next previous
Up: The Small Magellanic Cloud infrared


Subsections

5 The 12 $\mu $m IRAS map

5.1 General remarks

As was stated in SI89 already, a reliable source detection is most difficult in the case of the 12 $\mu $m IRAS data due to the noisy and irregular background structure. For our HIIphot analysis, we therefore chose S $/N \ge 6.0$. The PSF FWHM was assumed to be 4.0 pixels at a pixel scale of 15''/4.36 pc (at a distance of 60 kpc). The maximum extension of the sources was limited to 50 pc. The projected distance for the background estimation was limited to 25 pc.

The cutoff in surface brightness was adjusted to 1.4 MJy/sr which caused the program to limit the source growth to the visually determined size. The MIR emission in Fig. 5 (upper plot) is dominated by the presence of numerous discrete sources while nearly no diffuse emission is detected. The discrete sources are mainly located in two different regions: most of them cluster in a ring-like structure in the SW part of the SMC, and some of them are located in the NE part of the SMC bar. Most detections are found in the central IRAS field 0 (upper plot in Fig. 5). To summarize, at 12 $\mu $m the SMC appears as a patchy pattern of single discrete sources which are not a tracer of the morphology of this galaxy.

5.2 Source catalog and classification

73 sources were found and assigned flux densities up to $\approx $7 Jy by the HIIphot program with the vast majority of the 12 $\mu $m sources exhibiting flux densities below 1 Jy (as seen in Appendix A). The five sources labelled 23, 44, 50, 62, and 70 in Fig. 5 were assigned a growth luminosity of 1.1 Jy, 1.1 Jy, 1.5 Jy, 0.9 Jy, and 1.7 Jy, respectively. These sources were selected for labelling this and the following IRAS and ISO figures since they could be identified by the HIIphot program in all five SMC maps, thereby serving as "landmarks''. Noticeably, the two brightest 12 $\mu $m sources with luminosities of 7.1 Jy and 2.7 Jy (#25 and #57), respectively, are identified in the 25 $\mu $m data only.

As given in Table 4, 25 of our 73 sources listed in Appendix A were identified as IRAS PSC or FSC sources using the SIMBAD database within a correlation radius of 60'', 14 were classified as HII regions already in former studies. Finally, 38 sources fit into the category "emission line star/emission object''. The SIMBAD results for the identification of carbon stars are omitted in Appendix A and in Table 4, since nearly all of our sources (65) were associated to such a star within the correlation radius. 18 sources could be classified as type "C'' (cold) according to the classification criteria for the 170 $\mu $m catalog given above. 14 of the sources show a 170 $\mu $m/100 $\mu $m flux ratio greater than 1.0. Of the sources which were not found in the 170 $\mu $m ISO map but could be classified using the 60 $\mu $m and 100 $\mu $m data, 2 have flux ratios $f_{\rm 100~\mu
m}/f_{\rm 60~\mu m}\ge 1.0$, other two ones yield $f_{\rm 100~\mu
m}/f_{\rm 60~\mu m}< 1.0$.


next previous
Up: The Small Magellanic Cloud infrared

Copyright ESO 2003