Measuring distances using infrared surface brightness fluctuations

Surface brightness fluctuations (SBFs) are much brighter in the infrared th an they are at optical wavelengths, making it possible to measure greater d istances using IR SBFs. We report new K' (2.1 mu m) SBF measurements of nin e galaxies in the Fornax and Eridanus clusters using a 1024(2) pixel HgCdTe array. We used improved analysis techniques to remove contributions to the SBFs from globular clusters and background galaxies, and we assess the rel ative importance of other sources of residual variance. We applied the impr oved methodology to our Fornax and Eridanus images and to our previously pu blished Virgo cluster data. Apparent fluctuation magnitudes were used in co njunction with Cepheid distances to M31 and the Virgo cluster to calibrate the K' SBF distance scale. We find the absolute fluctuation magnitude (M) o ver bar(K') = -5.61 +/- 0.12, with an intrinsic scatter of 0.06 mag to the calibration. No statistically significant change in (M) over bar(K') is det ected as a function of (V-I). Our calibration is consistent with simple (co nstant age and metallicity) stellar population models. The lack of a correl ation with (V-I) in the context of the stellar population models implies th at elliptical galaxies bluer than (V-I) = 1.2 have SBFs dominated by younge r (5-8 Gyr) populations and metallicities comparable to redder ellipticals. Significant contributions to the SBFs from anomalous populations of asympt otic giant branch stars are apparently uncommon in giant ellipticals. K' SB Fs prove to be a reliable distance indicator as long as the residual varian ce from globular clusters and background galaxies is properly removed. Also , it is important that a sufficiently high signal-to-noise ratio be achieve d to allow reliable sky subtraction because residual spatial variance can b ias the measurement of the SBF power spectrum.