## 4. Structure of the Sagittarius dwarf galaxy## 4.1. Surface density of SgrA surface density map is constructed from RRab with the amplitude cuts stated above. The spatial distribution of these RRab has been convolved with a Gaussian on a grid with a step of and a variable filter size adapted to the local surface density , constrained between and . This map was then corrected for the different completeness in amplitude and crowding (see Sect. 3.2). The resulting map is shown on Fig. 7 where the elongated shape of Sgr is clearly visible. This is the first map of Sgr in these regions, showing that Sgr extends far beyond the outer limit of the map previously published by IWGIS. One of the most striking features of this map is the slow decrease (if any?) of the density along the main axis of Sgr for .
The main source of uncertainty in the surface density is the Poissonian noise in the star counts, which is variable over the field and tends to increase towards lower b. To estimate this noise we simulated 1 000 maps by injecting 1 400 stars (corresponding to the number of RRab actually used to construct the final map) onto the field with a probability density matching the surface density of the real map. These spatial distributions were then processed exactly in the same manner as the real one and a 1 "noise map" has been deduced. This map is shown on Fig. 8 where the contours are labelled in percent. The typical (relative) uncertainty is constrained between 10 and 15 over the main part of the field, but increases up to 40 towards the edges where the number of RRab drops.
## 4.2. Surface density profile of the main axisThe position angle of Sgr has been determined by fitting an
exponential to the surface density along various directions. The
highest scale length was reached for an angle of
, which we choosed as the direction
of the major axis. Fig. 9 displays the density profile of Sgr along
that axis. This figure is based on a map smoothed on a constant scale
of . The thick line is the density
after correction for completeness whereas the dotted line is the
density before that correction. The shaded region represents the
1 uncertainty issued from simulated
maps. A discontinuity in the slope is clearly visible at
from the centre. After this point
the surface density seems to be almost constant. It is however
disconcerting that this discontinuity occurs near the limit between
It is also possible to derive an upper limit for the extension of Sgr along the line of sight: the distance modulus histogram can be roughly fitted by a Gaussian with a width of 0.2 mag, corresponding to a depth of 4.5 kpc for an assumed distance of 24 kpc. ## 4.3. Model fittingWe define the following analytical functions to fit to the density profile: Where
The single function model that best fits Concerning the core of Sgr ( Finally, the best fit on © European Southern Observatory (ESO) 2000 Online publication: June 5, 2000 |