Feshbach resonant scattering of three fermions in one-dimensional wells

We study the weak-tunnelling limit for a system of cold 40K atoms trapped in a one-dimensional optical lattice close to an s-wave Feshbach resonance. We calculate the local spectrum for three atoms at one site of the lattice within a two-channel model. Our results indicate that, for this one-dimensional system, one- and two-channel models will differ close to the Feshbach resonance, although the two theories would converge in the limit of strong Feshbach coupling. We also find level crossings in the low-energy spectrum of a single well with three atoms that may lead to quantum phase transition for an optical lattice of many wells. We discuss the stability of the system to a phase with non-uniform density.Comment: 10 pages, 5 figure

Cosmic-Ray Production and the Role of Supernovae in NGC 6946

We present a study of 35 radio-selected supernova remnant (SNR) candidates in the nearby spiral galaxy NGC 6946. In this study, an optically selected sample of 27 SNRs from Matonick & Fesen is compared to our sample of radio-selected SNRs. The optically selected SNRs are found to have almost no overlap with the radio-selected sample. This dichotomy is further enhanced by the observation that the optically selected SNRs favor the interarm regions, while the radio-emitting SNRs lie predominately on the spiral arms in or near regions of high star formation. The separation of the two samples of SNRs is discussed in terms of selection effects and differences in cosmic-ray production. The optical sample of SNRs is probably biased toward those SNRs located in areas of NGC 6946 where the confusion of Hα emission by H II regions is relatively low and the SNRs are easier to identify by their emission-line signatures. The radio-selected sample is also subject to selection biases such that these SNRs favor the arms. However, the absence of radio emission from the optically selected, largely interarm SNRs and the relatively large number of radio-detected SNRs in the arms require additional explanation. The properties of the radio-selected SNRs are discussed in the context of diffusive shock acceleration theory. We find that the theory can account for the range in radio flux densities and the nondetection of the optically selected SNRs. The differences in the radio properties between the arm and interarm population of SNRs can be explained by differences in the average gas densities and magnetic fields since the latter affect both the cosmic-ray yield and the radio properties of the SNRs. The possibility that the arm and interarm SNRs arise from different stellar populations is also addressed

Spatially-resolved Thermal Continuum Absorption against the Supernova Remnant W49B

We present sub-arcminute resolution imaging of the Galactic supernova remnant W49B at 74 MHz (25") and 327 MHz (6"), the former being the lowest frequency at which the source has been resolved. While the 327 MHz image shows a shell-like morphology similar to that seen at higher frequencies, the 74 MHz image is considerably different, with the southwest region of the remnant almost completely attenuated. The implied 74 MHz optical depth (~ 1.6) is much higher than the intrinsic absorption levels seen inside two other relatively young remnants, Cas A and the Crab Nebula, nor are natural variations in the relativistic electron energy spectra expected at such levels. The geometry of the absorption is also inconsistent with intrinsic absorption. We attribute the absorption to extrinsic free-free absorption by a intervening cloud of thermal electrons. Its presence has already been inferred from the low-frequency turnover in the integrated continuum spectrum and from the detection of radio recombination lines toward the remnant. Our observations confirm the basic conclusions of those measurements, and our observations have resolved the absorber into a complex of classical HII regions surrounded either partially or fully by low-density HII gas. We identify this low-density gas as an extended HII region envelope (EHE), whose statistical properties were inferred from low resolution meter- and centimeter-wavelength recombination line observations. Comparison of our radio images with HI and H_2CO observations show that the intervening thermal gas is likely associated with neutral and molecular material as well.Comment: 18 pages, LaTeX with AASTeX-5, 5 figures in 7 PostScript files; accepted for publication in the Ap

A possible radio supernova in the outer part of NGC 3310

As part of an on-going radio supernova monitoring program, we have discovered a variable, compact steep spectrum radio source ~65 arcsec (~4 kpc) from the centre of the starburst galaxy NGC 3310. If the source is at the distance of NGC 3310, then its 5 GHz luminosity is ~3 x 10^{19} WHz^-1. The source luminosity, together with its variability characteristics, compact structure (<17 mas) and its association with a group of HII regions, leads us to propose that it is a previously uncatalogued type II radio supernova. A search of archival data also shows an associated X-ray source with a luminosity similar to known radio supernova.Comment: 5 pages, 3 figures. Accepted by MNRA

Clusters of Extragalactic Ultra Compact HII Regions

We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young, embedded star birth regions, which are likely to be clusters of ultracompact HII regions. All 35 of the sources presented in this article have positive radio spectral indices alpha>0 suggesting an optically thick thermal bremsstrahlung emission arising in the HII region surrounding hot stars. Energy requirements indicate a range of a several to >500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF, this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly half of the sources in our sample, there is no obvious optical counterpart, giving further support for their deeply embedded nature. Their luminosities and radio spectral energy distributions are consistent with HII regions having electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We suggest that the less luminous of these sources are extragalactic ultracompact HII region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, while the brightest will be counterparts to 30 Doradus. These objects constitute the lower mass range of extragalactic ``ultradense HII regions'' which we argue are the youngest stages of massive star cluster formation yet observed. This sample is beginning to fill in the continuum of objects between small associations of ultracompact HII regions and the massive extragalactic clusters that may evolve into globular clusters.Comment: 37 pages, uses AASTeX; scheduled to appear in ApJ v. 559 October 2001. Full postscript version available from http://www.astro.wisc.edu/~chip/Papers/Johnson_Kobulnicky_etal_ApJ559.ps.g

6.7 GHz methanol absorption toward the Seyfert 2 galaxy NGC 3079

The detection of the 6.7 GHz line of methanol (CH3OH) is reported for the first time toward an object beyond the Magellanic Clouds. Using the Effelsberg 100 m telescope, two absorption features were identified toward the Seyfert 2 galaxy NGC 3079. Both components probably originated on lines-of-sight toward the central region, presumably absorbing the radio continuum of the nuclear sources A, B, and E of NGC 3079. One absorption feature, at the systemic velocity, is narrow and may arise from gas not related to the nuclear environment of the galaxy. The weaker blue-shifted component is wider and may trace outflowing gas. Total A-type CH3OH column densities are estimated to be between a few times 10^13 and a few times 10^15 cm^-2. Because of a highly frequency-dependent continuum background, the overall similarity of HI, OH, and CH3OH absorption profiles hints at molecular clouds that cover the entire area occupied by the nuclear radio continuum sources ~ 4 pc.Comment: 4 pages, 1 figure, accepted for publication in A&A Letter

High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz

We present a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz. With a resolution of ~ 7" X 12" and an RMS noise of 1.6 mJy/beam, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small diameter sources in the region, has resulted in the detection of two new Non Thermal Filaments (NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range. A version of this paper containing full resolution images may be found at http://lwa.nrl.navy.mil/nord/AAAB.pdf.Comment: Astronomical Journal, Accepted 62 Pages, 21 Figure

High Latitude Radio Emission in a Sample of Edge-On Spiral Galaxies

We have mapped 16 edge-on galaxies at 20 cm using the VLA. For 5 galaxies, we could form spectral index, energy and magnetic field maps. We find that all but one galaxy show evidence for non-thermal high latitude radio continuum emission, suggesting that cosmic ray halos are common in star forming galaxies. The high latitude emission is seen over a variety of spatial scales and in discrete and/or smooth features. In general, the discrete features emanate from the disk, but estimates of CR diffusion lengths suggest that diffusion alone is insufficient to transport the particles to the high latitudes seen (> 15 kpc in one case). Thus CRs likely diffuse through low density regions and/or are assisted by other mechanisms (e.g. winds). We searched for correlations between the prevalence of high latitude radio emission and a number of other properties, including the global SFR, supernova input rate per unit star forming, and do not find clear correlations with any of these properties.Comment: 40 pages of text, 3 figures, 6 tables, and an appendix of 21 jpeg figures (which is a radio continuum catalogue of 17 galaxies). to appear in A. J. (around January 1999

High Resolution, High Sensitivity Imaging of the Galactic Center at 330 MHz

We present results derived from a wide field, sub-arcminute resolution VLA image of the Galactic Center region at 330 MHz (λ = 90 cm). With a resolution of ∼7″ × 12″ and an rms noise of 1.6 mJy beam−1, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency (eg. LaRosa et al. 2000). The improvement in sensitivity has significantly increased the census of small diameter sources in the region, resulted in the detection of two new Non-Thermal Filaments (NTFs) and 18 new NTF candidates, and resulted in the lowest frequency (tentative) detection of Sgr A*