6 The 25 $\mu$m IRAS map

6.1 General remarks

For the 25 $\mu$m data, identical parameter settings as in the 12 $\mu$m case were used, with the exception of the final S/N value for the source detection ($\ge$5.5). Especially the final surface brightness cutoff (1.4 MJy/sr) and the maximum size of the sources (50 pc) remained unchanged. At this longer wavelength, more sources emanate from the background, especially in the SW part of the SMC (Fig. 6). The ring-like structure, already recognizable at 12 $\mu$m, shows up as a plateau on a comparably high flux level. However, the underlying shape of the SMC still remains invisible in the central field (upper plot in Fig. 6), since there are no sources along the main body and in the outer edges of the SMC. In contrast to the 12 $\mu$m case, extended sources start to show up in the so-called "bridge'' region in the eastern part of the SMC (lower row in Fig. 6, left plot).

Figure 6: 25 $\mu$m IRAS HiRes data of the five examined SMC fields. Contours are plotted at the following flux levels: 1.8, 8, 16, 20, 28, 60, 120, 180 MJy/sr (solid lines), 3, 10, 20, 35, 80, 140, 200 MJy/sr (dotted lines), and 5, 12, 24, 40, 100, 160 MJy/sr (dashed lines). Upper contour plot: the central SMC field. Lower row, from left to right: adjacent SMC fields with numbers 1, 3, 5, and 7 in eastern, northern, western, and southern direction. As in Fig. 5, sources detected by HIIphot are marked with a cross (+). Labelling numbers refer to the source entries in Appendix B.

6.2 Source catalog and classification

In total, 135 sources are detected (Appendix B) of which 60 are classified as IRAS PSC or FSC sources. 31 sources are identified as HII regions according to SIMBAD, 79 of the detections fall into the "emission line star/emission object'' category. Using the 170 $\mu$m data which is essential for determining low temperatures below $\approx$30 K, 41 have FIR SEDs which are fitted best using a blackbody with temperatures $\le$30 K ("C''), 24 of which have $f_{\rm 170~\mu m}/f_{\rm 100~\mu m}\ge 1.0$, 12 fall within the range $f_{\rm 170~\mu m}/f_{\rm 100~\mu m}< 1.0$. Six are associated with warm ("W'') sources. 9 sources remain unclassified due to source confusion among the three FIR bands. 16 sources which are not detected in the 170 $\mu$m band have $f_{\rm 100}/f_{\rm 60}\ge 1.0$, 4 have $f_{\rm 100}/f_{\rm 60}< 1.0$. From this and Table 4 is is clearly visible that, compared to the 12 $\mu$m case, the number of cold sources as well as the number of sources with $f_{\rm 170~\mu m}/f_{\rm 100~\mu m}\ge 1.0$ have considerably increased.

The source counts resemble numbers and flux distribution of the 12 $\mu$m map: as can be seen from Fig. 13, the bulk of the flux densities lies in the interval $0.05~{\rm Jy}\ldots 1.0~{\rm Jy}$, although there is now a larger percentage of sources with even higher fluxes (17% of the 135 sources). Our five marked sources - again among the brightest ones - in Fig. 6 with numbers 34, 80, 87, 114, 127 were assigned fluxes of 10.0 Jy, 29.0 Jy, 12.5 Jy, 6.5 Jy, and 19.2 Jy. Four cases are found where two 100 $\mu$m sources lie within the correlation radius of 90'' around a 25 $\mu$m detection.