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[27.11] HST Far-Ultraviolet Photometry of Globular Clusters in M87

S.T. Sohn, R.W. O'Connell (UVa), A. Kundu (MSU), W.B. Landsman (GSFC, NASA), D. Burstein (ASU), R. Bohlin (STScI), J.A. Frogel (OSU), R.T. Rood (UVa), J.A. Rose (UNC)

The ``ultraviolet-upturn'' in elliptical galaxies -- a sharp rise in the spectrum shortward of 2000Å\ -- is believed to originate from extreme horizontal branch (EHB) stars with T_eff > 16000\,K. Properly calibrated, it could be the most sensitive probe of the ages and abundances of elliptical galaxies. The rich globular cluster systems around nearby galaxies offer the best means of relating the production of EHB stars to the characteristics of their parent populations since they are (1) well understood from the standpoint of evolution; and (2) mostly simple populations with a small internal dispersion in age and abundance.

We present HST/STIS far-UV photometry of globular clusters in the gE galaxy M87 in 3 different fields (FOV = 25\arcsec) at 23\arcsec, 38\arcsec, and 73\arcsec\ radius. Exposure times were 10,480 secs. The imaging was done in TIMETAG mode to maximize the low sky background observing periods. We detected 53 clusters with magnitudes in the range 22 \la m₁₄₆₀ \la 25. Comparing with the WFPC2 photometry of Kundu et al.\ (1999, ApJ, 513, 733), we detected 63% of the clusters brighter than V = 23 in our far-UV images. Five of the UV detections have no optical counterparts. The color distribution of the clusters has a strong peak at m₁₄₆₀ - V = 1.6 and a mean of 1.8. They appear to have significantly stronger EHB components than is typical of Milky Way globulars. Most of the detected clusters belong to the bluer of the two subpopulations found by Kundu et al., presumably the more metal-poor and older subsystem.

This work was supported by NASA through grant GO-08643 from the Space Telescope Science Institute.

The author(s) of this abstract have provided an email address for comments about the abstract: ss5fb@astsun.astro.virginia.edu

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.