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The densest galaxy
Astrophysical Journal Letters (2013)
  • J. Strader, Michigan State University
  • A. C. Seth, University of Utah
  • J. P. Brodie
  • D. A. Forbes
  • G. Fabbiano
  • Aaron J. Romanowsky, San Jose State University
  • C. Conroy, University of California, Santa Cruz
  • N. Caldwell
  • V. Pota
  • C. Usher
  • J. A. Arnold

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 2.0 × 108 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–1 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 (gsim 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–1) 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.

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Copyright © 2013 Institute of Physics. The published version of the article can be found online at:
Citation Information
J. Strader, A. C. Seth, J. P. Brodie, D. A. Forbes, et al.. "The densest galaxy" Astrophysical Journal Letters Vol. 775 Iss. 1 (2013)
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