Annu. Rev. Astron. Astrophys. 1991. 29: 499-541
Copyright © 1991 by Annual Reviews Inc. All rights reserved

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3.5 Targeted Surveys

Numerous surveys include objects selected according to criteria that do not conform to flux, size, or spatial completeness; their goals can vary greatly and a brief summary and categorization are difficult tasks. Below we discuss some examples of major surveys of such special objects.

BlNARIES AND GROUPS These systems are studied in detail for a variety of reasons: (a) because relative velocities of members are small, they provide the loci for the study of environmental effects of a milder nature than those observed in clusters of galaxies; (b) the distribution of mass can be probed beyond scales accessible by sampling rotation curves; and (c) the understanding of their dynamics can lead to insights in the frequency of mergers, the circumstances associated with starburst phenomena, and in other evolutionary concerns. The production of binary and group catalogs is a sort of cottage industry, initially carried out on the basis of angular separation and size criteria and, as more redshift data have become available, via the application of more sophisticated group-finding algorithms. Central contributions are the catalogs of binaries of Karachentsev (1983 and refs. therein) and Turner (1976). Humason et al (1956) and Sandage (1978) first identified groups in redshift catalogs, while variations on the ``friends- of-friends'' algorithm have been used by Press & Davis (1982), Huchra & Geller (1982), and Morgan & Hartwick (1988), and the dendogram method introduced by Materne (1978) has been most notably applied to generate a catalog of nearby groups [Tully (1987)]. Notable among the numerous analyses of binary and group properties are those of Peterson (1979), White et al (1983), Schneider et al (1986), Schweizer (1987 and refs. therein), Maia et al (1989), Ramella et al (1989), Soares (1989, based on a catalog developed by van Albada, cf. in Soares 1989), and Zepf & Koo (1989).

ISOLATED GALAXIES Isolated galaxies are thought to provide the most reliable statistical reference to study global properties of galaxy populations, unaffected as they might be by environmentally driven evolutionary processes. Several catalogs of isolated galaxies have been produced using varying criteria: the one most used is that of Karachentseva (1973). Haynes & Giovanelli (1984) and Davis & Seaquist (1983) have reported 21-cm results on subsamples of Karachentseva's catalog of, respectively, 324 and 113 galaxies.

DWARFS, LSB, AND EMISSION-LINE GALAXIES The determination of the faint end of the luminosity function, the understanding of galaxy formation, evolution processes and the effects of environment on the latter, and establishing whether segregation phenomena related to properties such as luminosity, surface brightness, or gas content are at work, have motivated many surveys of special objects. Surveys of low-surface-brightness dwarf systems have been carried out mainly via 21-cm line spectroscopy principally at Arccibo: Hoffman et al (1987) in the Virgo cluster region, Thuan and collaborators for an all-northern sky survey of UGC dwarfs (Thuan & Seitzer 1979a, b; Schneider et al 1990); Bothun et al (1985), Eder et al (1989), Salzer et al (1990), and Thuan et al (1987) for the investigation of surface brightness and morphological segregation with local galaxian density. A search for faint Local Supercluster members prompted a 6-m telescope survey of 92 blue galaxies by Karachentsev (1984) who, in addition to identifying 28 new supercluster members, also first suggested the existence of a 70 h-1 Mpc feature in the galaxian clustering spectrum. An exhaustive compilation of low surface brightness dwarf galaxies has been produced by Karachentseva & Sharina (1988).

Markarian and collaborators (1981 and refs. therein) at the Byurakan Observatory surveyed 15,000 deg2 of sky with objective-prism plates to obtain the best known sample of active galaxies. Efforts towards obtaining redshifts for this sample have been ongoing at the 6-m telescope of the Soviet Academy of Sciences (Markarian et al 1988a, b and refs. therein). Stepanian (1985) has reported on the Second Byurakan survey and on the space distribution of galaxies associated with this effort. Salzer et al (1989) have similarly reported reshifts for the University of Michigan objective-prism survey. Searches for emission-line galaxies that might populate low density regions in the galaxian distribution have been carried out by Tifft et al (1986), Moody et al (1987), and Weistrop & Downes (1988). Salzer (1989) has analyzed more generally the relationship between large-scale structure and emission-line objects.

ZONE OF AVOIDANCE Galactic extinction exceeds half a magnitude for about one fifth of the extragalactic sky at optical wavelengths, where catalogs of galaxies become severely incomplete. The task of mapping the nearby universe in those regions thus relies on painstaking efforts of identification of highly absorbed images in red survey plates (Kraan-Korteweg 1990 and refs. therein) or on the application of a combination of radio and infrared techniques. The IRAS point source catalog has served as a source of extragalactic candidates in the zone of avoidance, with 21-cm line observations used to establish a redshift. In this mode, several surveys have helped reduce the margins of darkness of the zone of avoidance, e.g. Dow et al (1988), Pfleiderer et al (1981), Chamaraux et al (1990), and Lu et al (1990). Kerr & Henning (1987) have carried out blind 21-cm searches in the galactic plane. Complete coverage of the zone of avoidance at 21 cm has often been invoked as a desirable goal. However, the necessary investment of telescope time is exceptionally large, and blind radio searches out to redshifts of 0.03 can realistically be limited to only restricted patches of sky.

SUPERCLUSTERS AND VOIDS Several studies targeted selected areas for redshift survey work, aiming at the definition of three-dimensional features in the large-scale structure of the galaxian distribution. They have often been stimulated by features in the projected galaxy counts, suggestive of coherent density enhancements. Oort (1983) reviews the early work. Kraan-Korteweg (1986) gives a useful catalog of 2810 galaxies in the Local Supercluster. The Coma region continues to receive special attention (Gavazzi 1987, 1989; Gregory et al 1988; Tifft & Gregory 1988), as have regions around Hercules (Freudling et al 1988), Corona Borealis (Postman et al 1988), Hydra-Centaurus (da Costa et al 1986, 1987), Centaurus-Pavo (Fairall 1988), Horologium-Reticulum (Lucey et al 1983, Chincarini et al 1984), and Lynx-Ursa Major (Giovanelli & Haynes 1982, Focardi et al 1986). Particularly notable in terms of the sample size are the surveys of the Cancer region by Bicay & Giovanelli (1986a, b, 1987), which included a contribution of 644 redshifts, that of the Great Attractor (Dressler 1988, 1990), which included 1314 redshifts, and those ongoing at Nancay [Chamaraux (personal communication) is completing a survey of MCG late spiral galaxies between 0° > delta > - 18°, while Fouque (personal communication) is targeting a similar sample of ESO galaxies south of delta = - 18°]. Haynes & Giovanelli (1990a) have presented 300-foot observations of 304 galaxies, mainly north of +38°, that complement their Arecibo supercluster survey work. Rood (1988) recently reviewed the efforts directed at the description of voids in the galaxian distribution. More recently, Pellegrini et al (1989) surveyed void regions in the southern galactic cap, and Burns et al (1988) and Willick et al (1990) studied voids in the Pisces region.

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