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An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy
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abstract
We present a detailed exploration of the most optically-luminous Little Red Dot ($L_{H\alpha}=10^{44}$erg/s, $L_V=10^{45}$erg/s, F444W=22AB) found to date. Located in the Abell 2744 field, source A744-45924 was observed by NIRSpec/PRISM with ultradeep spectroscopy reaching SNR$\sim$100pix$^{-1}$, high-resolution 3-4 micron NIRCam/Grism spectroscopy, and NIRCam Medium Band imaging. The NIRCam spectra reveal high rest-frame EW $W_{H\alpha,0,broad}>800$\r{A}, broad H$\alpha$ emission (FWHM$\sim$4500 km/s), on top of narrow, complex absorption. NIRSpec data show exceptionally strong rest-frame UV to NIR Fe II emission ($W_{FeII-UV,0}\sim$340\r{A}), N IV]$\lambda\lambda$1483,1486 and N III]$\lambda$1750, and broad NIR O I $\lambda$8446 emission. The spectra unambiguously demonstrate a broad-line region associated with an inferred $M_{BH}\sim10^9M_\odot$ supermassive black hole embedded in dense gas, which might explain a non-detection in ultradeep Chandra X-ray data (>$10\times$ underluminous relative to broad $L_{H\alpha}$). Strong UV Nitrogen lines suggest supersolar N/O ratios due to rapid star formation or intense radiation near the AGN. The continuum shows a clear Balmer break at rest-frame 3650\r{A}, which cannot be accounted for by an AGN power-law alone. A stellar population model produces an excellent fit with a reddened Balmer break and implying a massive ($M_*\sim8\times10^{10}M_\odot$), old $\sim$500 Myr, compact stellar core, among the densest stellar systems known ($\rho\sim3\times10^6M_\odot$/pc$^2$ for $R_{e,opt}=70\pm10$ pc), and AGN emission with extreme intrinsic EW $W_{H\alpha,0}\gg$1000\r{A}. However, although high $M_*$ and $M_{BH}$ are supported by evidence of an overdensity containing 40 galaxies at $z=4.41-4.51$, deep high-resolution spectroscopy is required to confirm stellar absorption and rule out that dense gas around the AGN causes the Balmer break instead.
Forward citations
Cited by 16 Pith papers
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Misaligned or chaotic? A strong break of axial symmetry in the local LRD J1025 revealed with VLT/FORS2 spectropolarimetry
The local Little Red Dot J1025 shows a 48° polarisation-angle offset between continuum and broad H-alpha, requiring broken axial symmetry.
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(Re)solving the Complex Multiscale Morphology and V-shaped Spectral Energy Distribution of a Newly Discovered Strongly Lensed Little Red Dot in A383
Lensed LRD A383-LRD1 resolves into blue and red components ~300 pc apart whose combined light produces the canonical V-shaped SED.
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What you see is what you get: empirically measured bolometric luminosities of Little Red Dots
Directly integrating the observed spectra of two Little Red Dots shows the bolometric luminosity is dominated by rest-frame optical light, lowering implied black hole masses to about 10^5 to 10^7 solar masses.
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BlackTHUNDER Reveals a Massive Filament around a Compact AGN at $z\simeq5.23$
GN-77652, a compact broad-line AGN at z=5.229, lies beside a 12 kpc multi-source filament at the same redshift whose coalescence will erase the compact AGN appearance within ~150–440 Myr.
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NEXUS: Spectral Variability of Little Red Dots and Blue Active Galactic Nuclei at $2 \lesssim z \lesssim 6$
Little Red Dots at z=2-6 show less than 4% intrinsic H-alpha variability on 1-3 month rest-frame timescales, a flat white-noise pattern unlike normal AGNs, implying different broad-line production.
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ATLAS. II. Extremely High Incidence of Balmer Line Absorption with Predominant Blueshifts in LRDs: Statistical Insights through Comparison with Type 1 AGNs
Balmer-line absorption occurs in ~35% (14/40) of JWST little-red-dot AGNs, roughly 850x the rate in SDSS type-1 AGNs, with mostly slow blueshifted absorber velocities.
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The Distinctive Evolution and Spectral Energy Distribution of Binary Massive Black Hole Accretion
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Optically Thick Outflow Driven by Supercritical Accretion May Explain Little Red Dots
Super-Eddington outflows from 10^5–10^7 M_sun black holes, with 3000–6000 K photospheres, can account for Little Red Dots' red continua, broad-line widths, and Balmer breaks.
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The z=6.68 Little Red Dot THRILS 46403 shows a 4.5 sigma [FeVII] line, the first robust coronal-line detection among z>5 LRDs, plus narrow Balmer absorption, supporting an AGN with direct sightlines to gas at or beyon...
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Do Little Red Dots Vary?
Super-Eddington accretion models can explain why little red dots show almost no variability, whereas standard sub-Eddington AGN variability models predict changes that should already have been seen.
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Spectroscopic follow-up of the completed HSC-SSP survey yields 43 new quasars, 11 candidate obscured quasars, and 29 galaxies at z 5.71 to 7.02.
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BASS. XLIX. Characterization of highly luminous and obscured AGNs: local X-ray and [NeV]$\lambda$3426 emission in comparison with the high-redshift Universe
A local sample of 21 luminous obscured AGNs shows a high [NeV] detection rate, and a stacked spectrum reveals strong [NeV] that is missing in JWST-selected high-redshift AGNs matched in [OIII].
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Lonely Little Red Dots: Challenges to the AGN-nature of little red dots through their clustering and spectral energy distributions
Little red dots sit in lower-density environments than typical galaxies, and Bayesian model comparison favors non-AGN interpretations for most of them.
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Tracing the Evolution of the Balmer Break from Cosmic Dawn to Cosmic Noon with JWST
Using JWST photometry of ~17,000 galaxies, the median Balmer break strength decreases from 1.5 to 1.1 (flux ratio) from z~3.5 to z~10, driven by stellar population age, with rare extremely strong breaks at both epochs...
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Little Red Dots as Obscured Little Blue Dots: A Super-Eddington Unification Model
Little Red Dots are proposed to be dust-reddened, edge-on views of the same super-Eddington accreting blue AGNs seen face-on as Little Blue Dots.
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Evolution of Size, Mass, and Density of Galaxies Since Cosmic Dawn
Under the author's CCC+TL cosmology, galaxy effective radii are larger by roughly (1+z)^0.93, reducing the inferred density and mass of early galaxies.
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