NGC 3628-UCD1: A possible $\omega$ Cen Analog Embedded in a Stellar Stream

TL;DR

This study identifies a compact star cluster, UCD1, embedded in a stellar stream around NGC 3628, suggesting it may be an omega Cen-like remnant formed through tidal stripping of a dwarf galaxy.

Contribution

The paper presents the discovery and detailed characterization of UCD1, a star cluster likely formed from a tidally stripped dwarf galaxy nucleus, providing evidence for such formation pathways.

Findings

UCD1 has a size of ~10 pc and luminosity similar to omega Cen.

The associated stellar stream indicates an accreted dwarf galaxy with luminosity ~4.1×10^8 L_sun.

Spectral analysis shows UCD1 is 6.6 Gyr old with subsolar metallicity.

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 $\omega$ 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 (${\rm L}_{\rm i}=1.4\times10^{6}~{\rm L}_{\odot}$). 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 ${\rm L}_i\sim4.1\times10^{8}~{\rm L}_{\odot}$. 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.