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A new census of dust and PAHs at z=0.7-2 with JWST MIRI

Power of SMILES in studying the mid-IR dust properties of galaxies at cosmic noon. Left: The top-left image shows a group of galaxies at z ~ 0.7 - 2.3 (matching the redshift range in this paper) in F444W/F200W/F090W NIRCam colors (JADES). The pre-JWST mid-IR view of these galaxies is shown in the top row from Spitzer IRAC and MIPS (GOODS survey; Dickinson & FIDEL Team 2007; Ashby et al. 2015). The bottom two rows show the mid-IR images with JWST MIRI at 5.6, 7.7, 10, 12.8, 15, 18, 21, and 25.5 µm (SMILES). The PSF sizes are shown with white circles in the corners (diameter of the circle is the FWHM). The unprecedented improvement of MIRI in spatial resolution (0.19 to 0.86'' from 5.6 to 25.5 µm) compared to that of IRAC (1.72 and 1.88'' at 5.8 and 8 µm) and MIPS 24 µm (6'') makes it possible to observe the mid-IR emission of individual typical galaxies at these redshifts. Right: The IR dust SEDs of typical galaxies at z = 1.8 with IR luminosities of 1010.5 - 1011.5 L? (Rieke et al. 2009; corresponding to obscured SFRs of ~ 5 - 50 M?/yr assuming the Kennicutt & Evans 2012 calibration) in grey shaded region, along with 3s sensitivity limits of the GOODS-S Herschel/PACS (green; Magnelli et al. 2013), Spitzer MIPS 24 µm confusion limit (red; 56 µJy, Dole et al. 2004), Spitzer IRAC Channels 3 and 4 (blue; Guo et al. 2013), and the MIRI sensitivities of SMILES in ~ 10 - 30 min exposures (orange; Section 2.1). MIRI filter transmission curves are shown with orange shaded regions. The high sensitivity and multi-band capability of MIRI enables measurements and characterizations of IR emission of typical galaxies at cosmic noon. Abstract: This paper utilizes the JWST MIRI multi-band imaging data from the SMILES survey (5-25micron), complemented with HST and NIRCam photometric and spectroscopic data from the JADES and FRESCO surveys for 443 star-forming (non-AGN) galaxies at z=0.7-2.0 to extend the study of dust and PAH emission to a new mass and SFR parameter space beyond our local universe. We find a strong correlation between the fraction of dust in PAHs (PAH fraction, q_PAH) with stellar mass. Moreover, the PAH fraction behavior as a function of gas-phase metallicity is similar to that at z~0 from previous studies, suggesting a universal relation: q_PAH is constant (~3.4%) above a metallicity of ~ 0.5Z? and decreases to <1% at metallicities <0.3Z?. This indicates that metallicity is a good indicator of the ISM properties that affect the balance between the formation and destruction of PAHs. The lack of a redshift evolution from z~0-2 also implies that above 0.5Z?, the PAH emission effectively traces obscured luminosity and the previous locally-calibrated PAH-SFR calibrations remain applicable in this metallicity regime. We observe a strong correlation between obscured UV luminosity fraction (ratio of obscured to total luminosity) and stellar mass. Above the stellar mass of >5×109M?, on average, more than half of the emitted luminosity is obscured, while there exists a non-negligible population of lower mass galaxies with >50% obscured fractions. At a fixed mass, the obscured fraction correlates with SFR surface density. This is a result of higher dust covering fractions in galaxies with more compact star forming regions. Similarly, galaxies with high IRX (IR to UV luminosity) at a given mass or UV continuum slope tend to have higher SFR surface density and shallower attenuation curves, owing to their higher effective dust optical depths and more compact star forming regions.