The 6dF Galaxy Survey: Baryon Acoustic Oscillations and the Local Hubble Constant

TL;DR

The paper reports a first robust detection of the Baryon Acoustic Oscillation peak in the low-redshift 6dF Galaxy Survey and uses it to derive a precise measurement of the distance scale D_V at z_eff ≈ 0.106, along with the BAO distance ratio r_s(z_d)/D_V(z_eff). Through a wide-angle, non-linear BAO model and log-normal covariance, it obtains H0 ≈ 67 km/s/Mpc with ~5–6% precision using only standard-ruler calibrations, and constrains the dark-energy equation of state to w ≈ -0.97 when combined with WMAP-7 data. The work highlights the low-redshift BAO’s independence from many cosmological parameters, its compatibility with ΛCDM, and its potential to sharpen future constraints with WALLABY and TAIPAN. These results demonstrate BAO as a competitive, ladder-free method for local H0 estimation and for breaking degeneracies in cosmic expansion history.

Abstract

We analyse the large-scale correlation function of the 6dF Galaxy Survey (6dFGS) and detect a Baryon Acoustic Oscillation (BAO) signal. The 6dFGS BAO detection allows us to constrain the distance-redshift relation at z_{\rm eff} = 0.106. We achieve a distance measure of D_V(z_{\rm eff}) = 456\pm27 Mpc and a measurement of the distance ratio, r_s(z_d)/D_V(z_{\rm eff}) = 0.336\pm0.015 (4.5% precision), where r_s(z_d) is the sound horizon at the drag epoch z_d. The low effective redshift of 6dFGS makes it a competitive and independent alternative to Cepheids and low-z supernovae in constraining the Hubble constant. We find a Hubble constant of H_0 = 67\pm3.2 km s^{-1} Mpc^{-1} (4.8% precision) that depends only on the WMAP-7 calibration of the sound horizon and on the galaxy clustering in 6dFGS. Compared to earlier BAO studies at higher redshift, our analysis is less dependent on other cosmological parameters. The sensitivity to H_0 can be used to break the degeneracy between the dark energy equation of state parameter w and H_0 in the CMB data. We determine that w = -0.97\pm0.13, using only WMAP-7 and BAO data from both 6dFGS and \citet{Percival:2009xn}. We also discuss predictions for the large scale correlation function of two future wide-angle surveys: the WALLABY blind H{\sc I} survey (with the Australian SKA Pathfinder, ASKAP), and the proposed TAIPAN all-southern-sky optical galaxy survey with the UK Schmidt Telescope (UKST). We find that both surveys are very likely to yield detections of the BAO peak, making WALLABY the first radio galaxy survey to do so. We also predict that TAIPAN has the potential to constrain the Hubble constant with 3% precision.

Figures (17)

• Figure 1: Redshift distribution of the data (black solid line) and the random catalogue (black dashed line). The weighted distribution (using weights from eq. \ref{['eq:weight']}) is shifted to higher redshift and has increased shot noise but a smaller error due to sample variance (blue solid and dashed lines).
• Figure 2: The large scale correlation function of 6dFGS. The best fit model is shown by the black line with the best fit value of $\Omega_mh^2 = 0.138\pm0.020$. Models with different $\Omega_mh^2$ are shown by the green line ($\Omega_mh^2 = 0.12$) and the blue line ($\Omega_mh^2 = 0.15$). The red dashed line is a linear CDM model with $\Omega_bh^2 = 0$ (and $\Omega_mh^2 = 0.1$), while all other models use the WMAP-7 best fit value of $\Omega_bh^2 = 0.02227$Komatsu:2010fb. The significance of the BAO detection in the black line relative to the red dashed line is $2.4\sigma$ (see Section \ref{['sec:sig']}). The error-bars at the data points are the diagonal elements of the covariance matrix derived using log-normal mock catalogues.
• Figure 3: Correlation matrix derived from a covariance matrix calculated from $200$ log-normal realisations.
• Figure 4: Likelihood contours of the distance $D_V(z_{\rm eff})$ against $\Omega_mh^2$. The corresponding values of $\alpha$ are given on the right-hand axis. The contours show $1$ and $2\sigma$ errors for both a full fit (blue solid contours) and a fit over $20-190h^{-1}$ Mpc (black dashed contours) excluding the first data point. The black cross marks the best fitting values corresponding to the dashed black contours with $(D_V,\Omega_mh^2) = (462,0.129)$, while the blue cross marks the best fitting values for the blue contours. The black solid curve corresponds to a constant $\Omega_m h^2D_V(z_{\rm eff})$ ($D_V \sim h^{-1}$), while the dashed line corresponds to a constant angular size of the sound horizon, as described in the text.
• Figure 5: The distance measurement $D_V(z)$ relative to a low redshift approximation. The points show 6dFGS data and those of Percival:2009xn.