A new population of extended, luminous star clusters in the halo of M31

We present three new clusters discovered in the halo of M31 which, although having globularlike colours and luminosities, have unusually large half-light radii, ∼30 pc. They lie at projected galactocentric distances of ≈15 to ≈35 kpc. These objects begin to fill the gap in parameter space between globular clusters and dwarf spheroidals, and are unlike any clusters found in the Milky Way, or elsewhere to date. Colour‐magnitude diagrams, integrated photometric properties and derived King profile fit parameters are given, and we discuss possible origins of these clusters and their relationships to other populations. Ke yw ords: galaxies: individual: M31 ‐ galaxies: star clusters.

[1]  M. Hilker,et al.  Fornax compact object survey FCOS: On the nature of Ultra Compact Dwarf galaxies , 2004, astro-ph/0401610.

[2]  Arlo U. Landolt,et al.  UBVRI Photometric Standard Stars in the Magnitude Range 11 , 1992 .

[3]  N. Suntzeff,et al.  The abundance spread among giants and subgiants in the globular cluster omega centauri , 1996, astro-ph/9601013.

[4]  Masashi Chiba,et al.  Formation of giant globular cluster G1 and the origin of the M 31 stellar halo , 2004, astro-ph/0401229.

[5]  Yale University,et al.  Disruption of a dwarf galaxy under strong shocking: the origin of ω Centauri , 2004 .

[6]  Taft E. Armandroff,et al.  Standard Globular Cluster Giant Branches in the (M(I), (V - I)o) Plane , 1990 .

[7]  Zeljko Ivezic,et al.  Andromeda IX: A New Dwarf Spheroidal Satellite of M31 , 2004 .

[8]  S. G. Djorgovski,et al.  Mayall II = G1 in M31: Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy? , 2001, astro-ph/0105013.

[9]  Francisco Prada,et al.  Where Are the Missing Galactic Satellites? , 1999, astro-ph/9901240.

[10]  F. Ferraro,et al.  The central density cusp of the Sagittarius dwarf spheroidal galaxy , 2004, astro-ph/0411107.

[11]  Michael J. West,et al.  Reconstructing galaxy histories from globular clusters , 2004, Nature.

[12]  G. Worthey.  Comprehensive stellar population models and the disentanglement of age and metallicity effects , 1994 .

[13]  Junichiro Makino,et al.  Formation of ω Centauri by Tidal Stripping of a Dwarf Galaxy , 2004, astro-ph/0408431.

[14]  Karl Gebhardt,et al.  A 20,000 M☉ Black Hole in the Stellar Cluster G1 , 2002, astro-ph/0209313.

[15]  Patrick Seitzer,et al.  The dwarf spheroidal companions to M31: Surface-brightness profiles , 1992 .

[16]  Chile,et al.  Ultra compact objects in the Fornax cluster of galaxies: Globular clusters or dwarf galaxies? , 2002, astro-ph/0201011.

[17]  S. Larsen.  A G1-like Globular Cluster in NGC 1023 , 2001, astro-ph/0107065.

[18]  K. C. Freeman,et al.  Formation of ω Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc , 2003 .

[19]  K. O'Neil.  A Wide Field CCD Survey for Low Surface Brightness Galaxies , 1996 .

[20]  Andrew A. West,et al.  A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite? , 2004, astro-ph/0410416.

[21]  L. Ho,et al.  A Population of Massive Globular Clusters in NGC 5128 , 2004, astro-ph/0404003.

[22]  HongSheng Zhao.  The Survival of the Sagittarius Dwarf Galaxy and the Flatnessof the Rotation Curve of the Galaxy , 1998, astro-ph/9804304.

[23]  J. P. Huchra,et al.  M31 Globular Clusters: Colors and Metallicities* ** *** , 2000 .

[24]  Sidney van den Bergh,et al.  Globular clusters and the formation of the outer Galactic halo , 2004 .

[25]  Nial R. Tanvir,et al.  Distances and metallicities for 17 Local Group galaxies , 2005 .

[26]  F. Ochsenbein,et al.  The VizieR database of astronomical catalogues , 2000, astro-ph/0002122.

[27]  M. Irwin,et al.  INT WFS pipeline processing , 2001 .

[28]  S. Larsen,et al.  New Members of the Cluster Family in Nearby Lenticular Galaxies , 2002, astro-ph/0203454.

[29]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[30]  Sidney van den Bergh.  Some integrated properties of galactic globular clusters , 1996 .

[31]  Mike Irwin,et al.  Structural parameters for the Galactic dwarf spheroidals , 1995 .

[32]  P. Hut,et al.  UvA-DARE ( Digital Academic Repository ) A Dynamical Model for the Globular Cluster G 1 , 2003 .

[33]  J. C. Muzzio,et al.  Are Globular Clusters the Nuclei of Cannibalized Dwarf Galaxies , 1994 .

[34]  M. Fellhauer,et al.  The Possible Origin of the Faint Fuzzy Star Clusters in NGC 1023 , 2002, astro-ph/0207046.

[35]  Nelson Caldwell,et al.  Surface Brightness Profiles of Three New Dwarf Spheroidal Companions to M31 , 1999, astro-ph/9905302.

[36]  Annette Ferguson,et al.  Evidence for Stellar Substructure in the Halo and Outer Disk of M31 , 2002, astro-ph/0205530.

[37]  L. Federici,et al.  2MASS NIR photometry for 693 candidate globular clusters in M 31 and the Revised Bologna Catalogue , 2003, astro-ph/0312448.

[38]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .