4 Source classification

Before we go in detail through the maps of each IR band (Sects. 5 to 9), we consider the source classification by colour temperature and other quantities, enabling us to list also the source class for most objects in the catalogs. Our classification scheme relies on the ISO 170 $\mu$m data as a primary indicator and is then extended to the other four IRAS catalogs as far as possible. The full cross-correlated tables are given in Appendices A-E, for illustration purposes, an excerpt of the ISO 170 $\mu$m catalog is presented in Table 4. Only 170 $\mu$m sources with flux densities $F_{\nu}>2.0$ Jy are further analysed and classified, which applies to 155 sources (in contrast to 243 sources in total). Our classification symbols always appear as the first letter in the "identifications'' column.

The general classification scheme is the following: a source is labelled "C'' (cold) when the color temperatures (assuming a modified black body with an emissivity of $\beta =2$) derived from both, the 170 $\mu$m/100 $\mu$m and the 100 $\mu$m/60 $\mu$m ratios, are $T_{\rm BB}<30~{\rm K}$, it is classified "W'' (warm) when $T_{\rm BB}>30~{\rm K}$ is obtained in both cases. "(C)'' denotes sources where a classification as "C'' remains uncertain due to missing 60 $\mu$m and/or 100 $\mu$m data points. A source is labelled "Q'' (questionable) when the precise shape of the SED cannot be derived due to source confusion. There are a few cases where the SED seems to "drop'' between 60 $\mu$m and 170 $\mu$m due to a lower 100 $\mu$m flux density value. This behaviour has to be attributed to different PSF sizes in the different wavelength bands, leading to correlations of physically independent sources in the three wavelength bands.

The first letter of the identification label is followed by a down- or uparrow ( $\Downarrow$ or $\Uparrow$) in those cases where F₁₇₀/F₁₀₀-ratios <1 and >1, respectively. Identical 170 $\mu$m and 100 $\mu$m flux densities are indicated by a =. Note that for a "cold'' BB-spectrum, i.e., $T_{\rm BB}<30~{\rm K}$, the F₁₇₀/F₁₀₀-ratio needs not necessarily exceed 1.0.

A cross-identification with the source catalogs given by SI89 leads to the next classification letter, which may be "(C)'' or "(W)'', hinting at their cold and warm sources. Note that this type of coding could in principle lead to an ambiguity in the meaning of the single-letter label "(C)'': this could either mean that a SI89 identification is referred to without any identification in our catalog or be the label for a probably cold source which is only found in our source study. In fact, all sources classified as "(C)'' are assigned this source type in our catalog only, so this ambiguity is avoided.

 

 

Table 3: This table is an excerpt from the final ISO 170 $\mu$m catalog as it is presented in Appendix E. The 170 $\mu$m source list (the master catalog, entry numbers n₅) was correlated with the four IRAS catalogs according to the source center coordinates (epoch 2000) in the 170 $\mu$m ISO catalog given in columns $\alpha$ and $\delta$. If the distance between an IRAS source from any of the four catalogs and a source from the ISO list was found to be less than 90'', the two were considered identical. F₅ and $\Delta F_{5}$ denote the 170 $\mu$m flux and the 1$\sigma$ error derived from the HIIphot algorithm. n₄, n₃, n₂, and n₁indicate the corresponding entry numbers in the 100 $\mu$m, 60 $\mu$m, 25 $\mu$m, and the 12 $\mu$m catalog, respectively. More than one number in these columns indicates the presence of two or more sources within the correlation radius of 90''. For the meaning of the letters and acronyms in the last column ("identifications'') see text.

ISO 170  $\mu\rm m$ source catalog
n5	$\alpha~~\rm [h~~ m~~ s]$	$\delta~~ [{^\circ}~~ {'}~~ {''}]$	F5	$\Delta F_{5}$	n4	n3	n2	n1	identifications
1	00 41 40	-73 02 16	2.2	0.4	40 42	20 24	9		C IRAS 00395-7317
2	00 42 06	-73 08 34	0.3	0.0
3	00 42 06	-72 59 58	4.1	0.4	50	32	14		C$\Uparrow$(C) IRAS 00403-7316 HII DEM S6
4	00 42 58	-72 59 36	5.5	0.5	58	39	17		Q(C) IRAS 00413-7316
5	00 43 25	-73 02 31	3.2	0.4	59	41	19		Q(W)
6	00 44 37	-72 58 06	0.7	0.3	71		20
7	00 45 18	-73 05 23	59.8	1.6	76	56	24	15	C$\Uparrow$IRAS 00436-7321 Em*(AzV7)
8	00 45 21	-73 16 16	228.8	2.7	74	54	22	16	C$\Uparrow$ HII DEM S14 Em*(LIN72/MA93 96)
9	00 45 28	-73 22 07	259.5	0.0	75	55	23	17	C$\Uparrow$(C) IRAS 00435-7339 EmO(LHA 115-N 13/A/B)
10	00 46 32	-73 05 59	105.8	1.8	82	64	29	20	C $\Downarrow$(C) IRAS 00447-7332 HII DEM S23 Em*(AM77 9)
11	00 46 34	-73 21 23	158.0	2.4	79		28	18	C$\Uparrow$(C) HII DEM S24
12	00 46 35	-73 15 28	84.6	1.7	81	65			C$\Uparrow$ Em*(LIN79)
13	00 46 45	-73 30 51	46.1	0.0	84	68	30		C $\Downarrow$(C) IRAS 00449-7347
14	00 46 46	-73 09 37	93.9	1.7	83				(C)$\Uparrow$ Em*(LIN84/MA93 143)
15	00 47 37	-73 05 26	329.3	3.3	89				C$\Uparrow$
16	00 47 52	-73 15 16	453.4	3.6	91	79	34	23	C$\Uparrow$(C) IRAS 00462-7331 EmO(LHA 115-N 20)
17	00 47 55	-73 35 21	1.7	0.0	90	81
18	00 47 56	-73 23 47	13.1	0.9		75 84			(C)(W) IRAS 00462-7339
19	00 47 58	-72 23 13	4.3	0.4	92	83			C$\Uparrow$
20	00 48 13	-73 08 01	186.2	2.4		77	32		C(C) X(HFP2/409)
21	00 48 34	-72 57 51	6.6	0.8	98	89	38		Q(W) IRAS 00467-7314
22	00 48 37	-72 26 54	0.8	0.3	97	88
23	00 48 39	-73 19 52	32.6	1.3	96				C$\Uparrow$
24	00 48 52	-72 48 38	65.2	1.8	99	93			C$\Uparrow$
25	00 48 52	-73 08 33	269.4	3.0	100	94		26	C$\Uparrow$(C) Em*(MA93 265/276)
26	00 48 55	-72 54 54	12.0	1.0	101	96	40		Q Em*(MA93 262)
27	00 49 23	-73 34 27	2.6	0.5	103	97			C$\Uparrow$(W)
28	00 49 27	-73 26 49	34.3	1.2	110	101	43		C$\Uparrow$(C) IRAS 00483-7250 Em*(LHA 115-N 33)EmO(MA93 301)
29	00 49 42	-73 23 45	9.3	0.8			46		(C) X(HFP2/468) Em*(LIN 139)
30	00 49 58	-73 12 00	26.5	1.2		106			(C) X(SHP2/SMC30) Em*(MA93 335)
...	...	...	...	...	...	...	...	...	...
...	...	...	...	...	...	...	...	...	...

Similar classification schemes apply to the four IRAS catalogs. In the 100 $\mu$m case, all available identifications were copied from the 170 $\mu$m catalog first. Other sources not found in the 170 $\mu$m list were then classified according to their 100 $\mu$m/60 $\mu$m ratio, but down to a flux limit of 1.0 Jy only (which applies to the 60 $\mu$m catalog as well). "$\uparrow$'', " $\downarrow$'', and "$\odot$'' indicate 100 $\mu$m/60 $\mu$m flux ratios >1, <1, and $\approx$1, respectively. For all following catalogs, identifications found for catalog entries at larger wavelengths were copied if and only if the referring source was directly identified in that catalog. When a source detected at 60 $\mu$m is identified with a source at 100 $\mu$m, the 170 $\mu$m counterpart identifications of the 100 $\mu$m source will be accepted for the 60 $\mu$m catalog only if the 60 $\mu$m source is directly correlated and therefore listed in the 170 $\mu$m catalog as well.

Our sources detected by HIIphot in all five bands were cross-correlated with the SIMBAD database in order to identify well-known HII regions, X-ray sources, etc. The SIMBAD database was searched using a correlation radius of 60'' for all catalogs (which is different from the coordinate correlation radius of 90'' used in our IR study). From a large list of possible identifications the most important catalogs were included:

• IRAS sources from the point source catalog (PSC) or the Faint Source Catalog (FSC) are referred to by the label IRAS and the corresponding catalog entry number (e.g., IRAS 00403-7316);
• HII region identifications are taken from the sample of Davies et al. (1976) and labelled HII DEM with the identification number from that catalog, e.g., HII DEM SS161n;
• Cross-identifications with several catalogs of emission-line stars are marked Em*. The following catalogs may occur: LIN or MA93 for a source from the catalog of emission-line stars and small nebulae in the SMC by Meyssonnier & Azzopardi (1993), AM77 for a catalog entry from the catalog of supergiant stars by Ardeberg & Maurice (1977), LHA for the catalog of H$_{\alpha}$ emission stars and nebulae in the MCs by Henize (1956), AzV for entries from the catalog of SMC star members by Azzopardi & Vigneau (1982);
• If one of the above catalogs classifies the source not as emission-line star, but as emission object, it is labelled EmO with the same catalog acronyms;
• X-ray identifications are labelled "X'' with the referring catalog identifiers in brackets: ROSAT sources are labelled "RX'', e.g., RX J0059.3-7223, other labels may be used for entries from the ROSAT HRI catalogue of X-ray sources in the SMC region by Sasaki et al. (2000, hereafter SHP), the PSPC catalogue of X-ray sources in the SMC region by Haberl et al. (2000, HFP), papers by Gotthelf et al. (1996, GHH), Inoue et al. (1983, IKT), Yokogawa et al. (2000, YIT), and Bruhweiler et al. (1987, BKGS).

In the following sections, we will discuss the general morphological aspects of the four IRAS and the ISO map, the spatial source distribution, and the corresponding source catalogs generated by HIIphot. The cross-correlation of the five catalogs and the comparison to older existing IRAS studies of the SMC are addressed in two further sections.

Figure 5: 12 $\mu$m IRAS HiRes data of the five examined SMC fields. Contours are plotted at the following flux levels: 2, 8, 16, 20, 28 MJy/sr (solid lines), 3, 10, 20, 35 MJy/sr (dotted lines), and 5, 12, 24, 40 MJy/sr (dashed lines). Upper contour plot: the central SMC field. Lower plots, from left to right: adjacent SMC fields with numbers 1, 3, 5, and 7 in eastern, northern, western, and southern direction. Sources detected by the HIIphot algorithm are marked with a cross (+). Labelling numbers refer to the catalog entries in Appendix A.

The question of the underlying physics in the SMC interstellar medium and the nature of single sources will be extensively discussed in Paper II where we will also put our SMC results in the context of already existing complementary observations and catalogs, e.g., the FUV spectroscopic explorer survey of interstellar molecular hydrogen in the SMC by Tumlinson et al. (2002), HI data as analysed in Stanimirovic et al. (2000), or CO data (see, e.g., the results from the ESO-SEST key programme in the SMC given in Israel et al. 1993 and subsequent papers).