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

ABSTRACT Using deep (11. 2\, h) VLT/MUSE data from the MEGAFLOW survey, we report the first detection of extended Mg\, II emission from a galaxy’s halo that is probed by a quasar sightline. The Mg\, II\, \, 2796, 2803 emission around the z = 0. 702 galaxy ( (M_*/M_) =10. 05-₀. ₁₁^+0. 15) is detected out to 25\, kpc from the central galaxy and covers 1. 0 10³\, kpc² above a surface brightness of 14 10^-19\, erg\, s^-1\, cm^-2\, arcsec^-2 (2\, ; integrated over 1200\, km\, s^-1= 19 A and averaged over 1. 5\, arcsec^2). The Mg\, II emission around this highly inclined galaxy (i ≃ 75 deg) is strongest along the galaxy’s projected minor axis, consistent with the Mg\, 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 Mg\, II absorption (EW₀^ 2796=1. 8\, A) at an impact parameter of 39\, 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 (vₒut=130\, km\, s^-1). We investigate potential origins of the extended Mg\, II emission using simple toy models. With continuum scattering models we encounter serious difficulties in explaining the luminosity of the Mg\, 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 Mg\, II (and non-resonant O\, II) emission.