A Peculiar Faint Satellite in the Remote Outer Halo of M31

A. D. Mackey, A. P. Huxor, N. F. Martin, A. M. N. Ferguson, A. Dotter, A. W. McConnachie, R. A. Ibata, M. J. Irwin, G. F. Lewis, C. M. Sakari, N. R. Tanvir, K. A. Venn

Research output: Contribution to journalArticlepeer-review

Abstract

We present Hubble Space Telescope imaging of a newly discovered faint stellar system, PAndAS-48, in the outskirts of the M31 halo. Our photometry reveals this object to be comprised of an ancient and very metal-poor stellar population with age greater than or similar to 10 Gyr and [Fe/H] less than or similar to -2.3. Our inferred distance modulus (m - M)(0) = 24.57 +/- 0.11 confirms that PAndAS-48 is most likely a remote M31 satellite with a three-dimensional galactocentric radius of 149(-8)(+19) kpc. We observe an apparent spread in color on the upper red giant branch that is larger than the photometric uncertainties should allow, and briefly explore the implications of this. Structurally, PAndAS-48 is diffuse, faint, and moderately flattened, with a half-light radius r(h) = 26(-3)(+4) pc, integrated luminosity M-V = -4.8 +/- 0.5, and ellipticity epsilon = 0.30(-0.15)(+0.08). On the size-luminosity plane it falls between the extended globular clusters seen in several nearby galaxies and the recently discovered faint dwarf satellites of the Milky Way; however, its characteristics do not allow us to unambiguously classify it as either type of system. If PAndAS-48 is a globular cluster then it is among the most elliptical, isolated, and metal-poor of any seen in the Local Group, extended or otherwise. Conversely, while its properties are generally consistent with those observed for the faint Milky Way dwarfs, it would be a factor of similar to 2-3 smaller in spatial extent than any known counterpart of comparable luminosity.

Original languageEnglish
Article number17
Number of pages6
JournalAstrophysical Journal Letters
Volume770
Issue number2
DOIs
Publication statusPublished - 20 Jun 2013
Externally publishedYes

Keywords

  • galaxies: dwarf
  • galaxies: individual (M31)
  • globular clusters: general
  • Local Group
  • MILKY-WAY SATELLITES
  • DISCOVERED GLOBULAR-CLUSTERS
  • DWARF GALAXIES
  • STAR-CLUSTERS
  • SPECTROSCOPIC SURVEY
  • ADVANCED CAMERA
  • LOCAL GROUP
  • ACS PHOTOMETRY
  • METALLICITY
  • SYSTEM

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