Evidence for non-stellar rest-frame near-IR emission associated with increased star formation in galaxies at $z \sim 1$

TL;DR

This study confirms that non-stellar near-IR emission at z~1 is common in star-forming galaxies and correlates with increased star formation rates, offering a new method to measure SFRs with JWST.

Contribution

It provides the first large-sample analysis linking near-IR excess to star formation activity, distinguishing it from AGN contributions.

Findings

Galaxies with near-IR excess are predominantly star-forming.

Near-IR excess correlates with higher IR and Hα emission.

AGNs are unlikely the main source of the near-IR excess.

Abstract

We explore the presence of non-stellar rest-frame near-IR ($2-5 \ \mu \mathrm{m}$) emission in galaxies at $z \sim 1$. Previous studies identified this excess in relatively small samples and suggested that such non-stellar emission, which could be linked to the $3.3 \ \mu \mathrm{m}$ polycyclic aromatic hydrocarbons feature or hot dust emission, is associated with an increased star formation rate (SFR). In this Letter, we confirm and quantify the presence of an IR excess in a significant fraction of galaxies in the 3D-HST GOODS catalogs. By constructing a matched sample of galaxies with and without strong non-stellar near-IR emission, we find that galaxies with such emission are predominantly star-forming galaxies. Moreover, star-forming galaxies with an excess show increased mid- and far-IR and H$\alpha$ emission compared to other star-forming galaxies without. While galaxies with a near-IR excess show a larger fraction of individually detected X-ray active galactic nuclei (AGNs), an X-ray stacking analysis, together with the IR-colors and H$\alpha$ profiles, shows that AGNs are unlikely to be the dominant source of the excess in the majority of galaxies. Our results suggest that non-stellar near-IR emission is linked to increased SFRs and is ubiquitous among star-forming galaxies. As such, the near-IR emission might be a powerful tool to measure SFRs in the era of the James Webb Space Telescope.