The Color-Magnitude Relation in Coma: Clues to the Age and Metallicity of Cluster Populations

Abstract

We have observed three fields of the Coma cluster of galaxies with a narrow band (modified Strömgren) filter system. Observed galaxies include 31 in the vicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839 complete to M 5500 = −18 in all three subclusters. Spectrophotometric classification finds all three subclusters of Coma to be dominated by red, E type (ellipticals/S0's) galaxies with a mean blue fraction, f B , of 0.10. The blue fraction increases to fainter luminosities, possible remnants of dwarf starburst population or the effects of dynamical friction removing bright, blue galaxies from the cluster population by mergers. We find the colormagnitude (CM) relation to be well defined and linear over the range of M 5500 = −13 to −22. The observational error is lower than the true scatter around the CM relation indicating that galaxies achieve their final positions in the mass-metallicity plane by stochastic processes. After calibration to multi-metallicity models, bright ellipticals are found to have luminosity weighted mean [Fe/H] values between −0.5 and +0.5, whereas low luminosity ellipticals have [Fe/H] values ranging from −2 to solar. The lack of CM relation in our continuum color suggests that a systematic age effect cancels the metallicity effects in this bandpass. This is confirmed with our age index (∆(bz − yz)) which finds a weak correlation between luminosity and mean stellar age in ellipticals such that the stellar populations of bright ellipticals are 2 to 3 Gyrs younger than low luminosity ellipticals. With respect to environmental effects, there is a slight decreasing metallicity gradient with respect to distance to each subcluster center, strongest around NGC 4874. Since NGC 4874 is the dynamic and x-ray center of the Coma cluster, this implies that environmental effects on low luminosity ellipticals are strongest at the cluster core compared to outlying subgroups.

Cluster Populations in Abell 2125 and 2218

“…Based on our metallicity calibration from globular clusters (Rakos & Schombert 2005), the galaxies with the reddest colors have mean metallicities just above solar, dropping to values of [Fe/ H ] between À0.5 and À1.0 for the galaxies 1 mag fainter than L Ã . In our previous work, we have demonstrated an age dependence for ellipticals (E class) with luminosity (Odell et al 2002), but this effect is minor in the vz À yz, bz À yz diagram compared to metallicity effects. While star-forming galaxies appear to follow the same linear relation in Figures 4 and 5, their colors are in fact dominated by a number of young, massive stars and not metallicity effects.…”

“…For comparison, the population fractions for the core of the Coma Cluster are also shown in Table 1 (taken from Odell et al 2002). Those objects classified as non-star-forming (E and SÀ class) comprise 74% of A2125 and A2218, compared to 95% for the core of Coma.…”

“…Early CMRs were observed in near-blue colors, such as U À V ; however, since then the CMR has been investigated from the far-UV to the near-IR (Pahre 1999; Ellis et al 1997). With respect to the uvby filter system, we have previously presented a full analysis of our filter system's behavior with galaxy luminosity in Coma and Fornax (Odell et al 2002).…”

“…The CMR for A2125 and A2218 is shown in Figures 6 and 7, where the top panel displays the near-UV color, uz À yz, the middle panel displays the metallicity color, vz À yz, and the bottom panel displays the continuum color, bz À yz, as a function of M 5500 . The solid lines are fits to the CMR for Coma, with the dotted lines in vz À yz displaying the 3 variation to the Coma fit (note that these fits were made only to those systems that were morphologically classified as ellipticals in Coma; see Odell et al 2002). While our discussion below is confined to the photometric E class objects, all the cluster data from E to S+ are shown for completeness.…”

“…9 and are discussed below). Of course, it is naive to expect that each galaxy arrives at its final metallicity based solely on mass given the numerous Odell et al (2002), and the dashed lines in the vz À yz diagram display the 3 errors on the fit. The similarity to Coma is striking, even reproducing the flat bz À yz relation, which is unexpected in Coma or this paper's data, as metallicity effects should be detectable in the bz À yz color.…”