Clustering of luminous red galaxies - IV. Baryon acoustic peak in the line-of-sight direction and a direct measurement ofH(z)

We study the clustering of LRG galaxies in the latest spectroscopic SDSS data releases, DR6 and DR7, which sample over 1 Gpc³/h³ to z=0. 47. The 2-point correlation function is estimated as a function of perpendicular and line-of-sight (radial) directions. We find a significant detection of a peak at r 110Mpc/h, which shows as a circular ring in the - plane. There is also significant evidence for a peak along the radial direction whose shape is consistent with its originating from the recombination-epoch baryon acoustic oscillations (BAO). A model with no radial BAO peak is disfavored at 3. 2, whereas a model with no magnification bias is disfavored at 2. The radial data enable, for the first time, a direct measurement of the Hubble parameter H (z) as a function of redshift. This is independent from earlier BAO measurements which used the spherically averaged (monopole) correlation to constrain an integral of H (z). Using the BAO peak position as a standard ruler in the radial direction, we find: H (z=0. 24) = 79. 69 2. 32 (1. 29) km/s/Mpc for z=0. 15-0. 30 and H (z=0. 43) = 86. 45 3. 27 (1. 69) km/s/Mpc for z=0. 40-0. 47. The first error is a model independent statistical estimation and the second accounts for systematics both in the measurements and in the model. For the full sample, z=0. 15-0. 47, we find H (z=0. 34) = 83. 80 2. 96 (1. 59) km/s/Mpc.