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

Karina I. Caputi, Ryan A. Cooper, Pierluigi Rinaldi, Rafael Navarro-Carrera, Edoardo Iani

First listed 2026-01-16 | Last updated 2026-01-16

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.

Short digest

Reports a highly magnified low-mass galaxy behind Abell 370 at z=5.96, dubbed Pseudo-LRD-NOM, whose red optical color is driven by an extreme Hα line with EW0 ≳ 800 Å. NIRSpec/PRISM reveals narrow Hα plus a minor broad component implying an active black hole of MBH ≈ 2.9×10^6 M⊙, a narrow Hβ (S/N=8) giving a striking Balmer decrement Hα/Hβ ≈ 11, and a non-detection of [O III] with [O III]/Hβ < 0.25. Photoionization/SED modeling requires a dusty, very dense (nH ≳ 10^6 cm−3), extremely metal-poor gas (Z ≈ 0.01–0.1 Z⊙) with E(B−V) ≈ 0.18–0.45 in a compact host (M* ≈ 1.6×10^7 M⊙; Σ* ≈ 4.2×10^2 M⊙ pc−2). This identifies a metal-poor, dusty starburst seed environment for early black-hole growth and a plausible progenitor stage of real LRDs.

Key figures to inspect

• Figure 1: Check the F444W and F460M postage stamps to see how the Hα line boosts the reddest bands and how compact the source is, supporting the pseudo-LRD color and size impression.
• Figure 2: Compare the original versus Hα-corrected F444W point to quantify how emission-line contamination drives the F277W–F444W color relative to LRD selection cuts (also inspect F115W–F200W).
• Figure 3: Inspect the PRISM spectrum to verify strong Hα and narrow Hβ while confirming non-detections at the expected [O III]4959,5007 (and [N II]/[S II]) wavelengths; derive the stringent [O III]/Hβ < 0.25 constraint and the large Hα EW.
• Figure 4: Use the line zooms to see the narrow+broad decomposition of Hα (for MBH ≈ 2.9×10^6 M⊙) versus the purely narrow Hβ; this underpins the Balmer decrement Hα/Hβ ≈ 11 and the inference of heavy attenuation with no accompanying metal lines.