Multiphase Analysis of (U)LIRG Nuclear Activity (MAUNA). I. Spatially Resolved Optical Emission-Line Diagnostics

We present resolved optical integral-field spectroscopy of the 32 luminous and ultraluminous infrared galaxy systems of the Multiphase Analysis of (U)LIRG Nuclear Activity (MAUNA) sample, drawn from the GOALS survey and selected to have JWST/MIRI medium-resolution spectrometer (MRS) observations. The data set combines new Keck/KCWI observations of 10 systems with archival VLT/MUSE cubes for 22 systems, covering H β to [S II] at 1 arcsecond spatial resolution (0.9 kpc at the sample median distance). Using the optical [N II]/H α BPT diagram, we find that composite and LINER-like excitation together account for most of the classified area (median fractions 0.44 and 0.26), while Seyfert-like gas covers a median fraction of 0.02 but carries a far larger proportion of the line luminosity where it is present. Both the projected size of the Seyfert-classified region, R Sy=0.08 - 7.5 kpc, and its [O III] luminosity increase with the bolometric AGN luminosity, with R Sy proportional to L AGN,bol 0.54 plus or minus0.18, as expected for gas photoionized at a constant ionization parameter. The scatter about the luminosity relation depends on dust opacity: the [O III] luminosity per unit AGN luminosity falls by 1.09 plus or minus0.25 dex per unit depth of the 9.7 μ m silicate absorption feature (Spearman ρ=-0.60, p=0.001). Optical excitation diagnostics therefore locate AGN-excited gas reliably but systematically underestimate the luminosity the AGN supplies to the host in the most obscured (U)LIRGs.