Investigating the physical properties of galaxies in the Epoch of
  Reionization with MIRI/JWST spectroscopy
  The James Webb Space Telescope will provide deep imaging and spectroscopy for
sources at redshifts above 6, covering the Epoch of Reionization (EoR, 6 < z <
10). The Mid-IR instrument (MIRI) integral field spectrograph (MRS) will be the
only instrument on board JWST able to observe the brightest optical emission
lines H$\alpha$ and [OIII]0.5007$\mu$m at redshifts above 7 and 9,
respectively. This paper presents a study of the H$\alpha$ fluxes predicted by
FIRSTLIGHT cosmological simulations for galaxies at redshifts of 6.5 to 10.5,
and its detectability with MIRI. Deep (40 ks) spectroscopic integrations with
MRS will be able to detect (S/N > 5) EoR sources at redshifts above 7 with
intrinsic star formation rates of more than 2 M$_{\odot}$ yr$^{-1}$, and
stellar masses above 4-9 $\times$ 10$^7$ M$_{\odot}$. In addition, the paper
presents realistic MRS simulated observations of the expected (rest-frame)
optical and near-infrared spectra for some spectroscopically confirmed EoR
sources detected by ALMA as [OIII]88$\mu$m emitters. The MRS simulated spectra
cover a wide range of low metallicities from about 0.2 to 0.02Z$_{\odot}$, and
different [OIII]88$\mu$m/[OIII]0.5007$\mu$m line ratios. The simulated 10ks MRS
spectra show S/N in the range of 5 to 90 for H$\beta$,
[OIII]0.4959,0.5007$\mu$m, H$\alpha$ and HeI1.083$\mu$m emission lines of
MACS1149-JD1 at z = 9.11, independent of metallicity. In addition, deep 40 ks
simulated spectra of the luminous merger candidate B14-65666 at z=7.15 shows
the MRS capabilities of detecting, or putting strong upper limits, on the
[NII]0.6584$\mu$m, [SII]0.6717,0.6731$\mu$m, and [SIII]0.9069,0.9532$\mu$m
emission lines. In summary, MRS will enable the detailed study of key physical
properties like internal extinction, instantaneous star formation, hardness of
the ionising continuum, and metallicity, in bright (intrinsic or lensed) EoR
sources.
