5,013 research outputs found

    The Temperature and Opacity of Atomic Hydrogen in Spiral Galaxies

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    We analyze the resolved neutral hydrogen emission properties of a sample of eleven of the nearest spiral galaxies. Between 60 and 90% of the total HI line flux within the optical disk is due to a high brightness network (HBN) of emission features which are marginally resolved in their narrow dimension at about 150 pc and have a face-on covering factor of about 15%. Averaged line profiles of this component are systematically non-Gaussian with a narrow core (less than about 6 km/s FWHM) superposed on broad Lorentzian wings (30 km/s FWHM). An upper limit to the gas temperature of 300 K follows directly from the narrow line profiles, while simple modeling suggests kinetic temperatures equal to the peak emission brightness temperature (80-200 K) in all cases but the outer disks of low mass galaxies, where the HBN becomes optically thin to the 21 cm line. Positive radial gradients in the derived kinetic temperature are found in all spiral galaxies. The distributions of brightness temperature with radius in our sample form a nested system with galaxies of earlier morphological type systematically displaced to lower temperature at all radii. The fractional line flux due to the HBN plummets abruptly near the edge of the optical disk where a diffuse outer gas disk takes over. We identify the HBN with the Cool Neutral Medium.Comment: 22 page LaTeX requires aastex, 10 PS figures. Accepted for publication in the Ap

    Conic bundles in projective fourspace

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    P. Ellia and G.Sacchiero have shown that if SS is a smooth surface in \Pn 4 which is ruled in conics, then SS has degree 4 or 5. In this paper we give a proof of this result combining the ideas of Ellia and Sacchiero as they are used in the paper of the second author on plane curve fibrations and the recent work of G. Fl\o ystad and the first author bounding the degree of smooth surfaces in \Pn 4 not of general type.Comment: 7 pages, Plain-Te

    Tiny HI Clouds in the Local ISM

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    Very sensitive HI absorption spectra (tau RMS about 10^-4 over 1 km/s) toward high latitude QSOs have revealed a population of tiny discrete features in the diffuse ISM with peak tau of 0.1 - 2% and core line-widths corresponding to temperatures as low as 20 K. Imaging detections confirm linear dimensions of a few 1000 AU. We suggest these structures may be formed by the stellar winds of intermediate mass stars. A more speculative origin might involve molecular "dark matter".Comment: 6 pages, 8 figures, to appear in "The IMF at 50", eds. E. Corbelli, F. Palla, and H. Zinnecker, ASSL (Kluwer

    On the Formation of Warped Gas Disks in Galaxies

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    We consider the most commonly occurring circumstances which apply to galaxies, namely membership in galaxy groups of about 1013h1M10^{13}h^{-1} M_\odot total mass, and estimate the accompanying physical conditions of intergalactic medium (IGM) density and the relative galaxy-IGM space velocity. We then investigate the dynamical consequences of such a typical galaxy-IGM interaction on a rotating gaseous disk within the galaxy potential. We find that the rotating outer disk is systematically distorted into a characteristic "warp" morphology, of the type that has been well-documented in the majority of well-studied nearby systems. The distortion is established rapidly, within two rotation periods, and is long-lived, surviving for at least ten. A second consequence of the interaction is the formation of a one arm retrograde spiral wave pattern that propagates in the disk. We suggest that the ubiquity of the warp phenomenon might be used to reconstruct both the IGM density profile and individual member orbits within galaxy groups.Comment: 7 pages, 7 figures, accepted for publication in MNRAS Letters. For supplementary material, see http://www.atnf.csiro.au/people/Sebastian.Haan/publications.htm

    A Kinematic Measurement of Ram Pressure in the Outer Disk of Regular Galaxies

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    While most ram pressure studies have focused on ram pressure stripping in galaxy clusters, we devise a novel approach based on a kinematic measurement of ram pressure perturbations in HI velocity fields for intergalactic material (IGM) densities and relative velocities that are one to two orders of magnitude lower than in galaxies showing ram pressure stripping. Our model evaluates ram pressure induced kinematic terms in gas disks with constant inclination as well as those with a warped geometry. Ram pressure perturbations are characterized by kinematic modes of even order, m=0 and m=2, corresponding to a ram wind perpendicular and parallel to the gas disk, respectively. Long-term consequences of ram pressure, such as warped disks as well as uncertainties in the disk geometry typically generate uneven modes (m=1 and m=3), that are clearly distinguishable from the kinematic ram pressure terms. We have applied our models to three nearby isolated galaxies, utilizing Markov Chain Monte Carlo fitting routines to determine ram pressure perturbations in the velocity fields of NGC 6946 and NGC 3621 of ~30km s1^{-1} (effective line-of-sight velocity change) at HI column densities below (4-10)×\times1020^{20}cm2^{-2} (at radial scales greater than ~15kpc). In contrast, NGC 628 is dominated by a strongly warped disk. Our model fits reveal the three-dimensional vector of the galaxies' movement with respect to the IGM rest-frame and provide constraints on the product of speed with IGM density, opening a new window for extragalactic velocity measurements and studies of the intergalactic medium.Comment: 34 pages, 28 figures, accepted for publication in MNRA