Dust temperature is an important property of the interstellar medium (ISM) of galaxies. It is required when converting (sub)millimetre broad-band flux to total infrared luminosity (LIR), and hence star formation rate, in high-redshift 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 ($M_{\rm star} \gt 10^{10}\, \mathrm{M}_{\odot }$) $z$ = 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, which is required to accurately estimate the total dust mass, 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 for converting (sub)millimetre 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.

We present a suite of 34 high-resolution cosmological zoom-in simulations consisting of thousands of halos up to M halo ∼ 10 12 M (M * ∼ 10 10.5 M ) at z 5 from the Feedback in Realistic Environments project. We post-process our simulations with a three-dimensional Monte Carlo dust radiative transfer code to study dust attenuation, dust emission, and dust temperature within these simulated z 5 galaxies. Our sample forms a tight correlation between infrared excess (IRX ≡ F IR /F UV ) and ultraviolet (UV)-continuum slope (β UV ), despite the patchy, clumpy dust geometry shown in our simulations. We find that the IRX-β UV relation is mainly determined by the shape of the attenuation law and is independent of its normalization (set by the dust-to-gas ratio). The bolometric IR luminosity (L IR ) correlates with the intrinsic UV luminosity and the star formation rate (SFR) averaged over the past 10 Myr. We predict that at a given L IR , the peak wavelength of the dust spectral energy distributions for z 5 galaxies is smaller by a factor of 2 (due to higher dust temperatures on average) than at z = 0. The higher dust temperatures are driven by higher specific SFRs and SFR surface densities with increasing redshift. We derive the galaxy UV luminosity functions (LFs) at z = 5-10 from our simulations and confirm that a heavy attenuation is required to reproduce the observed bright-end UVLFs. We also predict the IRLFs and UV luminosity densities at z = 5-10. We discuss the implications of our results on current and future observations probing dust attenuation and emission in z 5 galaxies.

We study the interstellar medium in a sample of 27 high-redshift quasar host galaxies at z ≳ 6, using the [C ii] 158 μm emission line and the underlying dust continuum observed at ∼1 kpc resolution with Atacama Large Millimeter Array. By performing uv-plane spectral stacking of both the high and low spatial resolution data, we investigate the spatial and velocity extent of gas and the size of the dust-emitting regions. We find that the average surface brightness profile of both the [C ii] and the dust continuum emission can be described by a steep component within a radius of 2 kpc and a shallower component with a scale length of 2 kpc, detected up to ∼10 kpc. The surface brightness of the extended emission drops below ∼1% of the peak at radius of ∼5 kpc, beyond which it constitutes 10%–20% of the total measured flux density. Although the central component of the dust continuum emission is more compact than that of the [C ii] emission, the extended components have equivalent profiles. The observed extended components are consistent with those predicted by hydrodynamical simulations of galaxies with similar infrared luminosities, where the dust emission is powered by star formation. The [C ii] spectrum measured in the mean uv-plane stacked data can be described by a single Gaussian, with no observable [C ii] broad-line emission (velocities in excess of ≳500 km s−1), which would be indicative of outflows. Our findings suggest that we are probing the interstellar medium and associated star formation in the quasar host galaxies up to radii of 10 kpc, whereas we find no evidence for halos or outflows.

The observable properties of galaxy groups, and especially the thermal and chemical properties of the intragroup medium (IGrM), provide important constraints on the different feedback processes associated with massive galaxy formation and evolution. In this, the first in a series of studies aimed at identifying and exploring these constraints, we present a detailed analysis of the global properties of simulated galaxy groups with X-ray temperatures in the range 0.5 − 2 keV over the redshift range 0 ≤ z ≤ 3. The groups are drawn from a cosmological simulation that includes a well-constrained prescription for galactic outflows powered by stars and supernovae, but no AGN feedback. Our aims are (a) to establish a baseline against which we will compare future models; (b) to identify model successes that are genuinely due to stellar/supernovae-powered outflows; and (c) to pinpoint features that not only signal the need for AGN feedback but also constrain the nature of this feedback.We find that even without AGN feedback, our simulation successfully reproduces the observed present-day group global IGrM properties such as the hot gas mass fraction, the various X-ray luminosity-temperature-entropy scaling relations, as well as the mass-weighted and emission-weighted IGrM iron and silicon abundance versus group X-ray temperature trends, for all but the most massive groups. We also show that these trends evolve self-similarly for z < 1, in agreement with the observations. Contrary to expectations, we do not see any evidence of the IGrM undergoing catastrophic cooling. And yet, the z = 0 group stellar mass is a factor of ∼ 2 too high. Probing further, we find that the latter is due to the build-up of cold gas in the massive galaxies before they are incorporated inside groups. This, in turn, indicates that other feedback mechanisms must activate in real galaxies as soon as their stellar mass grows to M * ≈ a few ×10 10 M . We show that these must be powerful enough to expel a significant fraction of the halo gas component from the galactic halos. Gentle "maintenance-mode" AGN feedback, as has been suggested to occur in galaxy clusters, will not do; it cannot bring the stellar and the baryonic fractions into agreement with the observations at the same time. Just as importantly, we find that stellar/supernovae-powered winds are vital for explaining the metal abundances in the IGrM, and these results ought to be relatively insensitive to the addition of AGN feedback.

Recent long wavelength observations on the thermal dust continuum suggest that the Rayleigh-Jeans (RJ) tail can be used as a time-efficient quantitative probe of the dust and ISM mass in high-z galaxies. We use high-resolution cosmological simulations from the Feedback in Realistic Environment (FIRE) project to analyze the dust emission of M * > ∼ 10 10 M galaxies at z = 2 − 4. Our simulations (MASSIVEFIRE) explicitly include various forms of stellar feedback, and they produce the stellar masses and star formation rates of high-z galaxies in agreement with observations. Using radiative transfer modelling, we show that sub-millimeter (sub-mm) luminosity and molecular ISM mass are tightly correlated and that the overall normalization is in quantitative agreement with observations. Notably, sub-mm luminosity traces molecular ISM mass even during starburst episodes as dust mass and mass-weighted temperature evolve only moderately between z = 4 and z = 2, including during starbursts. Our finding supports the empirical approach of using broadband sub-mm flux as a proxy for molecular gas content in high-z galaxies. We thus expect single-band sub-mm observations with ALMA to dramatically increase the sample size of high-z galaxies with reliable ISM masses in the near future.1 SMGs are sub-mm sources with observed flux density at 850µm (S 850µm ) larger than a few mJy.