MUSE searches for galaxies near very metal-poor gas clouds at z ~ 3: New constraints for cold accretion models

We report on the search for galaxies in the proximity of two very metal-poor gas clouds at z ~ 3 towards the quasar Q0956+122. With a 5-hour Multi-Unit Spectroscopic Explorer (MUSE) integration in a ~500 × 500 kpc2 region centred at the quasar position, we achieve a ≥ 80 per cent complete spectroscopic survey of continuum-detected galaxies with mR ≤ 25 mag and Lyα emitters with luminosity LLyα ≥ 3 × 1041 erg s-1. We do not identify galaxies at the redshift of a z ~ 3.2 Lyman limit system (LLS) with log Z/Z⊙ = -3.35 ± 0.05, placing this gas cloud in the intergalactic medium or circumgalactic medium of a galaxy below our sensitivity limits. Conversely, we detect five Lyα emitters at the redshift of a pristine z ~ 3.1 LLS with log Z/Z⊙≤-3.8, while~0.4 sources were expected given the z~3Lyα luminosity function. Both this high detection rate and the fact that at least three emitters appear aligned in projection with the LLS suggest that this pristine cloud is tracing a gas filament that is feeding one or multiple galaxies. Our observations uncover two different environments for metal-poor LLSs, implying a complex link between these absorbers and galaxy haloes, which ongoing MUSE surveys will soon explore in detail. Moreover, in agreement with recent MUSE observations, we detected a ~90 kpc Lyα nebula at the quasar redshift and three Lyα emitters reminiscent of a 'dark galaxy' population.