Frank N. Bash

Giant H II regions in M81

H-alpha and VLA radio continuum observations at wavelengths of 6 and 20 cm are used to study the distribution of extinction and the distribution of giant radio H II regions along the spiral arms in M81. The radial distribution of visual extinction Av in the plane of M81 shows no trend, with an upper limit 0.1 mag/kpc for the radial extinction gradient and a mean Av of 1.1 + or - 0.4 mag. Nearly all the giant radio H II regions lie along the spiral arms or the inner H I ring. The radial distribution of the set of giant radio H II regions exhibits a strong maximum at a galactocentric distance R roughly 300 arcsec. Unless molecular hydrogen in M81 is also concentrated near R roughly 300 arcsec, the data disagree with Vissers (1980) model for star formation associated with a density wave. Some suggestions are made about how to change the ballistic particle model of Leisawitz and Bash to agree with the observed radial distribution of giant radio H II regions. 47 references.

Kiloparsec-scale molecular gas excitation in spiral galaxies

We combine beam-matched (C-13)O, (C-12)O J = 3 - 2 and J = 2 - 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of IR-bright external galaxies: NGC 253, IC 342, M83, Maffei 2, and NGC 6946. We find that the central 170 to 530 pc diameter regions have typical molecular gas densities ranging from approximately less than 10,000/cu cm (in M83) to approximately greater than 100,000/cu cm (in NGC 253) and that, outside of these regions, the densities are likely to be approximately less than 10,000/cu cm. The molecular clouds outside the inner 170-530 pc are at least as warm as the molecular clouds in our Galaxy. Column densities derived from integrated (C-13)O line strengths and H-alpha surface brightnesses suggest that the star formation rate is enhanced in the central 170-530 pc diameters by an order of magnitude over that inferred for the outer star-forming disks in spiral galaxies.

A statistical study of the correlation of Galactic supernova remnants and spiral arms

The distribution of observed giant H II regions and supernova remnants (SNRs) in Galactic longitude is analyzed with Monte Carlo models to determine the possible correlation with spiral arms or disk populations. The novel feature of the current analysis is that it depends mainly on the angular distribution of the H II regions and SNRs and hence is independent of distance.

Very Large Array Limits For Intermediate-Mass Black Holes In Three Globular Clusters

The observational evidence for central black holes in globular clusters has been argued extensively, and their existence has important consequences for both the formation and evolution of the cluster. Most of the evidence comes from dynamical arguments, but the interpretation is difficult, given the short relaxation times and old ages of the clusters. One of the most robust signatures for the existence of a BH is radio and/or X-ray emission. We observed three globular clusters: NGC6093 (M80), NGC6266 (M62), and NGC7078 (M15), with the Very Large Array (VLA) in the A and C configuration with a 3-σ noise of 36, 36, and 25 μJy, respectively. We find no statistically significant evidence for radio emission from the central region for any of the three clusters. NGC6266 shows a 2-σ detection. It is difficult to infer a mass from these upper limits due to uncertainty about the central gas density, accretion rate, and accretion model.

Molecular gas excitation in NGC 253

The J=3→2 and J=2→1 lines of 13 CO and 12 CO were observed with 22″-24″ resolution in the central region of the spiral galaxy NGC 253. The 13 CO J=3→2 emissions is pointlike (<15″), whereas the emission in the other CO lines is consistent with emission from a ∼40″×10″ FWHM bar. The ratio of 13 CO J=3→2 to 13 CO J=2→1 line intensities toward the nucleus requires warm, dense molecular gas. Spatial coincidence of 3 mm continuum and [Ne II] 12.8 μm emission from H II regions and mid-IR emission from hot dust with the 13 CO J=3→2 emission implies a common source of excitation, possibly related to the intense star formation that has occurred in the nucleus

A comparison of spiral tracers in M81

High-resolution digitized images of M81 in H I, B and I bands, H-alpha, and the radio continuum are studied. The H I gas, the nonthermal radio emission, the dust and the narrow dust filaments, the young stars, and the giant radio H II regions are each distributed across a broad spiral compression zone that starts near the measured position of the spiral velocity shock front and extends 1-2 kpc downstream from the shock front. Old and young stars both collect near the gravitational potential minimum on the arms. The evidence favors an SNR origin for the cosmic ray electrons that produce the nonthermal radio arms. The location of an H II region above or below the midplane explains the observed dispersion in the value of the visual extinction for the set of giant radio H II regions. Many of the general features of the spiral arms can be explained by density wave models that emphasize the cloudy nature of the interstellar medium. 57 refs.

The global spiral structure of M81 - Radio continuum maps

VLA observations of the radio continuum emission from M81 at 6 and 20 cm are presented and used to check the predictions of density-wave theories. Both thermal and nonthermal radiation from the spiral arms are detected. Most of the bright knots along the radio arms are giant radio H II regions. The nonthermal emission defines spiral arms that are patchy and well-resolved, with a width of 1-2 kpc. The observed nonthermal arms are too broad to agree with the continuum gasdynamical calculations of Roberts (1969), Shu et al. (1972), and Visser (1978, 1980) for a classical density wave model. The observed arm widths appear consistent with the predictions of density-wave models that emphasize the clumpy nature of the ISM. The 20 cm arms appear to spiral outward from a faint inner H I ring, suggesting that the ring is produced by the inner Lindblad resonance. 36 references.

The Hobby-Eberly Telescope: performance upgrades, status, and plans

The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for

Density wave induced star formation: The optical surface brightness of galaxies

13.9M over the period 1994-1997. A series of extensive engineering upgrades and corrective actions have been completed recently, resulting in significantly improved delivered image quality and increased operational efficiency. The telescopes Spherical Aberration Corrector (SAC) optics were recoated with a highly reflective and durable broadband coating at Lawrence Livermore National Laboratory. The software mount model that maintains optical alignment of the SAC with the 11-m primary mirror array was recalibrated and improved. The acquisition and guiding optics for both the High Resolution Spectrograph (HRS) and the Low Resolution Spectrograph (LRS) were reworked and improved, allowing for better focus and SAC alignment monitoring and control. Recoating of the primary mirror segment array was begun. Telescope images of 0.82 arcseconds have been recorded for sustained periods in preliminary testing following the engineering upgrade, an improvement of 50% over previous best performance. Additional engineering upgrades are scheduled to consolidate these performance gains and to continue improving delivered image quality, throughput, and telescope operational efficiency. The HET is now capable of the science performance for which it was designed.

Extremely large telescope: a twenty-five meter aperture for the twenty-first century

A model for the galactic orbits of molecular clouds has been devised. The molecular clouds are assumed to be launched from the two-armed spiral-shock wave, to orbit in the Galaxy like ballistic particles with gravitational perturbations due to the density-wave spiral-potential, and each cloud is assumed to produce a cluster of stars. Each cloud radiates detectable /sup 12/C/sup 16/O (J=0..-->..1) spectral line radiation from birth for 40 million years. Stars are seen in the cloud about 25 million years after birth, and the star cluster is assumed to continue in ballistic orbit around the Galaxy.The model has been tested by comparing its predicted velocity-longitude diagram for CO against that observed for the Galaxy and by comparing the models predicted distribution of light in the UBV photometric bands against observed surface photometry for Sb and SC galaxies. The interpolation of the initial velocities in the model was corrected, and the model was examined to see whether preshock or postshock initial velocities better fit the observations. The model gives very good general agreement and reproduces many of the features observed in the CO velocity-longitude diagram.