Rebecca A. W. Elson and Basilio X. Santiago
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, England
Increasingly accurate measurements of the colors and metallicities of globular clusters in distant galaxies are beginning to shed new light on, and provoke new questions about, the origin of globular cluster systems. Color and, by inference, metallicity distributions are being revealed as bimodal or even multi-modal, and appear to vary significantly from one host/parent galaxy to the next (cf. Ajhar, Blakeslee & Tonry 1994). This has prompted the suggestion that distinct sub-systems of clusters formed in distinct episodes, perhaps during mergers (cf. Ashman & Zepf 1992). In this contribution we examine a field in M87 (NGC 4486), the central giant elliptical galaxy in the Virgo cluster, observed with HST as part of the Medium Deep Survey Key Project (cf. Griffiths et al. 1994). A full report of the results may be found in Elson & Santiago (1996).
Our study is based on a field located arcmin from the center of M87, obtained with HST's Wide Field Camera (WFC-2) on 1995 August 5. The F814W ( Cousins I) and F606W ( Cousins V) filters were used. Two V images were obtained, with total exposure time of 2600 seconds, and four I images, with total exposure time 4700 seconds.
Objects were detected automatically and measured using DAOPHOT. Instrumental HST magnitudes were transformed to the Johnson-Cousins system using the calibrations from Holtzman et al. (1995). Globular cluster candidates were selected on the basis of color and morphology. We corrected for contamination from compact, spherical background galaxies using a deep field from the Medium Deep Survey database. Our final sample contains clusters with .
Figure 1 shows a color distribution of the M87 globular clusters. Two narrow peaks are evident; one at , and one at . Fitting two Gaussian functions to the bimodal distribution for clusters with , yields mag for both the red and blue peaks; we find the same for only the brightest clusters, with . This value is several times greater than the Poisson errors which at are . There is, therefore, a small but real spread in the intrinsic colors of both the red and blue clusters.
Figure: A comparison of the color distributions of the M87 clusters () (N=254), with the color distribution of 91 Milky Way globular clusters from Cameron Reed et al. (1988). The M87 histogram has been divided by the ratio of sample sizes. It is clearly bimodal. The large population of red clusters evident in M87 is almost entirely absent in our Galaxy.
Figure 1 also shows such a comparison, between the M87 clusters with from both our samples, and 91 Milky Way globulars from Cameron Reed, Hesser, & Shawl (1988). Our histogram has been divided by the ratio of the sample sizes. The Galactic clusters have which, at the distance modulus of M87, corresponds to . Thus, the magnitude limits of the two samples are similar. There are relatively fewer blue clusters in M87, and the substantial population of red globulars present in M87 is almost entirely absent in the Milky Way.
Figure 2 shows histograms of color for the full sample divided into sub-samples of bright and faint clusters. The bright clusters () are clearly dominated by blue objects. 67% have , while only 23% have . For the fainter clusters () the fractions are 56% (blue) and 44% (red).
Figure: Histogram of colors for faint (N=126) and bright (N=128) clusters in the combined sample. The bright sample is clearly dominated by blue clusters, while the faint sample has similar numbers of red and blue clusters.
Our results suggest the presence of two distinct populations of globular clusters in M87, each with an intrinsic color spread, which probably originated during two distinct episodes, or in two different modes of cluster formation. The color distribution of the blue population in M87 is virtually indistinguishable from that of the globular clusters in our Galaxy. The red population on the other hand, is almost entirely absent in our Galaxy. One possible interpretation of this is that the color reflects metallicity, and that there is a dearth of metal rich globulars in our Galaxy. According to Couture et al. (1990), the red colors would imply approximately solar metallicity. They would, therefore, have formed out of gas that had already been significantly enriched. Indeed, the metallicity of M87 itself is [Fe/H] (Brodie & Huchra 1991), consistent with the inferred metallicity of the population of red clusters. If elliptical galaxies formed from the merger of spiral galaxies (cf. Toomre 1977), then the blue population may represent clusters native to the galaxies which merged, while the red population may have formed during the merger. Indeed, several galaxies currently undergoing mergers have recently been observed to contain blue point-like objects, which may be young globular clusters (cf. Holtzman et al. 1992, Whitmore et al. 1993).
An alternative interpretation of the color differences is that they reflect a difference in age, in the sense that the bluer clusters are younger. This seems unlikely, however, given the similarity in color between the blue M87 clusters and those in the Galaxy, which are known to be old. Certainly the blue clusters are not blue enough to be suggestive of a recent episode of cluster formation in M87. The 40 bright point-like objects observed in NGC 7252 by Whitmore et al. (1993) have mean color , considerably bluer than the blue population in M87, which peaks at .
The red clusters in our sample seem to be on average fainter than the blue clusters. This does not necessarily imply a difference in mass, however. In a metal poor globular cluster the giants, which dominate the integrated light, would be brighter than those in a metal rich cluster by as much as two magmitudes in for a change in metallicity from [Fe/H] to (cf. Bergbusch & VandenBerg 1992). The blue clusters may, therefore, only appear brighter because they are more metal poor. It is interesting that Whitmore et al. (1995) do not find a luminosity difference between the red and blue clusters in their sample. This may reflect a difference in the population of clusters near the center, and further out in M87.
In any case, the existence of two distinct populations of clusters with differing mean luminosity casts doubt on the popular concept of a universal luminosity function for globular clusters. And even if such a thing existed, measuring the peak luminosity accurately at these distances is very difficult, even with HST. Indeed, now that globular cluster color distributions have clearly been shown to vary from galaxy to galaxy, any detailed coincidence among the corresponding luminosity functions must be attributed to a conspiracy between the distributions of cluster masses and metallicities. This is particularly true between spirals, which appear to contain only blue globular clusters, and ellipticals, which may contain both red and blue clusters. It will be interesting to see whether increasingly accurate photometry reveals differences among luminosity functions, as suggested by our results, and indeed by Ajhar et al. (1994) who find some suggestion of a non-universal luminosity function for the one galaxy in their sample with clusters intermediate between red and blue. It would also be interesting to obtain spectra of a sample of red and blue clusters to determine whether their metallicities are indeed different, and whether there is any evidence for kinematic differences between the two populations.
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