Gravitational Redshift of Galaxies in Clusters from the Sloan Digital   Sky Survey and the Baryon Oscillation Spectroscopic Survey
  The gravitational redshift effect allows one to directly probe the gravitational potential in clusters of galaxies. Following up on Wojtak et al. [Nature (London) 477, 567 (2011)], we present a new measurement. We take advantage of new data from the tenth data release of the Sloan Digital Sky Survey and the Baryon Oscillation Spectroscopic Survey. We compare the spectroscopic redshift of the brightest cluster galaxies (BCGs) with that of galaxies at the outskirts of clusters, using a sample with an average cluster mass of $10^{14} M_{\odot}$. We find that these galaxies have an average relative redshift of -11 km/s compared with that of BCGs, with a standard deviation of +7 and -5 km/s. Our measurement is consistent with that of Wojtak et al. However, our derived standard deviation is larger, as we take into account various systematic effects, beyond the size of the dataset. The result is in good agreement with the predictions from general relativity. 
Iftach Sadeh, Low Lerh Feng and Ofer Lahav
astro-ph.CO