Cosmic web Lyα emission in a sample of overdense regions

Abstract

We present a complete and homogeneous analysis of the łya emission properties of the cosmic web at 3 łesssim z łesssim 5 as a function of the overdensity of Lyα emitters (LAEs) in the MUSE Ultra Deep Field (MUDF) and the MUSE Extremely Deep Field (MXDF). We identified 41 overdensities, probing environments that are δ ≈ 2-10 times denser than the field. We searched for extended emission down to surface brightness (SB) levels of 3-5 10^ -20 revealing filamentary structures beyond the scale of the circumgalactic medium, confirming that LAEs act as signposts of the cosmic web. A trend with overdensity emerges: non-detections are mainly at δ < 2, a ≈ 1 dex scatter in SB appears for 2 łesssim δ łesssim 3.5, and detections reach a maximum intrinsic SB of ≈ 2 10^ -19 for δ ≳ 3.5. Emitting regions occupy a fraction of projected area f_ s ≈ 0.20-0.25 inside filaments, yielding a cosmological incidence for łya emission in the cosmic web ell(̊m Lyα) of ≈ 1.22, which is similar to the one of partially neutral Lyman limit systems. This analysis indicates that the emitting gas is partially ionized at moderate densities (n_H ≈ 10^ -3 - 10^ -1 cm ^ -3 ), likely tracing the denser spines of intergalactic filaments and embedded substructures. Finally, we forecast how oriented stacking in larger samples of overdensities from shallower observations could yield an expanded view of the cosmic web before the next-generation wide-field spectroscopic instruments become operational.