Void replenishment: how voids accrete matter over cosmic history

TL;DR

This study reveals that cosmic voids, previously thought to be empty, actually experience significant matter inflows over cosmic time, challenging the idea that they are pristine environments for galaxy formation.

Contribution

The paper provides the first detailed analysis showing that voids accrete substantial matter from overdense regions, based on specialized cosmological simulations.

Findings

Void mass inflow can reach over 35% of total mass.

More than half of the matter in voids has been inside them for around 10 Gyr.

Void environments are not pristine but contain processed matter from overdense regions.

Abstract

Cosmic voids are underdense regions filling up most of the volume in the Universe. They are expected to emerge in regions comprising negative initial density fluctuations, and subsequently expand as the matter around them collapses and forms walls, filaments and clusters. We report results from the analysis of a cosmological simulation specially designed to accurately describe low-density regions, such as cosmic voids. Contrary to the common expectation, we find that voids also experience significant mass inflows over cosmic history. On average, $10\%$ of the mass of voids in the sample at $z \sim 0$ is accreted from overdense regions, reaching values beyond $35\%$ for a significant fraction of voids. More than half of the mass entering the voids lingers on periods of time $\sim 10 \, \mathrm{Gyr}$ well inside them, reaching inner radii. This would imply that part of the gas lying inside voids at a given time proceeds from overdense regions (e.g., clusters or filaments), where it could have been pre-processed, thus challenging the scenario of galaxy formation in voids, and dissenting from the idea of being pristine environments.