Extended Lyman alpha haloes around individual high-redshift galaxies
  revealed by MUSE
  We report the detection of extended Ly alpha emission around individual
star-forming galaxies at redshifts z = 3-6 in an ultradeep exposure of the
Hubble Deep Field South obtained with MUSE on the ESO-VLT. The data reach a
limiting surface brightness (1sigma) of ~1 x 10^-19 erg s^-1 cm^-2 arcsec^-2 in
azimuthally averaged radial profiles, an order of magnitude improvement over
previous narrowband imaging. Our sample consists of 26 spectroscopically
confirmed Ly alpha-emitting, but mostly continuum-faint (m_AB >~ 27) galaxies.
In most objects the Ly alpha emission is considerably more extended than the UV
continuum light. While 5 of the faintest galaxies in the sample show no
significantly detected Ly alpha haloes, the derived upper limits suggest that
this is just due to insufficient S/N. Ly alpha haloes therefore appear to be
(nearly) ubiquitous even for low-mass (~10^8-10^9 M_sun) star-forming galaxies
at z>3. We decompose the Ly alpha emission of each object into a compact
`continuum-like' and an extended halo component, and infer sizes and
luminosities of the haloes. The extended Ly alpha emission approximately
follows an exponential surface brightness distribution with a scale length of a
few kpc. While these haloes are thus quite modest in terms of their absolute
sizes, they are larger by a factor of 5-15 than the corresponding rest-frame UV
continuum sources as seen by HST. They are also much more extended, by a factor
~5, than Ly alpha haloes around low-redshift star-forming galaxies. Between
~40% and >90% of the observed Ly alpha flux comes from the extended halo
component, with no obvious correlation of this fraction with either the
absolute or the relative size of the Ly alpha halo. Our observations provide
direct insights into the spatial distribution of at least partly neutral gas
residing in the circumgalactic medium of low to intermediate mass galaxies at z
> 3.
