N. Geis

The 158 micron forbidden C II line: A measure of global star formation activity in galaxies

Some 158 micron forbidden C II fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the C II line is bright amounting to approximately 0.1 to 1 percent of the FIR luminosity of the nuclear regions of galaxies; the C II line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the C II line tracks the FIR continuum in a manner consistent with the PDR models; the integrated C II to isotope (C-12)D (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the C II line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given. 156 refs.

Neutral gas in the central 2 parsecs of the Galaxy

We have mapped the 1.1 mm HCN J=3→2 line with IRAM 30 m telescope at 12″ resolution and the 63 μm [O I] 3 P 1 → 3 P 2 line with the Kuiper Airborne Observatory at 20″ resolution within a projected distance of 2 pc of IRS 16/SgrA * . The HCN J=3→2 data, together with the previous HCN J=1→0 data of Gusten et al., indicate that the circumnuclear molecular material is arranged in several kinematically distinct streamers

158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium

A well-sampled map (23 x 17 kpc) of the strong 158 micron forbidden C II cooling line in the Scd galaxy NGC 6946 at 55 arcsec resolution is presented which was taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) in the Kuiper Airborne Observatory. It is concluded that the line emission in NGC 6946 is present in three spatially distinct components including nucleus, spiral arms, and extended region. An extended emission region is a source of most of the forbidden C II luminosity in NGC 6946. The 1 arcmin nuclear component has a line luminosity of 1.5 x 10 exp 7 solar luminosity and contributes 0.15 percent of the galaxys total FIR luminosity. An extended component of forbidden C II emission is found to exist past the molecular extent of the galaxy and to be present to at least the full dimensions of the map. This component is attributed to a mixture of neutral and atomic clouds.

158 micron forbidden C II mapping of the Orion molecular cloud

A fully sampled, 1000-point, 1-arcmin-resolution map of the inner 6.5 x 10 arcmin (alpha x delta) regions of the Orion Nebula in the 157.7409-micron forbidden fine-structure line is constructed. Large-scale strip maps in forbidden C II across the face of the Orion molecular cloud, and CO(17-16), (14-13), and (7-6) spectra are obtained at selected positions in the Orion H II region/molecular cloud interface. Strong forbidden C II line emission is observed across the face of the Orion molecular. The total forbidden C II luminosity from the Orion molecular cloud is about 1500 solar luminosities, or 0.3 percent of the FIR luminosity. The extended forbidden C II emission probably arises in either the UV-exposed surface of the molecular cloud or from the surfaces of UV-exposed clumps within the molecular cloud.

The Orion Molecular Clouds OMC-1 and OMC-2 Mapped in the Far-Infrared Fine-Structure Line Emission of C+ and O0

We have mapped the Orion Molecular Ridge (OMC-1 and OMC-2) in the fine-structure lines [C II] 158 μm and [O I] 63 and 146 μm. The maps cover a region of 7 × 18 (Δα × Δδ) and are fully sampled toward the Orion A H II region/molecular cloud interface. The emission maximum near the molecular region S6/FIR4 arises most probably from a photodissociation region (PDR) on the surface of that molecular condensation. The PDR has column densities that are most likely explained by an edge-on geometry. For the emitting material, we find a temperature between 300 and 500 K, a number density of 3 × 105 cm-3, and O0 and C+ column densities of 1 × 1019 and 4 × 1018 cm-2, respectively. In the Orion Bar region, the northern edge of the Orion interface region, and M43 west we find somewhat lower values for the temperature, number density, and column densities. Furthermore, the ratio of the oxygen to carbon column density is lower in this region and approaches a value of 1.7. The lower ratio could be due to the edge-on geometry and the lower temperature of these regions. In the Dark Lane region we find evidence for cooler oxygen partly absorbing the oxygen line emission from the warmer background material. For the cooler molecular material covering the warm medium in the Dark Lane, we estimate an atomic oxygen abundance [O]/[H] on the order of a few 10-4. In the northern part of OMC-1 the morphology of the [O I] 63 μm emission is consistent with moderately warm oxygen associated with the dense molecular ridge. From the oxygen emission arising from the ridge, we derive an atomic oxygen abundance [O]/[H] > 10-5. The [O I] line emission in OMC-2 may arise partly from the inner parts of the molecular cloud, but emission arising exclusively from PDRs cannot be excluded.

Stressed and unstressed Ge:Ga detector arrays for airborne astronomy

We have constructed and used two dimensional arrays of both unstressed and stressed Ge:Ga photoconductive detectors for far-infrared astronomy from the Kuiper Airborne Observatory (KAO). The 25 element (5×5) arrays are designed for a new cryogenically cooled spectrometer, the MPE/UCB Far-Infrared Imaging Fabry-Perot Interferometer (FIFI). All of the pixels for the stressed array performed well on the first flights with FIFI; 25% of the detectors in the array are more sensitive than our best single element detector, with background limited noise equivalent powers (NEPs)≲3.0×10−15 W Hz−1/2 at 158 μm and 40 km s−1 spectral resolution. The average array element performs within±15% of this value. With a bias field of 0.1 V/cm, the average detector response is 20±6 Amp/Watt at 158 μm. The cutoff wavelength and response also compare well with our single element detectors. The unstressed array delivers significantly better performance than our single element detector due to the lower thermal background in the new spectrometer. The average background limited NEp at 88 μm and 35 km s−1 spectral resolution is ∼7×10−15 W Hz−1/2. The least sensitive pixel is only 40% less sensitive. The unstressed array response at 88 μm with a bias field of 1 V/cm is 5±1 Amp/Watt. Twenty four of the 25 elements worked on the first flights-on subsequent flights all channels have worked. Some of the exciting new science possible with far-infrared detector arrays is also discussed.

[C II] Emission from NGC 4038/9 (the “Antennae”)

We present observations of NGC 4038/39 in the [CII] 158 micron fine structure line taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the KAO with 55 resolution. The total [CII] luminosity of the Antennae is 3.7x10^8 L(sun), which is about 1% of the FIR luminosity. The main part of the [CII] emission probably arises from photodissociation regions (PDRs). Up to 1/3 of the observed [CII] emission may originate from the CNM for higher temperatures and densities. From PDR models we derive densities and FUV intensities for the nuclei and the interaction zone. The minimum masses in the [CII] emitting regions in the interaction zone and the nuclei are a few x 10^(7) M(sun). We find that the [CII] to CO intensity ratio at the interaction zone is a factor of 2.6 lower than usually observed in starburst galaxies, but still a factor of about 1.3 to 1.4 higher than at the nuclei of NGC 4038/39, indicating no global starburst is taking place in the Antennae. [CII] emission arising partly from confined starburst regions and partly from surrounding quiescent clouds could explain the observed [CII] radiation at the interaction zone and the nuclei. There are small confined regions with high star formation activity in the interaction zone and with a lower star formation activity in the nuclei. This supports the high density and high FUV intensity for the PDRs in the interaction zone and the nuclei.

CII Emission From NGC 4038/39 (The Antennae)

We present observations of NGC 4038/39 in the [CII] 158 micron fine structure line taken with the MPE/UCB Far-infrared Imaging Fabry-Perot Interferometer (FIFI) on the KAO with 55 resolution. The total [CII] luminosity of the Antennae is 3.7x10^8 L(sun), which is about 1% of the FIR luminosity. The main part of the [CII] emission probably arises from photodissociation regions (PDRs). Up to 1/3 of the observed [CII] emission may originate from the CNM for higher temperatures and densities. From PDR models we derive densities and FUV intensities for the nuclei and the interaction zone. The minimum masses in the [CII] emitting regions in the interaction zone and the nuclei are a few x 10^(7) M(sun). We find that the [CII] to CO intensity ratio at the interaction zone is a factor of 2.6 lower than usually observed in starburst galaxies, but still a factor of about 1.3 to 1.4 higher than at the nuclei of NGC 4038/39, indicating no global starburst is taking place in the Antennae. [CII] emission arising partly from confined starburst regions and partly from surrounding quiescent clouds could explain the observed [CII] radiation at the interaction zone and the nuclei. There are small confined regions with high star formation activity in the interaction zone and with a lower star formation activity in the nuclei. This supports the high density and high FUV intensity for the PDRs in the interaction zone and the nuclei.

A survey of the 158 micron forbidden C II fine-structure line in the central 50 parsecs of the Galaxy

NASAs Kuiper Airborne Observatory has been used to conduct an extensive, velocity-resolved survey at 1-arcsec resolution of the 158-micron forbidden C II fine-structure line emission in the central 50 pc of the Galaxy. The emission is found to be strongest toward the rotating circumnuclear disk surrounding Sgr A West; a continuous bridge of the emission connects the Sgr A complex to the thermal radio filaments in the radio arc 10 arcmin north of the center. This suggests a direct physical connection. The brightest emission occurs preferentially near the edges of the massive Galactic center molecular clouds. It is concluded that these clouds are predominantly ionized by external UV photons. 29 refs.

Atomic Oxygen in Molecular Clouds? High-Resolution Spectroscopy of the [O I] 63 Micron Line toward DR 21

We report the first high-resolution spectra of the [O I] 63 μm fine-structure line toward DR 21. The observations were made from NASAs Kuiper Airborne Observatory using the MPE/UCB imaging spectrometer FIFI. The spectra show a pronounced dip, which may be due to absorption by a foreground molecular cloud, seen against the broad [O I] emission from DR 21. In this case we derive a column density of cold, atomic oxygen of 5 × 1018 cm-2, corresponding to a relative abundance of atomic oxygen of [O]/[H] 6 × 10-4. Therefore, most of the gas-phase oxygen in this cloud would be in atomic form. This result is in contrast to predictions by common chemistry models for a steady state cloud and may support models which predict a high abundance of atomic oxygen in dark cores of molecular clouds.