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 $omega$ 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 (${rm L}_{rm i}=1.4times10^{6}~{rm L}_{odot}$). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of $sim10$ 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 ${rm L}_isim4.1times10^{8}~{rm L}_{odot}$. Spectral analysis reveals that UCD1 has an age of $6.6$ Gyr , $[rm{Z}/rm{H}]=-0.75$, an $[{alpha}/rm{Fe}]=-0.10$. We propose that UCD1 is an example of an $omega$ 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.
Download