STELLAR POPULATIONS FROM ADAPTIVE OPTICS OBSERVATIONS - 4 TEST CASES

We describe a first attempt to apply adaptive optics to the study of r esolved stellar populations in galaxies. Advantages over traditional a pproaches are (i) improved spatial resolution and point-source sensiti vity through adaptive optics, and (ii) use of the near-infrared region , where the peak of the spectral energy distribution for old populatio ns is found. Disadvantages are the small area covered and the need for excellent seeing. We made observations with the ADONIS system at the European Southern Observatory of the peculiar elliptical galaxy NGC 51 28; the irregular galaxy IC 5152 (a possible outer member of the Local Group); the Sc galaxy NGC 300 (a member of the Sculptor group); and t he Sgr window in the bulge of the Milky Way. These different fields gi ve excellent test cases for the potential of adaptive optics. In the f irst two cases, we failed to obtain photometry of individual stars, wh ich would have required excellent seeing. For NGC 300 we measured magn itudes for nine individual supergiants (H = 18.3-20.2), but did not go deep enough to detect the tip of the RGB of an old population. For th e Sgr field we produced a infrared luminosity function and colour-magn itude diagram for 70 stars down to K similar or equal to 19.5. These a re the deepest yet measured for the Galactic bulge, reaching beyond th e turn-off.