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The MUSE eXtremely Deep Field: Individual detections of Lyα haloes around rest-frame UV-selected galaxies at z~2.9-4.4

Hydrogen Ly$α$ haloes (LAHs) are commonly used as a tracer of the circumgalactic medium (CGM) at high redshifts. In this work, we aim to explore the existence of Ly$α$ haloes around individual UV-selected galaxies, rather than around Ly$α$ emitters (LAEs), at high redshifts. Our sample was continuum-selected with F775W<=27.5, and spectroscopic redshifts were assigned or constrained for all the sources thanks to the deepest (100- to 140-hour) existing Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) data with adaptive optics. The final sample includes 21 galaxies that are purely F775W-magnitude selected within the redshift range z=2.9-4.4 and within a UV magnitude range -20<=M1500<= -18, thus avoiding any bias toward LAEs. We tested whether galaxy's Ly$α$ emission is significantly more extended than the MUSE PSF-convolved continuum component. We find 17 LAHs and four non-LAHs. We report the first individual detections of extended Ly$α$ emission around non-LAEs. The Ly$α$ halo fraction is thus as high as $81.0^{+10.3}_{-11.2}$%, which is close to that for LAEs at z=3-6 in the literature. This implies that UV-selected galaxies generally have a large amount of hydrogen in their CGM. We derived the mean surface brightness (SB) profile for our LAHs with cosmic dimming corrections and find that Ly$α$ emission extends to 5.4 arcsec (~40 physical kpc at the midpoint redshift z=3.6) above the typical 1$σ$ SB limit. The incidence rate of surrounding gas detected in Ly$α$ per one-dimensional line of sight per unit redshift, dn/dz, is estimated to be $0.76^{+0.09}_{-0.09}$ for galaxies with M1500<= -18 mag at z~3.7. Assuming that Ly$α$ emission and absorption arise in the same gas, this suggests, based on abundance matching, that LAHs trace the same gas as damped Ly$α$ systems (DLAs) and sub-DLAs.