The clustering of the SDSS-IV extended Baryon Oscillation Spectroscopic Survey DR14 quasar sample: First measurement of Baryon Acoustic Oscillations between redshift 0.8 and 2.2

Abstract: We present measurements of the Baryon Acoustic Oscillation (BAO) scale in redshift-space using the clustering of quasars. We consider a sample of 147,000 quasars from the extended Baryon Oscillation Spectroscopic Survey (eBOSS) distributed over 2044 square degrees with redshifts $0.8 < z < 2.2$ and measure their spherically-averaged clustering in both configuration and Fourier space. Our observational dataset and the 1400 simulated realizations of the dataset allow us to detect a preference for BAO that is greater than 2.8$\sigma$. We determine the spherically averaged BAO distance to $z = 1.52$ to 3.8 per cent precision: $D_V(z=1.52)=3843\pm147 \left(r_{\rm d}/r_{\rm d, fid}\right)\ $Mpc. This is the first time the location of the BAO feature has been measured between redshifts 1 and 2. Our result is fully consistent with the prediction obtained by extrapolating the Planck flat $\Lambda$CDM best-fit cosmology. All of our results are consistent with basic large-scale structure (LSS) theory, confirming quasars to be a reliable tracer of LSS, and provide a starting point for numerous cosmological tests to be performed with eBOSS quasar samples. We combine our result with previous, independent, BAO distance measurements to construct an updated BAO distance-ladder. Using these BAO data alone and marginalizing over the length of the standard ruler, we find $\Omega_{\Lambda} > 0$ at 6.6$\sigma$ significance when testing a $\Lambda$CDM model with free curvature.

• The paper reports the first BAO measurement in quasars between redshifts 0.8 and 2.2 with a detection confidence over 2.8σ.
• It employs advanced spectroscopic and statistical analyses, using 147,000 quasars and 1,400 mock datasets to isolate the BAO feature.
• The precise BAO distance measurement at z = 1.52 (3843 ± 147 Mpc) aligns with the Planck ΛCDM model, supporting high-redshift cosmology.

Overview of DR14 eBOSS Quasar BAO Measurements

This paper presents a thorough investigation of Baryon Acoustic Oscillations (BAOs) using data from the extended Baryon Oscillation Spectroscopic Survey (eBOSS), a component of SDSS-IV, focusing particularly on quasars. The objective is to measure the BAO scale within the redshift range of 0.8 to 2.2—regions primarily occupied by quasars. Through these measurements, the study bridges a significant observational gap in BAO data at high redshifts.

Data and Methodology

The research utilizes a sample of 147,000 quasars over 2,044 square degrees, employing a combination of configuration and Fourier space analyses to determine the observed BAO scale. This is cross-validated with 1,400 mock datasets to ensure the robustness of BAO detection and measurement.

The setup involves:

1. Spectroscopic Data: Quasar selection and redshift measurement techniques were optimized to handle challenges such as low space density and spectroscopic complexity, using advanced methods like the Baryon Oscillation Photometric Survey (BOSS) pipeline and visual inspection.
2. Statistical Analysis: Both the correlation function and power spectrum were modeled to isolate BAO features, focusing on the spherically averaged clustering signal. Specific parameters like the non-linear damping scale, $\Sigma_{\text{nl}}$, were adjusted to refine the BAO signal extraction.
3. Mock Simulations: The study leverages two distinct sets of mock simulations (EZmock and QPM), each exhibiting minor variations from fiducial cosmological models. These simulations aid in constructing covariance matrices for parameter estimation processes.

Key Findings

• The study presents the first-time measurement of the BAO feature within a redshift range of 1 to 2 using quasar data, with a significant detection confidence exceeding $2.8\sigma$.
• A precise determination of the spherically averaged BAO distance at $z = 1.52$ was achieved, yielding $$D_V(z=1.52) = 3843 \pm 147(r_{\text{d}}/r_{\text{d, fid}}) \, \text{Mpc}$$, reflecting a fractional distance uncertainty of around 3.8%.
• The results are consistent with the Planck best-fit flat $\Lambda$CDM model, underscoring the reliability of quasars as tracers of large-scale structure and enhancing confidence in eBOSS's data for future cosmological tests.

Implications and Future Prospects

The paper underscores the importance of extending BAO measurements to higher redshifts using quasars, thus cementing their role in constructing a precise distance ladder. The findings contribute valuable high-redshift data for cosmological parameter estimation, particularly in probing the Universe's expansion history and geometry.

The research paves the way for more comprehensive studies that integrate eBOSS data with larger galaxy surveys, enriching our understanding of the cosmic microwave background and gravitational phenomena. Furthermore, the study enhances the methodological pipeline for utilizing quasar-based data in high-redshift cosmology, marking significant progress ahead of future ambitious projects like DESI and Euclid.

In conclusion, this paper signifies a remarkable step in utilizing quasar data to augment high-redshift BAO measurements, aligning with existing cosmological models while paving avenues for future explorations to refine our understanding of the Universe's expansion and its underlying physics.