MUSE Analysis of Gas around Galaxies (MAGG) -- I: Survey design and the environment of a near pristine gas cloud at z~3.5

TL;DR

The MAGG survey uses MUSE at VLT to study the environment of gas-rich structures around high-redshift quasars, revealing inhomogeneous metal enrichment and galaxy-gas associations at z~3-4.

Contribution

This paper introduces the MAGG survey design and presents the first detailed analysis of a metal-poor gas cloud and its galaxy environment at z~3.5.

Findings

Detection of three Lyman alpha emitters near the gas cloud

Evidence of inhomogeneous metal enrichment at high redshift

Proposal that the gas cloud resides in a filament possibly accreting onto a galaxy

Abstract

We present the design, methods, and first results of the MUSE Analysis of Gas around Galaxies (MAGG) survey, a large programme on the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) which targets 28 z > 3.2 quasars to investigate the connection between optically-thick gas and galaxies at z~3-4. MAGG maps the environment of 52 strong absorption line systems at z > 3, providing the first statistical sample of galaxies associated with gas-rich structures in the early Universe. In this paper, we study the galaxy population around a very metal poor gas cloud at z~3.5 towards the quasar J124957.23-015928.8. We detect three Lyman alpha emitters within <200km/s of the cloud redshift, at projected separations <185 kpc (physical). The presence of star-forming galaxies near a very metal-poor cloud indicates that metal enrichment is still spatially inhomogeneous at this redshift. Based on its very low metallicity and the presence of nearby galaxies, we propose that the most likely scenario for this LLS is that it lies within a filament which may be accreting onto a nearby galaxy. Taken together with the small number of other LLSs studied with MUSE, the observations to date show a range of different environments near strong absorption systems. The full MAGG survey will significantly expand this sample and enable a statistical analysis of the link between gas and galaxies to pin down the origin of these diverse environments at z~3-4.