AAS 197, January 2001
Session 119. The Cosmic Distance Scale
Oral, Thursday, January 11, 2001, 10:30am-12:00noon, San Diego

## [119.01] Infrared Surface Brightness Fluctuation Measurements Using the Hokupaa Adaptive Optics System on Gemini

J. B. Jensen, M. R. Chun (Gemini), J. L. Tonry (Institute for Astronomy)

Infrared measurements of surface brightness fluctuations (SBFs) can be used to accurately measure extragalactic distances and probe the properties of the brightest stars in unresolved stellar populations. Observations using NICMOS on the Hubble Space Telescope demonstrated that fluctuations can be measured to distances greater than 100 Mpc in modest exposure times because of the excellent spatial resolution attainable from space (Jensen et al. 2000). Using the Gemini 8-m telescope and University of Hawaii adaptive optics system Hokupaa'', we collected even sharper H-band (1.6 micron) images of the central galaxies in the Abell 262 and 2666 clusters that have 65 milliarcsecond spatial resolution. We show that although the PSF varies spatially, it is still possible to achieve a good fit to the data. These galaxies were selected to provide overlap with WFPC-2 and NICMOS SBF measurements, and the Hokupaa results are consistent with the HST results. We find that the required exposure time in sec using Hokupaa on Gemini is approximately t = 200 \times (cz/1000)2 \times s where s is the seeing FWHM in arcsec and cz is measured in km/s. At a distance of 100 Mpc (or a redshift of 7500 km/s) with 0.1 arcsec seeing, an exposure time of 20 minutes is sufficient to make a reliable SBF measurement. These are the first SBF measurements made using adaptive optics; additional Hokupaa observations are scheduled for the last quarter of 2000 to explore the limits of this technique.

The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation on behalf of the international Gemini partnership. Hokupaa was built at the University of Hawaii Institute for Astronomy with support from the National Science Foundation.