MusE GAs FLOw and Wind (MEGAFLOW) VIII. Discovery of a Mgii emission halo probed by a quasar sightline

Using deep ($11.2, m{h}$) VLT/MUSE data from the MEGAFLOW survey, we report the first detection of extended $ m{Mg, small {II}}$ emission from a galaxy's halo that is probed by a quasar sightline. The $ m{Mg, small {II}}, lambda lambda , 2796, 2803$ emission around the z = 0.702 galaxy ($log (M_∗/ m{M_odot })=10.05_{-0.11}^{+0.15}{}$) is detected out to $approx 25, hbox{kpc}$ from the central galaxy and covers $1.0 imes 10^3, hbox{kpc}^2$ above a surface brightness of $14 imes 10^{-19}, m{erg}, m{s}^{-1}, m{cm}^{-2}, m{arcsec}^{-2}{}$ ($2, sigma$; integrated over $1200, m{km, s}^{-1}= 19mathring{ m A}$ and averaged over $1.5, m{arcsec}^{2}$). The $ m{Mg, small {II}}$ emission around this highly inclined galaxy (i ? 75 deg) is strongest along the galaxy's projected minor axis, consistent with the $ m{Mg, small {II}}$ gas having been ejected from the galaxy into a bi-conical structure. The quasar sightline, which is aligned with the galaxy's minor axis, shows strong $ m{Mg, small {II}}$ absorption ($hbox{$EW_0^{lambda 2796}$}{}=1.8{}, mathring{ m A}$) at an impact parameter of $39{}, hbox{kpc}$ from the galaxy. Comparing the kinematics of both the emission and the absorption - probed with VLT/UVES - to the expectation from a simple toy model of a bi-conical outflow, we find good consistency when assuming a relatively slow outflow ($v_ m{out}=130{}, m{km, s}^{-1}$). We investigate potential origins of the extended $ m{Mg, small {II}}$ emission using simple toy models. With continuum scattering models we encounter serious difficulties in explaining the luminosity of the $ m{Mg, small {II}}$ halo and in reconciling density estimates from emission and absorption. Instead, we find that shocks might be a more viable source to power the extended $ m{Mg, small {II}}$ (and non-resonant $[ m{O, small {II}}]$) emission.