Deep narrow band imagery of the diffuse ISM in M33

Very deep narrow band images were obtained for several fields in the local group spiral galaxy M33 using a wide field reimaging Charge Coupled Device (CCD) camera on the 1.5 m telescope at Palomar Observatory. The reimaging system uses a 306 mm collimator and a 58 mm camera lens to put a 16 minute by 16 minute field onto a Texas Instruments 800 x 800 pixel CCD at a resolution of 1.2 arcseconds pixel (-1). The overall system is f/1.65. Images were obtained in the light of H alpha (S II) lambda lambda 6717, 6731, (O III) lambda 5007, and line-free continuum bands 100A wide, centered at 6450A and 5100A. Assuming a distance of 600 kpc to M33 (Humphreys 1980, Ap. J., 241, 587), this corresponds to a linear scale of 3.5 pc pixel (-1), and a field size of 2.8 kpc x 2.8 kpc. Researchers discuss the H alpha imagery of a field centered approx. equal to 8 minutes NE of the nucleus, including the supergiant HII region complex NGC 604. Two 2000 second H alpha images and two 300 second red continuum images were obtained of two slightly offset fields. The fields were offset to allow for discrimination between real emission and possible artifacts in the images. All images were resampled to align them with one of the H alpha frames. The continuum images were normalized to the line images using the results of aperture photometry on a grid of stars in the field, then the rescaled continuum data were directly subtracted from the line data

The interstellar halo of spiral galaxies: NGC 891

Researchers have detected the Warm Ionized Medium (WIM) phase in the galaxy NGC 891. They found that the radial distribution of the WIM follows the molecular or young star distribution - an expected dependence. The amount of the WIM in this galaxy exceeds that in our Galaxy. The major surprize is the large thickness of the WIM phase - about 9 kpc instead 3 kpc as in our Galaxy. Clearly, this is the most significant result of the observations. The presence of low ionization gas at high z as well as at large galactocentric radii (where young stars are rare) is an important clue to the origin of the halo and observations such as the one reported here provide important data on this crucial question. In particular, the ionization of gas at high absolute z implies that either the UV photons manage to escape from the disk of the galaxy or that the extragalactic UV background plays an important role. The bulk of the WIM in spiral galaxies is a result of star-formation activity and thus these results can be understood by invoking a high star formation rate in NGC 891. Only the concerted action of supernovae can get the gas to the large z-heights as is observed in this galaxy. Support for this view comes from our detection of many worms i.e., bits and pieces of supershells in the form of kilo-parsec long vertical filaments. Researchers also saw a 600-pc size supershell located nearly one kpc above the plane of the galaxy

M51: Molecular spiral arms, GMAs and superclouds

Researchers present an aperture synthesis image of M51 in the CO 1 to 0 line at 9 seconds x 7 seconds resolution made with the Owens Valley Millimeter Interferometer. The image is a mosaic of 30 one-arcminute fields. The image shows narrow spiral arms which are coincident with the optical dust lanes and non-thermal radio emission, but are offset from the ridges of H alpha emission. Many dense concentrations of CO emission, termed Giant Molecular Associations (GMAs), are seen both along and between the arms. The typical GMA mass is about 3 times 10(exp 7) solar mass. Most of the on-arm GMAs appear to be gravitationally bound. These GMAs consist of several spectral components (Molecular Superclouds) with typical mass 10(exp 7) solar mass, which also appear to be bound. The observed streaming motions in the GMAs are consistent with density wave theory. The interarm GMAs are not gravitationally bound, and are likely to be due to a secondary compression of the density wave

An Analysis of the Statistics of the Hubble Space Telescope Kuiper Belt Object Search

We calculate statistical limits to the detection of Kuiper belt objects in the Hubble Space Telescope (HST) data of Cochran et al., in which they report the discovery of a population of Halley-sized objects in Pluto-like orbits. Detection of a population of faint objects in these data is limited by the number of false objects that appear owing only to random noise; the number of real objects must exceed the uncertainty in the number of these false objects for the population to be observable. We determine the number of false objects expected owing to random noise in the data of Cochran et al. by measuring the pixel-to-pixel noise level in the raw HST data and propagating this noise through the detection method employed by Cochran et al. We find that the uncertainty in the number of false objects exceeds by 2 orders of magnitude the reported number of objects detected by Cochran et al. The detection of such a population of Halley-sized Kuiper belt objects with these data is therefore not possible

Prediction of biopore- and matrix-dominated flow from X-ray CT-derived macropore network characteristics

Peer reviewedPublisher PD

Star formation and the distribution of HI and infrared emission in M51

H I, infrared, CO, H alpha and beta band observations of M51, the prototypical grand-design spiral galaxy, are used to study the consequences of star formation for the distribution of H I and dust. Using the H I and CO data sets new tests of the idea that the H I is largely a dissociation product in star-forming regions were performed. It was confirmed that the H I spiral arms are generally coincident with the H II region arms, and offset downstream from the CO arms. The radial distributions of total gas, H alpha and H I surface density have a simple explanation in the dissociation picture. The distributions also demonstrate how the surface density of H I might be related to the star formation efficiency in molecule-rich galaxies. The large width of the H I regions along the arms compared to that of the H II regions can be understood in terms of a simple Stroemgren sphere calculation. The longer lifetime of the stars producing dissociating radiation vs. those producing ionizing radiation will also contribute to the greater width of the H I arms if stars are continuously forming on the arms. The lack of detailed coincidence of the H I and H II regions along the inner arms has a variety of possible explanations within the dissociation scenario. Two simple tests to probe the origin of the IRAS emission in M51 were performed