Abstract.We describe an aperture synthesis radio telescope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide fields. The telescope produces images of atomic hydrogen emission using the 21-cm H i spectral line, and, simultaneously, continuum emission in two bands centred at 1420 MHz and 408 MHz, including linearly polarized emission at 1420 MHz, with synthesized beams of 1 and 3.4 at the respective frequencies. A full synthesis can achieve a continuum sensitivity (rms) of 0.28 mJy/beam at 1420 MHz and 3.8 mJy/beam at 408 MHz, and the 256-channel H i spectrometer has an rms sensitivity of 3.5B −0.5 sin δ K per channel, for total spectrometer bandwidth B MHz and declination δ. The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide fields of view of 3.1• and 9.6 • (at the 10% level), at the two frequencies, and also allow data to be gathered on short baselines, yielding extremely good sensitivity to extended structure. Single-antenna data are also routinely incorporated into images to ensure complete coverage of emission on all angular scales down to the resolution limit. In this paper we describe the telescope and its receiver and correlator systems in detail, together with calibration and observing strategies that make this instrument an efficient survey machine.

The high-latitude supernova remnant (SNR) DA 530 (G93.3]6.9), apparently a typical shell remnant, has highly polarized radio continuum emission and a very uniform circumferential magnetic Ðeld. We present new radio continuum (408 and 1420 MHz) and H I line observations, made with the Dominion Radio Astrophysical Observatory Synthesis Telescope, and we have made the Ðrst detection of X-ray emission from the SNR, using the ROSAT Position-Sensitive Proportional Counter. The SNR lies within a shell of H I, possibly created by an earlier stellar wind, whose kinematic distance is nominally 2.5 kpc but whose actual distance may be larger. The X-ray emission is extremely faint. A Raymond-Smith ionization-equilibrium model Ðts the data and suggests a very low density, D0.05 cm~3, consistent with the occurrence of the supernova in a stellar wind cavity, but this model yields an explosion energy 100 times lower than the accepted value. A nonequilibrium shock model, incorporating a range of ionization timescales, is able to give more realistic physical parameters for the supernova remnant. On the balance of the evidence, we place DA 530 at a distance of 3.5 kpc, the largest distance permitted by the H I observations, where it lies 420 pc above the Galactic plane. The explosion, probably a Type Ia supernova, in a low-density cavity has resulted in weak X-ray emission and slow evolution. The explosion energy was 3.9 ] 1050 ergs and the age is D5000 years. The remnant, having swept up 3.9in an M _ ambient density of D0.01 cm~3, is only now in the adiabatic phase, and this explains the absence of detected optical emission. Despite the low ambient density the efficiency of generation of synchrotron radio emission is D0.4%, higher than in some historical SNRs. The ratio of radio to X-ray Ñux is about 100 times that for the remnant of SN 1006, which has comparable radio continuum properties. The very uniform magnetic Ðeld is not explained. DA 530 joins a small group of remnants at high Galactic latitude with unusual features, perhaps resulting from low ambient densities. Inhomogeneous nonequilibrium ionization models may be required for the interpretation of the X-ray emission from many other older SNRs.

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We present a new method based on H i column densities for determination of distances within the disk of the Galaxy. The technique is useful for all Galactic plane objects, including H ii regions and supernova remnants (SNRs), provided a line-of-sight velocity can be assigned to the object. Our method uses 21 cm spectral-line data to find the atomic hydrogen column density to an object, and beyond it to the Galactic edge. A model of the smooth large-scale Galactic distribution of H i material seen in emission (which principally traces the smooth structure of the Galaxy) is constructed. Our model accounts for scale-height flaring with increasing Galactocentric radius and includes the Galactic warp, which is prominent in the first and second quadrants of the Galaxy. The model's ability to trace the observed distribution of H i is demonstrated on lines of sight toward SNR DA 530 (l ¼ 93=3, b ¼ 7) and H ii region Sh 121 (l ¼ 90=2, b ¼ 1=7). We then apply the new technique to 29 Sharpless H ii regions with known photometric distances across the second quadrant. We measure line-of-sight velocities for the H ii regions from associated 12 CO emission, using 1 0 resolution 12 CO (J ¼ 1 0) data from the Canadian Galactic Plane Survey. Our distance method yields distances to these objects that are consistent with their photometric distances and which are markedly smaller than the kinematic distances found from a flat Galactic rotation curve.