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Astron. Astrophys. 357, 871-880 (2000)

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1. Introduction

The Sagittarius dwarf galaxy is the closest known member of the Local Group orbiting around the Milky Way ([FORMULA]25 kpc from the sun, [FORMULA]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 ([FORMULA]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 [FORMULA] 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 ([FORMULA] mag.arcsec-2), contamination by foreground Galactic stars and its large spatial extent (at least [FORMULA]) (Ibata et al. 1997, hereafter IWGIS). Evidence for the presence of Sgr has been established over [FORMULA] from b[FORMULA] (Alard 1996, hereafter A96; Alcock et al. 1997, hereafter Alc97) down to b[FORMULA] (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 [FORMULA] deg2 from [FORMULA] down to [FORMULA]. However, their method based on statistical decontamination fails at low Galactic latitudes [FORMULA] where differential reddening and high density of foreground stars (only [FORMULA]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 [FORMULA] RRab were detected between [FORMULA] and [FORMULA] (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 [FORMULA]1 500 RRab members of Sgr and located in its Northern extension. We present a surface density map of Sgr covering [FORMULA]50 deg2 between [FORMULA] and [FORMULA], 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.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000