1 Introduction

The Small Magellanic Cloud (SMC hereafter) is the smallest member of an interacting threesome of galaxies (Murai & Fujimoto 1980), the two other members of which are our Galaxy and the Large Magellanic Cloud (LMC hereafter). Due to its low mass, the SMC suffers most from the mutual disruption by gravitational forces, and as a consequence, the large scale morphology and structure exhibit distinct signs of close interactions, of which a more recent one (200 million years ago) is widely believed to be responsible for the present appearance of the SMC (Putman et al. 1998). The most prominent morphological features are the so-called bridge (connecting the SMC and the LMC), and the Magellanic stream which extends far beyond the SMC and the LMC covering roughly 100 square degrees in the sky (Westerlund 1997).

As the nearest dwarf irregular galaxies, the SMC and the LMC serve as excellent laboratories for studying their detailed compositions since all objects in them can be assigned the same distance (50...60 kpc, see Westerlund 1997). At the same time, radio and optical data can well be obtained down to parsec and sub-parsec scales.

Both galaxies have been found to be low in dust and heavy-element abundances in comparison to the solar neighbourhood, however, high star formation rates lead to significant infrared emission from warm and cool dust. Numerous studies focus on the structural peculiarities, e.g., the extremely rich and complex HI morphology with numerous apparently expanding shells, filaments, and arcs (for results see, e.g., Martin et al. 1989, and Staveley-Smith 1997, and references therein). The SMC also hosts several giant HII regions, the referring catalog of which was produced by Davies et al. (1976).

The first infrared study by Schwering & Israel 1989 (SI89 hereafter) was based on IRAS data. They found numerous new objects not listed in the IRAS published catalogs (PSC and SSS, see Beichmann et al. 1985; Helou & Walker 1985, and references therein) and compared the infrared results to catalogs obtained in different wavelength bands. The near-infrared (NIR) DENIS-based point source catalog (PSC) towards both Magellanic Clouds has recently been published by Cioni et al. (2000).

A comprehensive radio continuum study (e.g., Haynes et al. 1991; Filipovic et al. 1997, 1998a,b) addressed the question of the possible existence of magnetic fields in the SMC, the distribution of radio sources, and the comparison with other surveys (e.g., X-ray and IR data).

The LMC and SMC also became part of an ESO-SEST key programme which performed CO survey observations in both galaxies and focused on the molecular gas content and the physical properties of the molecular clouds (see, e.g., Israel et al. 1993; Rubio et al. 1993, and Lequeux et al. 1994). An X-ray source catalog can be found in Haberl et al. (2000). Comprehensive overviews of both Magellanic Clouds as well as a list of further references can be found in Westerlund (1990) and Westerlund (1997).

Since the IRAS mission the question has been raised how much (if any) cold dust exists in certain objects (e.g., star forming galaxies) which escapes an detection due to its low temperatures. The latter would cause the FIR SEDs to keep on rising significantly beyond the IRAS 100 $\mu$m band. In order to accurately determine the temperature of the cold dust in the SMC, we therefore made use of ISO's long wavelength capability and obtained a complete 170 $\mu$m map. This is the first of two papers in which the FIR properties of the SMC are discussed. In this paper, we put special emphasis on the overall SMC morphology at 170 $\mu$m, the statistical results of the ISO SMC observations, the comparison with the IRAS data, and former studies based on IRAS source catalogs. In a subsequent paper we will discuss the quantitative and detailed properties of the warm and colder dust, ranging from color temperatures and integrated FIR luminosities over dust properties of single sources to the properties of the interstellar radiation field and the star formation rates.