Annu. Rev. Astron. Astrophys. 1989. 27: 235-277
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11. cD AND BRIGHTEST CLUSTER GALAXIES

The most luminous galaxies in rich clusters are giant ellipticals. Some of these are classified as D (possessing a large, diffuse envelope) or cD (extralarge D) (Matthews et al. 1964, Morgan & Lesh 1965, Morgan et al. 1975, Albert et al. 1977). The distinction between cD and E galaxies is useful, but we argue below that the term "D galaxy" is not. When we wish to discuss brightest cluster members without regard to their morphology, we will refer to them as BCMs.

The importance of BCMs is twofold. First, because of their large luminosities, they are used as standard candles for cosmological studies. Secdnd, because of their large masses and special locations, they are believed to be the sites of interesting evolutionary phenomena (e.g. dynamical friction, galactic cannibalism, interactions with the intracluster medium, and cooling flows). These subjects are reviewed in detail by Dressler (1984b) and Tonry (1987); here we discuss them briefly in the context of recent photometry. Extensive photometric studies are reported by Oemler (1976), Thuan & Romanishin (1981), Schneider et al. (1983), Lugger (1984), Malumuth & Kirshner (1985), Hoessel & Schneider (1985), Schombert (1986, 1987, 1988), Lauer (1988a), and Porter (1988). Except for Valentijn (1983) and Morbey & Morris (1983), most authors are in fairly good agreement.

The classification of galaxies as D and cD is often done loosely and may be misleading. More luminous ellipticals have shallower brightness profiles (Section 7.1). Also, galaxies can have SO disks and tidally stretched halos. All of these satisfy the definition of a D galaxy but are not new phenomena, nor even a single class of phenomena. For this reason, we recommend that the term D galaxy not be used (Kormendy 1987b). On the other hand, the cD classification is useful: cDs are physically different from ellipticals.

Finding an objective way to recognize cDs is nontrivial. The most objective way is to look for an inflection in the outer brightness profile that is independent of the way the profile is plotted. This is interpreted as the signature of a halo that is a distinct dynamical subsystem. In practice, not all halos are prominent enough to produce inflection points in the profiles. Then, less objective identification criteria are necessary, such as extra light compared with mean profiles of comparably bright ellipticals. We use the term cD only for bright ellipticals with such extra envelopes. It is not clear whether a cD envelope belongs to the galaxy or whether it was formed by the parent cluster independently of whether there happened to be a bright galaxy at the bottom of the cluster potential well.

BCM and cD galaxies are generally believed to have formed or been modified by mergers (e.g. VZw311; Schneider & Gunn 1982, and references therein). The luminosities of BCMs are weakly correlated with some properties of their parent clusters, like Abell richness class or Bautz-Morgan type (Sandage 1972, Sandage & Hardy 1973, Schneider et al. 1983, Morbey 1984, Hoessel & Schneider 1985, Schombert 1987). Schombert (1987) also finds weak correlations with cluster velocity dispersion and X-ray luminosity. Beers & Geller (1983) find that cD galaxies are always found in local density maxima, even if they are not the brightest or central cluster members. Such correlations suggest that environment - dependent processes are responsible for at least some of the luminosity of BCMs. Mergers are a natural candidate, but other options are possible. Examples include a gradual accumulation of cluster tidal debris (Malumuth & Richstone 1984; and references therein) or star formation in (now possibly extinguished) cooling flows (Sarazin 1986, and references therein). Or the large luminosities of BCMs may be a consequence of environment-dependent initial conditions. In the language of biased galaxy formation, BCMs may originate from unusually large primordial fluctuations, which are most likely to occur in dense environments.

An argument often cited in favor of mergers is the high frequency of secondary nuclei in BCMs. These may be semidigested cluster members. Schneider et al. (1983) and Hoessel & Schneider (1985) find that about half of the BCMs in their sample of Abell clusters are multiple-nucleus systems, considerably more than would be expected from chance projections if the clusters have cores. This argument is weakened by the conclusion of Beers & Tonry (1986) that rich clusters do not have cores, but instead have steep number density profiles. Then many nuclei are predicted to be near the central galaxy. Tonry (1984a, 1985; see also Hoessel et al. 1985) shows that many of the secondary nuclei move too quickly to be gravitationally bound to the BCM core. This effect was explained independently by Merritt (1984) and Tonry (1984a) as a natural outcome of the evolution of galaxy orbits in a rich cluster. A possible way to distinguish between bound and unbound secondary nuclei is to decompose BCM images into elliptically symmetric components and look for tidal distortions. Lauer (1986, 1988a) made such a study of 17 multiple-nucleus systems and found that ~ 50% of the secondary nuclei show isophote distortions. However, these distortions do not correlate as expected with the kinematics. Even nuclei with large relative velocities show distortions. Therefore, the problem of which nuclei are currently being accreted is not settled. Based only on the observed distortions, Lauer estimates that material is being cannibalized at an average rate of ~ 4 L* (primary galaxy)-1 (10 Gyr)-1. [Here L* is the characteristic luminosity of the Schechter (1976) luminosity function.] This is in rough agreement with models by Merritt (1985), which imply accretion rates of ~ 1 L* (primary galaxy)-1 (10 Gyr)-1. Since the total luminosity of a cD galaxy is typically ~ 12 L*, this argues that not all of a cD originates through cannibalism.

The structural properties of BCMs are often discussed in the framework of the homologous merger picture (e.g. Ostriker & Tremaine 1975, Ostriker & Hausman 1977, Hausman & Ostriker 1978, Malumuth & Richstone 1984, Merritt 1985; see also White 1982, and references therein). In this picture, the kinetic energy per unit mass is preserved. Then, if the orbital structure of the cannibal galaxy stays the same, its projected central velocity dispersion does not change, even though the mass and luminosity increase. That is, merger products should deviate from the Faber-Jackson relation by being too luminous for their velocity dispersions. Such an effect was found by Malumuth & Kirshner (1985). Because of the conversion of galaxy orbital energy into internal random motions in the merger remnant, the envelope of the cannibal should get shallower after every merger [as measured, say, by the Gunn-Oke (1975) structure parameter alpha]. Therefore, more luminous merger remnants should have shallower profiles. This is the sense of the luminosity dependence of profile shapes for all ellipticals (Section 7.1; Schneider et al. 1983, Hoessel & Schneider 1985). Finally, compared with the color luminosity relation for normal galaxies, merger remnants should be too blue for their luminosities. This prediction is not confirmed: Gallagher et al. (1980), Lugger (1984), Lachieze-Rey et al. (1985), and Schombert (1988) find that integrated colors and color gradients in the envelopes of cD galaxies are consistent with those in normal ellipticals of comparable luminosities.

Of more interest is the radius-surface brightness relation (cf. Section 8.2). Kormendy (1980), Thomsen & Frandsen (1983), Lugger (1984), and Romanishin (1986b) find strong correlations consistent with those for normal ellipticals. With larger data sets, Schneider et al. (1983), Schombert (1987), and Hoessel et al. (1987) conclude that the relations between re and µe or L are steeper for BCM than for non-BCM ellipticals: Hoessel et al. (1987) find that re propto Ie-0.80 for BCMs and re propto Ie-0.55 for non-BCMs in the Gunn r band. BCMs are larger at a given surface brightness than normal ellipticals; However, since the relation for non-BCM ellipticals that are well resolved is re propto Ie-0.83 (Section 8.2), the above difference may be due partly to seeing effects. (The non-BCM ellipticals in the Hoessel et al. sample typically have re appeq 2 - 5".)

An even more informative comparison of BCMs and other ellipticals can be made using fundamental plane solutions. Hoessel et al. (1987) find that the R - sigma - µ solutions for the galaxies in their sample are consistent with solutions for normal ellipticals (Djorgovski & Davis 1987), but with a hint of a different slope. S. Djorgovski & R.R. de Carvalho (in preparation), using data from Malumuth & Kirshner (1985), obtain different solutions for BCMs and normal ellipticals. At a given effective radius or luminosity, the range of velocity dispersions is much smaller than for normal ellipticals; as a result, the scatter in the re - µe relation is smaller for BCMs than for non-BCM ellipticals. This can be understood in the homologous merger picture, because velocity dispersions are not changed much by mergers, while luminosities and radii increase. Different fundamental planes for BCMs and other ellipticals imply different formation histories.

The re - µe relation and especially the fundamental plane solutions for BCMs are promising distance indicators (Thomsen & Frandsen 1983, Hoessel et al. 1987; S. Djorgovski & R.R. de Carvalho, in preparation). They may also lead to an improved angular size-redshift cosmological test.

The origin of cD halos remains murky. They are purely a rich cluster phenomenon: Thuan & Romanishin (1981) and Schombert (1986) find that cD halos do not occur in poor clusters. Struble (1988) has discovered what appears to be a cD envelope without a central galaxy in the rich cluster Abell 545. It would be interesting to know whether more such cases exist. They are difficult to find because of their low surface brightnesses (µV ~ 24 mag arcsec-2) and unimposing luminosities (L ~ L* for Struble's "star pile").

The most systematic photometric study of cD envelopes to date is by Schombert (1988), building on work by Oemler (1976). He subtracted template brightness profiles of ellipticals from those of cD galaxies and measured the properties of the envelopes. He found that envelopes have brightness, profiles I(r) propto r-1.6 similar to those of their galaxies and any X-ray halos. Envelope luminosities are comparable to those of the central galaxies (~ 1012 Lsun). Therefore, if the theoretical models are correct, mergers are an insufficient source of material to build either cD galaxies or their envelopes. Envelope luminosities correlate with parent galaxy luminosities, which suggests that similar processes may be responsible for both. They also correlate with cluster richness (Lenv propto N1.6, where N is the Abell galaxy count) and weakly with Bautz-Morgan type. Finally, there is a good correlation with the cluster X-ray luminosity (Lenv propto LX1.06±0.18).

Other connections between BCMs and their clusters include alignment effects. In clusters with well-defined orientations, BCM isophotes tend to align with cluster major axes (Carter & Metcalfe 1980, Binggeli 1982, Porter 1988, Rhee & Roos 1989; see also the review by Djorgovski 1987c). Cluster position angles are uncertain, but Porter (1988) also finds a tendency for alignment with cluster X-ray gas isodensity contours. Ellipticity tends to increase strongly with radius in BCMs. Porter finds that BCMs tend to have larger ellipticities and ellipticity gradients and smaller isophote twists than normal ellipticals.

All of these correlations suggest that cD envelopes are products of their clusters. They may be the accumulated debris of all tidal interactions. Further support for this interpretation comes from the kinematics: The projected velocity dispersions of cDs increase with radius and approach cluster velocity dispersions (Dressler 1979, Carter et al. 1981, 1985).

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