Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations

Abstract

To empirically calibrate the IR surface brightness fluctuation (SBF) distance\nscale and probe the properties of unresolved stellar populations, we measured\nfluctuations in 65 galaxies using NICMOS on the Hubble Space Telescope. The\nearly-type galaxies in this sample include elliptical and S0 galaxies and\nspiral bulges in a variety of environments. Absolute fluctuation magnitudes in\nthe F160W (1.6 micron) filter were derived for each galaxy using\npreviously-measured I-band SBF and Cepheid variable star distances. F160W SBFs\ncan be used to measure distances to early-type galaxies with a relative\naccuracy of ~10% provided that the galaxy color is known to ~0.035 mag or\nbetter. Near-IR fluctuations can also reveal the properties of the most\nluminous stellar populations in galaxies. Comparison of F160W fluctuation\nmagnitudes and optical colors to stellar population model predictions suggests\nthat bluer elliptical and S0 galaxies have significantly younger populations\nthan redder ones, and may also be more metal-rich. There are no galaxies in\nthis sample with fluctuation magnitudes consistent with old, metal-poor (t>5\nGyr, [Fe/H]<-0.7) stellar population models. Composite stellar population\nmodels imply that bright fluctuations in the bluer galaxies may be the result\nof an episode of recent star formation in a fraction of the total mass of a\ngalaxy. Age estimates from the F160W fluctuation magnitudes are consistent with\nthose measured using the H-beta Balmer line index. The two types of\nmeasurements make use of completely different techniques and are sensitive to\nstars in different evolutionary phases. Both techniques reveal the presence of\nintermediate-age stars in the early-type galaxies of this sample.\n