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 1 Gpc3 h−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≃ 110 Mpc h−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) km s−1 Mpc−1 for z= 0.15–0.30 and H(z= 0.43) = 86.45 ± 3.27 (±1.69) km s−1 Mpc−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) km s−1 Mpc−1.