Baryon acoustic oscillations from the cross-correlation of Ly$α$ absorption and quasars in eBOSS DR14

Abstract: We present a measurement of the baryon acoustic oscillation (BAO) scale at redshift $z=2.35$ from the three-dimensional correlation of Lyman-$\alpha$ (Ly$\alpha$) forest absorption and quasars. The study uses 266,590 quasars in the redshift range $1.77<z<3.5$ from the Sloan Digital Sky Survey (SDSS) Data Release 14 (DR14). The sample includes the first two years of observations by the SDSS-IV extended Baryon Oscillation Spectroscopic Survey (eBOSS), providing new quasars and re-observations of BOSS quasars for improved statistical precision. Statistics are further improved by including Ly$\alpha$ absorption occurring in the Ly$\beta$ wavelength band of the spectra. From the measured BAO peak position along and across the line of sight, we determined the Hubble distance $D_{H}$ and the comoving angular diameter distance $D_{M}$ relative to the sound horizon at the drag epoch $r_{d}$: $D_{H}(z=2.35)/r_{d}=9.20\pm 0.36$ and $D_{M}(z=2.35)/r_{d}=36.3\pm 1.8$. These results are consistent at $1.5\sigma$ with the prediction of the best-fit spatially-flat cosmological model with the cosmological constant reported for the Planck (2016) analysis of cosmic microwave background anisotropies. Combined with the Ly$\alpha$ auto-correlation measurement presented in a companion paper, the BAO measurements at $z=2.34$ are within $1.7\sigma$ of the predictions of this model.

Baryon Acoustic Oscillations from the Cross-Correlation of Ly$\alpha$ Absorption and Quasars in eBOSS DR14

The paper presents an analysis of the Baryon Acoustic Oscillations (BAO) at redshift $z=2.35$ through cross-correlation of Lyman-$\alpha$ (Ly$\alpha$) forest absorption and quasars, utilizing data from the Sloan Digital Sky Survey (SDSS) Data Release 14 (DR14). The research incorporates 266,590 quasars in the redshift range $1.77$\alpha$ absorption in the Ly$\beta$ spectral region.

Key Findings

• Measurements: The authors provide a measurement of the BAO peak position in both the line-of-sight and transverse directions. Specifically, they determine the Hubble distance $H(z=2.35)/r_{d}=9.20\pm 0.36$ and the comoving angular diameter distance $M(z=2.35)/r_{d}=36.3\pm 1.8$. These results are consistent at the 1.5$\sigma$ level with the spatially-flat $\Lambda$CDM cosmology derived from Planck (2016) data, which incorporates cosmic microwave background (CMB) anisotropies.
• BAO Peak Position: The measurement indicates a shift of approximately 0.3$\sigma$ toward the Planck cosmology prediction compared to earlier DR12 results, primarily due to inclusion of the Ly$\beta$ region absorption.
• Model Validation: The cross-correlation includes several model components to account for the quasar-lensed Ly$\alpha$ effect, metal absorbers, high column density systems (HCDs), and transverse proximity effects. The study presents robust testing methods, ensuring that statistical analysis sufficiently captures potential moisture in fitted data without substantial deviation from expected peak positions.
• Combination with Auto-Correlation: Integrating these results with auto-correlation measurements improves parameter constraints. The combined data yields $M(z=2.34)/r_{d}=37.0~_{-1.2}^{+1.3}$ and $H(z=2.34)/r_{d}=9.00~_{-0.22}^{+0.22}$, further aligning with $\Lambda$CDM predictions.

Implications and Future Directions

Practically, the research refines our understanding of the universe's expansion history and the effective use of large-scale structures as cosmic rulers. Theoretically, it underscores the consonance between high-redshift quasar data and CMB-derived cosmological models, reinforcing current paradigms about the consistency of large-scale structure measurements across cosmic time scales.

Looking forward, upcoming surveys such as DESI and WEAVE-QSO promise further advancements by expanding quasar datasets and improving spectral resolution, which are critical for reducing uncertainties in BAO measurements and testing cosmological models. This study exemplifies the potential in leveraging next-generation datasets to validate or challenge fundamental cosmological assumptions.

Overall, this research enhances comprehension of cosmological distances at high redshifts, providing a valuable complement to galaxy surveys at lower redshifts and CMB temperature fluctuations, thereby contributing to a more precise characterization of the universe's expansion dynamics.