The MUSE Hubble Ultra Deep Field Survey: VII. FeII* Emission in
  Star-Forming Galaxies
  Non-resonant FeII* 2365, 2396, 2612, 2626 emission can potentially trace
galactic winds in emission and provide useful constraints to wind models. From
the 3'x3' mosaic of the Hubble Ultra Deep Field (UDF) obtained with the
VLT/MUSE integral field spectrograph, we identify a statistical sample of 40
FeII* emitters and 50 MgII 2796, 2803 emitters from a sample of 271 [OII] 3726,
3729 emitters with reliable redshifts from z = 0.85 - 1.5 down to 2E-18 (3
sigma) ergs/s/cm^2 (for [OII]), covering the stellar mass range 10^8 - 10^11
Msun. The FeII* and MgII emitters follow the galaxy main sequence, but with a
clear dichotomy. Galaxies with masses below 10^9 Msun and star formation rates
(SFRs) of <1 Msun/year have MgII emission without accompanying FeII* emission,
whereas galaxies with masses above 10^10 Msun and SFRs >10 Msun/year have FeII*
emission without accompanying MgII emission. Between these two regimes,
galaxies have both MgII and FeII* emission, typically with MgII P-Cygni
profiles. Indeed, the MgII profile shows a progression along the main sequence
from pure emission to P-Cygni profiles to strong absorption, due to resonant
trapping. Combining the deep MUSE data with HST ancillary information, we find
that galaxies with pure MgII emission profiles have lower star formation rate
surface densities than those with either MgII P-Cygni profiles or FeII*
emission. These spectral signatures produced through continuum scattering and
fluorescence, MgII P-Cygni profiles and FeII* emission, are better candidates
for tracing galactic outflows than pure MgII emission, which may originate from
HII regions. We compare the absorption and emission rest-frame equivalent
widths for pairs of FeII transitions to predictions from outflow models and
find that the observations consistently have less total re-emission than
absorption, suggesting either dust extinction or non-isotropic outflow
geometries.
