6 Radio-to-infrared source flux density comparison

The IR and the radio continuum images of the MCs are found to be closely correlated, in the sense of an overall coincidence of peaks of emission and a strong resemblance around giant star-formation regions such as 30 Doradus (Xu et al. 1992). Similar correlations are found in other disk galaxies (Wunderlich & Klein 1988 and references therein). Here, we will concentrate on the comparison between discrete source flux densities from these two wavelengths.

The comparison between the source radio flux density at the 4.75-GHz survey (4.85-GHz survey for the SMC) and the IR survey at 60 $\mu$m is shown in Figs. 3a and 3b for the LMC and SMC respectively. Here, three groupings of sources are clearly identified. A group of Hii regions, SNRs and background sources which are strong at both frequencies and with a good correlation of flux densities with a spread of $\sim$20% (after allowing for the radio flux density to be $\sim$1% of the IR due to the mean spectra between these frequencies). From the 53 previously studied Hii regions in the LMC, 49 IR sources were found to be associated with these sources. Also, from the 26 known SMC Hii regions, 23 IR sources have counterparts in radio surveys. A similar situation can be found for SNRs embedded in Hii regions.

  

Figure 3: The comparison of IR ($60~\mu \rm {m}$) and radio flux density at 4.75 GHz (4.85 GHz for the SMC) for different classes of sources towards a) the LMC and b) the SMC. Asterisks represent SNRs; filled squares - Hii Regions; and open circles - background sources. The dotted line represent the approximate threshold in radio survey. Sources below the dotted lines represents non-detections. The diagonal lines represent a ratio of $S_{60\mu\rm {m}}/S_{4.75\,\rm {GHz}}$=100

Two other groups representing strong sources at IR or radio frequencies, but which are not detected at radio and IR frequencies, are also present. These sources fall below the sensitivity limits of the catalogues.

Most of the sources that fall below the IR cut-off are "uncontaminated'' radio SNRs that are known to be low in IR emission. This type of graphical representation serves well to identify these objects. The ratio of IR-quiet to IR-loud SNRs is 21 to 37 ($\sim$36%) for the LMC and 11 to 9 ($\sim$55%) for the SMC. A small number of Hii regions were also not found, probably because of confusion within the IR surveys.

Several radio-loud objects are plotted as lower limits, as exact flux densities at 4.75 GHz (4.85 GHz for the SMC) are not known (Sect 3.1). In addition, there are many IR detections ($\sim$1000 for the LMC) for which there is no confidently ascribed radio counterpart. We treat these objects less seriously as many of these detections may be spurious (as noted above) and they are not plotted in Figs. 3a and 3b.