A young star-forming galaxy at z = 3.5 with an extended Ly\,$\alpha$
  halo seen with MUSE
  Spatially resolved studies of high redshift galaxies, an essential insight
into galaxy formation processes, have been mostly limited to stacking or
unusually bright objects. We present here the study of a typical (L$^{*}$,
M$_\star$ = 6 $\times 10^9$ $M_\odot$) young lensed galaxy at $z=3.5$, observed
with MUSE, for which we obtain 2D resolved spatial information of Ly$\alpha$
and, for the first time, of CIII] emission. The exceptional signal-to-noise of
the data reveals UV emission and absorption lines rarely seen at these
redshifts, allowing us to derive important physical properties (T$_e\sim$15600
K, n$_e\sim$300 cm$^{-3}$, covering fraction f$_c\sim0.4$) using multiple
diagnostics. Inferred stellar and gas-phase metallicities point towards a low
metallicity object (Z$_{\mathrm{stellar}}$ = $\sim$ 0.07 Z$_\odot$ and
Z$_{\mathrm{ISM}}$ $<$ 0.16 Z$_\odot$). The Ly$\alpha$ emission extends over
$\sim$10 kpc across the galaxy and presents a very uniform spectral profile,
showing only a small velocity shift which is unrelated to the intrinsic
kinematics of the nebular emission. The Ly$\alpha$ extension is $\sim$4 times
larger than the continuum emission, and makes this object comparable to
low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies
and Ly$\alpha$ emitters usually studied at high redshift. We model the
Ly$\alpha$ line and surface brightness profile using a radiative transfer code
in an expanding gas shell, finding that this model provides a good description
of both observables.
