Measuring distances using infrared surface brightness fluctuations

Surface brightness fluctuations (SBFs) are much brighter in the infrared than they are at optical wavelengths, making it possible to measure greater distances using IR SBFs. We report new K' (2.1 mu m) SBF measurements of nine 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 relative importance of other sources of residual variance. We applied the improved methodology to our Fornax and Eridanus images and to our previously published Virgo cluster data. Apparent fluctuation magnitudes were used in conjunction with Cepheid distances to M31 and the Virgo cluster to calibrate the K' SBF distance scale. We find the absolute fluctuation magnitude (M) over 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 detected as a function of (V-I). Our calibration is consistent with simple (constant age and metallicity) stellar population models. The lack of a correlation with (V-I) in the context of the stellar population models implies that elliptical galaxies bluer than (V-I) = 1.2 have SBFs dominated by younger (5-8 Gyr) populations and metallicities comparable to redder ellipticals. Significant contributions to the SBFs from anomalous populations of asymptotic giant branch stars are apparently uncommon in giant ellipticals. K' SBFs prove to be a reliable distance indicator as long as the residual variance from globular clusters and background galaxies is properly removed. Also, it is important that a sufficiently high signal-to-noise ratio be achieved to allow reliable sky subtraction because residual spatial variance can bias the measurement of the SBF power spectrum.