NGC 3628-UCD1: A POSSIBLE ω CEN ANALOG EMBEDDED in A STELLAR STREAM

Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we identify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to ω Cen, the most luminous Milky Way globular cluster, which has long been suspected to be the stripped remnant of an accreted dwarf galaxy. The object has a magnitude of i = 19.3 mag (Li = 1.4 × 106 L⊙). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of ∼10 pc. We measure an integrated brightness for the stellar stream of i = 13.1 mag, with (g-i) = 1.0. This would correspond an accreted dwarf galaxy with an approximate luminosity of Li∼4.1×108 L⊙. Spectral analysis reveals that UCD1 has an age of 6.6 Gyr, and We propose that UCD1 is an example of an ω Cen-like star cluster possibly forming from the nucleus of an infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population may be created through tidal stripping.