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T.M. Brown, C.W. Bowers, R.A. Kimble, A.V. Sweigart (GSFC), H.C. Ferguson (STScI)
We present the deepest near-UV image of M32, which for the first time resolves hot horizontal branch (HB) stars in an elliptical galaxy. Given the near-solar metallicity of M32, much larger than that of globular clusters, the existence of an extended horizontal branch is a striking example of the second parameter effect, and, most importantly, provides direct evidence that hot HB stars are the major contributors to the UV upturn phenomenon observed in elliptical galaxies. Our image, obtained with the Space Telescope Imaging Spectrograph (STIS), detects approximately 8000 stars in a 25x25 arcsec field, centered 7.7 arcsec from the galaxy core. These stars span a range of 21-28 mag in the STMAG system, and in the deepest parts of the image, our catalog is reasonably complete (> 25%) to a magnitude of 27. The hot HB spans a magnitude range of 25-27 at effective temperatures hotter than 8500 K. We interpret the luminosity function with an extensive set of HB and post-HB evolutionary tracks.
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