First measurement of the correlation between cosmic voids and the Lyman-$α$ forest

Abstract

We report the first detection at a median redshift $z = 2.49$ of large-scale matter flows around cosmic voids. Voids are identified within a tomographic map of large-scale Lyman-$α$ (Ly$α$) transmissions, built from the eBOSS Ly$α$ forest sample in the SDSS Stripe 82 field. We measure the imprint of flows around voids, known as redshift-space distortions (RSD), with a statistical significance of $8\,σ$. The observed quadrupole of the void-forest cross-correlation is described by a linear RSD model. The derived RSD parameter of the Ly$α$ forest around voids is $β= 1.21 \pm 0.18$. Our model accounts for the tomographic effect induced by the Ly$α$ data being located along parallel quasar lines of sight. This work presents a novel approach to observing the growth of cosmic structures at redshifts currently inaccessible to galaxy surveys.

Figures (4)

• Figure 1: Left: measurement of $r\times \xi_(r_{\perp}, r_{\parallel})$ from eBOSS Stripe 82 data, where $r_\perp$ (resp. $r_\parallel$) is the transverse (resp. longitudinal) component of the separation vector with respect to the line of sight. Right: associated multipoles for $\ell=0, 2, 4$, from data (black points) and mock realizations including RSD (blue) or not (orange). Thin curves represent individual mock realizations, and their average is shown with thick curves. Black dashed curves show the average multipoles measured from shuffled eBOSS data. The black continuous curve shows the fit of the eBOSS quadrupole with Eqn. \ref{['eq:fit_model']}.
• Figure 2: The cross-correlation $\xi$ as a function of $\mu^2$ for mock realizations, for the bins $20< r<22$ and $26<r<28~h^{-1} \mathrm{Mpc}$. Top: results for RSD-mocks (continuous, $\xi_{\rm RSD}$) and noRSD-mocks (dotted, $\xi_{\rm noRSD}$). Bottom: difference $\xi_{\rm RSD}-\xi_{\rm noRSD}$, with statistical error bars. Dashed lines show associated linear fits.
• Figure 3: Measured monopoles, quadrupoles, and hexadecapoles of $\xi$ in mock data, including different instrumental and astrophysical effects. "Raw" mocks (blue) include only the absorption law from Eqn. \ref{['eq:fgpa']}. Statistical error bars are estimated using sub-sample variance. For the quadrupole, they are only represented on the red curve.
• Figure 4: Points: measurements of the shuffling-corrected monopole and quadrupole functions for the void-forest correlation reported in Fig. 1 of the main article. Continuous lines: measurements of $\xi_0(r/r_v)$ and $\xi_2(r/r_v)$ for the LRG-void correlation, as reported from the SDSS DR16 sample in Aubert2020. We arbitrarily rescaled the LRG measurements along the $y$-axes by a factor of $-0.1$ (resp. $-0.29$) for $\xi_0$ (resp. $\xi_2$). The radial coordinate shown here is $r/r_v$ where we arbitrarily set $r_v=25\,h^{-1} \mathrm{Mpc}$ in the case of the $\mathrm{Ly}\alpha$-void correlation.