Spatially Resolved Emission Line Ratios for Nuclear AGN Selection

Differentiating between AGN activity and star formation in z 2 galaxies is difficult because traditional methods such as line ratio diagnostics change with redshift, while multiwavelength 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, quantifying AGN activity through the spatial gradient of the [O III]/HBeta line ratio. Through detailed simulations, we have shown that our novel line-ratio gradient approach identifies low-mass and obscured AGN well beyond the limits of classic methods. In the course of this archival program, we will extend the current simulations beyond stellar mass, star formation rate, and AGN accretion, additionally modeling the effects of different galaxy sizes and morphologies. We will then use the two-orbit 3D-HST G141 grism observations and 12- and 40-orbit G102 grism observations from FIGS and CLEAR in the CANDELS fields of several thousand 0.6 < z <2.3 galaxies to take a complete AGN census at cosmic noon. The HST grism is the ideal tool for this work because its unique spatial abilities applied to such a large dataset are unparalleled. Our simulations of spatially resolved AGN+galaxy emission lines will inform many related studies of resolved galaxy properties, such dust, star formation rate, metallicity.