Baryon Acoustic Oscillations in the Lyα forest of BOSS DR11 quasars

TL;DR

This study reports a definitive detection of the baryon acoustic oscillation feature in the Lyα forest autocorrelation at z≈2.34 using DR11 BOSS quasar data, delivering precise measurements of the angular- and radial-distance combinations D_A/r_d and D_H/r_d. By modeling the full 2D correlation function and validating the approach with 100 mock catalogs, the authors achieve about 3% precision on D_H/r_d and 6% on D_A/r_d, with a 2% optimal combination. The results are broadly consistent with the Lyα cross-correlation measurements but show a ~2.5σ tension with flat ΛCDM predictions from Planck, suggesting potential hints of new physics or the need for refined modeling. The analysis includes extensive systematics checks (continuum fitting, metals, DLAs, calibration artifacts) and emphasizes the complementarity of autocorrelation and cross-correlation BAO measurements at high redshift. The paper also outlines prospects for improved constraints with future DR12 data and joint analyses with lower-redshift BAO and CMB data.

Abstract

We report a detection of the baryon acoustic oscillation (BAO) feature in the flux-correlation function of the Lyα forest of high-redshift quasars with a statistical significance of five standard deviations. The study uses 137,562 quasars in the redshift range $2.1\le z \le 3.5$ from the Data Release 11 (DR11) of the Baryon Oscillation Spectroscopic Survey (BOSS) of SDSS-III. This sample contains three times the number of quasars used in previous studies. The measured position of the BAO peak determines the angular distance, $D_A(z=2.34)$ and expansion rate, $H(z=2.34)$, both on a scale set by the sound horizon at the drag epoch, $r_d$. We find $D_A/r_d=11.28\pm0.65(1σ)^{+2.8}_{-1.2}(2σ)$ and $D_H/r_d=9.18\pm0.28(1σ)\pm0.6(2σ)$ where $D_H=c/H$. The optimal combination, $\sim D_H^{0.7}D_A^{0.3}/r_d$ is determined with a precision of $\sim2\%$. For the value $r_d=147.4~{\rm Mpc}$, consistent with the CMB power spectrum measured by Planck, we find $D_A(z=2.34)=1662\pm96(1σ)~{\rm Mpc}$ and $H(z=2.34)=222\pm7(1σ)~{\rm km\,s^{-1}Mpc^{-1}}$. Tests with mock catalogs and variations of our analysis procedure have revealed no systematic uncertainties comparable to our statistical errors. Our results agree with the previously reported BAO measurement at the same redshift using the quasar-Lyα forest cross-correlation. The auto-correlation and cross-correlation approaches are complementary because of the quite different impact of redshift-space distortion on the two measurements. The combined constraints from the two correlation functions imply values of $D_A/r_d$ and $D_H/r_d$ that are, respectively, 7% low and 7% high compared to the predictions of a flat $Λ$CDM cosmological model with the best-fit Planck parameters. With our estimated statistical errors, the significance of this discrepancy is $\approx 2.5σ$.

Figures (18)

• Figure 1: Hammer-Aitoff projection of the BOSS DR11 footprint (dec. vs. r.a.). The light areas show the DR9 subregion available for the earlier studies of busca13 and slosar13. The red-dashed line shows the location of the galactic plane.
• Figure 2: Top: redshift distribution of pixel pairs contributing to $\xi$ in the region $80<r<120~h^{-1}{\rm Mpc}$. Bottom: distribution of all pixel redshifts.
• Figure 3: Measured correlation function averaged over three angular regions: $\mu>0.8$ (top), $0.8>\mu>0.5$ (middle), and $0.5>\mu>0.0$ (bottom), where $\mu$ is the central value of $r_\parallel/\sqrt{r_\parallel^2+r_\perp^2}$ in each $(r_\parallel,r_\perp)$ bin. The gray lines show individual sets of mocks, the solid blue line represents the mean of the 100 mock sets. The dashed blue lines are the $1\sigma$ variations of the mocks. The red points show the data.
• Figure 4: Example of a BOSS quasar spectrum of redshift 2.91 The red and blue lines cover the forest region used here, $104.0<\lambda_{\rm rf}<120.0$. This region is sandwiched between the quasar's Ly$\beta$ and Ly$\alpha$ emission lines at 400.9 and 475.4 nm The blue (green) line is the C2 (C3) continuum model, $C_q(\lambda)$, and the red line is the C1 model of the product of the continuum and the mean absorption, $C_q(\lambda)\bar{F}(z)$. (See text.)
• Figure 5: The measured correlation functions (continuum C2) in three angular regions: $\mu>0.8$ (top), $0.8>\mu>0.5$ (middle), and $0.5>\mu>0.$ (bottom), where $\mu$ is the central value of $r_\parallel/\sqrt{r_\parallel^2+r_\perp^2}$ in each $(r_\parallel,r_\perp)$ bin. The curves show the results of fits as described in Sect. \ref{['fitssec']}. The full curve is best fit and the dashed curve is best fit when the parameters $\alpha_\perp$ and $\alpha_\parallel$ (Eq. \ref{['eq:alpha']}) are both set to unity. The irregularities in the fits are due to the use of $(r_\parallel,r_\perp)$ bins rather than $(r,\mu)$ bins.