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

C. Gallart, P. B. Stetson, E. Hardy, F. Pont, R. Zinn

Published 2004-09-01Version 1

We present a high quality CMD for a 36'x 36' field located 8 degrees (7 kpc) from the LMC center, as well as a precise determination of the LMC surface brightness derived from the resolved stellar population out to this large galactocentric radius. This deep CMD shows for the first time the detailed age distribution at this position, where the surface brightness is V=26.5 mag/sq". At a radius R=474' the main sequence is well populated from the oldest turnoff at I=21.5 to the 2.5 Gyr turnoff at I=19.5. Beyond this radius, a relatively strong gradient in the density of stars with ages in the 2.5-4 Gyr range is apparent. There are some stars brighter and bluer than the main population, quite uniformly distributed over the whole area surveyed, which are well matched by a 1.5 Gyr isochrone and may be indicative of a relatively recent star formation, or merger, event. The surface brightness profile of the LMC remains exponential to this large galactocentric radius and shows no evidence of disk truncation. Combining the information on surface brightness and stellar population we conclude that the LMC disk extends (and dominates over a possible stellar halo) out to a distance of at least 7 kpc. These results confirm that the absence of blue stars in the relatively shallow off-center CMDs of dIrr galaxies is not necessarily evidence for an exclusively old stellar population resembling the halo of the Milky Way.