Spectroscopic Metallicities for Fornax Ultracompact Dwarf Galaxies, Globular Clusters, and Nucleated Dwarf Elliptical Galaxies

Various formation channels for the puzzling ultracompact dwarf galaxies (UCDs) have been proposed in the last few years. To better judge some of the competing scenarios, we present spectroscopic [Fe/H] estimates for a sample of 26 compact objects in the central region of the Fornax Cluster, covering the magnitude range of UCDs and bright globular clusters (18 mag < V < 21 mag or -13.4 mag < MV < -10.4 mag). We find a break in the metallicity distribution of compact objects at MV ≃ -11 mag (≃3 × 106 M⊙): for MV < -11 mag the mean metallicity is [Fe/H] = -0.62 ± 0.05 dex, 0.56 ± 0.15 dex higher than the value of -1.18 ± 0.15 dex found for MV > -11 mag. This metallicity break is accompanied by a change in the size-luminosity relation for compact objects, as deduced from Hubble Space Telescope imaging: for MV < -11 mag, rh scales with luminosity, while for MV > -11 mag, rh is almost luminosity-independent. In our study we therefore assume a limiting absolute magnitude of MV = -11 mag between UCDs and globular clusters. The mean metallicity of five Fornax nucleated dwarf elliptical galaxy (dE,N) nuclei included in our study is about 0.8 dex lower than that of the UCDs, a difference significant at the 4.5 σ level. This difference is marginally higher than expected from a comparison of their (V - I) colors, indicating that UCDs are younger than or at most coeval to dE,N nuclei. Because of the large metallicity discrepancy between UCDs and nuclei, we disfavor the hypothesis that most of the Fornax UCDs are the remnant nuclei of tidally stripped dE,Ns. Our metallicity estimates for UCDs are closer to but slightly below those derived for young massive clusters (YMCs) of comparable masses. We therefore favor a scenario in which most UCDs in Fornax are successors of merged YMCs produced in the course of violent galaxy-galaxy mergers. It is noted that, in contrast, the properties of Virgo UCDs are more consistent with the stripping scenario, suggesting that different UCD formation channels may dominate in either cluster.

[1]  N. Bastian,et al.  The transition between star clusters and dwarf galaxies - On the existence of a mass-radius relation for star clusters of masses >$\mathsf{10^7}$ M$_\odot$: are these objects formed in mergers of stellar systems? , 2005, astro-ph/0512360.

[2]  C. Maraston,et al.  Dynamical mass estimates for two luminous star clusters in galactic merger remnants , 2005, astro-ph/0511033.

[3]  J. Tonry,et al.  The ACS Virgo Cluster Survey. IX. The Color Distributions of Globular Cluster Systems in Early-Type Galaxies , 2005, astro-ph/0509654.

[4]  S. M. Fall,et al.  The Age Distribution of Massive Star Clusters in the Antennae Galaxies , 2005, astro-ph/0509293.

[5]  C. Maraston,et al.  Young star cluster complexes in NGC 4038/39 - Integral field spectroscopy using VIMOS-VLT , 2005, astro-ph/0509249.

[6]  Q. A. Parker,et al.  Discovery of Ultracompact Dwarf Galaxies in the Virgo Cluster , 2005, astro-ph/0509120.

[7]  J. Tonry,et al.  The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation , 2005, astro-ph/0508219.

[8]  SRON,et al.  Hierarchical Star Formation in M51: Star/Cluster Complexes , 2005, astro-ph/0508110.

[9]  S. Djorgovski,et al.  The ACS Virgo Cluster Survey. VII. Resolving the Connection between Globular Clusters and Ultracompact Dwarf Galaxies , 2005, astro-ph/0503566.

[10]  M. Gregg,et al.  A Comparison of Surface Brightness Profiles for Ultracompact Dwarfs and Dwarf Elliptical Nuclei: Implications for the “Threshing” Scenario , 2005, astro-ph/0503377.

[11]  M. Fellhauer,et al.  A possible formation scenario for the ultramassive cluster W3 in NGC 7252 , 2005, astro-ph/0502086.

[12]  N. Benı́tez,et al.  Discovery of two M32 twins in Abell 1689 , 2004, astro-ph/0412263.

[13]  R. Klessen,et al.  Formation of Globular Clusters in Galaxy Mergers , 2004, astro-ph/0407248.

[14]  W. Couch,et al.  Cluster Cannibalism and Scaling Relations of Galactic Stellar Nuclei , 2004, astro-ph/0406442.

[15]  H. Ferguson,et al.  The Colors of Dwarf Elliptical Galaxy Globular Cluster Systems, Nuclei, and Stellar Halos , 2004, astro-ph/0406002.

[16]  J. Tonry,et al.  The ACS Virgo Cluster Survey. I. Introduction to the Survey , 2004, astro-ph/0404138.

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

[18]  E. Grebel,et al.  The Globular Cluster System of NGC 1399. III. VLT Spectroscopy and Database , 2004, astro-ph/0401218.

[19]  E. Grebel,et al.  The Globular Cluster System of NGC 1399. II. Kinematics of a Large Sample of Globular Clusters , 2004, astro-ph/0401175.

[20]  S. Zepf,et al.  The Globular Cluster Systems of the Early-Type Galaxies NGC 3379, NGC 4406, and NGC 4594 and Implications for Galaxy Formation , 2003, astro-ph/0310277.

[21]  G. Bruzual,et al.  Stellar population synthesis at the resolution of 2003 , 2003, astro-ph/0309134.

[22]  W. Couch,et al.  Galaxy threshing and the origin of ultra-compact dwarf galaxies in the Fornax cluster , 2003, astro-ph/0308243.

[23]  S. Phillipps,et al.  A class of compact dwarf galaxies from disruptive processes in galaxy clusters , 2003, Nature.

[24]  A. Kravtsov,et al.  Formation of Globular Clusters in Hierarchical Cosmology , 2003, astro-ph/0305199.

[25]  M. Gregg,et al.  The surface brightness and colour–magnitude relations for Fornax cluster galaxies , 2003, astro-ph/0301463.

[26]  Doug Geisler,et al.  The Globular Cluster System of NGC 1399. I. A Wide-Field Photometric Study , 2003, astro-ph/0301223.

[27]  Santiago,et al.  Faint dwarf spheroidals in the Fornax Cluster. A flat luminosity function , 2002, astro-ph/0212044.

[28]  C. Maraston,et al.  Integrated spectroscopy of bulge globular clusters and fields - I. The data base and comparison of individual Lick indices in clusters and bulge , 2002, astro-ph/0209238.

[29]  S. Larsen.  The Luminosity Function of Star Clusters in Spiral Galaxies , 2002, astro-ph/0206295.

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

[31]  S. Okamura,et al.  A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. V. Dependence of the Spectroscopic Properties on Location in the Cluster , 2001, astro-ph/0111189.

[32]  M. Fellhauer,et al.  The formation of ultracompact dwarf galaxies , 2001, astro-ph/0110621.

[33]  M. Gregg,et al.  Ultracompact Dwarf Galaxies in the Fornax Cluster , 2001, astro-ph/0106377.

[34]  J. Schombert,et al.  Ages and Metallicities of Fornax Dwarf Elliptical Galaxies , 2000, astro-ph/0012517.

[35]  Astrophysics,et al.  Substructure and Dynamics of the Fornax Cluster , 2000, astro-ph/0012415.

[36]  D. McLaughlin.  Binding Energy and the Fundamental Plane of Globular Clusters , 2000, astro-ph/0002086.

[37]  S. Phillipps,et al.  Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies? , 2000, Publications of the Astronomical Society of Australia.

[38]  K. Gebhardt,et al.  Globular Cluster Systems. I. V−I Color Distributions , 1999, astro-ph/9906499.

[39]  B. Whitmore,et al.  Wide Field Planetary Camera 2 Imaging of the Globular Cluster System of the S0 Galaxy NGC 3115 , 1998 .

[40]  H Germany,et al.  The central region of the Fornax cluster II. Spectroscopy and radial velocities of member and background galaxies , 1998, astro-ph/9807144.

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

[42]  S. Faber,et al.  Old Stellar Populations. VI. Absorption-Line Spectra of Galaxy Nuclei and Globular Clusters , 1997, astro-ph/9712258.

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

[44]  C. Grillmair,et al.  Keck Spectroscopy of Globular Clusters around NGC 1399 , 1997, astro-ph/9710117.

[45]  C. Grillmair,et al.  On the origin of globular clusters in elliptical and cD galaxies , 1997, astro-ph/9702146.

[46]  P. Hut,et al.  Merger Rate of Equal-Mass Spherical Galaxies , 1996, astro-ph/9608159.

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

[48]  W. Harris,et al.  Supergiant Molecular Clouds and the Formation of Globular Cluster Systems , 1994 .

[49]  J. Mould,et al.  Spectroscopy of dwarf elliptical galaxies in the fornax cluster , 1993, astro-ph/9312042.

[50]  B. Elmegreen,et al.  An interaction model for the formation of dwarf galaxies and 10 exp 8 solar mass clouds in spiral disks , 1993 .

[51]  J. Huchra,et al.  Extragalactic Globular Clusters. III. Metallicity Comparisons and Anomalies , 1991 .

[52]  W. Harris,et al.  BVI Photometry of Globular Clusters in M87 , 1990 .

[53]  H. Ferguson,et al.  The spatial distributions and intrinsic shapes of dwarf elliptical galaxies in the Virgo and Fornax Clusters , 1989 .

[54]  Michael J. West,et al.  The globular cluster system of the Galaxy. III: measurements of radial velocity and metallicity for 60 clusters and a compilation of metallicities for 121 clusters , 1984 .

[55]  Helmut Jerjen,et al.  Near-field Cosmology with Dwarf Elliptical Galaxies , 2005 .