* Astronomy

Title: NGC 3628-UCD1: A possible w Cen Analogue Embedded in a Stellar Stream
Author: Zachary G. Jennings, Aaron J. Romanowsky, Jean P. Brodie, Joachim Janz, Mark A. Norris, Duncan A. Forbes, David Martinez-Delgado, Martina Fagioli, Samantha J. Penny

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 w 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 (L_i = 1.4 x 10⁶ solar luminosity). 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 L_i ~4.1 x 10⁸ solar luminosity. Spectral analysis reveals that UCD1 has an age of 6.6 Gyr , [Z/H]=-0.75, an [alpha/Fe]=-0.10. We propose that UCD1 is an example of an w 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.

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