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

To empirically calibrate the IR surface brightness fluctuation (SBF) distancescale and probe the properties of unresolved stellar populations, we measuredfluctuations in 65 galaxies using NICMOS on the Hubble Space Telescope. Theearly-type galaxies in this sample include elliptical and S0 galaxies andspiral bulges in a variety of environments. Absolute fluctuation magnitudes inthe F160W (1.6 micron) filter were derived for each galaxy usingpreviously-measured I-band SBF and Cepheid variable star distances. F160W SBFscan be used to measure distances to early-type galaxies with a relativeaccuracy of ~10% provided that the galaxy color is known to ~0.035 mag orbetter. Near-IR fluctuations can also reveal the properties of the mostluminous stellar populations in galaxies. Comparison of F160W fluctuationmagnitudes and optical colors to stellar population model predictions suggeststhat bluer elliptical and S0 galaxies have significantly younger populationsthan redder ones, and may also be more metal-rich. There are no galaxies inthis sample with fluctuation magnitudes consistent with old, metal-poor (t>5Gyr, [Fe/H]<-0.7) stellar population models. Composite stellar populationmodels imply that bright fluctuations in the bluer galaxies may be the resultof an episode of recent star formation in a fraction of the total mass of agalaxy. Age estimates from the F160W fluctuation magnitudes are consistent withthose measured using the H-beta Balmer line index. The two types ofmeasurements make use of completely different techniques and are sensitive tostars in different evolutionary phases. Both techniques reveal the presence ofintermediate-age stars in the early-type galaxies of this sample.Comment: 38 pages, 8 figures; accepted for publication in Ap