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

TL;DR

This work measures the baryon acoustic oscillation scale at $z=2.35$ from the cross-correlation of Ly$\alpha$ forest absorption with quasars in SDSS DR14, extending the forest analysis to include Ly$\beta$ absorption to boost statistical power. The analysis employs a distortion matrix to correct biases from continuum fitting, a comprehensive physical model including metal absorbers, HCDs, proximity effects, and relativistic corrections, and a robust covariance treatment via subsampling. The results yield the BAO distance scales $D_M(z)/r_d$ and $D_H(z)/r_d$, with $D_M/r_d=36.3\pm1.8$ and $D_H/r_d=9.20\pm0.36$ at $z_{\rm eff}=2.35$, consistent with Planck-based flat $\Lambda$CDM. Combining with Ly$\alpha$ auto-correlation strengthens the constraints to $D_M/r_d=37.0^{+1.3}_{-1.2}$ and $D_H/r_d=9.00^{+0.22}_{-0.22}$ at $z_{\rm eff}=2.34$, and yields an Alcock-Paczyński parameter $F_{AP}=4.11^{+0.21}_{-0.19}$. These BAO measurements, independent of CMB data, support a concordant cosmology with Planck, constrain late-time geometry, and demonstrate the potential of next-generation surveys like DESI and WEAVE-QSO to improve these constraints further.

Abstract

We present a measurement of the baryon acoustic oscillation (BAO) scale at redshift $z=2.35$ from the three-dimensional correlation of Lyman-$α$ (Ly$α$) 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$α$ absorption occurring in the Ly$β$ 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 $1.5σ$ 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$α$ auto-correlation measurement presented in a companion paper, the BAO measurements at $z=2.34$ are within $1.7σ$ of the predictions of this model.

Figures (14)

• Figure 1: Sky distribution for sample of 266,590 tracer quasars ($1.77 < z_{\rm q} < 3.5$) from DR14Q in J2000 equatorial coordinates. The solid black curve is the Galactic plane. The high-density regions are the eBOSS and SEQUELS observations (for the northern regions of the two Galactic hemispheres) and SDSS-stripe 82 (for declination $\delta\sim0$). The discontiguous small areas contain only SDSS DR7 quasars.
• Figure 2: Normalized redshift distributions for tracer quasars (black) and Ly$\alpha$ forest absorption pixels of Ly$\alpha$ region (blue) and Ly$\beta$ region (red). The histograms include 266,590 tracer quasars, $30.2\times10^{6}$ pixels in the Ly$\alpha$ region, and $4.0\times10^{6}$ pixels in the Ly$\beta$ region. The vertical dashed lines show the mean value of each distribution: $\overline{z}= 2.40$ (tracer quasars), 2.37 (in Ly$\alpha$), 2.26 (in Ly$\beta$).
• Figure 3: Example spectrum of DR14Q quasar identified by $(\mathrm{Plate,MJD,FiberID})=(7305,56991,570)$ at $z_{\rm q}=3.0$. The blue line indicates the best-fit model $\overline{F}(z)C_{\rm q}(\lambda)$ for the Ly$\alpha$ region covering the rest-frame wavelength interval $104.0<\lambda_{\rm rf}<120.0~\mathrm{nm}$. The red line indicates the same for the Ly$\beta$ region over the range $97.4<\lambda_{\rm rf}<102.0~\mathrm{nm}$. The Ly$\alpha$ and Ly$\beta$ emission lines are located at $\lambda_{\alpha}=121.567$ nm and $\lambda_{\beta}=102.572$ nm in the quasar rest-frame. The spectrum has not been rebinned into analysis pixels in this figure.
• Figure 4: Redshift distribution of $9.7\times10^{9}$ correlation pairs. The dashed vertical black line indicates the effective redshift of the BAO measurement, $z_{\rm eff}=2.35$, calculated as the weighted mean of the pair redshifts for separations in the range $80<r<120~h^{-1}~\mathrm{Mpc}$.
• Figure 5: Smoothed correlation matrix from sub-sampling as a function of $\Delta r_{\parallel}=|r_{\parallel,A}-r_{\parallel,B}|$. The curves are for constant $\Delta r_{\perp}=|r_{\perp,A}-r_{\perp,B}|$ for the three lowest values $\Delta r_{\perp}=\left[0,4,8\right]~h^{-1}~\mathrm{Mpc}$. The right panel shows an expansion of the region $\Delta r_{\parallel}<140~h^{-1}~\mathrm{Mpc}$.