Disentangling AGN and Star Formation Activity at High Redshift Using Hubble Space Telescope Grism Spectroscopy

TL;DR

This paper introduces a new spatially resolved emission line gradient method using HST grism spectra to distinguish AGN activity from star formation in high-redshift galaxies, improving detection of obscured AGN.

Contribution

The authors develop a novel line-ratio gradient technique that enhances AGN identification at z~2, surpassing classical methods, and establish a relationship linking galaxy properties to AGN activity.

Findings

Identifies ~40% more low-mass and obscured AGN than traditional methods.

Develops a mass, SFR, and gradient-based AGN Eddington ratio mapping.

Consistent results with previous stacked galaxy studies at z~2.

Abstract

Differentiating between active galactic nuclei (AGN) activity and star formation in z ~ 2 galaxies is difficult because traditional methods, such as line ratio diagnostics, change with redshift while multi-wavelength methods (X-ray, radio, IR) are sensitive to only the brightest AGN. We have developed a new method for spatially resolving emission lines in HST/WFC3 G141 grism spectra and quantifying AGN activity through the spatial gradient of the [O III]/H$\beta$ line ratio. Through detailed simulations, we show that our novel line-ratio gradient approach identifies ~ sim 40% more low-mass and obscured AGN than obtained by classical methods. Based on our simulations, we developed a relationship that maps stellar mass, star formation rate, and measured [O III]/H$\beta$ gradient to AGN Eddington ratio. We apply our technique to previously studied stacked samples of galaxies at z ~2 and find that our results are consistent with these studies. Using this gradient method will also be able to inform other galaxy evolution science, such as inside-out quenching and metallicity gradients, and will be widely applicable to future spatially resolved JWST data.