An Automated Method for the Detection and Extraction of HI Self-Absorption in High-Resolution 21cm Line Surveys

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.Comment: 39 pages, including 14 figure pages; to appear in June 10 ApJ, volume 626; figure quality significantly reduced for astro-ph; for full resolution, please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey

A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey

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.Comment: 44 pages, including 13 figure pages; to appear in June 10 ApJ, volume 626; figure quality significantly reduced for astro-ph; for full resolution, please see http://www.ras.ucalgary.ca/~gibson/hisa/cgps1_survey

Jamelie Hassan ; Material Knowledge : A Moral Art of Crisis

Dewdney outlines Hassan's early life, the influences on her interest in cultural and political displacement, and the development of her work and concerns. Biographical notes. 6 bibl. ref

Casa de Las Américas : Ron Benner, Greg Curnoe, Christopher Dewdney, Murray Favro, Jamelie Hassan, Fern Helfand

Text by Dewdney on five artists from London, Ontario, followed by biographical notes (in Spanish)

Blue Book #8

With reference to Curnoe's letter works and lists, Dewdney focuses on the artist's representation of the subject "I", destabilized through a series of negated attributes in the rubber stamped book. Includes an index of attributes as well as an addendum listing Curnoe's works (1961-1989)

Nether Mind

Images by the eleven members of the Nether Mind Artist Collective and Dewdney's essay on "Boskopian cyborg" are scattered through postcard-size sheets cut out from various fabrics, magazines, books, etc

Judith Schwarz

The four pieces exhibited by Schwarz constitute, for essayist Dewdney, artifacts of the artist's progression which allude to the human body; the impressions made in concrete and organic shapes formed out of steel plates, are discussed in their relationship to the viewer. Biographical notes. 11 bibl. ref

Brian Groombridge

Dewdney describes the formal qualities of Groombridge's steel sculpture and elaborates on its meanings. Artist's statement. Biographical notes. 7 bibl. ref