Measurement of Baryon Acoustic Oscillations in the Lyman-alpha Forest Fluctuations in BOSS Data Release 9

TL;DR

This paper demonstrates a robust detection of BAO in the Lyman-$\alpha$ forest fluctuations using BOSS DR9 data at $z_{\rm eff}\approx2.4$, achieving a significant peak at 3–5$\sigma$ and consistency with $\Lambda$CDM. It develops and validates a sophisticated analysis pipeline based on continuum fitting, 1D power spectrum estimation, and a quasi-optimal 3D correlation estimator with careful treatment of continuum-distortion and covariance. The authors report isotropic and anisotropic dilation measurements, $100\times(\alpha_{\rm iso}-1)\!=\!-1.6^{+2.0+4.3+7.4}_{-2.0-4.1-6.8}$ (stat.) $\pm1.0$ (syst.), and $100\times(\alpha_{\parallel}-1)=-1.3^{+3.5+7.6+12.3}_{-3.3-6.7-10.2}$ (stat.) $\pm2.0$ (syst.), $100\times(\alpha_{\perp}-1)=-2.2^{+7.4+17}_{-7.1-15}$ (stat.) $\pm3.0$ (syst.). These results refine high-$z$ BAO tests and provide a consistency check for the standard cosmology, while highlighting the importance of robust covariance estimation and continuum marginalization. The analysis also compares to previous DR9 analyses and demonstrates improved precision, supporting the use of Lyman-$\alpha$ BAO as a probe of the expansion history at $z\sim2-3$.

Abstract

We use the Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 (DR9) to detect and measure the position of the Baryonic Acoustic Oscillation (BAO) feature in the three-dimensional correlation function in the Lyman-alpha forest flux fluctuations at a redshift z=2.4. The feature is clearly detected at significance between 3 and 5 sigma (depending on the broadband model and method of error covariance matrix estimation) and is consistent with predictions of the standard LCDM model. We assess the biases in our method, stability of the error covariance matrix and possible systematic effects. We fit the resulting correlation function with several models that decouple the broadband and acoustic scale information. For an isotropic dilation factor, we measure 100x(alpha_iso-1) = -1.6 ^{+2.0+4.3+7.4}_{-2.0-4.1-6.8} (stat.) +/- 1.0 (syst.) (multiple statistical errors denote 1,2 and 3 sigma confidence limits) with respect to the acoustic scale in the fiducial cosmological model (flat LCDM with Omega_m=0.27, h=0.7). When fitting separately for the radial and transversal dilation factors we find marginalised constraints 100x(alpha_par-1) = -1.3 ^{+3.5+7.6 +12.3}_{-3.3-6.7-10.2} (stat.) +/- 2.0 (syst.) and 100x(alpha_perp-1) = -2.2 ^{+7.4+17}_{-7.1-15} +/- 3.0 (syst.). The dilation factor measurements are significantly correlated with cross-correlation coefficient of ~ -0.55. Errors become significantly non-Gaussian for deviations over 3 standard deviations from best fit value. Because of the data cuts and analysis method, these measurements give tighter constraints than a previous BAO analysis of the BOSS DR9 Lyman-alpha forest sample, providing an important consistency test of the standard cosmological model in a new redshift regime.

Figures (21)

• Figure 1: Distribution of 58,227 quasars used in this work on the sky in J2000 equatorial coordinates, shifted by $180\deg$ to more clearly show the main survey area.
• Figure 2: Distribution of quasar redshifts (red, solid) and Lyman-$\alpha$ forest pixels (blue, dotted).
• Figure 3: Fitted mean continuum, $C(\lambda_{{\rm rest}})$ in arbitrary units per unit wavelength. Solid red curve corresponds to data, while the dashed green line is for synthetic data.
• Figure 4: Fitted mean transmission in the forest ($\bar{F}$) with arbitrary normalization. Solid red curve corresponds to data, while the dashed green line is for synthetic data. Dotted lines show the Balmer series. The blue dashed lines are positions of Galactic calcium extinction.
• Figure 5: The amplitude and slope distributions when fitting data and synthetic data. The left-hand slide plot shows the histogram of fitted amplitudes for data (red,solid) and synthetic data (green, dashed). The right-hand side plot shows the distribution of amplitudes and slopes for data (red points) and synthetic data (green points). See text for further discussion.