# A high baryon fraction in massive haloes at z~3

## TL;DR

This study examines the baryon content in the circumgalactic medium of massive haloes at z~3, using Lyα nebulae observations, and finds evidence for a high baryon fraction close to the cosmic value, challenging some feedback models.

## Contribution

It provides new constraints on the baryon fraction in high-redshift galaxy haloes by modeling Lyα emission, considering different emission mechanisms and their implications.

## Key findings

- CGM baryon fractions are close to the cosmic value, with cold gas occupying less than 1% of volume.
- Reconciliation of low baryon fractions with observations requires more massive haloes or colder gas than standard models predict.
- Scattered QSO broad line region photons imply a baryon fraction of at least 70%, indicating substantial baryonic content in these haloes.

## Abstract

We investigate the baryon content of the circumgalactic medium (CGM) within the virial radius of $M_h \sim 10^{12} \; M_\odot$ haloes at z ~ 3, by modelling the surface brightness profile of the giant Ly$\alpha$ nebulae recently discovered by MUSE around bright QSOs at this redshift. We initially assume fluorescent emission from cold photo-ionized gas confined by the pressure of a hot halo. Acceptable CGM baryon fractions (equal or smaller than the cosmological value) require that the cold gas occupies $\lesssim$ 1% of the volume, but is about as massive as the hot gas. CGM baryon fractions as low as 30% of the cosmic value, as predicted by some strongly ejective feedback models at this redshift, are not easy to reconcile with observations, under our assumptions, unless both the QSO-hosting haloes at $z\sim3$ are more massive than recent BOSS estimates based on clustering and the photo-ionized gas is colder than expected in a standard QSO ionizing radiation field. We also consider the option that the emission is dominated by photons scattered from the QSO broad line region. In this scenario, a very stringent lower limit to the baryon fraction can be obtained under the extreme assumption of optically thin scattering. We infer in this case a baryon fraction of at least 70% of the cosmic value, for fiducial parameters. Lower values require halo masses or gas temperatures different than expected, or that some mechanism keeps the cold gas systematically over-pressured with respect to the ambient medium.