| Clustering of Luminous Red Galaxies IV: Baryon Acoustic Peak in the Line-of-Sight Direction and a Direct Measurement of H(z) (2008) | |||||||||
Abstract | |||||||||
| This is the 4th paper in a series where we study the clustering of LRG galaxies in the latest spectroscopic SDSS data release, DR6, which has 75000 LRG galaxies sampling 1.1 (Gpc/h)^3 to z=0.47. Here we study the 2-point correlation function, separated in perpendicular (sigma) and line-of-sight (pi) directions. We find a significant detection of a peak at r=110 Mpc/h, which shows as a circular ring in the sigma-pi plane. There is also a significant detection of the peak along the line-of-sight (LOS) direction both in sub-samples at low, z=0.15-30, and high redshifts, z=0.40-0.47. The overall shape and location of the peak is consistent with baryon acoustic oscillations (BAO). The amplitude in the line-of-sight direction, however, is larger than conventional expectations. We argue this is due to magnification bias. Because the data is shot noise dominated, a lensing boost in signal translates into a boost in S/N. We take advantage of this high S/N to produce, for the first time, a direct measurement of the Hubble parameter H(z) as a function of redshift. This differs from earlier BAO measurements which used the spherically averaged (monopole) correlation function to constrain an integral of H(z). Using the BAO scale purely as a standard ruler in the LOS direction, we find: H(z=0.24)= 79.7 +- 2.1 (+- 1.0) km/s/Mpc for z=0.15-0.30, and H(z=0.43)= 86.5 +- 2.5 (+- 1.0) km/s/Mpc for z=0.40-0.47. For a flat universe with a cosmological constant, our two measurements of H(z) extrapolate to H_0=71.7 +-1.6 km/s/Mpc. This is in remarkable agreement with previous, independent, estimates of H_0 based on fitting the cosmological constant model. Combining our measurements with external constraints on w and dark energy abundance, we find w = -0.96 +- 0.05.. Comment: v3: Replace with minor changes. v2: We would like to thank Eiichiro Komatsu for pointing out a small error in the value of r_{WMAP} that we used for Eq.11 in the first version of this paper (v1). After this correction, there is very good agreement with the cosmological constant model. Precision cosmology seems at work | |||||||||
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