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$_esim$15600 K, n$_esim$300 cm$^{-3}$, covering fraction f$_csim0.4$) using multiple diagnostics. Inferred stellar and gas-phase metallicities point towards a low metallicity object (Z$_{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.
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