Annu. Rev. Astron. Astrophys. 1984. 22: 185-222
Copyright © 1984 by . All rights reserved

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5.2 Verifying and Interpreting the Butcher-Oemler Effect by Spectroscopy

Low-resolution (10-20 Å FWHM) spectroscopy of Lstar galaxies at z ~ 0.3-0.5 has recently become possible with the high throughput CCD spectrographs available on the Palomar 5-m, the KPNO 4-m, and the AAT 4-m telescopes. Depending on the strengths of emission and absorption features in the spectra, a redshift of a typical cluster galaxy can be obtained in 1-4 hr with these systems. Multiaperture masks, which allow the simultaneous observation of ~ 10 galaxies over a field of a few arcminutes, have made it feasible, though by no means easy, to obtain redshifts for the brightest 30-50 galaxies in such a cluster.

At the present time, redshifts of 20 or more galaxies have been determined for only 3 fields. These are the 3C 295 field (Dressler & Gunn 1982, 1983), C10024+1654 (Dressler & Gunn 1982, Dressler et al. 1984a), and C11446+2619 (Butcher & Oemler 1984b). In the 3C 295 field, Dressler & Gunn obtained redshifts for 6 red and 17 blue galaxies. All 6 of the red galaxies were cluster members at z ~ 0.46, but only 6 of the 17 blue galaxies were actually at the cluster redshift. This indicates a blue cluster population of about 20%, or about half of Butcher & Oemler's estimate (1978a; see Table 3 therein), in agreement with Mathieu & Spinrad's claim (1981) of a larger field correction. Dressler et al. (1984a) have found 11 out of 12 red galaxies in C10024 + 1654 to be cluster members at z ~ 0.39, and 13 out of 21 blue galaxies to be at the cluster redshift. This implies a blue galaxy population of ~ 45%, much larger than in the 3C 295 cluster, though somewhat smaller than Butcher & Oemler's field-corrected estimate of 56%. Butcher & Oemler (1984b) have a similar result for the irregular cluster C11446 + 2619, where they find 4 out of 4 red galaxies to be cluster members and at least 7 out of 12 blue galaxies to be at the cluster redshift of z ~ 0.37. In these last two cases, then, there does appear to be a significant blue galaxy population at the cluster redshift, but an assessment of how unusual these populations are compared with present-epoch clusters is more difficult.

In their first paper, Butcher & Oemler (1978a) assumed that the blue galaxies in the distant clusters were normal spirals of types Sb and later, although there was no evidence for this other than the single broadband color. (Of course, it will require the spatial resolution of the Space Telescope to obtain direct images of these galaxies suitable for morphological classification.) Since there were more data on morphological types than colors of the galaxies in nearby clusters, they attempted to use the mean relation of colors and morphological types for nearby field galaxies to predict a ``spiral fraction'' from the blue galaxy fraction of the distant clusters. This approach is more fully developed in Butcher & Oemler's second paper (1978b), where the evolution of the spiral fraction as a function of look-back time is presented. However, this tack of normalizing to the spiral fraction is not a safe procedure. Dressler & Gunn (1982) were able to show from a relation between the [O II] strength and galaxy color for nearby spirals that most of the blue galaxies in the 3C 295 field could not be spirals at the cluster redshift, a result later verified through extensive spectroscopy (Dressler & Gunn 1983). Secondly, Wirth & Gallagher (1980) showed that the spiral fraction of nearby clusters is sensitive to the resolution and depth of the images used for the classification.

A better procedure has now been adopted by Butcher & Oemler (1984a). They compare the fraction of blue galaxies in both nearby and distant clusters. The new definition of fB, as described earlier, requires that the galaxy be quite blue, in the central part of the cluster, and brighter than Mv = -20.0. With this more stringent definition, the fraction of blue galaxies in the distant clusters has dropped from the ~ 50% level of the early papers (which implies an even higher spiral fraction due to a correction for redspirals) to values ~ 20%. One is tempted to conclude that the Butcher-Oemler effect has virtually disappeared, since many nearby clusters, even concentrated ones, have spiral fractions of this order. But Butcher & Oemler now argue (1984a) that most of the spirals in the central regions of nearby clusters are red. From their definition and new data sample, they claim that 3 out of 4 field spirals are blue, but only 1 out of 4 of the nearby cluster spirals is blue. This result, which might be called the ``Oemler-Butcher effect,'' is perhaps as surprising as the Butcher-Oemler effect, and although it seems to be in qualitative agreement with the H I deficiency data (e.g. Giovanelli & Haynes 1983), it is in marked disagreement with the color-H I-morphology relations of Bothun et al. (1982). These relations indicate that spirals in clusters are normally indistinguishable from field spirals. Cast in this new way, the Oemler-Butcher effect might be described as follows: Distant, concentrated clusters have spirals with the colors of today's field galaxies, but these spirals have become red by the present epoch.

If Butcher & Oemler's new data are correct, it might be that the ancestors of S0s and early-type spirals in today's clusters were actively forming stars at z ~ 0.5, and that they have only begun to ``run down'' in recent times as a result of cluster influences and/or simply the exhaustion of gas by star formation. Observations of many more distant clusters with a range of environments, as well as more color and morphological data for nearby clusters, are essential to test even these simplest notions.

But even the few data available at this time seem to defy any attempt at a consistent picture of the evolution of ordinary spirals. From the study of the 3C 295 cluster, Dressler & Gunn (1982) concluded that the blue population was small and made up of only Seyfert and starburst galaxies, with no normal spirals detected. These observations imply that Seyfert and starburst galaxies occurred with a frequency about an order of magnitude higher in z ~ 0.5 clusters than in nearby clusters. It is tempting, therefore, to regard this evolution of ``active galaxies'' as a possible explanation of the Butcher-Oemler effect (see also Henry et al. 1983). But Dressler et al.'s (1984a) study of C10024 + 1654 picked up only one definite Seyfert and no starburst galaxies, and, in contrast to the 3C 295 cluster, the blue cluster members do have the spectra of spirals (proper emission-line strengths as a function of integrated color). Butcher & Oemler (1984b) also find typical spiral spectra in C11446 + 2619. Thus, the strong activity in the 3C 295 cluster may be connected to a global event in this cluster instead of a more general evolution with look-back time; without this activity, the cluster population might be very red like the Coma cluster. Similarly, Koo's photometry of C10016 + 16 also identifies a distant cluster with a very small percentage of blue galaxies, although confirming spectroscopy is not yet available.

Clearly, then, no consistent interpretation of the data is possible at this time. Even the data for clusters at the same epoch contain some puzzling inconsistencies. Much more photometry is needed to decide if both red and blue clusters inhabited the z ~ 0.5 epoch and to resolve the question of whether nearby cluster spirals have different colors than their field counterparts. The approach by Couch et al. (1983) of narrow-band photometry in 7 bandpasses may be an effective compromise between spectroscopy and broadband photometry, since it provides more reliable information on the spectral energy distribution than either of these methods and therefore is best suited to look for evidence of spectral evolution. Unfortunately, the more time-consuming spectroscopy is necessary to provide the redshifts that separate field galaxies from cluster members.

We-can expect the study of galaxy populations as a function of look-back time to make a substantial impact on the field of galaxy evolution in the near future. This is an area of research where new instruments and larger telescopes, both on the Earth and in space, will make an enormous difference on the type, quality, and quantity of the data needed to advance the subject. It is a time of great promise.

ACKNOWLEDGMENTS

It is a pleasure to thank James Binney, Dave Burstein, Roger Davies, Alfonso Cavaliere, George Efstathiou, Sandra Faber, Mike Fall, Gus Oemler, Paul Schechter, Don Schneider, Steve Shectman, and Roberto Terlevich for valuable discussions and comments about the material in this review.

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