5.2. The Coma Ellipticals: New Anomalies and Challenges

As a final example of particularly interesting case studies, we will turn to the E galaxies in the Coma cluster. Coma (at redshift cz 7000 km s^-1, about 4 magnitudes more distant than Virgo or Fornax) represents the nearest example of a genuinely rich Abell-cluster environment. Its central supergiant, NGC 4874, is an extremely luminous cD galaxy and the ~ 1 Mpc core of the cluster contains dozens of large ellipticals and hundreds of dwarfs.

At present, we have information for GCSs in three Coma giants: NGC 4874 itself (Harris 1987; Thompson & Valdes 1987; Blakeslee & Tonry 1995; Kavelaars et al. 1999), NGC 4881 (Baum et al. 1995), and IC 4051 (Baum et al. 1997; Woodworth & Harris 1999), primarily from HST imaging. The latter two galaxies have luminosities similar to the brighter Virgo ellipticals. These three cases, though they represent only a small fraction of the Coma population, are already showing us GCS characteristics that we have not seen before.

IC 4051: The halo of this galaxy is spatially quite centrally concentrated, possibly because it has been tidally trimmed by repeated passages through the Coma central potential (its radial velocity relative to the Coma center is -2100 km s^-1, suggesting that it is now passing through the cluster core; see Woodworth & Harris 1999). More notable, however, is the MDF for the globular clusters (Figure 13). It is almost entirely metal-rich, and unimodal (in fact, the observed dispersion of the histogram, [Fe/H] = -0.65, is scarcely larger than the 0.5 dex spread expected from observational scatter alone, suggesting that the intrinsic dispersion is closer to ~ 0.3 dex). If more than one ``mode'' is indeed present in this MDF, the two peak locations must be close enough to be almost entirely obscured by the observational scatter. The only other galaxy we have found in which the metal-poor component is virtually absent is the cD in Hydra I, NGC 3311 (Secker et al. 1995). IC 4051 is, however, not a ``central cluster'' elliptical.

Still more remarkable is its specific frequency, S_N = 12 ± 3, at a level comparable with some of the brightest BCGs despite the fact that it is not a cD, not exceptionally luminous, and not at the center of any identifiable group or subgroup of galaxies.

Figure 13. Metallicity distribution function for the globular clusters in the Coma giant elliptical IC 4051 (adapted from Woodworth & Harris 1999). The Gaussian fitted line to the histogram has a mean at [Fe/H] = -0.3 and a dispersion [Fe/H] = 0.65 dex. The broken solid line shows the MDF for the globular clusters in the Virgo elliptical NGC 4472; in IC 4051, the metal-poor component is almost entirely missing.

NGC 4881: This galaxy is similar in size and structure to IC 4051, but as shown by Baum et al. (1995), the globular cluster system has an extremely low specific frequency (S_N 2) and an MDF which is entirely metal-poor (with a mean [Fe/H] = -1.2).

NGC 4874: This central cD in Coma has the expected high S_N 10 - 15 and a very broad MDF like M87 (Virgo) or NGC 1399 (Fornax). Notably, however, it has an extremely extended spatial structure, with a core radius approaching ~ 15 kpc, almost three times larger than the GCS in M87 and an order of magnitude larger than in the Milky Way. Are we looking at a globular cluster population which, at least partially, belongs to the extended potential well of the Coma cluster itself rather than the central galaxy?

All three of these interesting galaxies do, however, display the same GCLF shape that we have come to expect: a Gaussian-like distribution by magnitude (Figure 14) very similar to M87 and the Virgo and Fornax ellipticals, despite their extreme differences in metallicity distribution and specific frequency. If we assume that the peak luminosity is M_V⁰ = -7.3, consistent with the distance calibrations to Virgo and Fornax through various stellar standard candles including Cepheids, planetary nebulae, and red-giant stars, then we obtain a Hubble constant H₀ 70 km s^-1 Mpc^-1 (see H99 for more detailed discussion).

Figure 14. Luminosity function for the globular clusters in three Coma ellipticals. The Gaussian curve (solid line) has a turnover (peak point) at V0 = 27.8 ± 0.2 and a dispersion = 1.5 magnitudes.

Interpreting these results from the Coma galaxies presents new challenges to all the galaxy formation scenarios. For example, if the in situ approach is basically correct, then in IC 4051 we would have to ask how the galaxy was able to bypass the first, low-metallicity enrichment stage almost totally. If its high S_N is, as argued above, due to gas ejection during the major star formation stage, then we would expect rather low effective yield y during this stage and thus a low-metallicity GCS, contrary to what we see. On the other hand, if IC 4051 formed almost entirely from mergers, then where is the low-[Fe/H] component from the smaller progenitor galaxies? How, also, did the high specific frequency arise, since all the observed merger cases seem to produce low S_N?

In NGC 4881, we have more or less the opposite set of problems. It is, apparently, entirely missing the high-metallicity component that is present in all other gE's. Thus, a merger formation would have had to be almost entirely gas-free (though this would be consistent with its low S_N). If, instead, it formed through accretion, then where is the high-[Fe/H] cluster component from the original E galaxy? Finally, if most of the galaxy formed in situ, then it is difficult to understand the large (~ 1 dex) mean metallicity difference between the GCS and the field-star light. All the current models are left with significant problems - which means that we have much to learn!