A. Russell Taylor
Director, University of Calgary Centre for Radio Astronomy
Professor, Department of Physics and Astronomy
The Outer Disk of the Milky Way Seen in 21-cm Absorption
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Authors: John M. Dickey, Simon Strasser, B.M. Gaensler, Marijke Haverkorn, Dain Kavars, N. M. McClure-Griffiths, Jeroen Stil, A. R. Taylor
(Submitted on 8 Jan 2009) on arXiv
Abstract: Three recent surveys of 21-cm line emission in the Galactic plane, combining single dish and interferometer observations to achieve resolution of 1 arcmin to 2 arcmin, 1 km/s, and good brightness sensitivity, have provided some 650 absorption spectra with corresponding emission spectra for study of the distribution of warm and cool phase H I in the interstellar medium. These emission-absorption spectrum pairs are used to study the temperature of the interstellar neutral hydrogen in the outer disk of the Milky Way, outside the solar circle, to a radius of 25 kpc.
The cool neutral medium is distributed in radius and height above the plane with very similar parameters to the warm neutral medium. In particular, the ratio of the emission to the absorption, which gives the mean spin temperature of the gas, stays nearly constant with radius to 25 kpc radius. This suggests that the mixture of cool and warm phases is a robust quantity, and that the changes in the interstellar environment do not force the H I into a regime where there is only one temperature allowed. The mixture of atomic gas phases in the outer disk is roughly 15% to 20% cool (40 K to 60 K), the rest warm, corresponding to mean spin temperature 250 to 400 K.
The Galactic warp appears clearly in the absorption data, and other features on the familiar longitude-velocity diagram have analogs in absorption with even higher contrast than for 21-cm emission. In the third and fourth Galactic quadrants the plane is quite flat, in absorption as in emission, in contrast to the strong warp in the first and second quadrants. The scale height of the cool gas is similar to that of the warm gas, and both increase with Galactic radius in the outer disk.
An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys
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Authors: Steven J. Gibson, A. Russell Taylor, Lloyd A. Higgs, Christopher M. Brunt, Peter E. Dewdney
(Submitted on 5 Mar 2005)
Abstract: We describe algorithms that detect 21cm line HI self-absorption (HISA) in large data sets and extract it for analysis. Our search method identifies HISA as spatially and spectrally confined dark HI features that appear as negative residuals after removing larger-scale emission components with a modified CLEAN algorithm. Adjacent HISA volume-pixels (voxels) are grouped into features in (l,b,v) space, and the HI brightness of voxels outside the 3-D feature boundaries is smoothly interpolated to estimate the absorption amplitude and the unabsorbed HI emission brightness. The reliability and completeness of our HISA detection scheme have been tested extensively with model data. We detect most features over a wide range of sizes, linewidths, amplitudes, and background levels, with poor detection only where the absorption brightness temperature amplitude is weak, the absorption scale approaches that of the correlated noise, or the background level is too faint for HISA to be distinguished reliably from emission gaps. False detection rates are very low in all parts of the parameter space except at sizes and amplitudes approaching those of noise fluctuations. Absorption measurement biases introduced by the method are generally small and appear to arise from cases of incomplete HISA detection. This paper is the third in a series examining HISA at high angular resolution. A companion paper (Paper II) uses our HISA search and extraction method to investigate the cold atomic gas distribution in the Canadian Galactic Plane Survey.
A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
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Authors: Steven J. Gibson, A. Russell Taylor, Lloyd A. Higgs, Christopher M. Brunt, Peter E. Dewdney
(Submitted on 5 Mar 2005)
Abstract: We present a 21cm line HI self-absorption (HISA) survey of cold atomic gas within Galactic longitudes 75 to 146 degrees and latitudes -3 to +5 degrees. We identify HISA as spatially and spectrally confined dark HI features and extract it from the surrounding HI emission in the arcminute-resolution Canadian Galactic Plane Survey (CGPS). We compile a catalog of the most significant features in our survey and compare our detections against those in the literature. Within the parameters of our search, we find nearly all previously detected features and identify many new ones. The CGPS shows HISA in much greater detail than any prior survey and allows both new and previously-discovered features to be placed into the larger context of Galactic structure. In space and radial velocity, faint HISA is detected virtually everywhere that the HI emission background is sufficiently bright. This ambient HISA population may arise from small turbulent fluctuations of temperature and velocity in the neutral interstellar medium. By contrast, stronger HISA is organized into discrete complexes, many of which follow a longitude-velocity distribution that suggests they have been made visible by the velocity reversal of the Perseus arm’s spiral density wave. The cold HI revealed in this way may have recently passed through the spiral shock and be on its way to forming molecules and, eventually, new stars. This paper is the second in a series examining HISA at high angular resolution. A companion paper (Paper III) describes our HISA search and extraction algorithms in detail.
The Canadian Galactic Plane Survey
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Author: Taylor, A. R., Gibson, S. J., Peracaula, M., Martin, P. G., Landecker, T. L., Brunt, C. M., Dewdney, P. E., Dougherty, S. M., Gray, A. D., Higgs, L. A., Kerton, C. R., Knee, L. B. G., Kothes, R., Purton, C. R., Uyaniker, B., Wallace, B. J., Willis, A. G., & Durand, D.
2003, Astronomical Journal, 125, 3145.
Abstract: The Canadian Galactic Plane Survey (CGPS) is a project to combine radio, millimetre and infrared surveys of the Galactic Plane to provide arc-minute scale images of all major components of the interstellar medium over a large portion of the Galactic disk. We describe in detail the observations for the low-frequency component of the CGPS, the radio surveys carried out at the Dominion Radio Astrophysical Observatory (DRAO), and summarize the properties of the merged database of surveys that comprises the CGPS.
The DRAO Synthesis Telescope surveys have imaged a 73 degree section of the Galactic Plane, using ~85% of the telescope time between April 1995 and June 2000. The observations provide simultaneous radio continuum images at two frequencies, 408 MHz and 1420 MHz, and spectral-line images of the 21cm transition of neutral atomic hydrogen. In the radio continuum at 1420 MHz dual-polarization receivers provide images in all four Stokes parameters. The surveys cover the region 74.2 < l < 147.3 degrees, with latitude extent of -3.6 < b < +5.6 degrees at 1420 MHz and -6.7 < b < +8.7 degrees at 408 MHz. By integration of data from single-antenna observations, the survey images provide complete information on all scales of emission structures down to the resolution limit, which is just below 1' x 1' cosec(DEC) at 1420 MHz, and 3.4' x 3.4' cosec(DEC) at 408 MHz. The continuum images have dynamic range of several thousand, yielding essentially noise-limited images with rms of ~0.3 mJy/beam at 1420 MHz and ~3 mJy/beam at 408 MHz. The spectral-line data are noise limited with rms brightness temperature dTB ~ 3 K in a 0.82 km/s channel.
The complete CGPS data set, including the DRAO surveys and data at similar resolution in 12CO (1–0) and in infrared emission from dust, all imaged to an identical Galactic co-ordinate grid and map projection, are being made publicly available through the Canadian Astronomy Data Centre.
A New View of Cold HI Clouds in the Milky Way
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Author: S. J. Gibson, A. R. Taylor, L. A. Higgs, & P. E. Dewdney
2000, Astrophysical Journal, 540, 851.
Abstract: We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the Solar circle. These objects vary in size, shape, and contrast against the background H I. All display a high level of angular and velocity structure, and most would appear significantly diluted, if not invisible, in lower-resolution H I surveys. A number of Perseus arm features remain unresolved by the 1′ beam of our survey, with apparent diameters < 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious 12CO emission counterparts. This suggests either HISA is not found predominantly in molecular clouds, as has often been presumed in the past, or CO is not a good tracer of H2. Some HISA lacking CO shows far-infrared dust emission, though whether this arises from shielded molecular gas or diffuse atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method which aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships, and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very different appearance. We find spin temperatures ~102 cm-3.
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