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

TL;DR

The paper presents JWST/NIRSpec spectroscopy of a highly magnified, low-mass galaxy behind Abell 370 at $z=5.96$, named Pseudo-LRD-NOM, notable for a prominent $H\alpha$ emission line driving a red rest-frame optical color and the absence of detectable metal lines. The $H\alpha$ line shows a dominant narrow component and a minor broad component, indicating active black hole growth with a mass $M_\mathrm{BH}=2.9\times10^{6}\,M_\odot$, and a detected narrow $H\beta$ line yields a Balmer decrement $H\alpha/H\beta\approx11$. The rest-frame UV slope is $\beta_{UV}^{spec}=-1.2$, implying substantial dust attenuation, but the non-detection of $[OIII]5007$ requires extremely low metallicity ($Z=0.01-0.1\,Z_\odot$) that cannot be explained by dust alone. Modeling favors a compact, high-density starburst with color excess $E(B-V)=0.18-0.45$, gas density $n_H\gtrsim10^6\,\mathrm{cm^{-3}}$, and stellar mass $M_\ast=1.62^{+1.54}_{-0.79}\times10^7\,M_\odot$, yielding a stellar-mass surface density $\Sigma_*\approx418^{+725}_{-310}\,M_\odot\,\mathrm{pc^{-2}}$, indicating massive black-hole growth in a dense, dusty, early chemical enrichment phase and suggesting a precursor to a bona fide LRD.

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.

Figures (1)

• Figure 1: Postage stamps of Pseudo-LRD-NOM in different JWST/NIRCam bands. The two reddest filters shown, i.e., F444W and F460M, encompass the bright H$\alpha$ line.