Strong Rest-UV Emission Lines in a "Little Red Dot" Active Galactic Nucleus at z = 7: Early Supermassive Black Hole Growth alongside Compact Massive Star Formation?

Abstract

JWST has now revealed a population of broad-line active galactic nuclei (AGN) at z ≳ 4 characterized by a distinctive spectral energy distribution shape, with a very red rest-frame optical and very blue rest-frame UV continuum. While the optical continuum is thought to originate from the accretion disk, the origin of the UV continuum has been largely unclear. We report the detection of the strong rest-frame UV emission lines of C III] λλ1907, 1909 and C IV λλ1549, 1551 in a "little red dot" AGN, COS-66964. Spectroscopically confirmed at z = 7.0371, COS-66964 exhibits broad Hα emission (FWHM ∼ 2000 km s^‑1) and weak broad Hβ, implying significant dust attenuation to the broad-line region ( ). The Hα line width implies a central supermassive black hole (SMBH) mass of M_⊙ and an Eddington ratio λ ∼ 0.3–0.5. While marginal He II λ4687 and [Fe X] λ6376 detections further indicate that the AGN dominates in the rest-frame optical, the nondetection of He II λ1640 in the UV despite high EW C III] and C IV (∼35 Å) is more consistent with photoionization by massive stars. The nondetection of Mg II λλ2800 is similarly inconsistent with an AGN scattered light interpretation. Assuming the rest-frame UV is dominated by stellar light, we derive a stellar mass of , implying an elevated M_BH/M_⋆ ratio ∼2 orders of magnitude above the local relation, but consistent with other high-z AGN discovered by JWST. The source is unresolved in all bands, implying a very compact size ≲200 pc in the UV. This suggests that the simultaneous buildup of compact stellar populations (i.e., galaxy bulges) and the central SMBH is ongoing even at z ≳ 7.