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NOAO Newsletter - NOAO Highlights - September 2000 - Number 63


M32 --- a Galaxy Becomes a Star Cluster

Tod R. Lauer

Tim Davidge (National Research Council of Canada), François Rigaut (Gemini), Wolfgang Brandner, and Dan Potter (Hawaii) used the Hokupa'a/QUIRC adaptive optics camera on the Gemini North 8-m reflector to obtain superbly sharp near-IR images of the center of the Local Group galaxy M32. The images, obtained in the H and K bands, have 0.13" FWHM resolution, which exceeds the resolving power of the Hubble Space Telescope at these wavelengths.

The most luminous stars appear to start to be resolved outside the central 2" of the nucleus. Already in the HST I-band image, the granular texture shows that surface brightness fluctuations (point-to-point random variation in the number of stars contributing to any pixel) are the dominant noise source. Comparison of the Hokupa'a H and K images is likely to reveal new information about the evolved stellar population of M32. The nucleus of M32 itself is extremely dense, exceeding densities of 107 M/pc3 in stars alone.

HST in the I-band has ~30% better resolution than the K-band Hokupa'a image. The Hokupa'a image, however, has over two times finer sampling; it has thus been rotated and binned to match the HST image in the figure below. The Hokupa'a data comprises sixteen 30s exposures, while the HST data is the sum of four 26s WFPC2 exposures. The region shown for both images is a 19.2 x 19.2" square field offset slightly from the M32 nucleus. Both have been deconvolved and are displayed with a logarithmic stretch spanning a factor of one thousand in surface brightness. The M32 data were obtained in engineering time in support of early observations being conducted with Gemini, and will be released publicly this month.

Star cluster-like core of M32
Caption: The star cluster-like appearance of the core of M32 in the Hokupa'a K-band image (left above) illustrates the potential that this instrument offers for crowded-field photometry. Comparison with an HST I-band (F814W) image (right) shows the increasing contribution in the K-band of light from giant stars, causing the dramatic change in appearance of the galaxy as one moves further out into the near-IR.


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