Galaxy clusters in the Perseus-Pisces region -- II. The peculiar velocity field

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Abstract
We have measured the mean peculiar motions of 103 early-type galaxies in seven clusters in the Perseus-Pisces (PP) ridge or PP background, and a further 249 such galaxies in nine calibrating clusters from the literature, using the inverse Fundamental Plane relation. This relation is found to have a distance error of 20 per cent per galaxy. None of the six clusters in the PP ridge has a significant motion with respect to the cosmic microwave background (CMB) frame, but the PP background cluster J8 shows marginal evidence of ‘backside infall’ into the PP supercluster. We find that the full 16-cluster sample has a mean CMB-frame bulk motion of 420 ± 280 km s⋆1 towards l = 262°, b = −25°. This result is consistent both with no bulk motion in the CMB frame and with the ~350 km s⋆1 bulk motion found by Courteau et al. It is inconsistent at the 98 per cent confidence level with the ~700 km s⋆1 bulk flow found by Lauer & Postman. The PP ridge clusters are found to have a small and statistically insignificant mean radial motion with respect to the CMB frame: − 60 ± 220 km s⋆1. Our error analysis fully accounts for the uncertainties in the mean Hubble flow, as well as the errors due to the merging of different spectroscopic data sets. A comparison between our cluster peculiar velocities and the predicted peculiar velocities from the IRAS 1.2-Jy density field, smoothed on a 500 km s⋆1 scale, yields ²I= Ω0.6/bI = 0.95 ± 0.48, consistent with previous results. We find agreement between our peculiar motions and published Tully-Fisher results for the same clusters. The disagreement between the 11 clusters common to our sample and that of Lauer & Postman, based on brightest cluster galaxies (BCGs), is statistically significant at the ≳99.7 per cent confidence level, indicating that the errors of one or both of these data sets are underestimated. When the BCG distances corrected for the X-ray luminosity of the host cluster are used, the disagreement is reduced to the ~94 per cent confidence level.
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