Discovery of a stellar system in the background of 47 Tucanae.pdf
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Astronomy Astrophysics manuscript no. final February 2, 2008
(DOI: will be inserted by hand later)
Discovery of a stellar system in the background of 47 Tucanae
?
A new cluster of the Small Magellanic Cloud?
M. Bellazzini
1
, E. Pancino
1
and F.R. Ferraro
2
1
INAF - Osservatorio Astronomico di Bologna Via Ranzani 1, I-40127, Bologna, Italy
e-mail: michele.bellazzini, elena.pancino@bo.astro.it
2
Universita? di Bologna, Dipartimento di Astronomia, Via Ranzani 1, I-40127, Bologna, Italy
e-mail: francesco.ferraro3@unibo.it
Received ; accepted
Abstract. We report on the discovery of a stellar system in the background of the Galactic globular cluster 47 Tucanae (NGC
104), located 14.8′ North-West of the cluster center. The object, whose apparent diameter is D ? 30′′ , is partially resolved into
stars on the available CCD images, reaching a limiting magnitude of V ～ 22.5, and is detected as a significant (more than 5σ)
overdensity of blue stars (B ? V 0.7). The color magnitude diagram of the system, its characteristic projected size and its
position in the sky suggest that it is an intermediate-old age cluster belonging to the Small Magellanic Cloud, whose outskirts
lie in the background of 47 Tuc. Although less likely, the possibility that the object is an unknown dwarf galaxy in the outskirts
of the Local Group cannot completely be ruled out by the present data.
Key words. globular clusters: individual: Bo A
1. Introduction
The advent of modern surveys exploring large swaths of the
sky at different wavelenghts, that provided deeper insight into
poorly explored regions of the Galaxy and its surroundings,
has lead to a burst of discoveries and cataloguing of new stel-
lar systems and structures that has no precedent in the past
three decades. For example, the insight into high extinction re-
gions provided by infrared imaging (2MASS, Spitzer) has al-
lowed the recognition of several stellar systems that
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