Pseudo Little Red Dot: an Active Black Hole Embedded in a Dense and Dusty, Metal-Poor Starburst Galaxy at z=5.96

Abstract

We present a study of Pseudo-LRD-NOM (Pseudo little red dot with no metal lines), a highly magnified low-mass galaxy behind the lensing cluster Abell 370 at z=5.96. We classify this object as a pseudo-LRD because its red rest-frame optical colour is mainly driven by a prominent Halpha line (with EW0 >~ 800 Angstroms) present in its JWST NIRSpec spectrum. Halpha is dominated by a narrow component and also has a minor broad component indicative of an active black hole with M_BH = 2.9x10^6 Msun. A narrow Hbeta emission line is also detected (with S/N = 8), producing a Balmer decrement (narrow) Halpha/Hbeta = 11. The rest-frame UV spectral slope is beta_UVspec = -1.2. All these features can be ascribed to high dust attenuation. However, no [OIII]5007 or any other metal lines are detected in the spectrum, so [OIII]5007/Hbeta < 0.25, at odds with a simple dust-attenuation explanation. Accounting for all the spectral properties requires the model of a starburst with moderate colour excess E(B-V)=0.18-0.45, high gas density (n_H >~ 10^6 cm^{-3}) and extremely low gas/stellar metallicities (Z = 0.01-0.1 Zsun). The demagnified stellar mass is 1.62^{+1.54}_{-0.79} x10^7 Msun and the stellar-mass surface density is Sigma* = 418^{+725}_{-310} Msun/pc^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 which is at the early stages of its chemical enrichment, and is likely a precursor to a real LRD.