The metallicity gradient and distribution function of globular clusters around NGC 1399

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Abstract

We have obtained integrated Washington photometry (C,M,T_1_) of globular cluster candidates in the field of NGC 1399. This galaxy, the dominant object in the Fornax cluster, shares with NGC 4486 (M87) the property of possessing one of the largest known specific frequencies of globulars. Conflicting evidence on the presence of a B - V color gradient as a function of galactocentric distance has been reported for this cluster system. Here we investigate the existence of a gradient using the C-T_1_, index, which is twice as metallicity sensitive as B - V. A small gradient is detected. This color gradient, in the sense that outer clusters are bluer, amounts to ~0.1 mag in C - T_1_ over a galactocentric range from about 0.5 to 3.5 arcmin, Corresponding to a difference of ~0.2 dex in metallicity. This confirms qualitatively the B - V gradient suggested by Bridges et al. [AJ, 101, 469 (1991)]. Photometry for 420 globular cluster candidates is presented. Metal abundances internally precise to ~0.25 dex and accurate to ~0.3 dex have been obtained for some 250 of the best candidates, after statistically subtracting a field contamination estimated at 30%. The mean abundance of -0.9+/-0.2 confirms previous indications that the NGC 1399 globular cluster system is, on average significantly more metal rich than that of the Galaxy, as expected from the relative parent galaxy luminosities. Approximately 10% of the clusters exceed solar abundance, similar to the case for NGC 5128. The signature of possible globular cluster formation in mergers, as suggested by Ashman and Zepf [ApJ, 384, 50 (1992)], is found in the distinct peaks in the metallicity distribution function. However, their prediction (based on a simple theory designed only for application to normal elliptical galaxies and not the more complicated cD galaxies represented by NGC 1399) that the color difference between the clusters and the background halo light monotonically increases with radius is not borne out by the observations. The difference decreases with radius from the center to ~2'; beyond this radius, the difference does appear to increase, however. At all radii, the mean globular cluster abundance is some 0.25-0.5 dex more metal poor than that of the background halo.

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