The cold circumgalactic medium in emission: MgII halos in TNG50
  We outline theoretical predictions for extended emission from MgII, tracing
cool ~10^4 K gas in the circumgalactic medium (CGM) of star-forming galaxies in
the high-resolution TNG50 cosmological magnetohydrodynamical simulation. We
synthesize surface brightness maps of this strong rest-frame ultraviolet metal
emission doublet (2796, 2803), adopting the assumption that the resonant
scattering of MgII can be neglected and connecting to recent and upcoming
observations with the Keck/KCWI, VLT/MUSE, and BlueMUSE optical integral field
unit spectrographs. Studying galaxies with stellar masses 7.5 < log(M*/M_sun) <
11 at redshifts z=0.3, 0.7, 1 and 2 we find that extended MgII halos in
emission, similar to their Lyman-alpha counterparts, are ubiquitous across the
galaxy population. Median surface brightness profiles exceed 10^-19
erg/s/cm^2/arcsec^2 in the central ~10s of kpc, and total halo MgII luminosity
increases with mass for star-forming galaxies, reaching 10^40 erg/s for M* ~
10^9.5 Msun. MgII halo sizes increase from a few kpc to > 20 kpc at the highest
masses, and sizes are larger for halos in denser environments. MgII halos are
highly structured, clumpy, and asymmetric, with isophotal axis ratio increasing
with galaxy mass. Similarly, the amount and distribution of MgII emission
depends on the star formation activity of the central galaxy. Kinematically,
inflowing versus outflowing gas dominates the MgII luminosity at high and low
galaxy masses, respectively, although the majority of MgII halo emission at
z~0.7 traces near-equilibrium fountain flows and gas with non-negligible
rotational support, rather than rapidly outflowing galactic winds.
