Weekly archive
LRDigestWeekly digests of little red dots and JWST-selected AGN papers, with curated reading notes and archive pages.
654 indexed papers · 430 research notes
JWST identifies 11 ultra-red, mostly resolved galaxies in the z=4.3 SPT2349-56 core. Their extreme concentration and ALMA-constrained low star formation suggest a likely protocluster population transitioning beyond the dusty-starburst phase, pending spectroscopy.
This forecasting study proposes using free–free spectral turnovers to infer the column density and characteristic radius of dense ionized gas around LRDs. Mock radio-to-mm photometry suggests useful constraints, but no new free–free detection is reported.
A single z=7.03 galaxy–absorber pair links a dusty, intensely star-forming galaxy to unusually strong multiphase metal absorption at 16 kpc. Blueshifted gas is consistent with an outflow, while abundance evidence for Population III enrichment remains suggestive rather than decisive.
For 142 optically unobscured, FIR-bright Stripe 82 quasars at z=1–4, ALMA deblending lowers inferred host SFRs yet leaves extreme starbursts. The authors find weak instantaneous starburst–accretion coupling and UV-line differences consistent with a transitional interpretation.
JWST spectra of the single dusty galaxy AzTEC-1 at z=4.3419 reveal a broad Hα component blueshifted by 1245 km s⁻¹. The authors favor a candidate sub-pc SMBH binary or recoiling SMBH over an ionized outflow, but confirmation requires resolved follow-up.
A semi-analytic post-quenching model predicts that delayed TP-AGB dust, grain growth, and AGN-regulated gas loss can produce wide dust and PAH diversity in z~1 quiescent galaxies, motivating joint ALMA–JWST constraints rather than any single ISM tracer.
JWST and ALMA reclassify two z=6.63–6.80, MUV≈−23.5 SHELLQs “quasars” as extreme star-forming galaxies. Their UV wind and broad He II features are better matched by VMS-inclusive population models, although inferred IMF cutoffs remain strongly model dependent.
JADES-GS 209777 (“the Hatchling”) is a z=3.711 red quasar with exposed broad-line/X-ray emission yet nuclear absorption and multiphase disturbed gas. The authors interpret this combination as a partially cleared, dusty buried-quasar phase.
JWST/NIRSpec and ALMA resolve LID-1166 into two accreting, gas-associated nuclei separated by ~1.5 kpc at z~4.5. The authors interpret the system as an obscured, gas-rich late-stage merger undergoing super-Eddington SMBH growth.
This paper uses spatially resolved CIGALE SED fitting to separate the ∼1 kpc nuclear core of GN20 from the full 9.9 kpc galaxy, combining JWST NIRCam, MIRI, NIRSpec PRISM pseudo-continuum, HST, and millimeter data. The central result is that the galaxy-wide SED is overwhelmingly star-formation powered, with only a marginal integrated AGN fraction of 0.09 ± 0.02, while the nucleus requires a much stronger AGN contribution of 0.34 ± 0.05 to explain a rest-frame 2.5–3.6 μm mid-infrared excess from torus-heated dust. That means the AGN contributes about one-third of the nuclear infrared luminosity but only about 9% of the total L_IR, naturally explaining why integrated diagnostics and earlier Spitzer limits made the source look starburst-dominated. The broader significance is that GN20 looks like a buried-black-hole case where only JWST-resolution, nuclear-scale decomposition can uncover ongoing early SMBH growth inside an extreme z = 4.055 dusty starburst.
Spatially resolved JWST, ALMA and KCWI data support a multiphase, minor-axis biconical outflow in the dusty local merger IRAS F01364-1042. The authors infer faster ionized than molecular gas and argue a nuclear starburst can energetically drive it despite an obscured AGN.
In lensed A2744, a rest-frame red cut identifies most robust ALMA sources while separating luminous, extended dusty star-forming galaxies from compact LRDs; ALMA non-detections of LRDs constrain, but do not alone establish, their dust content.
For the single z=8.312 galaxy MACS0416-Y1, deep JWST/NIRSpec plus archival IFU/ALMA data support a broad-line AGN interpretation, low metallicity and dust mass ratios, and a spatially non-coincident origin for optical and far-infrared [OIII].
Deep NOEMA observations of three z>6 JWST AGN—two Little Red Dots and one blue Type-I source—yield no significant [C II] or dust-continuum detection, tightening host-galaxy and SED constraints while leaving the physical origin of their [C II] deficit unresolved.
Multi-line JWST/NIRSpec IFU spectroscopy moves dusty massive galaxy S1 from z=5.58 to z=3.2439, tracing the earlier single-line grism assignment to spectral overlap. ALMA continuum and revised SED fitting instead describe a compact, obscured, massive star-forming galaxy.
JWST/NIRSpec spectroscopy of 16 ALMA-selected HUDF faint SMGs finds dusty, massive z=1–4 galaxies with mostly obscured AGN classifications, yet star-formation, metallicity, and bulk-ISM properties broadly consistent with comparable high-redshift star-forming populations.
In local merging (U)LIRGs, 230-GHz continuum combined with X-ray luminosity and IR-based SFR can conservatively separate star-formation-compatible systems from candidate obscured AGN; the authors identify one confirmed AGN and two strong candidates, but calibration remains preliminary.
Resolved ALMA gas kinematics and JWST stellar light in three ALPAKA discs reveal inner circular speeds exceeding fiducial mass-model predictions. The authors infer missing central mass, plausibly involving bulges, massive black holes, or non-standard inner halo concentrations.
This mini-review argues that little red dots are probably heterogeneous: broad Balmer lines and compact, V-shaped SEDs support accretion in some, whereas pervasive multiwavelength non-detections and viable stellar fits prevent a population-wide AGN identification.
ALMA CH–H2O absorption toward two lensed quasars finds no significant differential velocity and, with independent μ bounds, limits |Δα/α| to 0.55 ppm at zabs=0.88582, a sub-ppm cosmological constraint.
BRAHMA mock observations suggest that overmassive, dense-gas-enshrouded AGN can yield AGN-dominated LRD colors and an LRD density of (2.04 ± 0.32) × 10^-4 Mpc^-3 at z=5–8; this is a model-dependent explanation, not a direct detection of the proposed environments.
JWST/NIRSpec plus ALMA data indicate that the single z=4.2603 galaxy DLA0817g1 has matched Hα–[C II] disk morphology and rotation, enriched but weakly excited gas, and a revised molecular-gas budget consistent with a dynamically relaxed “smouldering” system.
![Figure 5 from Highly-ionized gas in lensed z = 6.027 Little Red Dot seen through [OIII] 88$μ$m with ALMA](/lrdigest/research/figures/2512.05097v1/figure-5-1.png?v=6630572a1f813580)
Deep ALMA spectroscopy detects [O III] 88 μm from the lensed z=6.027 Little Red Dot A383-5.1. Its high [O III]/[C II] ratio and dust-continuum limits favor a dust-poor, strongly ionized ISM, but do not directly measure ionizing-photon escape.
JWST resolves the lensed z=6.027 system A383-LRD1 into a compact red LRD candidate and a blue companion separated by ~380 pc. Lensing exposes a faint intrinsic LRD, but its AGN/BH* interpretation and association with the companion remain photometric inferences.
A 27-object z~6 SHELLQs sample is interpreted as very young, ionizing starbursts; JWST/ALMA observations of J0217-0208 link its exceptional UV brightness to compact star formation and a dusty ionized outflow, though the proposed causal link remains model-dependent.
This Letter median-stacks NIRCam, MIRI, and ALMA imaging for 302 photometrically selected little red dots across CEERS, JADES(+SMILES), and PRIMER (z≈2–11; median z≈6.2). The stacked SED rises to ≈3 µm in the rest frame, and CIGALE fits require a dusty AGN component in most (≳50%) LRDs; combined with ALMA non-detections and deep Chandra stack null results, this points to Compton-thick gas (NH>3×10^24 cm^-2) likely confined within Rsub≈0.1 pc. The inferred geometry leaves optical/UV and broad-line emission largely unobscured, matching blue UV slopes and strong Balmer features. While LRDs are heterogeneous, the prevalence of AGN-heated dust implies obscured black-hole growth is common in this population.
Empirically integrating X-ray–to–far-IR SEDs for two of the most luminous Little Red Dots (A2744-45924 and RUBIES-BLAGN-1), the authors directly measure bolometric luminosities rather than adopting standard AGN corrections. They find that over half of Lbol emerges in the rest-frame optical with Lbol/L5100≈5, while X-ray, UV, and reprocessed mid/far-IR components are sub-dominant, and deep ALMA/MIRI limits rule out a standard dust-reddened AGN SED. New bolometric corrections lower previously inferred luminosities by ~10×, implying BH masses of ~10^5–10^7 Msun (and ~10^8 Msun hosts) if radiating near Eddington and easing tensions with clustering and BH mass density. A key assumption is that far-IR luminosities lie well below current observational limits.
This Concept/Survey paper lays out PRIDES, a 1.6 deg^2 COSMOS-field program with PRIMA to fill the mid‑IR gap (24–240 μm) between JWST and ALMA and trace dust and black‑hole growth out to z≈4. With hyperspectral imaging and spectroscopy, PRIDES will constrain IR SED shapes, dust temperatures, and obscured SFRs while identifying dust‑obscured AGN via PAH and silicate features, detecting mid‑IR emission even beyond z>3. Simulations indicate photometric redshifts from PAH features with accuracy that degrades as the AGN contribution increases, and silicate strengths track AGN hardness. Together, these data will map the fraction of obscured star formation and the prevalence of buried SMBH accretion across environments in COSMOS.
High-resolution (∼0.3″) ALMA Band-3 CO(4–3) and 3‑mm continuum mapping of the MQN01 node isolates a massive companion, MQN01‑QC, ∼10 kpc and −300 km s⁻¹ from the hyperluminous quasar CTS G18.01 (z=3.251). Kinematic modeling of the cold gas yields a dynamical mass of 2.5×10^11 M⊙ within ≈4 kpc and a dynamically cold disk with Vrot/σ≈11, establishing the first quasar companion confirmed as a massive rotating disk at this epoch. Despite the proximity, the disk shows no clear tidal disturbances, while the quasar’s CO(4–3) profile exhibits a broad blueshifted wing consistent with a powerful molecular outflow or interaction. The system suggests dynamically cold disks can persist even in dense, early cosmic‑web nodes and hints the quasar may be a satellite in an incipient merger.
Using JWST/MIRI MRS, the authors detect ro‑vibrational H2 in the z≈2.2 X‑ray AGN cid_346, measuring 1‑0 S(1) (and S(0)) and inferring a hot H2 mass of ≈8×10^5 M⊙. They map hot molecular emission and CO(3‑2) to >10 kpc, and NIRCam imaging reveals two nearby satellites at ≈3.3 and 7.4 kpc, pointing to an interacting environment. The hot phase is 10^5–10^6× less massive than the cold reservoir, but the authors argue H2 ro‑vibrational lines effectively trace locally heated/shocked gas where CO may be faint. Ionized gas shows [O III] extension and a blue wing toward the SE, consistent with outflow activity shaping the multiphase medium.
Ranks 190 z>5.9 QSOs by host mass using ALMA [CII] luminosity as a gas/host-mass proxy, boosted by CISTERN cycle-10 coverage of UV-faint sources, and defines “massive hosts” at L[CII]>1.8x10^9 Lsun. From the resulting UV-luminosity distribution and [CII] luminosity functions, only ~3% of massive-host QSOs are UV-bright (Muv<-26) while >85% are UV-faint (Muv>-24.5). At Muv≈-23 the median extrapolated log10(MBH/Msun)=8.1±0.4, implying UV-bright QSOs host SMBHs ~15x more massive than typical massive-host systems at z~6. This points to most SMBH growth in massive galaxies being missed by UV-bright QSO selections; spectroscopy is still needed to confirm the MBH interpretation.
Uniform ALMA [CII] 158 μm size measurements for a curated sample of 22 z≈6 quasars show their hosts are systematically compact, with a median Re≈1.58 kpc versus 2.26 kpc for main-sequence SFGs measured in the same line. The hosts also have elevated MBH/M* and, for the most compact systems, reduced cold-gas content and sizes akin to massive quiescent galaxies at z≈4–5. Simple evolution over a gas-depletion timescale moves them toward the local MBH–M* relation as stellar mass builds. The compact, gas-poor subset likely marks the late stage of rapid BH growth and are prime progenitors of the first quiescent galaxies.
Two extreme, dust-enshrouded starbursts at z=6.6 are identified in ASPIRE quasar fields: they are bright in ALMA 1.2 mm continuum and [C II] 158 µm, yet essentially disappear in JWST/NIRCam (F356W >28 AB). SEDs anchored to ALMA fluxes imply heavier obscuration than Arp 220 and star formation rates of ≃80–250 M⊙ yr⁻¹. Their inferred star-formation histories make them plausible progenitors of massive quiescent galaxies at z≳4 and descendants of UV-luminous z>10 systems, with an abundance as high as n≈10⁻⁵⋅⁵ Mpc⁻³ under a light halo-occupation model (~30% of those comparison populations). A key caveat is that number densities come from quasar-companion fields; analogous z∼8 systems may remain undetected (“JWST-dark”) at current survey depths.
Re-assessing LRD dust, the authors forward-model UV–IR SEDs across multiple extinction laws and dust geometries, then demand consistency with JWST/MIRI, Herschel, and ALMA fluxes and limits. They find A_V ≲ 1.0–1.5 mag under an SMC law for A2744-45924, RUBIES-BLAGN-1, and stacked LRD SEDs, with only slightly weaker bounds for gray UV extinction. These moderate extinctions alleviate the “dust budget crisis” for hostless/low-mass systems and imply ≈10% radiative efficiencies, easing Sołtan-argument tension. The work predicts testable far-IR reprocessed spectra, reframing early black-hole growth pathways for LRDs.
ALMA Band-6 (1.3 mm) continuum follow-up of 60 of the reddest and brightest COSMOS-Web little red dots—preselected with MIRI F770W detections—finds no individual detections and a non-detection in the stack. The data reach a mean σ_rms≈22 μJy, with a stacked flux of S_1.3mm=2.1±2.9 μJy, implying 3σ limits of Mdust<10^6 M⊙ and Ldust≲10^11 L⊙, about 10× deeper than previous submm stacks. These results disfavor large, cold dust reservoirs in typical LRDs. They are consistent with compact, modest dust columns (A_V~2–4) or extreme Balmer breaks from very dense gas (n>10^9 cm^-3) enshrouding a central black hole.
SCUBADive II systematically compiles 60 F150W-dropout, far-IR–bright galaxies in COSMOS-Web by cross-matching JWST non-detections at 1.5 μm with SCUBA-2 and ALMA counterparts, motivated by the benchmark AzTECC71. A core subset of 16 AzTECC71-like sources shows faint F444W (>24 mag) and no COSMOS2020 IDs. SED fits give ⟨SFR⟩=550+500−360 M⊙ yr⁻1 and ⟨log10(M⋆)⟩=11.2+0.5−0.4, with offsets indicating they are not unusually elevated above the star-forming main sequence. Crucially, such heavily obscured systems constitute roughly 20% across mass bins and could account for up to 60% of the z>4 high-mass end (log10 M⋆>11), quantifying a major, previously missed component of the early galaxy census.
Deep MIRI imaging plus new ALMA continuum and [C II] spectroscopy on two luminous LRDs at z=3.1 (A2744-45924) and z=4.47 (RUBIES-BLAGN-1) set the tightest IR constraints yet. Rest-frame 1–4 μm detections show a flat IR slope, excluding hot (T≳500 K) dust, while far-IR non-detections exclude a cold (T≲75 K) component and typical [C II]-bright dusty starbursts. Empirical maximal SEDs require log(L_IR/L_sun) ≲ 12.2 (3σ), in strong tension with energy-balance predictions from reddened galaxy, AGN, or composite fits. The authors favor intrinsically red LRD SEDs over heavily dust-reddened blue engines, reshaping how these compact sources are interpreted.
Two z_spec > 7 Little Red Dots in FRESCO GOODS-N (ID9094, ID2756) show v‑shape colors, compact morphologies, and broad Hβ (FWHM 2280±490 and 1070±240 km/s), marking them as Type‑I AGN candidates. Deep NOEMA follow-up finds no [CII] 158 μm or 1.3 mm continuum; a DSFG interpretation would have yielded strong detections (>16σ [CII] and >3σ 1.3 mm for ID9094; >5σ [CII] for ID2756). Using the 3σ FIR limits in UV–FIR SED fitting (with/without AGN) and comparison to the L_[CII]–SFR_tot relation, the data favor an AGN contribution, as a dust‑only model would require unusually high dust temperatures. The work shows FIR non‑detections can discriminate AGN‑dominated LRDs from DSFG impostors at z > 7, though a purely dusty origin cannot be fully excluded.
ALMA Band 3/4/5 spectroscopy targets CO(7–6) and [CI](2–1) in three Little Red Dots at z=3.10, 4.46, and 7.04, finding no CO and no far-IR continuum, implying conservative molecular gas limits ≲1–5×10^10 Msun. A2744-45924 (z=4.4655) shows a tentative 4.9σ [CI](2–1) line with a narrow FWHM ~80 km s−1. The [CI]-inferred dynamical mass is ≲10^10 Msun for conservative size assumptions, well below the local MBH–Mdyn relation and an order of magnitude under SED-based stellar mass. If confirmed, this points to LRD hosts unlike massive, gas-rich quasar hosts and lends weight to a non-stellar origin for the Balmer break.