A tomographic map of the large-scale matter distribution using the eBOSS Stripe 82 Ly-$\alpha$ forest

Abstract

The Lyman-$\alpha$ (hereafter Ly-$\alpha$) forest is a probe of large-scale matter density fluctuations at high redshift, $z>2.1$. It consists of HI absorption spectra along individual lines-of-sight. If the line-of-sight density is large enough, 3D maps of HI absorption can be inferred by tomographic reconstruction. In this article, we investigate the Ly-$\alpha$ forest available in the Stripe 82 field ($220\mathrm{de}{\mathrm{g}}^2$), based on the quasar spectra from SDSS Data Release DR16. The density of observed quasar spectra is $37\mathrm{de}{\mathrm{g}}^{-2}$ with a mean pixel signal-to-noise ratio of two per angstrom. This study provides an intermediate case between the average SDSS density and that of the much denser but smaller CLAMATO survey. We derive a 3D map of large-scale matter fluctuations from these data, using a Wiener filter technique. The total volume of the map is $0.94{\mathrm{h}}^{-3}\mathrm{Gp}{\mathrm{c}}^3$. Its resolution is $13{\mathrm{h}}^{-1}\mathrm{Mpc}$, which is related to the mean transverse distance between nearest lines-of-sight. From this map, we provide a catalog of voids and protocluster candidates in the cosmic web. The map-making and void catalog are compared to simulated eBOSS Stripe 82 observations. A stack over quasar positions provides a visualization of the Ly-$\alpha$ quasar cross-correlation. This tomographic reconstruction constitutes the largest-volume high-redshift 3D map of matter fluctuations.