Globular Clusters in the dE,N Galaxy NGC 3115 DW1: New Insights from Spectroscopy and Hubble Space Telescope Photometry

摘要

The properties of globular clusters in dwarf galaxies are key to understanding the formation of globular cluster systems and in particular in verifying scenarios in which globular cluster systems of larger galaxies formed (at least partly) from the accretion of dwarf galaxies. Here, we revisit the globular cluster system of the dE,N galaxy NGC 3115 DW1—a companion of the nearby S0 galaxy NGC 3115—adding Keck/LRIS spectroscopy and Hubble Space Telescope (HST) Wide Field Planetary Camera 2 imaging to previous ground-based photometry. Spectra for seven globular clusters reveal normal abundance ratios with respect to the Milky Way and M31 clusters, as well as a relatively high mean metallicity ([Fe/H] ≈ -1.0 ± 0.1 dex). Crude kinematics indicate a high velocity dispersion within 10 kpc that could be caused either by dark matter–dominated outer regions or by the stripping of outer globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass out to 3 and 10 kpc lies between 1 × 1010 and 1 × 1011 M⊙ and 2 × 1010 and 4 × 1011 M⊙, respectively, depending on the mass estimator used and the assumptions on cluster orbits and systemic velocity. The HST imaging allows measurement of sizes for two clusters, returning core radii around 2.0 pc, similar to the sizes observed in other galaxies. Spectroscopy allows an estimate of the degree of contamination by foreground stars or background galaxies for the previous ground-based photometry but does not require a revision of previous results: NGC 3115 DW1 hosts around NGC = 60 ± 20 clusters, which corresponds to a specific frequency of SN = 4.9 ± 1.9, on the high end for massive dE's. Given its absolute magnitude (MV = -17.7 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.