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4.1. ISO Surveys of Galaxies

While ISO was an observatory rather than a survey mission, many surveys were carried out using various capabilities of its 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 (C. 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, Coma and other clusters. All galaxies were surveyed in the LW2 (6.75 µm) and LW3 (15 µm) filters, and some in other filters within the 3 to 18 µm 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 mostly with 3" pixels, with an effective resolution of 7 to 9" half-maximum width.

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

  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 175 µm, most notably of M31, M33 and M 101.

  4. Photometry at lambda geq 60 µm was also carried out under the ISO-PHOT GT program for several samples, including 75 bright (B < 12 mag) galaxies from the Revised Shapley-Ames Catalog, and selected objects in the Virgo Cluster. These samples were observed (PI R. Joseph) at 60, 100 and 175 µm, as well as 12 µm (ISO-CAM filter LW10), with additional data collected from the ground in the near-infrared and the submm. The purpose was to extend the spectral energy distribution, look for cold dust, and investigate nuclear activity.

  5. The surveys above were coordinated by J. Lequeux, resulting in several coherent data sets of great interest to normal galaxy studies. The imaging with CAM at 6.75 and 15 µm, sampled galaxies at different distances, from the Magellanic Clouds out to the Virgo Cluster, under programs ``CAMSPIR'' and ``VIRGO''; program ``CAMSFR'' imaged star-forming regions at additional wavelengths, and with the CAM-CVF mode. In Virgo, at least thirty galaxies were imaged with CAM, and measured with PHOT at lambda geq 60 µm, while their [CII] lambda 157.7 µm line was targeted with LWS (PI K. Leech).

  6. 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.

  7. 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 M 101. For most galaxies, maps were obtained at 7 and 15 µm with ISO-CAM, spectro-photometry was obtained with ISO-PHOT-S between 3 and 12 µm, 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] lambda 157.7 µm, [OI] lambda 63.2 µm, [NII] lambda 121.9 µm, [OIII] lambda 88.4 µm, [NIII] lambda 57.3 µm, [OIII] lambda 51.8 µm, [NII] lambda 145 µm.

  8. The ISO-PHOT Serendipity Survey gathered data during satellite slews between target observations with the 170 µm 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 fnu (100 µm) gtapprox 2 Jy.

  9. 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 questions. 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 can also be constructed after the fact by selecting objects out of the ISO archive once it becomes available in the summer of 1999.

Though not addressed directly in this review, ground-based infrared surveys will make fundamental contributions to our view of normal galaxies, especially the Two-Micron All-Sky Survey (2MASS, Skrutskie et al. 1999) and Deep European Near-Infrared Survey (DENIS, Epchtein et al. 1999), Apart from these large systematic surveys, 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).

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