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

ABSTRACT

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