HST imaging of the globular clusters in the Fornax cluster: NGC1379

March 1998 • 1998MNRAS.295..240E

Authors • Elson, Rebecca A. W. • Grillmair, Carl J. • Forbes, Duncan A. • Rabban, Mike • Williger, Gerard M. • Brodie, Jean P.

Abstract • We present B and I photometry for ~300 globular cluster candidates in NGC1379, an E0 galaxy in the Fornax cluster. Our data are from both Hubble Space Telescope (HST) and ground-based observations. The HST photometry (B only) is essentially complete and free of foreground/background contamination to ~2 mag fainter than the peak of the globular cluster luminosity function. Fitting a Gaussian to the luminosity function, we find <B>=24.95+/-0.30 and sigma_B=1.55+/-0.21. We estimate the total number of globular clusters to be 436+/-30. To a radius of 70 arcsec we derive a moderate specific frequency, S_N=3.5+/-0.4. At radii r~3-6 kpc the surface density profile of the globular cluster system is indistinguishable from that of the underlying galaxy light. At r<~2.5 kpc the profile of the globular cluster system flattens, and at r<~1 kpc, the number density appears to decrease. The (B-I) colour distribution of the globular clusters (from ground-based data) is similar to that for Milky Way globulars, once corrected for background contamination. It shows no evidence for bimodality or for the presence of a population with [Fe/H]>~-0.5. Unlike in the case of larger, centrally located cluster ellipticals, neither mergers nor a multiphase collapse are required to explain the formation of the NGC1379 globular cluster system. We stress the importance of corrections for background contamination in ground-based samples of this kind: the area covered by a globular cluster system (with radius ~30 kpc) at the distance of the Virgo or Fornax cluster contains >~200 background galaxies unresolved from the ground, with magnitudes comparable to brighter globular clusters at that distance. The colour distribution of these galaxies is strongly peaked slightly bluer than the peak of a typical globular cluster distribution. Such contamination can thus create the impression of skewed colour distributions, or even of bimodality, where none exists.

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Carl Grillmair

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