PAH emission from star-forming galaxies in JWST mid-infrared imaging of the lensing cluster SMACS J0723.3$-$7327

TL;DR

This study uses JWST mid-infrared imaging to analyze PAH emission in star-forming galaxies within a lensing cluster, revealing redshift-dependent color variations and demonstrating the impact of lensing on galaxy detection.

Contribution

First observational demonstration of PAH emission effects on galaxy colors in JWST data, with simulated and real data showing redshift-dependent color variations in star-forming galaxies.

Findings

SFGs show significant color variations due to PAH features at different redshifts.

Lensing effects boost the number of detectable galaxies in the observed field.

Detected high-redshift PAH emitter at z=1.45 demonstrates JWST's capability.

Abstract

The mid-infrared spectra of star-forming galaxies (SFGs) are characterized by characteristic broad PAH emission features at 3-20 $\mu$m. 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 broad-band 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 $\mu$m) 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 towards 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, which 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.