A 39 kpc Spatially Extended Clumpy and Asymmetric Galactic Outflow
  Imaged with Mg II Emission
  We image the spatial extent of a cool galactic outflow with resonant Mg II
emission in a gravitationally lensed star-forming galaxy at $z \sim 1.7$ using
VLT/MUSE observations. We observe Mg II residual (continuum-subtracted)
emission out to an observed radial distance of $26.5_{-0.4}^{+0.5}$ kpc from
the galaxy, with an observed maximum spatial extent of
$\approx39_{-0.6}^{+0.8}$ kpc ($30_{-0.5}^{+0.7}$ kpc after correcting for
seeing). Mg II residual emission is patchy and covers a total area of ${\rm
184_{-10}^{+5} kpc^2}$, constraining the minimum area covered by the outflowing
gas to be $13.27_{-1.02}^{+0.55}$ % of the total area. The spatial extent of
the Mg II emission is asymmetric and shows an observed $27.6_{-0.7}^{+0.8}$ %
($20.9_{-0.6}^{+0.7}$ % after seeing correction) larger extent along the
declination direction. We constrain the covering fraction of the Mg II emission
as a function of radial distance and characterize it with a power-law model
convolved with the seeing with an index $\gamma= -1.25_{-0.02}^{+0.02}$. We
find two kinematically distinct Mg II emission components ($\Delta v
\approx400\ {\rm km\ s^{-1}}$) which extend out to similar distances, and may
correspond to two distinct shells of outflowing gas. By using multiple images
with different magnifications of the galaxy in the image plane, we trace the Mg
II residual emission in three individual star-forming regions inside the galaxy
out to $6.0_{- 0.2 }^{+0.2}$ , $7.0_{-0.2}^{+0.3}$ , and $8.5_{-0.1}^{+0.1}$
kpc. Both the Fe II* fine structure emission, and the nebular [O II] emission
are not spatially extended relative to the stellar continuum. These findings
provide robust constraints on the spatial extent of the outflowing gas and
combined with outflow velocity and column density measurements will give
stringent constraints on mass outflow rates of the galaxy.
