On the dust temperatures of high redshift galaxies
  Dust temperature is an important property of the interstellar medium (ISM) of
galaxies. It is required when converting (sub)millimeter broadband flux to
total infrared luminosity (L_IR), and hence star formation rate, in high-z
galaxies. However, different definitions of dust temperatures have been used in
the literature, leading to different physical interpretations of how ISM
conditions change with, e.g., redshift and star formation rate. In this paper,
we analyse the dust temperatures of massive (M* > 10^10 Msun) z=2-6 galaxies
with the help of high-resolution cosmological simulations from the Feedback in
Realistic Environments (FIRE) project. At z~2, our simulations successfully
predict dust temperatures in good agreement with observations. We find that
dust temperatures based on the peak emission wavelength increase with redshift,
in line with the higher star formation activity at higher redshift, and are
strongly correlated with the specific star formation rate. In contrast, the
mass-weighted dust temperature does not strongly evolve with redshift over
z=2-6 at fixed IR luminosity but is tightly correlated with L_IR at fixed z.
The mass-weighted temperature is important for accurately estimating the total
dust mass. We also analyse an 'equivalent' dust temperature for converting
(sub)millimeter flux density to total IR luminosity, and provide a fitting
formula as a function of redshift and dust-to-metal ratio. We find that
galaxies of higher equivalent (or higher peak) dust temperature ('warmer dust')
do not necessarily have higher mass-weighted temperatures. A 'two-phase'
picture for interstellar dust can explain the different scaling relations of
the various dust temperatures.
