Baryon Acoustic Oscillations in the Ly-α forest of BOSS quasars

TL;DR

This study reports the first detection of baryon acoustic oscillations in the Lyα forest using 48,640 BOSS quasars at z~2.3, measuring the 3D flux-correlation function via two continuum-filling approaches. By decomposing the correlation into monopole and quadrupole and carefully estimating covariances, the authors detect the BAO peak with ~4σ significance and constrain the angular diameter distance and expansion rate at z=2.3. The results yield a high-z H(z) measurement, showing deceleration during matter domination and, when combined with CMB data, a consistent transition to dark-energy–driven acceleration. This work demonstrates the Lyα forest as a competitive high-redshift BAO probe and highlights its potential for refining cosmological parameters with future data releases.

Abstract

We report a detection of the baryon acoustic oscillation (BAO) feature in the three-dimensional correlation function of the transmitted flux fraction in the \Lya forest of high-redshift quasars. The study uses 48,640 quasars in the redshift range $2.1\le z \le 3.5$ from the Baryon Oscillation Spectroscopic Survey (BOSS) of the third generation of the Sloan Digital Sky Survey (SDSS-III). At a mean redshift $z=2.3$, we measure the monopole and quadrupole components of the correlation function for separations in the range $20\hMpc<r<200\hMpc$. A peak in the correlation function is seen at a separation equal to $(1.01\pm0.03)$ times the distance expected for the BAO peak within a concordance $Λ$CDM cosmology. This first detection of the BAO peak at high redshift, when the universe was strongly matter dominated, results in constraints on the angular diameter distance $\da$ and the expansion rate $H$ at $z=2.3$ that, combined with priors on $H_0$ and the baryon density, require the existence of dark energy. Combined with constraints derived from Cosmic Microwave Background (CMB) observations, this result implies $H(z=2.3)=(224\pm8){\rm km\,s^{-1}Mpc^{-1}}$, indicating that the time derivative of the cosmological scale parameter $\dot{a}=H(z=2.3)/(1+z)$ is significantly greater than that measured with BAO at $z\sim0.5$. This demonstrates that the expansion was decelerating in the range $0.7<z<2.3$, as expected from the matter domination during this epoch. Combined with measurements of $H_0$, one sees the pattern of deceleration followed by acceleration characteristic of a dark-energy dominated universe.

Figures (24)

• Figure 1: Hammer-Aitoff projection in equatorial coordinates of the BOSS DR9 footprint. The observations cover $\sim 3000~{\rm deg^2}$.
• Figure 2: Top: weighted distribution of absorber redshifts used in the calculation of the correlation function in the distance range $80~{\rm h^{-1}Mpc}<r<120~{\rm h^{-1}Mpc}$. Bottom: distribution of signal-to-noise ratio for analysis pixels (triplets of pipeline pixels) averaged over the forest region.
• Figure 3: An example of a BOSS quasar spectrum of redshift 3.239. The red and blue lines cover the forest region used here, $104.5<\lambda_{\rm rf}<118.0$. This region is sandwiched between the quasar's Ly$\beta$ and Ly$\alpha$ emission lines respectively at 435 and 515 nm. The blue line is an estimate of the continuum (unabsorbed flux) by method 2 and the red line is the estimate of the product of the continuum and the mean absorption by method 1.
• Figure 4: The mean of $\delta(\lambda)$ plotted as a function of observed wavelength (method 1). Systematic offsets from zero are seen at the 2% level. The calcium lines (393.4,396.8 nm) is present. The features around the hydrogen lines H$\gamma$, $\delta$ and $\epsilon$ (434.1, 410.2, 397.0 nm) are artifacts from the use of F-stars for the photocalibration of the spectrometer.
• Figure 5: Mean transmitted flux fraction as a function of redshift obtained from the continuum fits with method 2. Data are shown in black, mock-000 in red and the input mean transmitted flux fraction in blue.