The dust-continuum size of TNG50 galaxies at z = 1–5: a comparison with the distribution of stellar light, stars, dust, and H2

Abstract

We present predictions for the extent of the dust-continuum emission of main-sequence galaxies drawn from the TNG50 simulation between z = 1–5. We couple the radiative transfer code SKIRT to the output of the TNG50 simulation and measure the dust-continuum half-light radius of the modeled galaxies, assuming a Milky Way dust type and a metallicity dependent dust-to-metal ratio. The dust-continuum half-light radius at observed-frame 850 μm is up to ∼75 per cent larger than the stellar half-mass radius, but significantly more compact than the observed-frame 1.6 μm (roughly corresponding to H-band) half-light radius, particularly towards high redshifts: the compactness compared to the 1.6 μm emission increases with redshift. This is driven by obscuration of stellar light from the galaxy centres, which increases the apparent extent of 1.6 μm disk sizes relative to that at 850 μm. The difference in relative extents increases with redshift because the observed-frame 1.6 μm emission stems from ever shorter wavelength stellar emission. These results suggest that the compact dust-continuum emission observed in z > 1 galaxies is not (necessarily) evidence of the buildup of a dense central stellar component. We find that the dust-continuum half-light radius closely follows the radius containing half the star formation and half the dust mass in galaxies and is ∼80 per cent of the radius containing half the H2 mass. The presented results are a common feature of main-sequence galaxies.

A$^{3}$COSMOS: A census on the molecular gas mass and extent of main-sequence galaxies across cosmic time

“…As discussed in Sect. 5, this apparent discrepancy between optical and FIR sizes of MS galaxies does not, however, necessarily translate into stellar half-mass radius discrepancy, as complex obscuration biases need to be accounted for when converting half-light stellar radius into halfmass stellar radius (e.g., Lang et al 2019;Suess et al 2019;Popping et al 2022). For example, the FIR sizes inferred in our study agree quantitatively with the mean redshift-independent half-mass stellar radius of SFGs measured by Suess et al (2019).…”

“…Using high-resolution ALMA and Hubble Space Telescope observations of 20 submillimeter-selected galaxies, Lang et al (2019) argues instead that the discrepancy between FIR and optical sizes is mostly due to observational biases in which important radial color gradients yield very discrepant half-light and half-mass radii. This observational finding has recently been supported by TNG50 simulations coupled with state-of-the-art radiative transfer code to study the FIR, optical, and half-mass radius of thousands high-redshift 10 9 − 10 11 M MS galaxies (Popping et al 2022). Indeed, in these simulations it is found that while the FIR half-light radius correlates with the radius containing half the star formation in galaxies, strong and un-corrected obscuration of the stellar light toward the galaxy centre increases significantly the apparent extent of the disk sizes in the optical.…”

“…Indeed, in these simulations it is found that while the FIR half-light radius correlates with the radius containing half the star formation in galaxies, strong and un-corrected obscuration of the stellar light toward the galaxy centre increases significantly the apparent extent of the disk sizes in the optical. Popping et al (2022) conclude that the compact dust-continuum emission of MS galaxies with respect to the optical size is not necessarily evidence of the buildup of a dense central stellar component. Future high-resolution near-infrared observations performed by the James Webb Space Telescope will certainly play a key role in validating or invalidating these later findings.…”

“…Note that we take here the FIR size of galaxies as a proxy of their SFR and gas mass distributions (under the hypothesis that the dust and gas are co-spatial). This assumption is justified by recent simulations in which the FIR half-light radius of galaxies is found to be consistent with the radius containing half their star formation and to be only slightly more compact than the radius containing half their molecular gas mass, at least in z 2 galaxies (Popping et al 2022).…”

“…The gray data points are the FIR sizes of MS galaxies from Barro et al (2016, dots), Rujopakarn et al (2016, pluses), Elbaz et al (2018, stars), Lang et al (2019, diamonds), Chang et al (2020, pentagons), andTadaki et al (2020, triangles); while light blue stars are radio sizes of MS galaxies from Jiménez-Andrade et al ( 2019). The black crosses are FIR sizes of M = 10 10.5 M MS galaxies from the simulations of Popping et al (2022). The green triangles and stars are optical half-light sizes of M = 10 10.25 M and M = 10 11.25 M MS galaxies from van der Wel et al ( 2014).…”