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A remarkable Ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a Little Red Dot at $z=3.5$
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abstract
The origin of the rest-optical emission of compact, red, high-redshift sources known as `little red dots' (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs would constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at $z=3.55$ with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad Hydrogen (H$\alpha\ \rm FWHM\sim1500$km/s) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation, or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size ($M_*\sim10^{10.5}\,M_\odot,\ r_{e}\sim40\,$pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and H$\alpha$ absorption line are instead most plausibly explained by a `black hole star' (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense, massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas.
Forward citations
Cited by 12 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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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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Spatial decomposition of Little Red Dots with JWST/NIRSpec IFU into broad-line red cores and narrow-line blue host galaxies
Spatially resolved JWST/NIRSpec IFU spectroscopy of five z~5 Little Red Dots shows blue continuum and narrow lines from extended hosts and red continuum plus broad Balmer features from compact cores.
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Quenching of X-ray emission in little red dots by both Compton-thick gas and high accretion rates
X-ray non-detection of little red dots requires both Compton-thick gas columns (~1e25 cm^-2) and intrinsically weak X-ray emission with bolometric correction k_bol,X ≳ 30.
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Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot
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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MEGA: Spectrophotometric SED Fitting of Little Red Dots Detected in JWST MIRI
Standard AGN and star-forming templates cannot fully reproduce the SEDs of eight MIRI-detected little red dots; dense-gas models plus extra hot dust partly work but fail on UV and narrow emission lines.
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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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Another view into JWST-discovered X-ray weak AGNs via radiative dusty feedback
Low-metallicity, dust-poor gas stalls under weak radiation pressure and blocks X-rays, explaining the X-ray weakness of early JWST-detected AGN.
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On the ages of the stellar populations of galaxies at $z=0.1$-7
A combined JWST/HST sample shows that most galaxies from z=0.1 to z=7 have mass-weighted ages younger than the age of the flat-ΛCDM universe, supporting galaxy age evolution.
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Little Red reionization factories
LRDs may drive cosmic reionization: tidal fields are said to funnel intergalactic hydrogen into colliding streams at LRD sites, igniting starbursts that ionize the gas.
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How similar are narrow-line Seyfert 1 galaxies and high-z type 1 AGN?
NLS1s and high-z JWST AGN share low black hole mass, high Eddington ratio, and narrow broad lines, so NLS1s are useful analogs, though host-to-BH ratios and line profiles differ.
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