Introduction

The Magellanic Clouds have been studied in detail because they are close (50 - 75 kpc) and, unlike the Galactic Plane, there is little interstellar obscuration for much of the electromagnetic spectrum and most lines of sight intersect only one object. Consequently many southern surveys at radio wavelengths have included the Clouds. Though nearly all the detected sources are presumed to be background objects, each Cloud does contain a number of extended sources which are either supernova remnants (SNRs) or H II regions. Less luminous Cloud sources such as planetary nebulae and most pulsars are below the present limits of detection.

The history of star formation in the Clouds is known to be different from that in the Galaxy; in particular, far more SNRs (per unit mass) have been discovered in the Clouds. Observations which clearly separate SNRs from nearby H II regions and have sufficient sensitivity to reveal faint features will assist in the study of this anomaly. Similarly the comparison of detailed radio images of the H II regions with images made at other wavelengths may provide more accurate estimates of the recent rate of star formation.

The Molonglo Observatory Synthesis Telescope (MOST) is well suited for such observations. It operates at a frequency of 843 MHz with a resolution of about 45 arcsec and sensitivity of around 1 mJy (1 mJy = WHzm ). It has completed radio surveys of two areas, one of 36 square degrees containing the Small Magellanic Cloud (SMC) and the other of 65 square degrees containing the Large Magellanic Cloud (LMC). Here we report on the MOST survey of the SMC covering an area with a scalloped boundary (see Figure 1) which includes the region to in right ascension and to in declination (J2000). The survey area is similar to that used for continuum observations of the SMC by the Parkes 64 m telescope (Haynes et al. 1986, 1991). Various catalogues are in preparation.

Other radio surveys which include information on the discrete sources in the direction of the SMC are Clarke, Little and Mills (1976) at 408 MHz, McGee, Newton and Butler (1976) at 5 and 8.8 GHz, Mills et al. (1982) at 843 MHz and Loiseau et al. (1987) at 1.4 GHz. More recently, Filipović et al. (1997) have presented a catalogue of the flux densities of 224 sources at five frequencies between 1.42 and 8.55 GHz. The present survey has better sensitivity and angular resolution and the final image contains many more sources with over a thousand exceeding a flux density of 7 mJy at 843 MHz.

Only a few of these sources (63 H II regions, 15 supernova remnants (SNRs) and 3 new SNR candidates) are thought to lie within the SMC. These sources are discussed in detail in subsequent papers in this series (in preparation) and previously in Ye 1988 and Ye, Turtle & Kennicutt 1991. The remaining sources do not possess the radio morphologies or optical and x-ray counterparts that characterise typical H II regions or SNRs. A few may be members of an unrecognised class of object that occurs in galactic disks but sources such as pulsars or counterparts of SS433 would be less than 10 mJy at the distance of the SMC. The majority of the detected sources have a larger flux density and are likely to be background objects lying beyond the SMC. The number of these sources is not significantly in excess of those expected from a background population.

Though the sources in the SMC are our primary interest, the background sources do provide a valuable sample of distant extragalactic objects. At this frequency there is little general obscuration due to the SMC and so hardly any sources stronger than 10 mJy should have been missed because of this or overlap with the extended sources of the SMC (which occupy only 0.04 square degrees in total). A preliminary list of sources has already been used to subtract the smoothed background contribution of these sources from the large-scale non-thermal radio emission from the SMC (Ye & Turtle 1991). In addition, the spectra of the brightest background sources may be used in other investigations to study the interstellar medium of the SMC.

In Sections 2 and 3, we describe the observations and the data reduction and, in Section 4, we present the images of the survey.

© Copyright Astronomical Society of Australia 1997