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Register a new userRadiative equilibrium estimates of dust temperature and mass in high-redshift galaxies
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Authors
Akio K. Inoue
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Estimating the temperature and mass of dust in high-$z$ galaxies is essential for discussions of the origin of dust in the early Universe. However, this suffers from limited sampling of the infrared spectral-energy distribution. Here we present an algorithm for deriving the temperature and mass of dust in a galaxy, assuming dust to be in radiative equilibrium. We formulate the algorithm for three geometries: a thin spherical shell, a homogeneous sphere, and a clumpy sphere. We also discuss effects of the mass absorption coefficients of dust at ultraviolet and infrared wavelengths, $kappa_{rm UV}$ and $kappa_{rm IR}$, respectively. As an example, we apply the algorithm to a normal, dusty star-forming galaxy at $z=7.5$, A1689zD1, for which three data points in the dust continuum are available. Using $kappa_{rm UV}=5.0times10^4$ cm$^2$ g$^{-1}$ and $kappa_{rm IR}=30(lambda/100mu m)^{-beta}$ cm$^2$ g$^{-1}$ with $beta=2.0$, we obtain dust temperatures of 38--70~K and masses of $10^{6.5-7.3}$ M$_odot$ for the three geometries considered. We obtain similar temperatures and masses from just a single data point in the dust continuum, suggesting the usefulness of the algorithm for high-$z$ galaxies with limited infrared observations. In the clumpy-sphere case, the temperature becomes equal to that of the usual modified black-body fit, because an additional parameter describing the clumpiness works as an adjuster. The best-fit clumpiness parameter is $xi_{rm cl}=0.1$, corresponding to $sim10$% of the volume filling factor of the clumps in this high-$z$ galaxy if the clump size is $sim10$ pc, similar to that of giant molecular clouds in the local Universe.
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We post-process galaxies in the IllustrisTNG simulations with SKIRT radiative transfer calculations to make predictions for the rest-frame near-infrared (NIR) and far-infrared (FIR) properties of galaxies at $zgeq 4$. The rest-frame $K$- and $z$-band galaxy luminosity functions from TNG are overall consistent with observations, despite a $sim 0.4,mathrm{dex}$ underprediction at $z=4$ for $M_{rm z}lesssim -24$. Predictions for the JWST MIRI observed galaxy luminosity functions and number counts are given. We show that the next-generation survey conducted by JWST can detect 500 (30) galaxies in F1000W in a survey area of $500,{rm arcmin}^{2}$ at $z=6$ ($z=8$). As opposed to the consistency in the UV, optical and NIR, we find that TNG, combined with our dust modelling choices, significantly underpredicts the abundance of most dust-obscured and thus most luminous FIR galaxies. As a result, the obscured cosmic star formation rate density (SFRD) and the SFRD contributed by optical/NIR dark objects are underpredicted. The discrepancies discovered here could provide new constraints on the sub-grid feedback models, or the dust contents, of simulations. Meanwhile, although the TNG predicted dust temperature and its relations with IR luminosity and redshift are qualitatively consistent with observations, the peak dust temperature of $zgeq 6$ galaxies are overestimated by about $20,{rm K}$. This could be related to the limited mass resolution of our simulations to fully resolve the porosity of the interstellar medium (or specifically its dust content) at these redshifts.
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.
We present a novel framework to self-consistently model the effects of radiation fields, dust physics and molecular chemistry (H$_2$) in the interstellar medium (ISM) of galaxies. The model combines a state-of-the-art radiation hydrodynamics module with a non-equilibrium thermochemistry module that accounts for H$_2$ coupled to a realistic dust formation and destruction model, all integrated into the new stellar feedback framework SMUGGLE. We test this model on high-resolution isolated Milky-Way (MW) simulations. We show that photoheating from young stars makes stellar feedback more efficient, but this effect is quite modest in low gas surface density galaxies like the MW. The multi-phase structure of the ISM, however, is highly dependent on the strength of the interstellar radiation field. We are also able to predict the distribution of H$_2$, that allow us to match the molecular Kennicutt-Schmidt (KS) relation, without calibrating for it. We show that the dust distribution is a complex function of density, temperature and ionization state of the gas which cannot be reproduced by simple scaling relations often used in the literature. Our model is only able to match the observed dust temperature distribution if radiation from the old stellar population is considered, implying that these stars have a non-negligible contribution to dust heating in the ISM. Our state-of-the-art model is well-suited for performing next generation cosmological galaxy formation simulations, which will be able to predict a wide range of resolved ($sim 10$ pc) properties of galaxies.
We utilize archival far-infrared maps from the Herschel Space Observatory in four Local Group galaxies (Small and Large Magellanic Clouds, M31, and M33). We model their Spectral Energy Distribution (SED) from 100 to 500 $mu$m using a single-temperature modified blackbody emission with a fixed emissivity index of $beta = 1.8$. From the best-fit model, we derive the dust temperature, $T_{rm d}$, and the dust mass surface density, $Sigma_{rm d}$, at 13 parsec resolution for SMC and LMC, and at 167 parsec resolution for all targets. This measurement allows us to build the distribution of dust mass and luminosity as functions of dust temperature and mass surface density. We compare those distribution functions among galaxies and between regions in a galaxy. We find that LMC has the highest mass-weighted average $T_{rm d}$, while M31 and M33 have the lowest mass-weighted average $T_{rm d}$. Within a galaxy, star forming regions have higher $T_{rm d}$ and $Sigma_{rm d}$ relative to the overall distribution function, due to more intense heating by young stars and higher gas mass surface density. When we degrade the resolutions to mimic distant galaxies, the mass-weighted mean temperature gets warmer as the resolution gets coarser, meaning the temperature derived from unresolved observation is systematically higher than that in highly resolved observation. As an implication, the total dust mass is lower (underestimated) in coarser resolutions. This resolution-dependent effect is more prominent in clumpy star-forming galaxies (SMC, LMC, and M33), and less prominent in more quiescent massive spiral (M31).
We present high-resolution (0.16$$) 870um Atacama Large Millimeter/submillimeter Array (ALMA) imaging of 16 luminous (L_IR ~ 4 x 10^12 L_sun) submillimeter galaxies (SMGs) from the ALESS survey of the Extended Chandra Deep Field South. This dust imaging traces the dust-obscured star formation in these z~2.5 galaxies on ~1.3 kpc scales. The emission has a median effective radius of $R_e=0.24 pm 0.02$, corresponding to a typical physical size of $R_{e}=1.8pm$0.2 kpc. We derive a median Sersic index of $n=0.9pm0.2$, implying that the dust emission is remarkably disk-like at the current resolution and sensitivity. We use different weighting schemes with the visibilities to search for clumps on 0.12$$ (~1.0 kpc) scales, but we find no significant evidence for clumping in the majority of cases. Indeed, we demonstrate using simulations that the observed morphologies are generally consistent with smooth exponential disks, suggesting that caution should be exercised when identifying candidate clumps in even moderate S/N interferometric data. We compare our maps to comparable-resolution HST H$_{160}$-band images, finding that the stellar morphologies appear significantly more extended and disturbed, and suggesting that major mergers may be responsible for driving the formation of the compact dust disks we observe. The stark contrast between the obscured and unobscured morphologies may also have implications for SED fitting routines that assume the dust is co-located with the optical/near-IR continuum emission. Finally, we discuss the potential of the current bursts of star formation to transform the observed galaxy sizes and light profiles, showing that the $zsim0$ descendants of these SMGs are expected to have stellar masses, effective radii, and gas surface densities consistent with the most compact massive (M_* ~ 1-2 x 10^11 M_sun) early-type galaxies observed locally.
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