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

We study the clustering of luminous red galaxies in the latest spectroscopic Sloan Digital Sky Survey data releases (DR), DR6 and DR7, which sample over 1Gpc³h^-3 to z = 0.47. The two-point correlation function ξ(σ, π) is estimated as a function of perpendicular σ and line-of-sight π (radial) directions. We find significant detection of a peak at r ~= 110Mpch^-1, which shows as a circular ring in the σ-π plane. There is also significant evidence of a peak along the radial direction whose shape is consistent with its origination from the recombination-epoch baryon acoustic oscillations (BAO). A ξ(σ, π) model with no radial BAO peak is disfavoured at 3.2σ, whereas a model with no magnification bias is disfavoured at 2σ. 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 of 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)kms^-1Mpc^-1 for z = 0.15-0.30 and H(z = 0.43) = 86.45 +/- 3.27 (+/-1.69)kms^-1Mpc^-1 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)kms^-1Mpc^-1.