# Are we seeing accretion flows in a 250kpc-sized Ly-alpha halo at z=3?

## TL;DR

This study uses deep MUSE observations to reveal a vast Ly-alpha halo around a z=3.12 radio galaxy, showing arc-like structures and filamentary gas flows that may indicate accretion shocks and gas inflows into the halo.

## Contribution

First detailed analysis of a 250 kpc Ly-alpha halo at z=3, linking observed structures to accretion shocks and gas inflows, supported by comparison with simulations.

## Key findings

- Ly-alpha emission extends to ~250 kpc from the AGN.
- Arc-like structures are observed at 150-250 kpc, possibly indicating accretion shocks.
- Filamentary structures suggest gas inflows into the halo.

## Abstract

Using MUSE on the ESO-VLT, we obtained a 4 hour exposure of the z=3.12 radio galaxy MRC0316-257. We detect features down to ~10^-19 erg/s/cm^2/arcsec^2 with the highest surface brightness regions reaching more than a factor of 100 higher. We find Ly-alpha emission out to ~250 kpc in projection from the active galactic nucleus (AGN). The emission shows arc-like morphologies arising at 150-250 kpc from the nucleus in projection with the connected filamentary structures reaching down into the circum-nuclear region. The most distant arc is offset by 700 km/s relative to circum-nuclear HeII 1640 emission, which we assume to be at the systemic velocity. As we probe emission closer to the nucleus, the filamentary emission narrows in projection on the sky, the relative velocity decreases to ~250 km/s, and line full-width at half maximum range from 300-700 km/s. From UV line ratios, the emission on scales of 10s of kpc from the nucleus along a wide angle in the direction of the radio jets is clearly excited by the radio jets and ionizing radiation of the AGN. Assuming ionization equilibrium, the more extended emission outside of the axis of the jet direction would require 100% or more illumination to explain the observed surface brightness. High speed (>300 km/s) shocks into rare gas would provide sufficiently high surface brightness. We discuss the possibility that the arcs of Ly-alpha emission represent accretion shocks and the filamentary emission represent gas flows into the halo, and compare our results with gas accretion simulations.