4 Radio-to-infrared comparison of the sources towards the SMC

4.1 Source identification

In the same manner as for the LMC, radio and IR surveys of the SMC have been compared, resulting in the identification of 38 sources common to both surveys. Each radio source and IR counterpart lie within 2.5$^\prime$ of each other. This criterion was chosen according to the upper limit of positional uncertainties in our 1.42-GHz survey (Paper V).

Data for all 38 sources in common are presented in Table 2. Entries of this table follow the format of Table 1 except for Col. 4 where the 4.85-GHz flux is listed rather than the 4.75-GHz flux. Radio spectral indices are not given for eight sources which were detected at only one radio frequency and for two sources for which radio data are only available at two close frequencies (4.75 and 4.85 GHz). These sources are flagged in Col. 4. No IR spectral index is given for four SMC sources, because of their detection at only one IR frequency.

  

Table 2: Catalogue of the SMC radio sources identified in the IRAS survey. The IR source number (Col. 3) is taken from Schwering & Israel (1990)

4.2 Positional differences between the SMC radio and infrared sources

Positions for all 38 SMC sources common to the radio and IR surveys have been compared in the same way as for the LMC. The results of this comparison are shown in Fig. 2. The mean differences are -7$^{\prime\prime}$$\pm$9$^{\prime\prime}$ (radio-IR) in RA with SD=55$^{\prime\prime}$ and -9$^{\prime\prime}$$\pm$11$^{\prime\prime}$ in Dec with SD=68$^{\prime\prime}$. This uncertainty is also consistent with the combined positional uncertainties for the radio (defined in Paper V) and the IR sources, and justifies the initial identification criterion of 2.5$^\prime$ (equivalent to 2.7$\sigma$ in RA and 2.2$\sigma$ in Dec). The distribution of radio-IR separations towards the SMC shows a significant excess for small separations like that seen for the LMC comparison (Fig. 1b), but with a smaller number of sources.

  

Figure 2: The differences between radio and IR source positions for the SMC. The mean offset (radio-IR) is -7$^{\prime\prime}$$\pm$9$^{\prime\prime}$  and -9$^{\prime\prime}$$\pm$11$^{\prime\prime}$  in RA and Dec, respectively. The standard deviations are 55$^{\prime\prime}$  and 68$^{\prime\prime}$, respectively. The outer circle (selection criterion) is 2.5$^\prime$  radius and the inner dashed circle is 1$\hbox{$^\prime$}$  radius