A population of ultraviolet-dim protoclusters detected in absorption

TL;DR

This study uses Lyman-alpha absorption in galaxy spectra to identify and analyze distant protoclusters, revealing many are underluminous in UV and may be influenced by early environmental effects.

Contribution

It introduces a novel absorption-based method to detect protoclusters independent of galaxy brightness, uncovering many previously unknown structures at high redshift.

Findings

Most absorption-selected structures are underluminous in UV.

Many candidate protoclusters contain fewer galaxies than expected.

Environmental effects may suppress star formation or increase dust in protocluster galaxies.

Abstract

Galaxy protoclusters, which will eventually grow into the massive clusters we see in the local universe, are usually traced by locating overdensities of galaxies. Large spectroscopic surveys of distant galaxies now exist, but their sensitivity depends mainly on a galaxy's star formation activity and dust content rather than its mass. Tracers of massive protoclusters that do not rely on their galaxy constituents are therefore needed. Here we report observations of Lyman-$\alpha$ absorption in the spectra of a dense grid of background galaxies, which we use to locate a substantial number of candidate protoclusters at redshifts 2.2-2.8 via their intergalactic gas. We find that the structures producing the most absorption, most of which were previously unknown, contain surprisingly few galaxies compared to the dark matter content of their analogs in cosmological simulations. Nearly all are expected to be protoclusters, and we infer that half of their expected galaxy members are missing from our survey because they are unusually dim at rest-frame ultraviolet wavelengths. We attribute this to an unexpectedly strong and early influence of the protocluster environment on the evolution of these galaxies that reduced their star formation or increased their dust content.