NGC 3628-UCD1: A possible $\ddot{I}‰$ 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 $\ddot{I}‰$ 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 (${\mathrm{L}}_{\mathrm{i}}=1.4\times 10^6{\mathrm{L}}_{\odot }$). UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of $\sim 10$ 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 ${\mathrm{L}}_i\sim 4.1\times 10^8{\mathrm{L}}_{\odot }$. Spectral analysis reveals that UCD1 has an age of $6.6$ Gyr , $[\mathrm{Z}/\mathrm{H}]=-0.75$, an $[\hat{I}\pm /\mathrm{Fe}]=-0.10$. We propose that UCD1 is an example of an $\ddot{I}‰$ 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.

6 Pages, 3 Figures, Accepted to ApJ Letters