Detection of surface brightness fluctuations in elliptical galaxies imaged with the advanced camera for surveys:: B- and I-band measurements

Taking advantage of the exceptional capabilities of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), we derive surface brightness fluctuation (SBF) measurements in the B and I bands from images of six elliptical galaxies with 1500 <= cz <= 3500. Given the low signal-to-noise ratio (S/N) of the SBF signal in the blue-band images, the reliability of the measurements is verified both with numerical simulations and with experimental data tests. This paper presents the first published B- and I-band SBF measurements for distant (>= 20 Mpc) galaxies, which are essential for comparisons of models to observations of normal ellipticals. By comparing I-band datawith our new simple stellar population (SSP) models, we find an excellent agreement, and we confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of the dominant stellar components in the galaxies and are not strongly affected by the contribution of possible secondary stellar components. As a consequence, I-band fluctuation magnitudes are ideal for distance studies. On the other hand, we show that standard SSP models do not reproduce the B-band SBF magnitudes of red [(B-I)(0) greater than or similar to 2.1] galaxies in our sample. We explore the capability of two noncanonical models in properly reproducing the high sensitivity of B SBFs to the presence of even small fractions of bright, hot stars (metal-poor stars, hot evolved stars, etc.). The disagreement is solved both by taking into account hot (post-asymptotic giant branch [AGB]) stars in SSP models and/or by adopting composite stellar population models. Finally, we suggest a limit value of the S/N for the B-band SBF signal required to carry out a detailed study of stellar population properties based on this technique.