The Lyman-alpha forest in three dimensions: measurements of large scale flux correlations from BOSS 1st-year data

TL;DR

This work presents the first three-dimensional measurement of Lyman-α forest flux correlations using about 14,600 z>2.1 quasars from BOSS Year One, detecting cross-sightline correlations up to 60 h⁻¹ Mpc and a robust redshift-space distortion signal. The authors develop and validate a comprehensive analysis pipeline, including continuum fitting, δF estimation, and a linear-theory framework with bias b, redshift-space distortion β, and evolution α, using extensive synthetic data to gauge errors and systematics. They find b in the range 0.16–0.24 and β between 0.44 and 1.20 at z≈2.25, with β=0 excluded at >5σ; their mocks indicate that high column-density systems and metal lines can reduce β from theoretical expectations. The results demonstrate the viability of 3D Lyman-α forest clustering for cosmological parameter inference, including BAO measurements, and highlight the need to carefully model LLS/DLA and metal contamination in future, larger data sets. Overall, this study validates a path to precision high-redshift cosmology with the Lyman-α forest and foreshadows tight constraints on dark energy from BAO in upcoming BOSS data releases.

Abstract

Using a sample of approximately 14,000 z>2.1 quasars observed in the first year of the Baryon Oscillation Spectroscopic Survey (BOSS), we measure the three-dimensional correlation function of absorption in the Lyman-alpha forest. The angle-averaged correlation function of transmitted flux (F = exp(-tau)) is securely detected out to comoving separations of 60 Mpc/h, the first detection of flux correlations across widely separated sightlines. A quadrupole distortion of the redshift-space correlation function by peculiar velocities, the signature of the gravitational instability origin of structure in the Lyman-alpha forest, is also detected at high significance. We obtain a good fit to the data assuming linear theory redshift-space distortion and linear bias of the transmitted flux, relative to the matter fluctuations of a standard LCDM cosmological model (inflationary cold dark matter with a cosmological constant). At 95% confidence, we find a linear bias parameter 0.16<b<0.24 and redshift-distortion parameter 0.44<beta<1.20, at central redshift z=2.25, with a well constrained combination b(1+β)=0.336 +/- 0.012. The errors on beta are asymmetric, with beta=0 excluded at over 5 sigma confidence level. The value of beta is somewhat low compared to theoretical predictions, and our tests on synthetic data suggest that it is depressed (relative to expectations for the Lyman-alpha forest alone) by the presence of high column density systems and metal line absorption. These results set the stage for cosmological parameter determinations from three-dimensional structure in the Lyman-alpha forest, including anticipated constraints on dark energy from baryon acoustic oscillations.

Figures (22)

• Figure 1: Survey area of the quasars used in this paper in equatorial coordinates, in the Aitoff projection.
• Figure 2: The geometry of the BOSS survey for a thin slice in the equatorial plane. Our galaxy is at the origin. The dark dots are the galaxies measured in the BOSS survey and the blue markers show the positions of quasars whose Lyman-$\alpha$ forests we use. The actual Lyman-$\alpha$ forest regions are shown in as red lines. Apparent differences between geometry of quasar and galaxy distribution arise from small differences in slice thickness and time-span.
• Figure 3: Redshift distribution of quasars for the sample used in this analysis (left panel), and weighted distribution of Lyman-$\alpha$ forest pixel redshifts for the three redshift bins considered in this paper (right panel). The quasar redshifts are cut to be between $2.1<z<3.5$. The pixel weights are limited by the UV coverage of the spectroscope at low redshift end and by the redshift of quasars at high redshift. We show the sample of quasars without DLA flag; however, the two plots are virtually identical for the sample in which DLA flagged quasars are included.
• Figure 4: A quasar spectrum at redshift $z=3.276$ from the first year of BOSS data. The Lyman-$\alpha$ forest is the shaded region between the Lyman-$\alpha$ and the Lyman-$\beta$ emission lines. Other strong emission lines are also indicated.
• Figure 5: The result of the $\chi^2$ distribution exercise for radial bins at $r>10h^{-1}{\rm Mpc}$, which is used in our cosmological fitting. The thick black histogram correspond to Monte-Carlos with realizations, while the thick red histogram is the distribution for the actual 30 realizations of synthetic data. The thin histograms of the same colors shows the effect of ignoring off-diagonal elements of the covariance matrix. The number of bins here is 300 ($10\times 10 \times 3$). See text for discussion.