Baryon acoustic oscillations at z = 2.34 from the correlations of Ly$α$ absorption in eBOSS DR14

TL;DR

The paper measures baryon acoustic oscillations at $z\approx2.34$ using Ly$\alpha$ absorption in quasar spectra from SDSS-IV DR14, including Ly$\beta$ absorption to boost statistics. A physically motivated model of Ly$\alpha$ auto-correlation plus metal contamination and HCD effects is fitted to the data, with a distortion treatment to connect to the measured field. The inferred distance ratios $D_H(2.34)/r_d$ and $D_M(2.34)/r_d$ are $8.86^{+0.29}_{-0.29}$ and $37.41^{+1.96}_{-1.77}$, respectively, and combined with quasar-Ly$\alpha$ cross-correlation results yield BAO constraints consistent with a flat $\Lambda$CDM model within about $1-2\sigma$, demonstrating the viability of Ly$\alpha$ BAO as a high-$z$ cosmological probe. The work also highlights improved precision over prior analyses and sets the stage for future DESI/WEAVE-QSO improvements, with careful handling of systematics from metals, HCDs, and continuum distortions.

Abstract

We measure the imprint of primordial baryon acoustic oscillations (BAO) in the correlation function of Ly$α$ absorption in quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS) and the extended BOSS (eBOSS) in Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS)-IV. In addition to 179,965 spectra with absorption in the Lyman-$α$ (Ly$α$) region, we use, for the first time, Ly$α$ absorption in the Lyman-$β$ region of 56,154 spectra. We measure the Hubble distance, $D_H$, and the comoving angular diameter distance, $D_M$, relative to the sound horizon at the drag epoch $r_d$ at an effective redshift $z=2.34$. Using a physical model of the correlation function outside the BAO peak, we find $D_H(2.34)/r_d=8.86\pm 0.29$ and $D_M(2.34)/r_d=37.41\pm 1.86$, within 1$σ$ from the flat-$Λ$CDM model consistent with CMB anisotropy measurements. With the addition of polynomial "broadband" terms, the results remain within one standard deviation of the CMB-inspired model. Combined with the quasar-Ly$α$ cross-correlation measurement presented in a companion paper Blomqvist19, the BAO measurements at $z=2.35$ are within 1.7$σ$ of the predictions of this model.

Figures (18)

• Figure 1: Sky distribution of the 216,163 quasars with redshift in the [2.0,3.5] range in the DR14 footprint of the BOSS and eBOSS surveys. The high-density regions are the eBOSS and SEQUELS observations (for the highest declinations in the two Galactic caps) and SDSS-stripe 82 (on the celestial equator in the south galactic cap).
• Figure 2: The Ly$\alpha$ and Ly$\beta$ spectral regions defined in Table \ref{['table:forests']}.
• Figure 3: Weighted distribution of the redshift of pairs used to measure the Ly$\alpha$(Ly$\alpha$) $\times$ Ly$\alpha$(Ly$\alpha$) and Ly$\alpha$(Ly$\alpha$) $\times$ Ly$\alpha$(Ly$\beta$) correlation functions. The mean redshift of the combined sample is $\langle z_{pairs}\rangle=2.34$.
• Figure 4: The one-dimensional correlation functions, $\xi_{\rm 1d}$, in the Ly$\alpha$ (red curve) and Ly$\beta$ (blue curve) regions as a function of the ratio of transition wavelengths. Peaks are due to absorption by the two labeled elements at zero physical separation (Table \ref{['table:metal_auto']}).
• Figure 5: Definition of the coordinates of pixels used in the computation of the correlation function. Absorbers $i$ and $j$ have angular separation $\theta_{ij}$ and distance separation $r_{ij}$. The radial separation $r_{\parallel,ij}$ is the projection of $r_{ij}$ on the median LOS and the transverse separation $r_{\perp,ij}$ is the LOS perpendicular component of $r_{ij}$, assuming the flat Pl2015 model (Table \ref{['tab:fidcosm']}).