WIDE-FIELD CAMERA OBSERVATIONS OF BAADE WINDOW

We have observed a field in Baade's Window using the Wide Field Camera of the Hubble Space Telescope (HST) and obtain V- and I-band photomet ry down to V approximately 22.5. These data go several magnitudes fain ter than previously obtained from the ground. The location of the brea k in the luminosity function suggests that there are a significant num ber of intermediate age (< 10 Gyr) stars in the Galactic bulge. This c onclusion rests on the assumptions that the extinction towards our fie ld is similar to that seen in other parts of Baade's Window, that the distance to the bulge is approximately 8 kpc, and that we can determin e fairly accurate zero points for the HST photometry. Changes in any o ne of these assumptions could increase the inferred age, but a conspir acy of lower reddening, a shorter distance to the bulge, and/or photom etric zero-point errors would be needed to imply a population entirely older than 10 Gyr. We infer an initial mass function slope for the ma in-sequence stars, and find that it is consistent with that measured i n the solar neighborhood; unfortunately, the slope is poorly constrain ed because we sample only a narrow range of stellar mass and because o f uncertainties in the observed luminosity function at the faint end.