Measurement of BAO correlations at $z=2.3$ with SDSS DR12 \lya-Forests

Abstract

We use flux-transmission correlations in \Lya forests to measure the imprint of baryon acoustic oscillations (BAO). The study uses spectra of 157,783 quasars in the redshift range $2.1\le z \le 3.5$ from the Sloan Digital Sky Survey (SDSS) Data Release 12 (DR12). Besides the statistical improvements on our previous studies using SDSS DR9 and DR11, we have implemented numerous improvements in the analysis procedure, allowing us to construct a physical model of the correlation function and to investigate potential systematic errors in the determination of the BAO peak position. The Hubble distance, $\DHub=c/H(z)$, relative to the sound horizon is $\DHub(z=2.33)/r_d=9.07 \pm 0.31$. The best-determined combination of comoving angular-diameter distance, $\DM$, and the Hubble distance is found to be $\DHub^{0.7}\DM^{0.3}/r_d=13.94\pm0.35$. This value is $1.028\pm0.026$ times the prediction of the flat-\lcdm model consistent with the cosmic microwave background (CMB) anisotropy spectrum. The errors include marginalization over the effects of unidentified high-density absorption systems and fluctuations in ultraviolet ionizing radiation. Independently of the CMB measurements, the combination of our results and other BAO observations determine the open-\lcdm density parameters to be $\om=0.296 \pm 0.029$, $\ol=0.699 \pm 0.100$ and $Ω_k = -0.002 \pm 0.119$.

Figures (19)

• Figure 1: SDSS DR12 footprint (in J2000 equatorial coordinates) used in this work. The survey covers one quarter of the sky ($10^4{\rm deg^2}$). The light blue regions are those added beyond the area covered by our previous study 2015AA...574A..59D. The dotted line is the Galactic plane.
• Figure 2: Example of a BOSS quasar spectrum of redshift 2.91 (smoothed to the width of analysis pixels). The red and blue lines cover the forest region used in our analysis, $104.0<\lambda_{\rm RF}<120.0$ nm. This region is sandwiched between the quasar's Ly$\beta$ and Ly$\alpha$ emission lines, respectively at 400.9 and 475.4 nm (restframe 102.572 and 121.567 nm). The blue line is the model of the continuum, $C_q(\lambda)$; the red line is the product of the continuum and the mean transmission, $C_q(\lambda)\times\overline{F}(z)$, as calculated by the method described in Sect. \ref{['xisec']}.
• Figure 3: Weighted redshift distribution of pairs of Ly$\alpha$ forest pixels. The mean is $\langle z\rangle=2.33$. Included in the distribution are the $\sim5\times10^{10}$ pairs within $20~h^{-1}{\rm Mpc}$ of the center of the BAO peak.
• Figure 4: Mean ratio, $R(\lambda)$, of observed flux to pipeline-model flux as a function of observed wavelength for quasar spectra to the red of the Ly$\alpha$ emission line ($\lambda_{\rm RF}>130~{\rm nm}$). (The mean is calculated by weighting each measurement by the inverse of the pipeline variance.) In this mostly unabsorbed region of quasar spectra, the percent-level wavelength-dependent deviations from unity are due to imperfect modeling of calibration stars and to the calcium H and K lines (393.4 and 396.9 nm) due to Galactic absorption.
• Figure 5: One-dimensional flux-correlation function, $\xi_{\rm 1d}$, for BOSS quasars showing correlations of $\delta_q(\lambda)$ within the same forest. The correlation function is shown as a function of wavelength ratio for the data and for the mocks (procedure Met1). Prominent peaks due to Ly$\alpha$-metal and metal-metal correlations are indicated. The peak at $\lambda_1/\lambda_2\sim1.051$, which is seen in the data but not in the mocks, is due to CII(133.5)-SiIV(140.277) at $z\sim1.85$, outside the redshift range covered by the mocks.