LRD host galaxies show average metallicity 0.08 Z_sun with narrow stable range, challenging pristine-gas formation models while ruling out typical local AGN.
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SPURS: Evidence for Clumpy Neutral Envelopes and Ionized IGM Surrounding Little Red Dots in Abell 2744 from Ultra-Deep Rest-UV Spectroscopy
16 Pith papers cite this work. Polarity classification is still indexing.
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abstract
Rest-frame ultraviolet (UV) spectra of Little Red Dots (LRDs) often show Ly$\alpha$ emission. Along with broad Balmer emission, LRDs are expected to produce broad Ly$\alpha$ emission. However, the large column density of neutral gas invoked to explain the Balmer break should significantly redshift and further broaden the Ly$\alpha$ line, making it challenging to detect without sensitive, moderate-resolution spectra. We present ultra-deep (29 hours) G140M JWST/NIRSpec observations covering the rest-UV of two LRDs in Abell2744 from the SPURS Cycle 4 Large Program. One of our targets is Abell2744-QSO1, a gravitationally-lensed LRD at $z=7.04$ with faint UV emission (M$_{\rm UV}=-16.9$), and the other source (UNCOVER-2476) is newly-confirmed at $z=4.02$ with a very bright UV continuum (M$_{\rm UV}=-19.6$). We find that Abell2744-QSO1 has a broad Ly$\alpha$ profile, along with narrow CIV, FeII$\lambda1786$, and OI$\lambda1302$ emission. The Ly$\alpha$ profile suggests an origin similar to the broad H$\alpha$, but the line is considerably less redshifted than expected from existing dense gas models. We show that the line profile can be explained if the dense neutral gas is clumpy, allowing Ly$\alpha$ to escape by scattering off of the clump surfaces. We find that UNCOVER-2476 has narrow [NeIV] emission, indicating either a hard radiation field or shocks. We confirm two close neighbors with Ly$\alpha$ emission around Abell2744-QSO1, indicating it traces a dense environment that may have ionized its surrounding IGM. We suggest that LRDs may preferentially trace bubbles carved by their dense environments, contributing to the prevalence of Ly$\alpha$ in the population.
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Lyα observations of Little Red Dots show luminosities and equivalent widths like normal star-forming galaxies but lower Lyα/Hα ratios and extended asymmetric emission, supporting a two-component model with host-scale gas.
High-resolution spectra show Balmer absorption in 4/10 LRDs with blue-shifted velocities and exponential wings, supporting a model of co-located partial-covering gas with inflow/outflow gradients.
The survey identifies 27 low-redshift LRDs with compact morphology, V-shaped continua, broad Balmer lines with extreme decrements, and ubiquitous outflows, matching high-z counterparts and yielding a number density lower limit of 7.5e-10 cMpc^-3.
Eight low-redshift Little Red Dots identified in DESI DR1 exhibit broad Balmer lines, steep decrements, compact shapes, and negligible variability, with a number density roughly 10,000 times lower than at z>4.
LRDs are reinterpreted as intermediate-mass super-Eddington systems with wind-driven pseudo-photospheres that explain their spectra and imply engine masses below 10^5 solar masses rather than overmassive black holes.
New stack-based strong-line calibrations from ~1500 spectra yield mass-metallicity relations at z=1-10 with decreasing metallicity toward higher redshift and no slope change, plus 50 EMPG candidates at 1-4% solar metallicity showing large scatter and opposite sSFR trends.
Analysis of ~100 JWST LRDs finds redder, compact UV emission with Fe II/Mg II ~8-10 and correlations suggesting central red continuum (β_UV~0) beyond host galaxy contribution.
A theoretical model of a magnetized black hole envelope is developed to explain the broad emission lines and X-ray faintness observed in little red dots using co-rotating plasma clumps and limited X-ray sources.
New JWST pure-parallel imaging over 400 arcmin² yields UV luminosity functions at z~7.5-10 consistent with pre-JWST models and significant clustering of bright galaxies implying they occupy more massive halos than previously modeled.
Narrow-line diagnostics on ~20 LRDs indicate that stellar photoionization alone cannot explain the observed ratios in many objects, implying anisotropic ionizing radiation from complex gas geometry.
JWST data on LRDs and LBDs show AGN-like excitation, strong Lyα with broad components, and X-ray weakness, implying clumpy or equatorial geometries around growing black holes rather than complete gas envelopes.
Red supergiant collisions with massive gaseous envelopes around SMBHs in LRDs can produce detectable transients at rates up to ~0.3 yr^{-1} per LRD for compact clusters of size ≲10 pc.
Little red dots are the dust-reddened, high-inclination counterparts of little blue dots under a super-Eddington unification model, with luminosity-dependent fractions peaking near 20% and obscured systems showing systematically higher black hole masses due to selection.
High-redshift AGN emitting UHE protons up to 10^19 eV generate a 50 PeV cosmogenic neutrino bump consistent with IceCube data from their JWST-measured average properties without fine-tuning.
Non-LTE wind atmosphere models computed with CMFGEN reproduce the SED and Balmer decrement of most Little Red Dots when dust-attenuated with Av ~2, while predicting Fe II, O I, and Ca lines, but struggle to produce both a genuine Balmer break and strong lines simultaneously.
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Transient Signatures of Star-Envelope Collisions in Little Red Dots
Red supergiant collisions with massive gaseous envelopes around SMBHs in LRDs can produce detectable transients at rates up to ~0.3 yr^{-1} per LRD for compact clusters of size ≲10 pc.
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Ultrahigh-energy cosmogenic neutrino emissions in the high-redshift universe
High-redshift AGN emitting UHE protons up to 10^19 eV generate a 50 PeV cosmogenic neutrino bump consistent with IceCube data from their JWST-measured average properties without fine-tuning.