Astron. Astrophys. 357, 871-880 (2000)
1. Introduction
The Sagittarius dwarf galaxy is the closest known member of the
Local Group orbiting around the Milky Way
( 25 kpc from the sun,
16 kpc from the Galactic Centre), but
as a consequence of its location behind the Galactic Centre, it has
been discovered only recently (Ibata et al. 1994, 1995). Since this
discovery it turned out that Sgr presents typical features of a dwarf
spheroidal: domination of an old
( 10 Gyr) metal poor stellar population
(Mateo et al. 1995; Fahlman et al. 1996; Marconi et al. 1998;
Bellazzini et al. 1999) and absence of gas (Burton & Lockman
1999). Its highest surface density region is centred on the Globular
Cluster M54 and it is oriented
roughly perpendicular to the Galactic plane so that its Northern
extension (in Galactic coordinates) is completely hidden by the
MW.
The mapping of Sgr is difficult to achieve because of the
combination of its low surface brightness
( mag.arcsec-2),
contamination by foreground Galactic stars and its large spatial
extent (at least ) (Ibata et al.
1997, hereafter IWGIS). Evidence for the presence of Sgr has been
established over from
b (Alard 1996, hereafter A96; Alcock
et al. 1997, hereafter Alc97) down to
b (Mateo et al. 1998, hereafter MOM),
but it is difficult to assess whether these regions still correspond
to the main body of Sgr or if we are merely encountering tidal debris
(as suggested by Johnston et al. 1999). IWGIS proposed a map of the
Southern part of Sgr based on the spatial distribution of the bright
main sequence stars in Sgr and covering an area of
deg2 from
down to
. However, their method based on
statistical decontamination fails at low Galactic latitudes
where differential reddening and
high density of foreground stars (only
1 star in 1 000 is in Sgr in these
regions) preclude any reliable decontamination, leaving the structure
of the Northern extension of Sgr almost unknown. To this point, the
detection of RR Lyrae constitutes an essential tool to trace the
structure of Sgr in these regions as they can be clearly separated
from the RR Lyrae of the MW. This method has already proven successful
and RRab were detected between
and
(A96; Alc97). However, a connection between these stars and the centre
of Sgr was necessary in order to offer a clear vision of this
important region strongly interacting with the MW.
In this paper we report the detection of
1 500 RRab members of Sgr and located
in its Northern extension. We present a surface density map of Sgr
covering 50 deg2 between
and ,
based on the spatial distribution of these variables.
The paper is organized as follows: in Sect. 2 we present our data
(observations and reduction). Sect. 3 is devoted to the description of
the selection process of RR Lyrae stars as well as a study of its
completeness. We then describe the structure of Sgr (Sect. 4). Finally
we summarize our results and conclude in Sect. 5.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000
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