A. Russell Taylor

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

A self-absorption census of cold H I clouds in the Canadian galactic plane survey

We present a 21 cm line H I self-absorption (HISA) survey of cold atomic gas within Galactic longitudes l = 75? to 146? and latitudes b = -3? to +5?. We identify HISA as spatially and spectrally confined dark H I features and extract it from the surrounding H I 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 H I 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 that they have been made visible by the velocity reversal of the Perseus arms spiral density wave. The cold H I 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.

An Automated Method for the Detection and Extraction of H I Self-Absorption in High-Resolution 21 Centimeter Line Surveys

We describe algorithms that detect 21 cm line H I self-absorption (HISA) in large data sets and extract it for analysis. Our search method identifies HISA as spatially and spectrally confined dark H I 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 H I brightness of voxels outside the three-dimensional feature boundaries is smoothly interpolated to estimate the absorption amplitude and the unabsorbed H I 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, line widths, 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.

Infrared Excess Sources in the Canadian Galactic Plane Survey

Using data sets from the Canadian Galactic Plane Survey, we have conducted a multiwavelength study of interstellar components, covering the region 102.5° < l < 141.5° and -3.03° < b < 5.41°. By comparing column density tracers of dust, atomic hydrogen, and molecular gas (traced by CO emission), we have found regions where the dust optical depth shows evidence of more gas than is predicted by the H I and CO observations. Within this population of infrared excess sources, it is possible to discriminate between sources associated with low and high dust temperatures. We interpret the colder temperature sources as molecular clouds/clumps not traced by the CO J = 1-0 transition. Possible reasons include the depletion of CO onto dust grains in the coldest, densest regions of molecular clouds, or photodissociation of CO on the outskirts of molecular clouds.

A Global View of Molecule-Forming Clouds in the Galaxy

We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Ways disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J=1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold HI in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.