Surface Brightness Fluctuations of Fornax Cluster Galaxies: Calibration of Infrared SBFs and Evidence for Recent Star Formation

We have measured K-band surface brightness fluctuations (SBFs) of 19 early-type galaxies in the Fornax cluster. Combining our results with published data, we find the absolute SBF magnitude Mbar_K = (-5.84)+(3.6)*[(V-I)-1.15]. The intrinsic cosmic scatter of Mbar_K appears to be larger than that of I-band SBFs. The discovery of a correlation between K-band SBFs and V-I color is a new clue into the star formation histories of early-type galaxies. This relation naturally accounts for galaxies previously claimed to have anomalously bright K-band SBFs, namely M32 and NGC 4489. Models indicate that the stellar populations dominating the SBF signal have a significant range in age; some scatter in metallicity may also be present. The youngest ages imply some galaxies have very luminous giant branches, akin to those in intermediate-age (few Gyr) Magellanic Cloud clusters. A few Fornax galaxies have unusually bright K-band SBFs, perhaps originating from a high metallicity burst of star formation in the last few Gyr. The increased spread and brightening of the K-band SBFs with bluer (V-I) colors suggest that the lower mass cluster galaxies may have had more extended and more heterogeneous star formation histories than those of the more massive galaxies. (Abridged)