Probing ISM conditions of high redshift galaxies with dust emission: A view from the FIRE simulations

Dust thermal emission is important tool for constraining ISM conditions of high redshift galaxies. However, the physical interpretation of its SED is very challenging as it is unclear how various galaxy properties shape the SED. Radiative transfer analysis of cosmological galaxy models provides many valuable insights. In the first part of my talk, I will discuss the method of using broadband fluxes in the Rayleigh-Jeans (RJ) tail as proxy for measuring molecular gas mass (Scoville+2014; Scoville+2016; Liang+2018). Compared to the traditional CO method, this approach is time-efficient. Using a galaxy sample that is extracted from the high-resolution cosmological simulations from the Feedback in Realistic Environment (FIRE) project, I will show that sub-millimeter (submm) luminosity and molecular ISM mass are tightly correlated and that the overall normalisation is in quantitative agreement with observations (Scoville+2016) from z~2 up to higher redshifts, including during starbursts. our result supports the empirical approach of using broadband submm flux as a reliable proxy for molecular gas content in high-z galaxies. [arXiv: 1804.02403] In the second part of the talk, I will focus on the topic of dust temperatures of high-z galaxies. The RJ approach of dust/ISM mass measurement depends on an assumed dust temperature, which is close to the mass-weighted dust temperature (Tmw). I will show that the observationally-derived dust temperatures generally differ from, and are even poor proxy for Tmw. We find that dust temperatures based on the peak emission wavelength (Tpeak) 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, Tmw does not strongly evolve with redshift over z = 2 − 6 at fixed IR luminosity but is tightly correlated with LIR at fixed z. We also analyse an ‘equivalent’ dust temperature (Teqv) that is commonly used for converting single (sub)millimeter flux density to total IR luminosity, and provide a fitting formula as a function of redshift and dust-to- gas ratio. I will show that galaxies of higher Teqv (or higher Tpeak) (‘warmer dust’) do not necessarily have higher Tmw. The rise of Teqv with redshift, which is implied by recent observational evidence, including the low number counts of submm sources in ALMA blind surveys and the unusual IRX-beta relation of high-z galaxies (Bouwens+2016, Casey+2018), is not simply a sign of dust being physically hotter but rather a consequence of the change in SED shape. A ‘two-phase’ picture for interstellar dust can explain the differences of the various dust temperatures. [arXiv: 1902.10727