Jacqueline A. Davidson

CO (J = 7→6) Observations of NGC 253: Cosmic-Ray-heated Warm Molecular Gas

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. Comparison of the CO line intensities with a large velocity gradient, escape probability model indicates that the bulk of the × 107 M☉ of molecular gas in the central 180 pc is highly excited. A model with T ~ 120 K, n ~ 4.5 × 104 cm-3, is consistent with the observed CO intensities, as well as the rotational H2 lines observed with the Infrared Space Observatory. 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 in which 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. We conclude that the best mechanism for heating the gas is cosmic rays, which provide a natural means of uniformly heating the full volume of molecular clouds. With the tremendous supernova rate in the nucleus of NGC 253, the cosmic-ray heating rate is at least ~800 times greater than that in the Galaxy, more than sufficient to match the cooling observed in the CO lines.

FAR-INFRARED POLARIMETRY OF GALACTIC CLOUDS FROM THE KUIPER AIRBORNE OBSERVATORY

In this paper we present a complete summary of the data obtained with the far-infrared polarimeter, Stokes, in flights of the Kuiper Airborne Observatory. We have observed 12 Galactic clouds and have made over 1100 individual measurements at 100 ?m and 60 ?m. The median P for all of the 60 ?m and 100 ?m measurements is 3.6% and 2.6% respectively. We also present flux maps obtained simultaneously with the polarimetry.

SPIFI: A direct-detection imaging spectrometer for submillimeter wavelengths

The South Pole Imaging Fabry-Perot Interferometer (SPIFI) is the first instrument of its kind-a direct-detection imaging spectrometer for astronomy in the submillimeter band. SPIFIs focal plane is a square array of 25 silicon bolometers cooled to 60 mK; the spectrometer consists of two cryogenic scanning Fabry-Perot interferometers in series with a 60-mK bandpass filter. The instrument operates in the short submillimeter windows (350 and 450 microm) available from the ground, with spectral resolving power selectable between 500 and 10,000. At present, SPIFIs sensitivity is within a factor of 1.5-3 of the photon background limit, comparable with the best heterodyne spectrometers. The instruments large bandwidth and mapping capability provide substantial advantages for specific astrophysical projects, including deep extragalactic observations. We present the motivation for and design of SPIFI and its operational characteristics on the telescope.

The far-infrared polarization of the Orion nebula

Polarization of the 100 micron thermal emission from 10 points in the Orion nebula has been measured. At one of the positions the degree, 5.7 percent, is the largest far-infrared polarization yet discovered. Except at a position in the barlike structure to the south, the position angles of the polarization vectors are well-ordered, suggesting that a uniform magnetic field threads the cloud. The magnetic field strength is estimated to be between 0.7 and 4 mG. The relationship of the degrees of polarization to the physical conditions in the cloud is discussed. 34 refs.

The magnetic field in the dust ring at the center of the Galaxy

Measurements of the polarization of the far-infrared thermal emission from six points in the dust ring at Sgr A are presented. The position angles are approximately perpendicular to the long axis of the ring as projected on the sky. The inferred magnetic field is therefore approximately in the plane of the ring. The pattern traced by the polarization vectors resembles that expected for a magnetic accretion disk. The measurements indicate a field in which the outward radial component is much greater than the axial component at the surface of the disk. The field thus appears to satisfy the condition proposed by Blandford and Payne (1982) for removing energy and angular momentum through centrifugal acceleration of surface material moving along the field lines. 31 refs.

Spatially Resolved Imaging at 350 μm of Cold Dust in Nearby Elliptical Galaxies

Continuum observations at 350 μm of seven nearby elliptical galaxies for which CO gas disks have recently been resolved with interferometry mapping are presented. These SHARC II mapping results provide the first clearly resolved far-infrared (FIR)-to-submillimeter continuum emission from cold dust (with temperatures 31 K T 23 K ) of any elliptical galaxy at a distance >40 Mpc. The measured FIR excess shows that the most likely and dominant heating source of this dust is not dilute stellar radiation or cooling flows, but rather star formation that could have been triggered by an accretion or merger event and fueled by dust-rich material that has settled in a dense region cospatial with the central CO gas disks. The dust is detected even in two cluster ellipticals that are deficient in H I, showing that, unlike H I, cold dust and CO in ellipticals can survive in the presence of hot X-ray gas, even in galaxy clusters. No dust cooler than 20 K, either distributed outside the CO disks or cospatial with and heated by the entire dilute stellar optical galaxy (or very extended H I), is currently evident.

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.

The Magnetic Field in the Dust Ring and Northern Arm of Sgr A West

The results of far-infrared polarimetry of the circumnuclear dust ring at Sgr A West are in agreement with predictions for a magnetic field with approximate axial symmetry except in the northwest quadrant where the field runs along an extension of the northern arm to a radius of at least 4 pc.

Stratospheric Observatory for Infrared Astronomy (SOFIA) science rationale

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a proposed 3-meter class telescope in a Boeing 747 aircraft, anticipated as a joint development by NASA and West German Science Ministry. SOFIA would have the capability to make astronomical observations over a wavelength range from 0.3 microns to 1.6mm. The concept is based on the design and 13 years of experience in the Kuiper Airborne Observatory (KAO), a lockheed C-141 jet transport with a 0.9-meter diameter telescope, which SOFIA would replace. Relative to the KAO the larger telescope on SOFIA would provide a factor of 10 improvement in sensitivity for compact sources and a factor of 3 improvement in (diffraction limited) angular resolution at wavelengths beyond 30 microns. In addition, SOFIA will retain the major features of the KAO which have made the airborne astronomy program so successful. Among these are: continuous in-flight access to focal plane instruments while flying at or above 41,000ft altitude; pointing stability of 0.2 arc seconds; mobility and scheduling flexibility to accommodate targets of opportunity such as comets, eclipses, occultations, and novae. In this paper we present the scientific background, the scientific potential, a comparison with other astronomy missions, and the overall justification for an expanded airborne observatory.

Mid-Infrared Spectral Assessment of Early-type Galaxies with Star Formation

Mid-infrared (mid-IR) spectra from ∼5 to 14 μm of five, nearby (< 70 Mpc) elliptical galaxies are presented that were observed with the Infrared Spectrograph on the Spitzer Space Telescope. The sample galaxies have a main stellar component that is typical for normal, passively evolving ellipticals; however, they are rich in cold gas and dust and have morphologicalmerger signatures from which a time order of the galaxies since the merger or accretion events can be estimated. The presented results are significant because (1) emission due to Polycyclic Aromatic Hydrocarbons (PAHs) and associated species is detected for the first time in these galaxies and (2) the detected mid-IR spectra are independently exploited as a probe of current or recent star-formation that, in this case, is assumed to be triggered by the merger. As shown in exemplary spectra of the early-age merger NGC 3656, the strength of the PAH emission is more centrally peaked in the earlier-age mergers, suggesting that the PAH data are indeed probing star-formation that is correlated with the time since the mergers and systematically depletes the centrally located gas, becoming weaker and more flatly distributed as the merger evolves.