2. OBSERVATIONS

While ISO was an observatory rather than a survey mission, many surveys were carried out using various capabilities of the versatile payload. Some prominent surveys that concern normal galaxies either directly or indirectly are listed below. This is by no means an exhaustive list, especially since most ISO data have yet to be published.

1. Mid-infrared maps of nearby galaxies were obtained under the ISO-CAM (Césarsky et al. 1996) guaranteed time (GT) program, targeting large angular-size galaxies in various categories, such as early-type, spirals barred and non-barred, dwarf irregulars, and active (Vigroux 1997). There were also surveys of galaxies in Virgo and other clusters. All galaxies were surveyed in the LW2 (6.75µm) and LW3 (15µm) filters, and some in other filters within the 3 to 18µm wavelength range of ISO-CAM. In addition, several were observed with the Circular Variable Filter (CVF), which yields images at a spectral resolution of about 20 over most of the same wavelength range. These data were taken with 3" pixels, with an effective resolution of 7 to 9" half-maximum width.

2. Far-infrared spectral surveys of a few samples of galaxies were carried out under the GT program of the ISO-LWS (Clegg et al. 1996), most notably for infrared-bright galaxies, meaning those with a flux density greater than 50Jy at 60µm, and of ultra-luminous galaxies (Fischer et al., 1999; Luhman et al. 1999). Most objects were observed with a LWS low-resolution full spectral scan covering 45 to 195µm.

3. Far-infrared maps of well-resolved nearby galaxies were obtained under the ISO-PHOT (Lemke et al. 1996) GT program at 60, 100 and 175µm, most notably of M31, M33 and M101.

4. Photometry at 60µm was also carried out under the ISO-PHOT GT program for several samples, including 75 bright (B < 12mag) galaxies from the Revised Shapley-Ames Catalog, and selected objects in the Virgo Cluster. These samples were observed (Joseph et al.) at 60, 100 and 175µm, as well as 12µm (ISO-CAM filter LW10), with additional data collected from the ground in the near-infrared and the submm.

5. Open time projects included several galaxy surveys, such as the Knapp et al. (1996) study of early type galaxies, the Lu et al. study of infrared-cold galaxies, and the Metcalfe et al. (1996) BCD/Irr survey.

6. The ISO Key Project on the Interstellar Medium of Normal Galaxies (Helou et al. 1996) under NASA GT collected data on a set of sixty galaxies that explore the full range of morphology, luminosity, infrared-to-blue ratio and far-infrared color among star-forming galaxies. These sixty objects were selected to be small in their IRAS emission size compared to the 80" LWS beam and the 3' ISO-CAM field of view, so as to allow studies of their global properties. In addition, nine nearby galaxies were mapped to the extent possible, including NGC 6946, NGC 1313, IC 10, and parts of M101. For most galaxies, maps were obtained at 7 and 15µm with ISO-CAM, spectro-photometry was obtained with ISO-PHOT-S between 3 and 12µm, and far-infrared fine-structure lines were targeted with ISO-LWS, attempting to measure as many as possible of the following lines, in the order listed: [CII] 157.7 µm, [OI] 63.2 µm, [NII] 121.9 µm, [OIII] 88.4 µm, [NIII] 57.3 µm, [OIII] 51.8 µm.

7. The ISO-PHOT Serendipity Survey collected data during satellite slews between target observations with the 170µm channel. By the end of the mission, data had been collected over 150,000° of slew track, with an estimated 4,000 galaxies detected (Stickel et al. 1998). This data set will be a unique source of far-infrared fluxes for thousands of galaxies with IRAS detections at f 1.5Jy.

8. By its nature as an observatory-class mission, ISO has generated a rich archive containing all the observations of individual galaxies, groups, or clusters of galaxies investigated by various observers for specific interests. This collection constitutes a de-facto survey of unique or peculiar objects from which one could learn much about the less exotic cases (e.g. Smith 1998; Smith & Madden 1997; Lu et al. 1996; Jarrett et al. 1999; Valentijn et al. 1996; Xu et al. 1999). Many useful survey samples could be constructed after the fact by selecting objects out of the ISO archive once it becomes available in the summer of 1999.

9. Ground-based infrared surveys: There is no question that the 2MASS and DENIS surveys will be making fundamental contributions to our view of normal galaxies, though their results are not reviewed here. Apart from these, several near-infrared imaging surveys of nearby galaxies are already revealing some surprising results. Grauer & Rieke (1998) for instance demonstrate that spiral arms are almost as contrasted in the K band as they are in the B band. See also Terndrup et al. (1994).