STRADER J., SETH A.C., FORBES D.A., FABBIANO G., ROMANOWSKY A.J., BRODIE J.P., CONROY C., CALDWELL N., POTA V., USHER C. and ARNOLD J.A.
Abstract (from CDS):
We report the discovery of a remarkable ultra-compact dwarf galaxy around the massive Virgo elliptical galaxy NGC 4649 (M60), which we call M60-UCD1. With a dynamical mass of 2.0x108 M☉ but a half-light radius of only ∼24 pc, M60-UCD1 is more massive than any ultra-compact dwarfs of comparable size, and is arguably the densest galaxy known in the local universe. It has a two-component structure well fit by a sum of Sérsic functions, with an elliptical, compact (rh= 14 pc; n ∼ 3.3) inner component and a round, exponential, extended (rh= 49 pc) outer component. Chandra data reveal a variable central X-ray source with LX∼ 1038 erg/s that could be an active galactic nucleus associated with a massive black hole or a low-mass X-ray binary. Analysis of optical spectroscopy shows the object to be old ( ≳ 10 Gyr) and of solar metallicity, with elevated [Mg/Fe] and strongly enhanced [N/Fe] that indicates light-element self-enrichment; such self-enrichment may be generically present in dense stellar systems. The velocity dispersion (σ ∼ 70 km/s) and resulting dynamical mass-to-light ratio (M/LV = 4.9±0.7) are consistent with–but slightly higher than–expectations for an old, metal-rich stellar population with a Kroupa initial mass function. The presence of a massive black hole or a mild increase in low-mass stars or stellar remnants is therefore also consistent with this M/LV. The stellar density of the galaxy is so high that no dynamical signature of dark matter is expected. However, the properties of M60-UCD1 suggest an origin in the tidal stripping of a nucleated galaxy with MB∼ -18 to -19.
galaxies: dwarf - galaxies: elliptical and lenticular, cD - galaxies: individual: M60 - galaxies: kinematics and dynamics - galaxies: star clusters: general