Frederick R. Chromey

Discovery of a Double Circumnuclear Ring and Minibar in the Starburst Galaxy M83

Our (J-K) color observations of the central regions of the starburst galaxy M83 reveal a double circumnuclear ring. The main dust lanes spiral into the outer nuclear ring at a radius of 150 pc. The two rings may coincide with two inner Lindblad resonances. The main hot spots occur in an arc that is between the rings. A dust bar that is offset 90? from the primary stellar bar connects the outer nuclear ring to the inner nuclear ring at a radius of 50 pc and may provide the path for gas to flow to the central starburst.

THE FLAT SKY: CALIBRATION AND BACKGROUND UNIFORMITY IN WIDE FIELD ASTRONOMICAL IMAGES

Gradients in the surface brightness of the twilight and night sky require careful consideration both in the flat-field calibration and in the subsequent reduction of images that cover a large angular field. We present observations of BVRI surface brightness gradients in the twilight sky, and discuss some strategies for the treatment of gradients in the moonlit and dark sky.

Circumnuclear Star Formation in the Spiral Galaxy NGC 3310

The star-forming properties of the circumnuclear ring in the starburst spiral galaxy NGC 3310 have been studied in B, I, J, H, and K bands using images from KPNO and from the HST archives. The colors and magnitudes of the star-forming regions indicate ages less than 10 Myr and masses of 104 to 105 M⊙ for the largest clumps. The luminosity distribution function of the diffuse circumnuclear clusters has a slope of about -2, which is typical of results both in circumnuclear rings and in the main disks of other galaxies. There are 17 candidate super star clusters (SSCs), primarily in the innermost southern spiral arm. The broad wavelength coverage allows a determination of reddening in the vicinity of the SSCs, which appears to be small. The locations of the SSCs in the circumnuclear ring and in an inner spiral are coincident with radio continuum and emission peaks, and may be the result of a suspected cannibalization of a dwarf galaxy in the last 10 Myr.

Near-Infrared Observations of a Nuclear Bar and Biconical Structure in the Starburst Galaxy NGC 6946

We use high-resolution near-infrared (J- and K-band) images of the nuclear starburst galaxy NGC 6946 to compare stellar morphology with the previously published distribution of CO in the central regions. We find a small stellar bar with a length of 210 pc, which lies perpendicular to an 1860 pc long stellar oval distortion. The small bar has a position angle of 140°, which coincides with a barlike feature in CO, as well as with the kinematic minor axis. It has a flat light distribution and a strong bar-interbar contrast of 1 mag; it may play a role in fueling the central starburst despite its small size. A biconical structure about 80 pc long, perpendicular to the bar, is evident in the (J - K) color map. The (J - K) contours match published high-resolution CO contours very well.

Near-Infrared Observations of Hot Spots in the Circumnuclear Rings of NGC 2997 and NGC 6951

Near-infrared observations of NGC 2997 and NGC 6951 reveal complete rings of symmetrically placed hot spots, which are star-forming complexes. The masses of the complexes range from a few times 104 to a few times 105 M? in both galaxies. The regular hot-spot spacings of about 200 pc in a ring 55 pc thick in NGC 2997 and 380 pc in a ring 150 pc thick in NGC 6951 are consistent with formation by a large-scale gravitational instability in the rings. Visual extinction is estimated to be up to 3 mag around the ring in NGC 2997 and up to 5 mag in NGC 6951, as dust lanes spiral in and cross the ring. Hubble Space Telescope observations reveal smaller super?star clusters within the large complexes. These clusters have power-law luminosity functions with an exponent of approximately -2 in both galaxy rings, the same as for star-forming complexes in the disks of normal spiral galaxies.

Atomic hydrogen in the Orion star-forming region

A large-scale survey of atomic hydrogen in Orion reveals low-density material with a total mass comparable to that in dense molecular clouds. The atomic gas is sufficiently dense that it can shield the molecular material from photodissociative radiation and provide a pressure link to the low-density intercloud medium. An excess of H I emission comes from photodissociation fronts near the bright stars and from a giant shell in the Orion Belt region. This shell may have caused the apparent bifurcation between the Orion A and B clouds, and the associated pressures may have induced peculiar motions and star formation in NGC 2023 and 2024. 49 refs.

Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of its Companion VCC 882

The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trail inside its optical halo that points toward the nucleated dwarf elliptical galaxy VCC 882. The trail seems to be stripped material from the dwarf. Extinction measurements suggest that the ratio of the total gas mass in the trail to the blue luminosity of the dwarf is about unity, which is comparable to such ratios in dwarf irregular galaxies. The ram pressure experienced by the dwarf galaxy in the hot gaseous halo of M86 was comparable to the internal gravitational binding energy density of the presumed former gas disk in VCC 882. Published numerical models of this case are consistent with the overall trail-like morphology observed here. Three concentrations in the trail may be evidence for the predicted periodicity of the mass loss. The evaporation time of the trail is comparable to its age obtained from the relative speed of the galaxies and the trail length. Thus the trail could be continuously formed from stripped replenished gas if the VCC 882 orbit is bound. However, the high gas mass and the low expected replenishment rate suggest that this is only the first stripping event. Implications for the origin of nucleated dwarf ellipticals are briefly discussed.

Observations of a Tidal Tail in the Interacting Galaxies NGC 4485/4490

NGC 4485 and NGC 4490 are closely interacting spiral galaxies in the NGC 4631 Group, separated by a projected distance of 7.7 kpc. Images in the B and I bands were obtained with the Burrell Schmidt telescope at Kitt Peak National Observatory in order to study star-forming regions in their tidal features, including a previously undetected faint tail whose color matches that of the outer disk in NGC 4490. A comparison of the tidal morphology with published simulations suggests that this is a prograde encounter with perigalacticon about 4 × 108 yr ago. This time is approximately the age of the youngest regions in the tail, based on a comparison of observed B - I colors with evolutionary models. Calculations of the Q instability parameter using a rotation curve derived from published H I observations support the conclusion that the tail is currently stable against star formation.