Dissecting the Obscured Core of GN20: an Active Galactic Nucleus Outshone by Young Stars

Abstract

We investigate the relative contributions of star formation and AGN activity to the total energy budget of GN20, one of the most luminous dusty star-forming galaxies known at $z>4$, through spatially resolved spectral energy distribution decomposition. We perform Bayesian SED fitting with CIGALE on two spatially distinct apertures: the nuclear core (r=0.14", $\sim$1kpc physical) and the full galaxy (r=1.4", 9.9 kpc), combining JWST/NIRCam and MIRI broadband imaging, JWST/NIRSpec PRISM IFU pseudo-continuum photometry spanning 42 wavelength bins across rest-frame $0.12$--$1.05\hat{I}¼$m, and archival HST and millimeter interferometry data from NOEMA and PdBI. The integrated SED is dominated by stellar-heated dust, with only a marginal AGN contribution at galaxy-wide scales ($f_{\mathrm{AGN}}^{\mathrm{int}}=0.09\pm 0.02$). The nuclear core, however, requires a significant AGN component ($f_{\mathrm{AGN}}=0.34\pm 0.05$) to account for a mid-infrared excess at rest-frame $\sim$2.5--3.6$\hat{I}¼$m characteristic of AGN-heated torus dust. The AGN accounts for $\sim 34\%$ of the nuclear infrared luminosity but only $\sim 9\%$ of the total integrated $L_{\mathrm{IR}}$, explaining its weak signature in integrated diagnostics and its consistency with existing upper limits from Spitzer spectroscopy. The inferred black hole mass places GN20 within the local $M_{\mathrm{BH}}$--$M_{\mathrm{bulge}}$ relation at the Eddington limit, and in the overmassive regime at sub-Eddington accretion rates, suggesting early and rapid black hole assembly concurrent with the dominant starburst. GN20 exemplifies a class of systems where nuclear-scale SED decomposition, enabled by the angular resolution and infrared sensitivity of JWST, is the only means to uncover a buried AGN overwhelmed by galaxy-wide star formation.

Submitted to A&A (29.05). 12 pages, 4 figures and 2 tables