NGC 3628-ucd1: a possible ω cen analog embedded in a stellar stream

Abstract

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 × 10^6 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 to an accreted dwarf galaxy with an
approximate luminosity of Li ~ 4.1 x 10^8 L. Spectral analysis reveals that UCD1 has an age of 6.6 Gyr,
[Z/H] = −0.75, and [α/Fe] = −0.10. 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.