Mark R. Calabretta

Representations of celestial coordinates in FITS

In Paper I, Greisen & Calabretta (2002) describe a generalized method for assigning physical coordinates to FITS image pixels. This paper implements this method for all spherical map projections likely to be of interest in astronomy. The new methods encompass existing informal FITS spherical coordinate conventions and translations from them are described. Detailed examples of header interpretation and construction are given.

A new look at the large-scale H I structure of the Large Magellanic Cloud

We present a Parkes multibeam H i survey of the Large Magellanic Cloud (LMC). This survey, which is sensitive to spatial structure in the range 200 pc ∼ 10 kpc, complements the Australia Telescope Compact survey, which is sensitive to structure in the range 15 pc ∼ 500 pc. With an rms column density sensitivity of 8 ×10 16 cm −2 for narrow lines and 4×10 17 cm −2 for typical linewidths of 40 km s −1 , emission is found to be extensive well beyond the main body of the LMC. Arm-like features extend from the LMC to join the Magellanic Bridge and the Leading Arm, a forward counterpart to the Magellanic Stream. These features, whilst not as dramatic as those in the SMC, appear to have a common origin in the Galactic tidal field, in agreement with recent 2MASS and DENIS results for the stellar population. The diffuse gas which surrounds the LMC, particularly at pa’s 90 ◦ to 330 ◦ , appears to be loosely associated with tidal features, but loosening by the ram pressure of tenuous Galactic halo gas against the outer parts of the LMC cannot be discounted. High-velocity clouds, which lie between the Galaxy and the LMC in velocity and which appear in the UV spectra of some LMC stars, are found to be associated with the LMC if their heliocentric velocity exceeds about +100 km s −1 . They are possibly the product of energetic outflows from the LMC disk. The H i mass of the LMC is found to be (4.8 ± 0.2) × 10 8 M⊙ (for an assumed distance of 50 kpc), substantially more than previous recent measurements.

Representations of world coordinates in FITS

The initial descriptions of the FITS format provided a simplified method for describing the physical coordinate values of the image pixels, but deliberately did not specify any of the detailed conventions required to convey the complexities of actual image coordinates. Building on conventions in wide use within astronomy, this paper proposes general extensions to the original methods for describing the world coordinates of FITS data. In subsequent papers, we apply these general conventions to the methods by which spherical coordinates may be projected onto a two-dimensional plane and to frequency/wavelength/velocity coordinates.

GASS: the Parkes Galactic all-sky survey: II. Stray-radiation correction and second data release

Context. The Parkes Galactic all-sky survey (GASS) is a survey of Galactic atomic hydrogen (H i) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release was published by McClure-Griffiths et al. (2009). Aims. We remove instrumental effects that affect the GASS and present the second data release. Methods. We calculate the stray-radiation by convolving the all-sky response of the Parkes antenna with the brightness temperature distribution from the Leiden/Argentine/Bonn (LAB) all sky 21-cm line survey, with major contributions from the 30-m dish of the Instituto Argentino de Radioastronomia (IAR) in the southern sky. Remaining instrumental baselines are corrected using the LAB data for a first guess of emission-free baseline regions. Radio frequency interference is removed by median filtering. Results. After applying these corrections to the GASS we find an excellent agreement with the Leiden/Argentine/Bonn (LAB) survey. The GASS is the highest spatial resolution, most sensitive, and is currently the most accurate H i survey of the Galactic H i emission in the southern sky. We provide a web interface for generation and download of FITS cubes.

Representations of Spectral Coordinates in FITS

Greisen & Calabretta (2002) describe a generalized method for specifying the coordinates of FITS data samples. Following that general method, Calabretta & Greisen (2002) describe detailed conventions for dening celestial coordinates as they are projected onto a two-dimensional plane. The present paper extends the discussion to the spectral coordinates of wavelength, frequency, and velocity. World coordinate functions are dened for spectral axes sampled linearly in wavelength, frequency, or velocity, linearly in the logarithm of wavelength or frequency, as projected by ideal dispersing elements, and as specied by a lookup table.

EVIDENCE FOR CHIMNEY BREAKOUT IN THE GALACTIC SUPERSHELL GSH 242 03+37

We present new high-resolution neutral hydrogen (H I) images of the Galactic supershell GSH 242-03+37. These data were obtained with the Parkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH 242-03+37 is one of the largest and most energetic H I supershells in the Galaxy, with a radius of 565 ± 65 pc and an expansion energy of 3 × 1053 ergs. Our images reveal a complicated shell with multiple chimney structures on both sides of the Galactic plane. These chimneys appear capped by narrow filaments about 1.6 kpc above and below the Galactic midplane, confirming structures predicted in simulations of expanding supershells. The structure of GSH 242-03+37 is extremely similar to the only other Galactic supershell known to have blown out of both sides of the plane, GSH 277+00+36. We compare the GASS H I data with X-ray and Hα images, finding no strong correlations.

A derivation of the free–free emission on the Galactic plane between ℓ= 20° and 44°

We present the derivation of the free–free emission on the Galactic plane between � = 20 ◦ and 44 ◦ and |b |≤ 4 ◦ , using radio recombination line (RRL) data from the H I Parkes All Sky Survey (HIPASS). Following an upgrade of the RRL data reduction technique, which improves significantly the quality of the final RRL spectra, we have extended the analysis to three times the area covered in Alves et al. The final RRL map has an angular resolution of 14.8 arcmin and a velocity resolution of 20 km s −1 . The electron temperature (T e) distribution of the ionized gas in the area under study at 1.4 GHz is derived using the line and continuum data from the present survey. The mean T e on the Galactic plane is 6000 K. The first direct measure of the free–free emission is obtained based on the derived T e distribution. Subtraction of this thermal component from the total continuum leads to the first direct measurement of the synchrotron emission at 1.4 GHz. A narrow component of width 2 ◦ is identified in the latitude distribution of the synchrotron emission. We present a list of H II regions and supernova remnants (SNRs) extracted from the present free–free and synchrotron maps, where we confirm the synchrotron nature of the SNRs G42.0−0.1 and G41.5+0.4 proposed by Kaplan et al. and the SNR G35.6−0.4 recently re-identified by Green. The latitude distribution for the RRL-derived free–free emission shows that the Wilkinson Microwave Anisotropy Probe (WMAP) maximum entropy method is too high by ∼50 per cent, in agreement with other recent results. The extension of this study to the inner Galaxy region � =− 50 ◦ to 50 ◦ will allow a better overall comparison of the RRL result with WMAP.

H I Clouds in the Lower Halo. I. The Galactic All-Sky Survey Pilot Region

We have detected over 400 H I clouds in the lower halo of the Galaxy within the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth quadrant that spans -->18° in longitude, -->40° in latitude, and is centered on the Galactic equator. These clouds have a median peak brightness temperature of 0.6 K, a median velocity width of 12.8 km s−1, and angular sizes 1°. The motion of these clouds is dominated by Galactic rotation with a random cloud-to-cloud velocity dispersion of 18 km s−1. A sample of clouds likely to be near tangent points was analyzed in detail. These clouds have radii on the order of 30 pc and a median H I mass of 630 -->M☉. The population has a vertical scale height of 400 pc and is concentrated in Galactocentric radius, peaking at -->R = 3.8 kpc. This confined structure suggests that the clouds are linked to spiral features, while morphological evidence that many clouds are aligned with loops and filaments is suggestive of a relationship with star formation. The clouds might result from supernovae and stellar winds in the form of fragmenting shells and gas that has been pushed into the halo rather than from a galactic fountain.

A new 1.4 GHz radio continuum map of the sky south of declination +25°

Archival data from the HI Parkes All-Sky Survey (HIPASS) and the HI Zone of Avoidance (HIZOA) survey have been carefully reprocessed into a new 1.4 GHz continuum map of the sky south of δ = +25°. The wide sky coverage, high sensitivity of 40 mK (limited by confusion), resolution of 14.4 arcmin (compared to 51 arcmin for the Haslam et al. 408 MHz and 35 arcmin for the Reich et al. 1.4 GHz surveys), and low level of artefacts make this map ideal for numerous studies, including: merging into interferometer maps to complete large-scale structures; decomposition of thermal and non-thermal emission components from Galactic and extragalactic sources; and comparison of emission regions with other frequencies. The new map is available for download.

AN INTERACTION OF A MAGELLANIC LEADING ARM HIGH-VELOCITY CLOUD WITH THE MILKY WAY DISK

The Leading Arm of the Magellanic system is a tidally formed H I feature extending ~60° from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (H I) data from the Galactic All-Sky Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic plane at a Galactic radius of -->R ≈ 17 kpc.