Clustering of Luminous Red Galaxies IV: Baryon Acoustic Peak in the Line-of-Sight Direction and a Direct Measurement of H(z)

TL;DR

This paper detects the baryon acoustic peak in galaxy clustering data, enabling a direct measurement of the Hubble parameter H(z) at multiple redshifts, which improves understanding of cosmic expansion.

Contribution

It presents the first direct radial measurement of H(z) using BAO peak position in galaxy clustering data, independent of previous spherically averaged methods.

Findings

Detection of a BAO peak at ~110 Mpc/h in galaxy correlation data.

First direct measurement of H(z) at multiple redshifts from BAO in the radial direction.

H(z=0.24)=79.69±2.32 km/s/Mpc, H(z=0.43)=86.45±3.27 km/s/Mpc.

Abstract

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 $\xisp$ is estimated as a function of perpendicular $\sigma$ and line-of-sight $\pi$ (radial) directions. We find a significant detection of a peak at $r\simeq 110$Mpc/h, which shows as a circular ring in the $\sigma-\pi$ 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 $\xisp$ model with no radial BAO peak is disfavored at $3.2\sigma$, whereas a model with no magnification bias is disfavored at $2\sigma$. 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: $H(z=0.24)= 79.69 \pm 2.32 (\pm 1.29)$ km/s/Mpc for z=0.15-0.30 and $H(z=0.43)= 86.45 \pm 3.27 (\pm 1.69)$ 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 $H(z=0.34)= 83.80 \pm 2.96 (\pm 1.59)$ km/s/Mpc.