THE ACS FORNAX CLUSTER SURVEY. V. MEASUREMENT AND RECALIBRATION OF SURFACE BRIGHTNESS FLUCTUATIONS AND A PRECISE VALUE OF THE FORNAX-VIRGO RELATIVE DISTANCE

Abstract

We present ( g 475 − z 850 ) color and z 850 -band surface brightness fluctuations (SBFs) measurements for 43 early-type galaxies in the Fornax cluster imaged with the Hubble Space Telescope Advanced Camera for Surveys. These are combined with our earlier measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z 850 -band SBF absolute magnitude as a function of ( g 475 − z 850 ) color, valid for the AB color range 0.8 < ( g 475 − z 850 ) < 1.6. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W ' group, and NGC 4697 in the Virgo Southern Extension. The calibration procedure yields a highly precise relative distance modulus for Fornax with respect to Virgo of Δ( m − M ) FV = 0.42 ± 0.03 mag, or a distance ratio d F / d V = 1.214 ± 0.017. The resulting Fornax distance modulus is ( m − M ) For = 31.51 ± 0.03 ± 0.15 mag, corresponding to d F = 20.0 ± 0.3 ± 1.4 Mpc, where the second set of error bars reflects the total systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax cluster galaxies is σ d = 0.49 +0.11 −0.15 Mpc, or a total line-of-sight depth of 2.0 +0.4 −0.6 Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or "cosmic," scatter σ cos in the SBF calibration, unlike the case for the larger Virgo cluster. As a result, we are able to place the first tight constraints on the value of σ cos . We find σ cos = 0.06 ± 0.01 mag, with a firm upper limit of σ cos < 0.08 mag, for the subsample of galaxies with ( g 475 − z 850 )>1.02, but it is about twice as large for bluer galaxies. We also present an alternative SBF calibration based on the "fluctuation count" parameter , a proxy for galaxy mass. This gives a consistent relative distance but with larger intrinsic scatter, and we adopt the result from the calibration on ( g 475 − z 850 ) because of its basis in stellar population properties alone. Finally, we find no evidence for systematic trends of the galaxy distances with position or velocity (e.g., no current infall); the Fornax cluster appears both compact and well virialized.