Resolved UV–Optical HST Imaging and Spectral Energy Distribution Modeling of Nearby BAT Active Galactic Nuclei

Abstract

We use high-resolution UV-to-optical imaging from the Hubble Space Telescope (HST) to construct spatially resolved spectral energy distributions (SEDs) for seven nearby (z < 0.07) hard (14–195 keV) X-ray-selected broad-line active galactic nuclei (AGN) with Lbol = 1043.26–1045.34 erg s−1. The high spatial resolution of HST, which physically resolves structures on the scale of ∼50 pc at z = 0.05, enables the separation of AGN and host-galaxy emission through morphological decomposition with GALFIT, yielding improved measurements of AGN properties compared to those obtained with lower-resolution Swift UV/Optical Telescope (UVOT) data. AGN UV magnitudes derived from HST imaging (e.g., F225W) can differ by more than a magnitude from those from Swift/UVOT UVM2 due to extended nuclear emission. Additionally, the inclusion of high-resolution data at longer wavelengths (e.g., F814W) can significantly affect the resulting SED fit. Comparing fits of accretion disk and extinction models using HST and Swift/UVOT data, we find significant differences in the resulting parameters, with average differences of 2.0 eV in the maximum disk temperature and 2.2 mag in the AGN host-galaxy extinction. These differences ultimately lead to significant changes in bolometric luminosities and X-ray bolometric corrections, with the HST-based fits yielding average increases of ∼0.57 dex and ∼0.66 dex, respectively. This demonstrates host-galaxy contamination in unresolved UV–optical data can strongly bias SED-based estimates of disk temperatures, extinction, bolometric luminosities, and X-ray bolometric corrections in AGN. Large-area, high-resolution imaging surveys from Euclid and the Nancy Grace Roman Space Telescope will extend these techniques to much larger AGN samples, enabling uniform, high-precision SED measurements in the near-IR.