The distance to Hydra and Centaurus from surface brightness fluctuations: Consequences for the Great Attractor model

Abstract

We present I-band Surface Brightness Fluctuation (SBF) measurements for 16 early-type galaxies (3 giants, 13 dwarfs) in the central region of the Hydra cluster, based on deep photometric data in 7 fields obtained with VLT FORS1. From the SBF-distances to the galaxies in our sample we estimate the distance of the Hydra cluster to be 41.2 ± 1.4 Mpc ((m - M) =33.07 ± 0.07 mag). Based on an improved correction for fluctuations from undetected point sources, we revise the SBF-distance to the Centaurus cluster from Mieske & Hilker (2003, A&A, 410, 455) upwards by 10% to 45.3 ± 2.0 Mpc ((m-M) =33.28 ± 0.09 mag). The relative distance modulus of the two clusters then is (m-M) Cen -(m-M) Hyd = 0.21±0.11 mag. With H 0 = 72 ± 4 km s -1 Mpc -1 , we estimate a positive peculiar velocity of 1225 ± 235 km s -1 for Hydra and 210 ± 295 km s -1 for the Cen30 component of Centaurus. Allowing for a thermal velocity dispersion of 200 km s -1 , this rules out a common peculiar flow velocity for both clusters at 98% confidence. We find that the 9 × 10 15 M ○. Great Attractor at a distance of ≃45 Mpc can explain the observed peculiar velocities if shifted about 15° towards the Hydra cluster position. Our results are inconsistent at 94% confidence with a scenario where the Centaurus cluster is identical to the GA. In order to better restrict partially degenerate Great Attractor parameters like its mass and distance, a recalculation of the local flow model with updated distance information over a larger area than covered by us would be needed.