Surface brightness fluctuations of Fornax Cluster galaxies: Calibration of infrared surface brightness fluctuations and evidence for recent star formation

We have measured K-s-band (2.0-2.3 mum) surface brightness fluctuations (SBFs) of 19 early-type galaxies in the Fornax Cluster. Fornax is ideally suited both for calibrating SBFs as distance indicators and for using SBFs to probe the unresolved stellar content of early-type galaxies. Combining our results with published data for other nearby clusters, we calibrate K-s-band SBFs using Hubble Space Telescope (HST) Cepheid cluster distances and I-band SBF distances to individual galaxies. With the latter, the resulting calibration is (M) over bar (Ks) = (-5.84 +/- 0.04) + (3.6 +/- 0.8)[(V - I-c)(o) - 1.15], valid for 1.05 < (V - I-c)(o) < 1.25 and not including any systematic errors in the HST Cepheid distance scale. The fit accounts for the covariance between V-I-c and (M) over bar (Ks) when calibrated in this fashion. The intrinsic cosmic scatter of (M) over bar (Ks) appears to be larger than that of I-band SBFs. S0 galaxies may follow a different relation, although the data are inconclusive. The discovery of correlation between K-s-band fluctuation magnitudes and colors with V-I-c 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. The inferred metallicities are roughly solar, although this depends on the choice of theoretical models. A few Fornax galaxies have unusually bright K-s-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-s-band SBFs with bluer V-I-c suggest that the lower mass cluster galaxies (less than or similar to 0.1 L*) may have had more extended and more heterogeneous star formation histories than those of the more massive galaxies.