CEERS: MIRI deciphers the spatial distribution of dust-obscured star formation in galaxies at 0.1 < z < 2.5

Abstract

Aims. We study the stellar (i.e., rest-optical) and dust-obscured star-forming (i.e., rest-mid-infrared) morphologies (i.e., sizes and Sérsic indices) of star-forming galaxies (SFGs) at 0.1 &lt; z &lt; 2.5. Methods. We combined Hubble Space Telescope (HST) images from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) with JWST images from the Cosmic Evolution Early Release Science (CEERS) survey to measure the stellar and dust-obscured star formation distributions of 69 SFGs. Rest-mid-infrared (rest-MIR) morphologies were determined using a Markov chain Monte Carlo (MCMC) approach applied to the sharpest Mid-InfraRed Instrument (MIRI) images (i.e., shortest wavelength) dominated by dust emission (Sνdust/Sνtotal &gt; 75%), as inferred for each galaxy from our optical-to-far-infrared spectral energy distribution fits with CIGALE. Rest-MIR Sérsic indices were only measured for the brightest MIRI sources, that is, with a signal-to-noise (S/N) greater than 75 (35 galaxies). At a lower S/N, simulations do indeed show that simultaneous measurements of both the size and Sérsic index become less reliable. We extended our study to fainter sources (i.e., S/N &gt; 10; 69 galaxies) by restricting our structural analysis to their rest-MIR sizes (ReMIR) and by fixing their Sérsic index to a value of one. Results. Our MIRI-selected sample corresponds to a mass-complete sample (&gt; 80%) of SFGs down to stellar masses 109.5, 109.5, and 1010 M⊙ at z ∼ 0.3, 1, and 2, respectively. The rest-MIR Sérsic index of bright galaxies (S/N &gt; 75) has a median value of 0.7−0.3+0.8 (the range corresponds to the 16th and 84th percentiles), which is in good agreement with their median rest-optical Sérsic indices. The Sérsic indices as well as the distribution of the axis ratio of these galaxies suggest that they have a disk-like morphology in the rest-MIR. Galaxies above the main sequence (MS) of star formation (i.e., starbursts) have rest-MIR sizes that are, on average, a factor ∼2 smaller than their rest-optical sizes (ReOpt.). The median rest-optical to rest-MIR size ratio of MS galaxies increases with their stellar mass, from 1.1−0.2+0.4 at ∼109.8 M⊙ to 1.6−0.3+1.0 at ∼1011 M⊙. This mass-dependent trend resembles the one found in the literature between the rest-optical and rest-near-infrared sizes of SFGs, suggesting that it is primarily due to radial color gradients affecting rest-optical sizes and that the sizes of the stellar and star-forming components of SFGs are, on average, consistent at all masses. There is, however, a small population of SFGs (∼15%) with a compact star-forming component embedded in a larger stellar structure, with ReOpt.c &gt; 1.8 × ReMIR. This population could be the missing link between galaxies with an extended stellar component and those with a compact stellar component, the so-called blue nuggets.

MIDIS: JWST/MIRI Reveals the Stellar Structure of ALMA-selected Galaxies in the Hubble Ultra Deep Field at Cosmic Noon

“…The inferred rest-frame NIR sizes are smaller compared to those observed in the rest-frame optical, implying a negative color gradient (i.e., the galaxies have redder centers). A similar trend has also been observed in other galaxy populations (e.g., Chen et al 2022;Suess et al 2022;Gillman et al 2023;Magnelli et al 2023), including local face-on spiral galaxies (e.g., Casasola et al 2017), and in studies of stellar-mass maps (e.g., Wuyts et al 2012;van der Wel et al 2024), and can be driven by differences in the properties of the stars, such as their age, or by dust extinction. The results can thus be attributed to the presence or formation of an older, more centrally concentrated stellar population (such as a bulge) and/or stronger dust extinction in the center (potentially linked to compact central star formation), where both lead to a "flatter" distribution in the rest-frame optical bands.…”

NGDEEP Epoch 1: Spatially Resolved Hα Observations of Disk and Bulge Growth in Star-forming Galaxies at z ∼ 0.6–2.2 from JWST NIRISS Slitless Spectroscopy

“…The mid-IR probes emission from polycyclic aromatic hydrocarbons (PAHs), a tracer of star formation from heating in photodissociation regions (e.g., Ronayne et al 2023). Early studies with JWST/MIRI (Magnelli et al 2023;Shen et al 2023) show that the effective radii of galaxies at 0.2 < z < 2.5 over a range of stellar mass measured in the mid-IR emission are more compact (with smaller effective radii) than the light in the rest-frame optical/ near-IR (which traces stellar light), the near-UV light (which traces the direct continuum from young stars with ∼100 Myr lifetimes), or the Hα emission. One explanation for this is that the mid-IR emission tracks star formation over longer timescales than either the UV or Hα emission, as longer-lived stars (with lifetimes of 500 Myr-1 Gyr) can heat PAHs efficiently (Salim et al 2009;Kennicutt & Evans 2012;Salim & Narayanan 2020).…”

CEERS Key Paper. VII. JWST/MIRI Reveals a Faint Population of Galaxies at Cosmic Noon Unseen by Spitzer

“…The weak mid-IR emission is then attributable to most of the dust in low-mass galaxies being colder. Morphological studies with MIRI and NIRcam may be able probe the extent of the dust and stars in z ∼ 1-2 galaxies (Magnelli et al 2023;Shen et al 2023).…”