Surface Brightness and Stellar Populations at the Outer Edge of the Large Magellanic Cloud: No Stellar Halo Yet

Abstract

We present a high-quality color-magnitude diagram (CMD) for a36^{'}{\times}36^{'} field located 8{\deg} (~=7 kpc) fromthe Large Magellanic Cloud (LMC) center, as well as a precisedetermination of the LMC surface brightness derived from the resolvedstellar population out to this large galactocentric radius. This deepCMD shows for the first time the detailed age distribution at thisposition, where the surface brightness is V~=26.5 magarcsec^{-2}. At a radius R~=474^{'} the main sequence iswell populated from the oldest turnoff at I~=21.5 to the 2.5 Gyr turnoffat I~=19.5. Beyond this radius, a relatively strong gradient in thedensity of stars with ages in the ~=2.5-4 Gyr range is apparent. Thereare some stars brighter and bluer than the main population, quiteuniformly distributed over the whole area surveyed, which are wellmatched by a 1.5 Gyr isochrone and may be indicative of a relativelyrecent star formation, or merger, event. The surface brightness profileof the LMC remains exponential to this large galactocentric radius andshows no evidence of disk truncation. Combining the information onsurface brightness and stellar population, we conclude that the LMC diskextends (and dominates over a possible stellar halo) out to a distanceof at least 7 kpc. These results confirm that the absence of blue starsin the relatively shallow off-center CMDs of dwarf irregular galaxies isnot necessarily evidence for an exclusively old stellar populationresembling the halo of the Milky Way.