Showing papers by "Michael W. Werner published in 1999"

High Spatial Resolution Imaging of Arp 220 from 3 to 25 Microns

Abstract: Images of Arp 220 from 3.45 to 24.5 μm with 0farcs5 resolution are presented that clearly separate the nucleus into at least two components. The western component is about 3 times more luminous than the eastern component, but the silicate absorption in the fainter, eastern component is roughly 50% greater than the absorption in the western component. Each component is marginally resolved. The two components seen at 24.5 μm are identified with the two radio components. The western source most likely coincides with the high-extinction disk previously suggested to exist in Arp 220, while the eastern nucleus is identified with a faint, highly reddened source seen in HST 2.2 μm NICMOS images. The two nuclei together account for essentially all of the measured 24.5 μm flux density. Two models are presented, both of which fit the observations. In one the majority of the total luminosity is produced in an extended star formation region, and in the other most of the luminosity is produced in the compact but extincted regions associated with the two nuclei seen at 24.5 μm. In both pictures, substantial luminosity at 100 μm emerges from a component having a diameter of 2"-3" (~1 kpc).

The Detached Dust Shell around the Massive Star HD 179821

Abstract: We have used the Keck I telescope to resolve at three mid-IR wavelengths the emission from HD 179821 (= RAFGL 2343), a G-type supergiant of perhaps 30 M☉ with a detached dust shell. The shell is very approximately circular in shape with an inner diameter of ~33, corresponding to 3.0 × 1017 cm. We estimate that the star was losing ~4 × 10-4 M☉ yr-1 until about 1800 yr ago, when the mass loss slowed dramatically. During the past ~104 yr, the star has lost ~10% of its initial mass. The star lies about 035 off center and is closer to the brighter, northern hemisphere of the nebula, which can be explained if the outflow velocity V∞ deviates by +20% from the average in the southern hemisphere and -20% from the average in the northern hemisphere. The mass-loss rate (θ) may have been inversely correlated with the outflow velocity so that the momentum outflow was isotropic during the mass-loss phase. It also seems that totalV∞ was within a factor of 2 of L*/c, where L* is the current luminosity of the star; the mass loss may have been driven by radiation pressure. These results may help characterize the asymmetric circumstellar winds into which supernova explosions propagate.

Hubble Space Telescope Imaging Polarimetry of the Gravitational Lens FSC 10214+4724

Abstract: We present imaging polarimetry of the extremely luminous, redshift 2.3 IRAS source FSC 10214+4724. The observations were obtained with HST's Faint Object Camera in the F437M filter, which is free of strong emission lines. The 0farcs7 long arc is unresolved to 0farcs04 FWHM in the transverse direction and has an integrated polarization of 28% ± 3%, in good agreement with ground-based observations. The polarization position angle varies along the arc by up to 35°. The overall position angle is 62° ± 3° east of north. No counterimage is detected to B = 27.5 mag (3 σ), giving an observed arc to counterimage flux ratio greater than 250, considerably greater than the flux ratio of 100 measured previously in the I band. This implies that the configuration of the object in the source plane at the B band is different from that at I band and/or that the lensing galaxy is dusty.

Keck mid-ir High-Resolution Imaging of arp 220

Abstract: To identify the main power source of an object, it is necessary to obtain high-resolution images of that object at the wavelengths at which most of the luminosity is coming out However, such observations had not been possible in the studies of ULIRGs, which release most of their luminosity in the mid/far-IR High-resolution imaging at 25 µm, for example, requires not only a sensitive mid-IR camera but also a 10-m class telescope to achieve a sub-arcsecond difraction limit Recently, such high-resolution imaging in the mid-IR has become possible with the MIRLIN mid-IR camera (Ressler 1994) on the Keck II telescope This camera is based on a 128 x 128 Si:As array, and has a field of view of 17" on a side with a pixel scale of 0"138/pixel The working wavelength range is 5-26 µm Using MIRLIN, we are now undertaking a program to perform high-resolution mid-IR imaging of nearby ultraluminous IR galaxies Here, we present our first results on Arp 220 More details can be found in Soifer et al (1999)

The molecular gas—star formation connection in an optically-selected sample of interacting galaxies

Abstract: We have obtained 12 CO (1–0) emission-line observations for a sample of 37 interacting galaxy systems, chosen from a parent sample of optically-selected interacting galaxies. The sample observed here spans a large range of interaction strengths and current star formation rates. Using the standard Galactic CO-to-H2 conversion factor we find that the interacting galaxies are, on average, marginally more rich in molecular gas than a comparison sample of isolated spiral galaxies, having mean MH2/LB and MH2/MHI ratios that are 20–40% higher than that of the isolated spirals. The interacting galaxies have a mean LIR/MH2 ratio that is a factor of ∼1.3 higher than the isolated spirals, indicating that the rate of highmass star formation per unit molecular gas—the star formation efficiency—is also enhanced within the interacting galaxy sample. There is a strong correlation between relative H2 content and star formation