-UV starlight. It is also not likely that the gas is heated by shocks in outflows or cloud-cloud collisions given the lack of evidence for substantial heating in fast shocks. We propose two mechanisms for heating the molecular gas: cosmic rays ionization and dissipation of supersonic turbulence, both of which which provide a natural means of uniformly heating the full volume of molecular clouds, and provide a good match to the cooling observed in the CO lines.

A 158μm [CII] Map of NGC 6946: Detection in Extragalactic Atomic and Ionized Gas

The first observations of the [CII] line toward the nuclei of gas-rich external galaxies, showed that the far-infrared line emission contributes up to 1% of the total luminosity and most likely originates from dense photon-dominated regions (PDRs) associated with the surfaces of molecular clouds exposed to FUV from external or embedded OB stars (Crawford et al. 1985, Lugten et al. 1986, Stacey et al. 1991). We have mapped the [CII] emission toward NGC 6946 over an 8′x 6′ (23 x 17 kpc) (Madden et al. 1991) using the Max-Planck Instutute/U.C.Berkeley Far- Infrared Imaging Fabry-Perot Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO).