High redshift JWST predictions from IllustrisTNG: II. Galaxy line and
  continuum spectral indices and dust attenuation curves
  We present predictions for high redshift ($z=2-10$) galaxy populations based
on the IllustrisTNG simulation suite and a full Monte Carlo dust radiative
transfer post-processing. Specifically, we discuss the ${\rm H}_{\alpha}$ and
${\rm H}_{\beta}$ + $[\rm O \,III]$ luminosity functions up to $z=8$. The
predicted ${\rm H}_{\beta}$ + $[\rm O \,III]$ luminosity functions are
consistent with present observations at $z\lesssim 3$ with $\lesssim 0.1\,{\rm
dex}$ differences in luminosities. However, the predicted ${\rm H}_{\alpha}$
luminosity function is $\sim 0.3\,{\rm dex}$ dimmer than the observed one at
$z\simeq 2$. Furthermore, we explore continuum spectral indices, the Balmer
break at $4000$\AA (D4000) and the UV continuum slope $\beta$. The median D4000
versus sSFR relation predicted at $z=2$ is in agreement with the local
calibration despite a different distribution pattern of galaxies in this plane.
In addition, we reproduce the observed $A_{\rm UV}$ versus $\beta$ relation and
explore its dependence on galaxy stellar mass, providing an explanation for the
observed complexity of this relation. We also find a deficiency in heavily
attenuated, UV red galaxies in the simulations. Finally, we provide predictions
for the dust attenuation curves of galaxies at $z=2-6$ and investigate their
dependence on galaxy colors and stellar masses. The attenuation curves are
steeper in galaxies at higher redshifts, with bluer colors, or with lower
stellar masses. We attribute these predicted trends to dust geometry. Overall,
our results are consistent with present observations of high redshift galaxies.
Future JWST observations will further test these predictions.
