1 Radio-continuum surveys of the Magellanic Clouds

During the last few decades, the Magellanic Clouds (MCs) have been investigated over almost all of the electromagnetic spectrum. Radio-continuum investigations of the MCs started with the detection by Mills & Little (1953). Since that time, numerous radio surveys have been undertaken. These include: Mills (1955) at 85.5 MHz; Mills & Little (1959) at 96.8 MHz and 158 MHz; Shain (1959) at 19.7 MHz; Mathewson & Healey (1964a) at 1400 MHz; Alvarez et al. (1987) at 45 MHz; and Mountfort et al. (1987) at 2300 MHz. The low-resolution observations helped to determine the large-scale structure and the total radio spectrum of the MCs (Mills & Little 1959; Haynes et al. 1991; Klein et al. 1991) whereas high-resolution studies concentrated on discrete sources in the Clouds.

1.1 High-frequency radio continuum surveys of discrete sources in the Magellanic Clouds

The first high-frequency radio-continuum detection of discrete sources in the MCs was made by Mathewson & Healey (1964b) at 1.4 GHz. In the past 30 years, numerous radio observations of many discrete sources in the MCs have been undertaken.

In 1972, McGee et al. (1972a) made detailed observations of the LMC at 5.00 GHz with the Parkes radio telescope (See Table 1 for a summary of the radio-source surveys of the LMC) and established the first catalogue of discrete radio sources in the LMC, known as the MC catalogue. Later, McGee et al. (1976) presented a list of sources towards the Small Magellanic Cloud (SMC) at 5.00 GHz (See Table 2 for a summary of the radio-source surveys of the SMC). McGee et al. (1972b) and Bolton & Butler (1975) observed the MCs at 2.70 GHz which became part of the PKSCAT-90 catalogue (Otrupcek & Wright 1991).

After Mills & Aller 1971 detected a few sources at 0.408 GHz, as a part of a whole sky survey, Clarke et al. (1976) observed the MCs with the Molonglo radio telescope and produced the MC4 catalogue of 227 sources towards the LMC and 75 towards the SMC. Also, some sources in the direction of the MCs (mainly background) appear in the MRC catalogue of Large et al. (1981).

  

Table 1: Summary of the previous high-frequency radio surveys of the LMC

  

Table 2: Summary of the previous high-frequency radio surveys of the SMC

McGee et al. (1976) and McGee et al. (1978) investigated the MCs with the Parkes telescope at 8.80 GHz. The PKSCAT-90 compilation (Otrupcek & Wright 1991) contains a number of MC sources with fluxes at 1.40, 2.70, 5.00 and 8.40 GHz.

New techniques and equipment evolved in the early 1980s, and with improved sensitivity, increased the interest in discrete sources in the MCs, such as Hii regions and supernova remnants (SNRs). Milne et al. (1980) observed a number of interesting LMC sources at 14.70 GHz to estimate their radio spectral index. Mills (1983), Mills et al. (1984a,b), Ye (1988) and Ye & Turtle (1993) observed selected fields in the MCs at 0.843 GHz with the Molonglo Synthesis Telescope (MOST), and Sabalisck & Abraham (1991) observed the 30 Doradus region at 22 GHz. Recently, Wright et al. (1994) surveyed the MCs as part of the PMN southern sky survey at 4.85 GHz.

Following the first radio classification of sources in the LMC based on studies of the spectral index (McGee & Newton 1972), Milne et al. (1980) and Mathewson et al. (1983,a, 1984, 1985) provided the first detailed analysis of the spectra of some 40 sources, most of which were SNRs. The radio spectral index study of the SMC sources was undertaken by Loiseau et al. (1987) for some 28 sources.

1.2 Other Surveys of discrete sources in the Magellanic Clouds

Numerous observations of the MCs in the optical, H$\alpha$, infrared (IR), far- infrared (FIR), and X-ray wavelengths have been undertaken together with the multi-waveband cross-correlation of the discrete sources.

Optical observations of the MCs made by Henize (1956) resulted in the first catalogue of emission-line objects. Hodge & Wright (1967) presented an optical atlas of the LMC and, 10 years later, of the SMC (Hodge & Wright 1977). Photographic H$\alpha$emission-line surveys by Henize (1956; N catalogue) and by Davies et al. (1976; DEM catalogue) have provided identifications, positions and sizes for several hundred sources in the two galaxies, ranging in scale from unresolved compact emission regions to the giant 30 Doradus complex in the LMC. Subsequent photometric surveys have provided quantitative information on the resolved MCs sources (Caplan & Deharveng 1985; Kennicutt & Hodge 1986; Bothun & Thompson 1988; Copetti & Dottori 1989; le Coarer et al. 1993; Caplan et al. 1996). Also some [SII] and [OIII] line surveys for detection of specific sources in the MCs, such as SNRs, were made by Mathewson et al. (1983, 1984, 1985). A number of MC objects appear in Sinnott's (1988) NGC (IC) catalogue of nebulae.

Lucke & Hodge (1970) published a list of 122 stellar associations in the LMC and Hodge (1985) a list of 70 stellar associations in the SMC. Up to 60 star clusters have been identified in the SMC and some 1600 in the LMC. Sanduleak et al. (1978) and Sanduleak (1984) published a list of planetary nebulae (PN), and Morgan (1994, 1995) updated these lists with 265 and 62 PN in the LMC and SMC, respectively.

The X-ray surveys of the LMC have been presented in Long et al. (1981; LHG catalogue) and in Wang et al. (1991; W catalogue). The SMC X-ray surveys can be found in Seward & Mitchell (1981; 1E catalogue), Inoue et al. (1983; IKT Catalogue), Bruhweiler et al. (1987) and in Wang & Wu (1992). All these surveys are based on observations made with the Einstein satellite. Almost half of the X-ray sources in the LMC field catalogued by Wang et al. (1991) were confirmed as SNRs, Hii regions or X-ray binaries. The other half are foreground stars or background objects (e.g. clusters of galaxies, radio galaxies, quasars). In the Wang & Wu (1992) survey, out of 70 sources towards the SMC, 24 are intrinsic. Trümper et al. (1991), Pietsch & Kahabka (1993), Snowden & Petre (1994), Kahabka & Pietsch (1993) and Schmidtke et al. (1994) also made X- ray observations of the MCs with the ROSAT satellite. In addition, X-ray ROSAT All Sky Survey (RASS) results are available from Pietsch et al. (in preparation) and Kahabka et al. (in preparation). The ASCA satellite X-ray observations of some LMC SNRs can be found in Hughes et al. (1995).

Vacuum ultraviolet (VUV) imagery of the LMC was obtained with an electrographic Schmidt camera during the Black Brant rocket mission in 1977 (Smith et al. 1987 and references therein) and the far-ultraviolet (FUV) survey with Apollo 16 mission (Page & Carruthers 1978). These surveys resulted in the discovery of a number of star-forming regions in the LMC. The SMC was surveyed with the ultraviolet imaging telescope (UIT) during the Astro-1 mission (December 1990) and the Astro-2 mission (March 1995) of space shuttle Endeavor (Cornett et al. 1994; 1997).

The IRAS catalogue of sources in the MCs at four infrared frequencies was published by Schwering (1989) and Schwering & Israel (1989, 1990). A complete CO millimetre-wave survey of the MCs was presented by Cohen et al. (1988), Israel et al. (1993) and Rubio et al. (1991, 1996). Following these millimetre- wave surveys, the first high-resolution SEST observations of some compact Hii regions in the MCs are presented in Rubio et al. (1993), Garay et al. (1993), Kutner et al. (1997), Chin et al. (1996, 1997) and in Hunt (in preparation).

1.3 New-generation surveys of discrete radio sources in the Magellanic Clouds

Parkes radio surveys by Haynes et al. (1991), Filipovic et al. (1995; hereafter Paper IV), Filipovic et al. (1996, hereafter Paper IVa) and Filipovic et al. (1997, hereafter Paper V) of the MCs are now available.

There are several other high-resolution radio surveys of the MCs in progress. Continuum surveys at 1.40 and 2.30 GHz of the MCs have been made with the Australia Telescope Compact Array (ATCA; L. Staveley-Smith et al. in preparation). The SMC HI observations with the ATCA have been published by Staveley-Smith et al. (1995, 1997) and some results of the LMC HI surveys can be found in Kim et al. (1997). Complete MOST surveys at 843 MHz of both Clouds are in preparation but some results were presented in Mills et al. (1984b), Ye (1988) and Ye & Turtle (1993).

In this paper we will present all sources found in the new Parkes radio surveys together with their fluxes and radio spectral indices. Also, we classify each source as either an Hii region, SNR or background object. In Sect. 2 we discuss briefly the Parkes radio surveys of discrete sources towards the MCs. In Sects. 3 and 4 we analyse, discuss and classify all discrete sources in the observed field of the MCs. We investigate the intrinsic source luminosity function in Sect. 5 and in Sect. 6 we discuss the source distribution throughout the MCs.