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 Gpc^3/h^3 to z=0.47. The 2-point correlation function is estimated as a function of perpendicular σ and line-of-sight π (radial) directions. We find a significant detection of a peak at r 110Mpc/h, which shows as a circular ring in the σ-π 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 3.2σ, whereas a model with no magnification bias is disfavored 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 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.