next previous
Up: A catalogue

4. Identifications

4.1. Comparison with radio observations

We compared our data with several single dish surveys of the LMC, the Molonglo observations at 408 MHz (Clarke et al. 1976) and the Parkes observations at 5 GHz (McGee et al. 1972a) and 4.75 GHz (Filipovic et al. 1995).

We identify sources of the ATCA catalogue with objects listed in the Molonglo Radio Source Catalogue 4, if the difference in position is smaller than the HPBW of the Molonglo telescope (tex2html_wrap_inline2429). Nearly all sources above 50 mJy can be identified with an MC4 object (see Table 3 (click here) Col. 9). To compare the peak flux densities of both surveys, we estimated the corresponding flux density of the MC4 source at 1.4 GHz by using our spectral indices between 1.4 and 2.4 GHz (Table 4 (click here) Col. 9). There are 21 usable objects for this comparison. Most of the estimated flux densities show a much higher value compared with the measured fluxes of the ATCA sources. This discrepancy may be accounted for solely by flux loss in the Compact Array observations of extended sources as compared with the filled aperture telescope. Furthermore many compact extragalactic objects are known to be variable. However, there may also be a component of this discrepancy arising from bias in the quoted spectral indices because of the different amount of the flux loss at 1.4 and 2.4 GHz. Only a few sources show similar flux densities in both surveys. These objects (MDM 1, MDM 64, MDM 84, MDM 90, MDM 100) seem to be point sources. Due to the high resolution of the ATCA, the strong source MC4(0524-708) was separated into two components.

There are only 10 sources which can be identified with objects found by McGee et al. (1972a) at 6 cm (Table 3 (click here) Col. 9), if we demand that the position difference from the ATCA object to the one of the MC-catalogue is smaller than the HPBW of the Parkes telescope, which is about 4' at 6 cm. The flux densities of all sources observed with the single dish telescope are much higher than those of the compact ATCA sources.

We identified a compact source in the snapshot field of 3tex2html_wrap_inline24354tex2html_wrap_inline1227 with an object observed with Parkes at 4.75 GHz (Filipovic et al. 1995), if the difference in position is smaller than the HPBW of tex2html_wrap_inline2439 (Table 3 (click here) Col. 9 abbreviation LMC). For all 53 identified sources the flux density of the Parkes object is higher than the value of the ATCA source at 1.4 GHz. There is also a large position difference for most objects.

The poor correspondance between compact sources detected with the ATCA and the Parkes sources is common also in galactic plane surveys (Garwood et al. 1988), and arises from the fact that most of the continuum flux from a galaxy disk comes from structures much larger than a few parsecs, which is our effective resolution at the distance of the LMC.

From the comparison of our snapshot sources with radio objects from single dish observations, we conclude that most of our discrete objects are compact cores of extended sources or background objects.

4.2. Identification with tex2html_wrap_inline2441 emitting objects

Comparison of the radio point source positions with tex2html_wrap_inline2443 emitting objects helps us to find sources intrinsic to the LMC. Figure 3 (click here) shows the distribution of the compact radio sources on the Htex2html_wrap_inline1175 map of Kennicutt et al. (1995).

Figure 3: Distribution of the compact radio sources on the Htex2html_wrap_inline1175 image of the LMC. The size of the crosses is proportional to the peak flux density of the source

A compact radio source is regarded as being identified with an Htex2html_wrap_inline1175 object of the Davies-Elliot-Meaburn catalogue (DEM), if the radio source lies within the extent of the DEM object. We find 32 radio point sources associated with an tex2html_wrap_inline2443 emitting object. These might be compact HII regions or young SNRs. Such objects can show high brightness knots embedded in the extended DEM object. Htex2html_wrap_inline1175 observations at high angular resolution performed with the aim at finding out which of the radio sources correspond to an optical emission knot, and which are background sources behind the DEM object, will be presented in a subsequent paper (Marx et al., in preparation).

next previous
Up: A catalogue

Copyright by the European Southern Observatory (ESO)