The ALMA REBELS Survey: cosmic dust temperature evolution out to z ∼ 7

Abstract

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature Td remains mostly unconstrained due to the few available FIR continuum data at redshift z > 5. This introduces large uncertainties in several properties of high-z galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [C $\scriptstyle \rm II$] 158μm line and underlying dust continuum measurements, we derive Td in the continuum and [C $\scriptstyle \rm II$] detected z ≈ 7 galaxies in the ALMA Large Project REBELS sample. We find 39 K < Td < 58 K, and dust masses in the narrow range Md = (0.9 − 3.6) × 107M⊙. These results allow us to extend for the first time the reported Td(z) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing Td(z) trend with the decrease of gas depletion time, tdep = Mg/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields Td∝(1 + z)0.4. The model also explains the observed Td scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, Td only depends on the gas column density (metallicity), $T_{\rm d} \propto N_{\rm H}^{1/6}$ (Td∝Z−1/6). REBELS galaxies are on average relatively transparent, with effective gas column densities around NH ≃ (0.03 − 1) × 1021cm−2. We predict that other high-z galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated Td ≫ 60 K, are significantly obscured, low-metallicity systems. In fact Td is higher in metal-poor systems due to their smaller dust content, which for fixed LIR results in warmer temperatures.

Analytic models of dust temperature in high-redshift galaxies

“…Bakx et al (2020) obtained an even higher dust temperature for a LBG at = 8.31 (> 80 K). Sommovigo et al (2021Sommovigo et al ( , 2022 indirectly derived dust temperatures by utilizing some empirical relations involving [C ] 158 m emission, obtaining similar dust temperatures to the above (∼ 30-70 K) for a sample of > 5 galaxies. These values imply not only systematically warmer dust than in nearby galaxies but also a large variety in dust temperature at 5.…”

Dual constraints with ALMA: new [O III] 88 μm and dust-continuum observations reveal the ISM conditions of luminous LBGs at $z \sim 7$

“…Liang et al 2019;Vijayan et al 2022). As pointed out by Sommovigo et al (2022), a large scatter in dust temperatures may be expected at early times owing to strongly varying physical conditions in the complex high-redshift ISM. This encourages detailed follow-up studies of objects with seemingly extreme dust properties such as COS-3018555981, as we will discuss in the following.…”

“…In the following, we move on to discuss the dust temperature, arguably one of its most important properties as it is essential in deriving other observable quantities such as the infrared luminosity and dust mass from a limited number of photometric data points (Sommovigo et al 2020(Sommovigo et al , 2022. Unfortunately, establishing the dust temperature is not trivial.…”

The ALMA REBELS Survey: The Cosmic H i Gas Mass Density in Galaxies at z ≈ 7

“…Of the surveyed galaxies, 23 were detected in [C II], with luminosities ranging from L [CII] = 1.3 × 10 8 L e to 1.7 × 10 9 L e (S. Schouws et al 2022, in preparation). Other recent results from the REBELS survey include constraints on the dust properties, masses and temperatures (Dayal et al 2022;Ferrara et al 2022;Inami et al 2022;Sommovigo et al 2022), sizes and morphologies of the [C II] emission components (Fudamoto et al 2022), and the efficiency of Lyα transmission (Endsley et al 2022) of galaxies during the epoch of reionization.…”