Detection of an ionized gas outflow in the extreme UV-luminous
  star-forming galaxy BOSS-EUVLG1 at z=2.47
  BOSS-EUVLG1 is the most ultraviolet (UV) and Ly$\alpha$ luminous galaxy
detected so far in the Universe, going through a very active starburst phase,
and forming stars at a rate (SFR) of 955 $\pm$ 118 M$_{\odot}$ yr$^{-1}$. We
report the detection of a broad H$\alpha$ component carrying 25% of the total
H$\alpha$ flux. The broad H$\alpha$ line traces a fast and massive ionized gas
outflow characterized by a total mass, $\log(M_{out}[M_{\odot}]),$ of 7.94
$\pm$ 0.15, an outflowing velocity (V$_{out}$) of 573 $\pm$ 151 km s$^{-1}$,
and an outflowing mass rate ($\dot{M}_{out}$) of 44 $\pm$ 20 M$_{\odot}$
yr$^{-1}$. The presence of the outflow in BOSS-EUVLG1 is also supported by the
identification of blueshifted UV absorption lines in low and high ionization
states. The energy involved in the H$\alpha$ outflow can be explained by the
ongoing star formation without the need for an Active Galactic Nucleus. The
derived low mass loading factor ($\eta$= 0.05 $\pm$ 0.03) indicates that
although massive, this phase of the outflow can not be relevant for the
quenching of the star formation. In addition, only a small fraction ($\leq$
15%) of the ionized outflowing material with velocities above 372 km s$^{-1}$
could escape the gravitational potential, and enrich the surrounding
circum-galactic medium at distances above tens of kpc. The ionized phase of the
outflow does not carry the mass and energy to play a relevant role neither in
the evolution of the host galaxy nor in the enrichment of the intergalactic
medium. Other phases of the outflow could be carrying most of the outflow
energy and mass in the form of hot X-ray emitting gas as predicted by some
recent simulations. The expected emission of the extended X-ray emitting halo
associated with the outflow in BOSS-EUVLG1 and similar galaxies could be
detected with the future X-ray observatory, {\it ATHENA} but could not be
resolved spatially.
