3 Radio-to-infrared comparison of the sources towards the LMC

3.1 Source identification

The comparison of the radio and IR surveys resulted in the discovery of 130 sources common to both surveys. The basic criterion for positive source identification is that a source must lie within 2.5$^\prime$ of its counterpart. The most accurate radio positions available are from the highest radio-frequency survey on which the source appears. These positions were compared with IR positions from Schwering & Israel (1990).

Data for these 130 sources in common are presented in Table 1. Columns 2 and 3 give the radio source and IRAS source names respectively. Column 4 lists the source radio flux density at 4.75 GHz. For 12 sources the flux density at 4.75 GHz was estimated by interpolation from other radio frequencies; these sources are flagged in Col. 4.

  

Table 1: Catalogue of the LMC radio sources identified in the IRAS survey. The IR source number (Col. 3) is taken from Schwering & Israel (1990)

 

Table 1: continued

The IR data, that is the flux at 60 $\mu$m and the IR spectral index ($\alpha_{\rm {IR}}$), are listed for each source in Col. 5 and Col. 7 respectively. Estimates of the $\alpha_{\rm {IR}}$ of each IR source are based on flux densities listed in Schwering & Israel (1990). The IR spectral index is defined by the relationship ${S_{\nu}\sim \nu^{\alpha}}$, where ${S_{\nu}}$ is the flux density and $\rm {\nu}$ is the frequency. The integrated flux densities at the various IR frequencies were plotted as Log(${S_{\nu}}$) versus Log($\nu_{\mu \rm {m}}$) and (for most sources) straight lines were fitted with a simple linear regression to produce the best estimates of spectral index.

Column 6 lists the source radio spectral index ($\alpha_{\rm {RAD}}$)and error ($\Delta\alpha_{\rm {RAD}}$) defined as for the IR spectral index in the previous paragraph. Radio spectral indices are not given for 13 sources which were detected at only one radio frequency. For one IR source no IR spectral index is given since it was detected at only one IR frequency.

Column 8 of Table 1 gives the "radio source type''. Sources are denoted as BG (background sources), Hii regions and SNRs. Note that upper case letters (BG, SNR and Hii) are used for classifications from previous works and lower case (bg, hii and snr) for sources classified here. The question-mark indicates probable but not certain classification. The criteria by which we classify these sources are discussed in Sect. 5 and in Filipovic et al. (1998a; hereafter Paper VII). Other source names can be found in Papers IV, V and VI.

3.2 Positional differences between the LMC radio and infrared sources

Positional comparisons were undertaken for all 130 sources common to the radio and IR surveys. The results of this comparison are shown in Figs. 1a and 1b. For the 130 sources, the mean difference in RA is 16$^{\prime\prime}$$\pm$5$^{\prime\prime}$ (radio-IR) with standard deviation (SD) of 54$^{\prime\prime}$. The difference in Dec is 4$^{\prime\prime}$$\pm$5$^{\prime\prime}$ (SD=53$^{\prime\prime}$). This uncertainty is consistent with the combined positional uncertainties for the radio (defined in Paper IV) and the IR sources, and retrospectively justifies the initial identification criterion of 2.5$^\prime$ (which is equivalent to 2.8$\sigma$ in both RA and Dec). Figure 1b shows that there is an excess in the number of sources with small radio-IR differences, as expected, well above the number due to random coincidence.

  

Figure 1: a) The differences between radio and IR source positions for the LMC. The mean offset (radio-IR) is 16$^{\prime\prime}$$\pm$5$^{\prime\prime}$  and 4$^{\prime\prime}$$\pm$5$^{\prime\prime}$  in RA and Dec, respectively. The standard deviations are 54$^{\prime\prime}$  and 53$^{\prime\prime}$, respectively. The outer circle (selection criterion) is 2.5$^\prime$  radius and the inner dashed circle is 1$\hbox{$^\prime$}$  radius. b) The number of sources as a function of radio-IR source separation towards the LMC per unit area of the radial bin (per arcmin2)