Weekly issue

Week 31, 2025

Jul 28 – Aug 3, 2025

Week 31, 2025 includes 10 curated papers, centered on high-z, LRD, spectroscopy.

2508.00057v1

Discovery of a Little Red Dot candidate at $z\gtrsim10$ in COSMOS-Web based on MIRI-NIRCam selection

Takumi S. Tanaka, Hollis B. Akins, Yuichi Harikane, John D. Silverman, Caitlin M. Casey, Kohei Inayoshi, Jan-Torge Schindler, Kazuhiro Shimasaku, Dale D. Kocevski, Masafusa Onoue, Andreas L. Faisst, Brant Robertson, Vasily Kokorev, Marko Shuntov, Anton M. Koekemoer, Maximilien Franco, Eiichi Egami, Daizhong Liu, Anthony J. Taylor, Jeyhan S. Kartaltepe, Sarah E. Bosman, Jaclyn B. Champagne, Koki Kakiichi, Santosh Harish, Zijian Zhang, Sophie L. Newman, Darshan Kakkad, Qinyue Fei, Seiji Fujimoto, Mingyu Li, Steven L. Finkelstein, Zi Jian Li, Erini Lambrides, Laura Sommovigo, Jorge A. Zavala, Kei Ito, Zhaoxuan Liu, Ezequiel Treister, Manuel Aravena, Ghassem Gozaliasl, Haowen Zhang, Hossein Hatamnia, Hiroya Umeda, Akio K. Inoue, Jinyi Yang, Makoto Ando, Junya Arita, Xuheng Ding, Suin Matsui, Yuki Shibanuma, Georgios Magdis, Ming-Yang Zhuang, Xiaohui Fan, Zihao Li, Weizhe Liu, Jianwei Lyu, Jason Rhodes, Sune Toft, Feige Wang, Siwei Zou, Rafael C. Arango-Toro, A. J. Battisti, Steven Gillman, Ali Ahmad Khostovan, Arianna S. Long, Bahram Mobasher, David B. Sanders

Theme match 5/5

Digest

Using COSMOS-Web’s joint NIRCam–MIRI imaging, the authors conduct the first targeted search for little red dots beyond z≈10, leveraging F770W to isolate the V‑shaped SED signature and cull Balmer‑break contaminants. They uncover one robust source, CW-LRD-z10, with zphot=10.5(+0.7/−0.6), MUV=−19.9(+0.1/−0.2), compact morphology, a clear V‑shaped SED, and an F115W non‑detection consistent with an LRD at z∼10. From this single detection they infer a number density at MUV≈−20 of 1.2(+2.7/−1.0)×10⁻⁶ Mpc⁻³ mag⁻¹, implying the LRD fraction rises with redshift to ≈3% at z∼10. If confirmed, this points to LRDs as a common early population potentially tied to the first SMBH growth; deep spectroscopy is still required to secure the redshift and nature of CW-LRD-z10.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect how the MIRI F770W point falls relative to NIRCam bands to form the V‑shaped SED, and compare against the modeled stellar Balmer‑break galaxy—this shows why F770W is decisive for separating true LRDs from contaminants at z>10.
Figure 2: Check the NIRCam–MIRI color–color planes, the red selection box, and the position of CW-LRD-z10 (star symbol with error bars and a lower‑limit arrow); compare against dusty SFG, Balmer‑break, and seed‑BH model tracks to see which loci are excluded once MIRI is included.
Figure 3: Review the HST+JWST cutouts for compactness and non‑detections (F606W, F814W, F115W) and the observed SED panel where F770W anchors the red side—confirming the dropout and the characteristic V‑shape of CW-LRD-z10.
Figure 4: Examine BAGPIPES fits and p(z): the LRD template provides the best fit near z≈10.5, while galaxy alternatives either favor lower redshift or yield poorer χ²—quantifying the preference for the LRD interpretation.

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2507.23774v1

Lord of LRDs: Insights into a "Little Red Dot" with a low-ionization spectrum at z = 0.1

Xihan Ji, Francesco D'Eugenio, Ignas Juodžbalis, Dominic J. Walton, Andrew C. Fabian, Roberto Maiolino, Cristina Ramos Almeida, Jose A. Acosta Pulido, Vasily A. Belokurov, Yuki Isobe, Gareth Jones, Claudia Maraston, Jan Scholtz, Charlotte Simmonds, Sandro Tacchella, Elena Terlevich, Roberto Terlevich

Theme match 5/5

Digest

A nearby LRD analog is uncovered: the broad-line AGN SDSS J102530.29+140207.3 at z=0.1 shows the hallmark V-shaped SED, broad Balmer emission (FWHM 1000–2000 km/s), and deep Balmer absorption. New GTC optical spectra reveal a G–K-giant-like continuum with a clear CH G-band and unusually deep low-ionization absorption, plus a series of [Fe II] emission lines; similar [Fe II] features are identified in a JWST-selected LRD at z=2.26. Multi-epoch data show small but significant Hα variability, and NuSTAR confirms extreme X-ray weakness consistent with Compton-thick obscuration (NH>10^24 cm^-2). Together these point to a dense, complex nuclear environment that reproduces LRD hallmarks locally and informs early black-hole growth, though the strength of the ionic/atomic/molecular absorptions remains challenging for standard stellar/AGN models.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

GTC optical spectrum around 3800–4500 Å: isolate the CH G-band near 4300 Å and compare its depth/shape to G–K giant templates to highlight the non-stellar excess absorption.
Balmer region montage (Hβ through Hα): show broad (1–2×10^3 km/s) emission profiles with superposed deep Balmer absorption; include multi-epoch Hα measurements or a line-flux light curve to illustrate the detected variability.
Low-ionization forest and [Fe II] series: annotate identified [Fe II] lines and other low-ionization atomic/ionic absorptions; compare equivalent widths to empirical stellar templates to emphasize the unusually deep features.
Broadband SED/X-ray constraints: overlay NuSTAR upper limits or spectrum with typical AGN SEDs to demonstrate the extreme X-ray weakness and the Compton-thick NH>10^24 cm^-2 regime.
Cross-redshift comparison: overlay SDSS J1025+1402 with the z=2.26 JWST-selected LRD to show matched V-shaped turnover and the shared [Fe II] emission signature.

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2507.20684v1

A deep dive down the broad-line region: permitted OI, CaII and FeII emission in an AGN Little Red Dot at z=5.3

Roberta Tripodi, Maruša Bradač, Francesco D'Eugenio, Nicholas Martis, Gregor Rihtaršič, Chris Willott, Laura Pentericci, Bianca Moreschini, Maxim Markevitch, Yoshihisa Asada, Antonello Calabrò, Guillaume Desprez, Giordano Felicioni, Gaia Gaspar, Anthony H. Gonzalez, Anishya Harshan, Xihan Ji, Jon Judež, Brian C. Lemaux, Alessandro Marconi, Vladan Markov, Rosa M. Merida, Lorenzo Napolitano, Gaël Noirot, Massimiliano Parente, Annika H. G. Peter, Luke Robbins, Andrew Robertson, Ghassan T. E. Sarrouh, Marcin Sawicki

Theme match 5/5

Digest

JWST/NIRCam imaging and NIRSpec prism spectroscopy of the Bullet‑cluster LRD Bz5.3 (zspec=5.2907) reveal broad Balmer emission plus strong low‑ionization Fe II, O I, and Ca II triplet lines, confirming a bona fide BLR around an accreting SMBH. The work reports the first high‑z detection of the O I λ1304 bump (O I+Si II), alongside prominent O I λ8446 and Ca II λλ8498,8542,8662. A markedly low O I λ1304/λ8446 photon ratio (0.1–0.3) implies internal dust with AV≈0.4–1.0 and points to Lyβ fluorescence as the dominant excitation, consistent with high EW O I λ8446 and weak O I λ7774. Photoionization modeling of O I and Ca II favors multiple BLR gas phases with distinct densities/ionization states, supporting a stratified BLR at early times.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

NIRSpec prism 2D/1D spectrum around the O I λ1304 bump: verify the blended O I+Si II ‘λ1304’ feature at z=5.2907, its breadth relative to nearby Fe II/ Balmer structure, and alignment with the systemic redshift.
Spectral window around O I λ8446 and the Ca II triplet (λ8498, λ8542, λ8662): check detection significance, relative fluxes, and whether line widths/centroids track each other—key for co-spatial LIL gas in the BLR.
OI excitation diagnostics: inspect the measured photon ratio O I λ1304/λ8446 and the weakness of O I λ7774 versus model expectations (fluorescence vs collisional), and how these map to the quoted AV≈0.4–1.0.
Fe II emission decomposition: look at the Fe II pseudo‑continuum/line template fit to judge whether iron is predominantly broad permitted versus any narrow forbidden contribution, and how its kinematics compare to O I/Ca II.
NIRCam cutouts with centroid overlays: assess compactness and any wavelength‑dependent centroid shifts relative to F444W, supporting a nuclear point source consistent with an LRD BLR.

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2508.01372v1

A Nitrogen-rich AGN Powering a Large Ionizing Bubble at z=8.63

Takahiro Morishita, Massimo Stiavelli, Charlotte A. Mason, Roberta Tripodi, Marco Chiaberge, Stefan Schuldt, Chris J. Willott, Yechi Zhang

Theme match 4/5

Digest

New NIRSpec/MSA G140H/F070LP data reveal broad Lyα in CANUCS-LRD-z8.6 (z=8.63) centered near systemic (FWHM 1540±260 km/s), implying transmission through a large reionized region. Simple Strömgren modeling of the line shape yields an ionized-bubble radius Rb=1.5^{+0.3}_{-0.2} pMpc and a Lyα escape fraction fesc=11±3%. The intrinsic width is even broader (≈2200±280 km/s), consistent with a BLR in a low-luminosity AGN, while high N IV]/C IV and N IV]/O III] ratios from PRISM indicate nitrogen enrichment. A mild local overdensity (δ≈1.8) suggests neighboring galaxies may help sustain the bubble, linking this LRD to the emerging class of nitrogen-rich systems like GN-z11 and GHZ2/GLASSz12.

Open paper page arXiv abstract arXiv PDF

Key figures to inspect

Figure 1 (NIRCam stamps): Check the blue-UV/red-optical LRD colors and the lensing environment near MACS J1149; verify that the western extended source is a lower-z interloper and does not contaminate the target.
Figure 2 (G140H Lyα): Inspect the 2D/1D Lyα profile peaking near systemic; compare the fitted model to the dashed intrinsic line to see how the inferred Rb≈1.5 pMpc and modest damping produce the observed shape and the measured FWHM≈1540 km/s.
Figure 3 (PRISM spectrum): Use the continuum-subtracted spectrum and line labels to verify strong N IV] λ1488 and measure N IV]/C IV and N IV]/O III] ratios that signal nitrogen enrichment in a metal-poor AGN.
Figure 4 (line widths): Confirm that all narrow UV lines are unresolved at PRISM resolution while the broad component stands out, supporting the BLR-derived intrinsic width ≈2200 km/s.

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2508.00774v1

A Novel Formation Channel for Supermassive Black Hole Binaries in the Early Universe via Primordial Black Holes

Saiyang Zhang, Boyuan Liu, Volker Bromm

Theme match 4/5

Digest

High-resolution hydrodynamics around ∼10^6 M⊙ primordial black holes show that accretion-driven LW fields plus strong baryon–DM streaming (v_bχ ≳ 0.8 σ_bχ) push gas onto an atomic-cooling track and trigger collapse in the PBH wake. The collapsing clouds sustain inflow rates ≳0.01–0.1 M⊙ yr⁻¹, enabling ∼10^5 M⊙ supermassive stars that rapidly form direct-collapse BHs at z ∼ 20–10. These secondary DCBHs pair with the primary PBH to yield in-situ SMBH binaries with initial q ≈ O(0.1) at ∼10 pc separations. The channel predicts compact, LRD-like sources and low-frequency GW emitters accessible to JWST/ALMA and LISA/TianQin.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Map the LW intensity vs. Eddington ratio for different BH masses to see where the flow transitions from ADAF to thin-disk (Eq. 4 scalings) and whether the LW level required to quench H2 is plausibly reached at the fiducial radius used in the simulations.
Figure 2: Inspect the z of first-collapse in PBH_LW_str_fd005 (ε_th = 0.005, v_bχ = 0.8 σ_bχ) to see the spatial offset between the PBH (black dot) and the collapsing cloud (blue star) and the coexistence of inflow and PBH-driven outflows that set up the wake-triggered collapse.
Figure 3: Compare T–n_H phase diagrams across CDM, PBH_fd005, PBH_LW_fd005, and PBH_LW_str_fd005 to verify that only when both LW feedback and streaming are included does the gas track the near-isothermal atomic-cooling branch toward runaway collapse.
Figure 4: Follow the time evolution of Ṁ_infall and collapsing-cloud mass for different streaming amplitudes to confirm sustained Ṁ ≳ 0.01–0.1 M⊙ yr⁻¹ and growth toward ∼10^5 M⊙ seeds, and to see how stronger streaming sharpens the conditions for DCBH formation relative to the primary ∼10^6 M⊙ PBH.

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2507.22206v1

The X-ray Link Between High Eddington Ratio Dust-Obscured Galaxies (DOGs) and Hot DOGs

Fan Zou, W. N. Brandt, Elena Gallo, Fabio Vito, Zhibo Yu

Theme match 3/5

Digest

Targets seven high-λEdd DOGs and combines new, deeper Chandra exposures with archival X-ray data to measure line-of-sight obscuration. Finds these systems are typically buried: 6/7 have N_H ≳ 10^23 cm^-2 and 3/7 are Compton-thick (N_H ≳ 10^24 cm^-2), with observed X-rays strongly suppressed relative to the mid-IR relation. Placing the sources on the λEdd–N_H plane shows they overlap the regime occupied by Hot DOGs. Authors argue both populations trace a post-merger phase where rapidly accreting SMBHs are enshrouded by dust-rich gas.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Compare LX,obs versus mid-IR (Stern 2015 relation) for the seven DOGs; the vertical offsets between LX,obs and LX,int for four sources visualize heavy suppression expected from high N_H, reinforcing the Compton-thick candidates.
Figure 2: Inspect the posterior PDFs of N_H for the four newly observed DOGs—note the 95% lower limits for J1042+2451 and J1513+1451, the 95% upper limit for J1235+4827, and the median/68% interval for J1210+6105—to see which are securely CT versus only lower-limited.
Figure 3: Position the sample on the λEdd–N_H plane against comparison sets (red type 1 quasars, DOGs, Hot DOGs) to confirm that high-λEdd DOGs lie in the same heavily obscured locus as Hot DOGs.
Figure 4: Use the λEdd–N_H diagram with radiation-pressure boundaries (single scattering vs. trapping), the long-lived-cloud region, and the host-obscuration band to judge whether the inferred columns require nuclear-scale enshrouding or can be explained by galaxy-scale dust; identify sources plausibly in the outflow/…
Cross-check specific cases: J1324+4501 (XMM; CT) and J0825+3002 (XMM+NuSTAR; CT) as anchors for the sample’s high-end N_H.

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2507.22014v1

Little Red Dots as self-gravitating discs accreting on supermassive stars: Spectral appearance and formation pathway of the progenitors to direct collapse black holes

Lorenz Zwick, Christopher Tiede, Lucio Mayer

Theme match 3/5

Digest

Models little red dots as supermassive stars fed by massive self‑gravitating discs assembled in gas‑rich major mergers, reproducing the V‑shaped SED as two blackbodies and attributing Balmer broadening to disc rotation—without invoking extra AGN, stellar, or dust components. MCMC fits to two representative LRDs (J0647‑1045 and COS‑756434) yield sub‑Eddington disc accretion and SMS masses of a few ×10^6 Msun with physically motivated parameters. The framework predicts a luminosity cut‑off of order few ×10^44 erg/s and ties the observed redshift distribution of LRDs to merger rates, matching current trends. Acknowledged caveat: reproducing the absolute abundance of LRDs remains challenging, though the fits are robust to observed diversity.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 2 — Use the inclination/temperature/size experiments to see how the SMS+SMD two‑BB superposition shapes the V‑spectrum; note which parameters most strongly tilt the red optical slope and the UV peak, clarifying degeneracies broken by physical constraints in Section 5.
Figure 3 — Inspect the MCMC posterior panels for correlations between SMS/SMD temperatures and radii, then check the overplotted fits to J0647‑1045 and COS‑756434 against the observed V‑shape and Balmer features, and verify consistency with the quoted X‑ray/IR limits without any added AGN/dust component.
Figure 4 — Compare gas‑mass vs stellar‑mass merger‑rate curves to the observed LRD redshift histogram; the gas‑weighted major‑merger track reproduces the full distribution, supporting a merger‑driven LRD formation path and indicating when LRD incidence should peak.
Figure 1 — Schematic pathway from gas‑rich major mergers to a sub‑pc self‑gravitating disc feeding a hot SMS; use it to map which structure sources the blue/UV vs red/optical blackbody components and where intrinsic rotation broadens Balmer lines.

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2507.21230v1

Can high-redshift AGN observed by JWST explain the EDGES absorption signal?

Alexandra Nelander, Christopher Cain, Jordan C J DSilva, Peter H Sims, Rogier A Windhorst, Judd D Bowman

Theme match 3/5

Digest

Tests whether a high‑z radio‑loud AGN population—calibrated to JWST UV luminosity functions and extrapolated to z≈20—can provide the excess radio background needed for the EDGES 78 MHz absorption. Matching the EDGES depth requires that essentially all UV‑bright sources down to M_UV ≳ −15 are radio‑loud AGN, with the UV number density declining by ≤1.5 dex from z=10→20, and an X‑ray escape fraction ≲1% to avoid pre‑heating that erases the trough. Only an extremely shallow UVLF evolution can achieve the amplitude, and reproducing the trough’s sharp edges and flat bottom demands fine‑tuned redshift dependence of the UVLF, AGN UV fraction, and X‑ray leakage. Verdict: physically possible but implausible given the extreme requirements.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Compare the steep, shallow, and very‑shallow UVLF parameterizations and their UV luminosity densities to see how extrapolations to z≈20 and the faint‑end limit drive the available radio‑photon budget.
Figure 2: Inspect the Lyα coupling coefficient split between Pop III and Pop II/AGN to verify that coupling is already strong by z≈17 and to see which component dominates in each UVLF model.
Figure 3: Read the thermal histories to judge how different X‑ray escape fractions and UV densities advance IGM pre‑heating; the tracks illustrating f_esc,X ≲1% show how the gas stays cold enough through z≈17.
Figure 4: Use the mK‑by‑z≈17 requirement map versus radio‑loud fraction and M_UV limit to pinpoint that only near‑unity radio‑loud fractions down to M_UV ≳ −15 (or the very‑shallow UVLF) reach the EDGES depth, highlighting the narrow, extreme corner of parameter space.

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2508.00209v1

Accretion disc reverberation mapping in a high-redshift quasar

F. Pozo Nuñez, E. Bañados, S. Panda, J. Heidt

Theme match 2/5

Digest

Six months of medium-band photometric reverberation mapping on the MPG/ESO 2.2 m captured inter-band lags in the z=2.66 quasar QSO J0455-4216, isolating continuum windows with ~7% variability and day-scale micro-variations. GPCC-derived lags follow a wavelength-dependent delay spectrum consistent with an SS73 thin disc plus lamp-post, placing the ultraviolet-emitting outermost radius at 3.02(+0.33/−0.57) light-days (~500 AU). Transfer-function modeling shows the light curves are disc-dominated, with only minor diffuse BLR continuum allowed; large DCE fractions oversmooth the variability. This delivers the first direct disc-size measurement at this redshift for an ~9×10^8 M⊙ black hole, validating thin-disc expectations and enabling faster high‑z mass estimates.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the multi-band light curves to see ~7% campaign-long variability, 2–3% day-scale micro-variations, and visually trace a few‑day inter-band lag while confirming the field star’s stability.
Figure 2: Read the GPCC lag posteriors versus wavelength and compare the peak lags to the SS73 lamp‑post curve to verify the disc-like delay spectrum and the model’s assumed MBH, luminosity, and redshift.
Figure 3: Examine the accretion-disc transfer functions broadening with wavelength and how the joint GPCC latent signal convolved with these kernels reproduces sharp features across all bands—key evidence for disc reprocessing.
Figure 4: Check the BLR/DCE transfer function (centered ~162 light-days for CIV) and the sequence of model curves; note how ≥50% DCE over-smooths the green-band light curve, constraining DCE to a minor (~few %) role.

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2507.23066v1

SHELLQs-JWST perspective on the intrinsic mass relation between supermassive black holes and their host galaxies at z > 6

John Silverman, Junyao Li, Xuheng Ding, Masafusa Onoue, Michael Strauss, Yoshiki Matsuoka, Takuma Izumi, Knud Jahnke, Tommaso Treu, Marta Volonteri, Camryn Phillips, Irham Andika, Kentaro Aoki, Junya Arita, Shunsuke Baba, Sarah Bosman, Anna-Christina Eilers, Xiaohui Fan, Seiji Fujimoto, Melanie Habouzit, Zoltan Haiman, Masatoshi Imanishi, Kohei Inayoshi, Kazushi Iwasawa, Nobunari Kashikawa, Toshihiro Kawaguchi, Chien-Hsiu Lee, Alessandro Lupi, Tohru Nagao, Jan-Torge Schindler, Malte Schramm, Kazuhiro Shimasaku, Yoshiki Toba, Benny Trakhtenbrot, Hideki Umehata, Marianne Vestergaard, Fabian Walter, Feige Wang, Jinyi Yang

Theme match 2/5

Digest

Using nine SHELLQs quasars at 6.18<z<6.4 with NIRCam host detections and NIRSpec Balmer-line M_BH, the authors infer the intrinsic M_BH–M_* relation via forward modeling that accounts for selection and measurement systematics, with the slope fixed to 1.61 and acknowledging parameter degeneracies. The sample (log M_BH≈8–9; log M_*≈9.5–11) is consistent with a non-evolving relation but with markedly larger intrinsic scatter, σ≈0.80 dex. The preferred model implies an active UV-unobscured AGN fraction of ~2.3% at z≈6 and predicts many lower-mass AGN soon to be found. An overmassive normalization is disfavored, as it would require unrealistically small scatter (~0.22 dex) and a very low AGN fraction (~0.6%).

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the SHELLQs selection completeness versus M1450 and redshift to see how the Cycle 1 targets populate 6.18<z<6.4 and how selection enters the forward model of biases.
Figure 2: Read the M_BH–M_* plane for the nine objects—note the best-fit line and 1σ band (slope fixed to 1.61), and compare directly to G20/KH13 and the high-z JWST assessments; the inset shows each quasar’s offset within 1–3σ.
Figure 3: Check the bi-variate model versus data in the M_BH–M_* plane and the observed M_BH–Eddington ratio distribution; confirm that the forward-model contours reproduce both spaces simultaneously.
Figure 4: Examine the forced overmassive scenario (normalization above KH13, slope 1.17) and note it only matches the data with σ≈0.22 dex and AGN fraction ≈0.6%, illustrating why such a normalization is disfavored.