Abstract
We examined the relation between extended Lyα halos around high-redshift galaxies and the main factors responsible for driving the emission in such halos, in particular at distances around and beyond one virial radius r_ erg cm To reach the required surface brightness sensitivity we take advantage of the MUSE eXtremely Deep Field (MXDF) survey, allowing us to probe levels as faint as ∼ 10^ -20 -2 $ s^-1 arcsec^-2 in individual Lyα halos. Our sample consists of the 21 apparently core- and halo-brightest (yet intrinsically low luminosity of < 42.3 erg s^-1) Lyα emitters (LAEs) in the MXDF at 3<z<4, with typical virial radii around 20 kpc. We measure their radial surface-brightness profiles out to 50 kpc (more than 2r_ Ly α vir ) and investigate the correlations between surface brightness and internal star formation rates of the host galaxies (SFR) or external influences (environmental density, δ+1). We find a clear break in these correlations at radii around or just below 1r_ vir . Below this break the emission correlates tightly with SFR (as expected) and not at all with δ+1. Beyond ∼ 1r_ (20 kpc) we observe the opposite trend with no dependence on SFR, but an emerging correlation with δ+1. We compare our measurements with the expected integrated surface brightness from ultrafaint, individually undetected LAEs and find that the latter is insufficient to drive the observed correlation. We conclude that Ly$α emission from the outer halos is regulated by the surrounding environment, but originates mostly from diffuse gas rather than discrete sources. vir