2023ApJ...946L..40L

The mid-infrared spectra of star-forming galaxies (SFGs) are characterized by characteristic broad polycyclic aromatic hydrocarbon (PAH) emission features at 3-20 μm. As these features are redshifted, they are predicted to dominate the flux at specific mid-infrared wavelengths, leading to substantial redshift-dependent color variations in broadband photometry. The advent of JWST for the first time allows the study of this effect for normal SFGs. Based on spectral energy distribution templates, we here present tracks in mid-infrared (4.4, 7.7, 10, 15, and 18 μm) color-color diagrams describing the redshift dependence of SFG colors. In addition, we present simulated color-color diagrams by populating these tracks using the cosmic star formation history and the star formation rate function. Depending on redshift, we find that SFGs stand out in the color-color diagrams by several magnitudes. We provide the first observational demonstration of this effect for galaxies detected in the JWST Early Release Observations of the field toward the lensing cluster SMACS J0723.3-7327. While the distribution of detected galaxies is consistent with the simulations, the numbers are substantially boosted by lensing effects. The PAH emitter with the highest spectroscopic redshift, detected in all bands, is a multiply imaged galaxy at z = 1.45. There is also a substantial number of cluster members that do not exhibit PAH emission except for one SFG at z = 0.38. Future wider-field observations will further populate mid-infrared color-color diagrams and provide insight into the evolution of typical SFGs.