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Register a new userThe Physical Drivers of the Luminosity-Weighted Dust Temperatures in High-Redshift Galaxies
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The underlying distribution of galaxies dust SEDs (i.e., their spectra re-radiated by dust from rest-frame $sim$3$mu$m-3mm) remains relatively unconstrained due to a dearth of FIR/(sub)mm data for large samples of galaxies. It has been claimed in the literature that a galaxys dust temperature -- observed as the wavelength where the dust SED peaks ($lambda_{peak}$) -- is traced most closely by its specific star-formation rate (sSFR) or parameterized distance to the SFR-M$_star$ relation (the galaxy main sequence). We present 0.24 resolved 870$mu$m ALMA dust continuum observations of seven $z=1.4-4.6$ dusty star-forming galaxies (DSFGs) chosen to have a large range of well-constrained luminosity-weighted dust temperatures. We also draw on similar resolution dust continuum maps from a sample of ALESS submillimeter galaxies from Hodge et al. (2016). We constrain the physical scales over which the dust radiates and compare those measurements to characteristics of the integrated SED. We confirm significant correlations of $lambda_{peak}$ with both L$_{IR}$ (or SFR) and $Sigma_{rm IR}$ ($propto$SFR surface density). We investigate the correlation between $log_{10}$($lambda_{peak}$) and $log_{10}$($Sigma_{rm IR}$) and find the relation to hold as would be expected from the Stefan-Boltzmann Law, or the effective size of an equivalent blackbody. The correlations of $lambda_{peak}$ with sSFR and distance from the SFR-M$_star$ relation are less significant than those for $Sigma_{rm IR}$ or L$_{IR}$; therefore, we conclude that the more fundamental tracer of galaxies luminosity-weighted integrated dust temperatures are indeed their star-formation surface densities in line with local Universe results, which relate closely to the underlying geometry of dust in the ISM.
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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 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.
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.
Obscured AGN represent a significant fraction of the entire AGN population, especially at high redshift (~70% at z=3--5). They are often characterized by the presence of large gas and dust reservoirs that are thought to sustain and possibly obscure vigorous star formation processes that make these objects shine at far-IR and sub-mm wavelengths. We exploit ALMA Cycle 4 observations of the continuum (~2.1mm) and high-J CO emission of a sample of six X-ray selected SMGs hosting an obscured AGN at z_spec>2.5 in the 7 Ms CDF-S. We measured the masses and sizes of the dust and molecular gas and we derived the gas density and column density on the basis of a uniform sphere geometry. Finally, we compared the measured column densities with those derived from the Chandra X-ray spectra. We detected both the continuum and line emission for three sources for which we measured both the flux density and size. For the undetected sources, we derived an upper limit on the flux density. We found that the detected galaxies are rich in gas and dust (molecular gas mass in the range <0.5 - 2.7 x 10^10 M_sun for {alpha}_CO=0.8 and up to ~2 x 10^11~M_sun for {alpha}_CO=6.5, and dust mass <0.9 - 4.9 x 10^8 M_sun) and compact (gas major axis 2.1-3.0 kpc, dust major axis 1.4-2.7 kpc). The column densities associated with the ISM are on the order of 10^(23-24) cm-2, which is comparable with those derived from the X-ray spectra. For the detected sources we also derived dynamical masses in the range 0.8 - 3.7 x 10^10 M_sun. We conclude that the ISM of high redshift galaxies can substantially contribute to nuclear obscuration up to the Compton-thick (>10^24 cm-2) regime. In addition, we found that all the detected sources show a velocity gradient reminding one rotating system, even though two of them show peculiar features in their morphology that can be associated with a chaotic, possibly merging, structure.
The chemical enrichment in the interstellar medium (ISM) of galaxies is regulated by several physical processes: stellar evolution, grain formation and destruction, galactic inflows and outflows. Understanding such processes is essential to follow the chemical enrichment of galaxies through the cosmic epochs, and to interpret the observations. Despite the importance of such topics, the efficiency of the different processes driving the evolution of baryons in galaxies, remain controversial. We revise the current description of metal and dust evolution in local low-metallicity dwarf galaxies and we develop a description for Lyman Break Galaxies. Our main goal is to reproduce i) the peak in the mass of dust over the mass of stars (sMdust) observed within few hundred Myrs; ii) the decrease of the sMdust at later time. The spectral energy distribution of the galaxies is fitted with the Code Investigating GALaxies Emission (CIGALE), through which the stellar and dust masses, and the star formation rate are estimated. For some of the dwarf galaxies, the metal and gas content are also available. We run different calculations of chemical evolution in galaxies, and we fit the observed properties through the model predictions. We show that i) a top-heavy initial mass function that favours massive stars and a dust condensation fraction for Type II Supernovae (SNe II) of 50% or more help to reproduce the peak of sMdust observed after 100 Myrs since the beginning of the cycle; ii) galactic outflows play a crucial role in reproducing the decline in sMdust with age, and they are more efficient than grain destruction from SNe II; iii) a star formation efficiency (mass of gas converted into stars) of few per cent is required to explain the metallicity of local dwarf galaxies; iv) dust growth in the ISM is not necessary to reproduce the sMdust and, if present, its effect is erased by galactic outflows.
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