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Ultra-Compact Dwarf Galaxies in the Fornax Cluster

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 نشر من قبل Michael Drinkwater
 تاريخ النشر 2001
  مجال البحث فيزياء
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By utilising the large multi-plexing advantage of the 2dF spectrograph on the Anglo-Australian Telescope, we have been able to obtain a complete spectroscopic sample of all objects in a predefined magnitude range, 16.5<Bj<19.7, regardless of morphology, in an area towards the centre of the Fornax Cluster of galaxies. Among the unresolved or marginally resolved targets we have found five objects which are actually at the redshift of the Fornax Cluster, i.e. they are extremely compact dwarf galaxies or extremely large star clusters. All five have absorption line spectra. With intrinsic sizes less than 1.1 arc second HWHM (corresponding to approximately 100 pc at the distance of the cluster), they are more compact and significantly less luminous than other known compact dwarf galaxies, yet much brighter than any globular cluster. In this letter we present new ground based optical observations of these enigmatic objects. In addition to having extremely high central surface brightnesses, these objects show no evidence of any surrounding low surface brightness envelopes down to much fainter limits than is the case for, e.g., nucleated dwarf ellipticals. Thus, if they are not merely the stripped remains of some other type of galaxy, then they appear to have properties unlike any previously known type of stellar system.



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We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5 to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of galaxies. Amo ng the unresolved targets are five objects which are members of the Fornax Cluster. They are extremely compact stellar systems with scale lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known type of stellar system, being more compact and significantly less luminous than other compact dwarf galaxies, yet much brighter than any globular cluster.
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