Pseudo Little Red Dot: An active black hole embedded in a dense and dusty, metal-poor starburst galaxy at z = 5.96

We present a study of a pseudo little red dot with no metal lines (Pseudo-LRD-NOM), a highly magnified lowmass galaxy behind the lensing cluster A370 at z = 5.96. We classify this object as a pseudo-LRD because its red rest-frame optical color is contaminated by a prominent Hα line (with EW0 ≳ 800 Å) present in its JWST NIRSpec spectrum. Hα is dominated by a narrow component and also has a minor broad component indicative of an active black hole with MBH ≈ 5.6 × 10^6 M⊙. A narrow Hβ emission line is also detected (where the signal-tonoise ratio, S/N, is 8), producing a Balmer decrement (narrow) Hα/Hβ = 11. The rest-frame UV spectral slope is ßspec/uv = - 1.20 ± 0.28. All these features can be ascribed to high dust attenuation. However, no [O III]λ5007 or any other metal lines are detected in the spectrum, so [O III]5007/Hβ < 0.25, at odds with a simple dust attenuation explanation. Accounting for all the spectral properties requires the model of a starburst with moderate color excess E(B − V )≈0.2–0.5, high gas density (nH ≳ 10^6 cm^−3 ), and low- to extremely-low gas/stellar metallicities (Z = 0.01–0.1 Z⊙). The demagnified stellar mass is × 2.25+ 0 1 . . 8 3 3 0 10^7 M , and the stellar-mass surface density is = + * 418 M pc 310 725 2, similar to that of massive/nuclear star clusters. Pseudo-LRD-NOM provides evidence of massive black-hole growth occurring in a high-density, dusty starburst that is at the early stages of its chemical enrichment, and is likely a precursor to a real LRD.