We have obtained deep photometric data in 24 fields along the southeast extension of the major axis of the Sagittarius dwarf spheroidal ( Sgr dSph ) galaxy , and in four fields along the northwest extension .
Using star counts at the expected position of the Sgr upper main-sequence within the resulting color-magnitude diagrams ( CMDs ) , we unambiguously detect Sgr stars in the southeast over the range 10–34 ^ { \circ } from the galaxy ’ s center .
If Sgr is symmetric , this implies a true major-axis diameter of at least 68 ^ { \circ } , or nearly 30 kpc if all portions of Sgr are equally distant from the Sun .
Star counts parallel to the galaxy ’ s minor-axis reveal that Sgr remains quite broad far from its center .
This suggests that the outer portions of Sgr resemble a stream rather than an extension of the ellipsoidal inner regions of the galaxy .
The inferred V-band surface brightness ( SB ) profile ranges from 27.3-30.5 mag arcsec ^ { -2 } over this radial range and exhibits a change in slope \sim 20 ^ { \circ } from the center of Sgr .
The scale length of the outer SB profile is 17.2 ^ { \circ } , compared to 4.7 ^ { \circ } in the central region of Sgr .
We speculate that this break in the SB profile represents a transition from the main body of Sgr to a more extended ‘ Sgr stream ’ .
By integrating the SB profile we estimate that the absolute visual magnitude of Sgr lies in the range -13.4 to -14.6 , depending on the assumed structure of Sgr ; an upper limit to the luminosity of Sgr is therefore L \sim 5.8 \times 10 ^ { 7 } L _ { \odot } .
This result lowers the M/L _ { V } ratio inferred for Sgr by Ibata et al .
( 1997 ) down to \sim 10 , consistent with values observed in the most luminous dSph companions of the Milky Way .

Subject headings : Galaxy : halo — Galaxy : structure — galaxies : individual ( Sgr dSph ) — galaxies : interactions — galaxies : Local Group