Globular clusters in the dE,N galaxy NGC 3115 DW1: New insights from spectroscopy and Hubble Space Telescope photometry

The properties of globular clusters in dwarf galaxies are key to understand ing the formation of globular cluster systems and in particular in verifyin g scenarios in which globular duster systems of larger galaxies formed (at least partly) from the accretion of dwarf galaxies. Here, we revisit the gl obular cluster system of the dE,N galaxy NGC 3115 DW1-a companion of the ne arby SO galaxy NGC 3115-adding Keck/LRIS spectroscopy and Hubble Space Tele scope (HST) Wide Field Planetary Camera 2 imaging to previous ground-based photometry. Spectra for seven globular clusters reveal normal abundance rat ios with respect to the Milky Way and M31 clusters, as well as a relatively high mean metallicity ([Fe/H] approximate to - 1.0 +/- 0.1 dex). Crude kin ematics indicate a high velocity dispersion within 10 kpc that could be cau sed either by dark matter-dominated outer regions or by the stripping of ou ter globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass out to 3 and 10 kpc lies between 1 x 10(10) and 1 x 10(11) M. and 2 x 10(10) and 4 x 10(11) M., respectively, depending on the mass estimator us ed and the assumptions on cluster orbits and systemic velocity. The HST ima ging allows measurement of sizes for two clusters, returning core radii aro und 2.0 pc, similar to the sizes observed in other galaxies. Spectroscopy a llows an estimate of the degree of contamination by foreground stars or bac kground galaxies for the previous ground-based photometry but does not requ ire a revision of previous results: NGC 3115 DW1 hosts around N-GC = 60 +/- 20 clusters, which corresponds to a specific frequency of S-N = 4.9 +/- 1. 9, on the high end for massive dE's. Given its absolute magnitude (M-V = -1 7.7 mag) and the properties of its cluster system, NGC 3115 DW1 appears to be a transition between a luminous dE and low-luminosity E galaxy.