Weekly issue

Week 41, 2025

Oct 6–12, 2025

Week 41, 2025 includes 12 curated papers, centered on spectroscopy, high-z, LRD.

2510.07376v1

The $M_{\rm BH}-M_{*}$ Relationship at $3<z<7$: Big Black Holes in Little Red Dots

Brenda L. Jones, Dale D. Kocevski, Fabio Pacucci, Anthony J. Taylor, Steven L. Finkelstein, Johannes Buchner, Jonathan R. Trump, Rachel S. Somerville, Michaela Hirschmann, L. Y. Aaron Yung, Guillermo Barro, Eric F. Bell, Laura Bisigello, Antonello Calabro, Nikko J. Cleri, Avishai Dekel, Mark Dickinson, Giovanni Gandolfi, Mauro Giavalisco, Norman A. Grogin, Kohei Inayoshi, Jeyhan S. Kartaltepe, Anton M. Koekemoer, Lorenzo Napolitano, Masafusa Onoue, Swara Ravindranath, Giulia Rodighiero, Stephen M. Wilkins

Theme match 5/5

Digest

Using NIRSpec/G395M spectra from CEERS, JADES, and RUBIES, the authors assemble 70 broad-line AGN at 3<z<7 (43% classified as little red dots) and derive MBH via single-epoch virial methods with host M* from 2D profile fitting plus SED modeling. Most sources (50/70) have MBH/M* ratios elevated by 1–2 dex relative to local AGN, and forward-modeling that includes uncertainties, scatter, and selection effects yields an MBH–M* relation >3σ above the local one with an intrinsic scatter of 0.9 dex that does not evolve. MBH/M* increases by 2.3 dex from z=3.5 to z=6.5, driven by a higher LRD fraction at z>4 whose hosts are ~1 dex less massive than non‑LRDs. The results argue for genuinely overmassive early black holes and sharpen constraints on early growth and seeding scenarios.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect how the LRD and non‑LRD redshift histograms differ; the LRD peak sits at higher z, illustrating the rising LRD fraction beyond z>4 that underpins the increasing MBH/M* trend.
Figure 2: Bolometric luminosity versus redshift with LRDs highlighted; check whether LRDs cluster at particular Lbol–z loci and whether luminosity differences could bias the inferred MBH–M* relation.
Figure 3: Example G395M broad-line decompositions; verify FWHM values (instrument-broadening corrected), the treatment of absorption components, and overall fit quality that feeds the single-epoch virial MBH estimates.
Figure 4: GALFIT image–model–residual panels; compare point-source–dominated LRDs versus extended hosts to see how host detection (or lack thereof) influences M* and thus MBH/M*.

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2510.07303v1

Confirming Near- to Mid-IR Photometrically-Identified Obscured AGNs in the JWST era

George H. Rieke, Yang Sun, Jianwei Lyu, Christopher N. A. Willmer, Yongda Zhu, Pierluigi Rinaldi, Meredith A. Stone, Kevin N. Hainline, Pablo G. Perez-Gonzalez

Theme match 5/5

Digest

Tests how well near–mid-IR photometric SED fitting picks out obscured AGN by comparing to JWST/NIRSpec MSA rest‑optical spectra for 17 MIRI‑selected candidates at z=1.24–3.33, and by auditing low‑mass interlopers. For massive hosts (log M*>9.5), the spectra and BPT diagnostics generally back the 1–6 μm excess criterion, with several sources showing outflow‑broadened [O III]/Balmer lines. Low‑mass galaxies can mimic a 4–6 μm AGN excess via warm dust; adopting Haro 11 as a limiting star‑forming SED curbs contamination but can miss some obscured AGN. Adding rest λ≈13.5 μm behavior restores completeness, feasible with MIRI out to z≈0.6, while emphasizing that wrong redshifts can undermine SED‑based IDs.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1 (BPT placement): Check where the 17 MIRI‑selected candidates land relative to Kewley/Kauffmann lines to see spectroscopic confirmation of obscuration and which lie in star‑forming vs AGN zones.
Figure 1 (example spectra A=87191, B=209962, C=196290): Inspect [O III] and Balmer line widths to separate outflow‑broadened narrow lines from genuine broad‑line components and link to the obscuration implied by the SEDs.
Figure 2 (SED decompositions): Verify that the AGN component is required by the 1–6 μm excess across acceptable fits, note how redshift shifts PAH features and drives degeneracy, and check how adding rest ~13.5 μm constrains the solution.
Figure 2 (continued panels): Compare massive vs dwarf hosts to see when a warm, non‑PAH stellar IR excess can masquerade as AGN and how the fit partitions flux among stellar, hot‑dust (star‑forming), and AGN components.
Figure 4 (WISE colors of dwarfs): Gauge how local low‑metallicity dwarfs intrude into AGN color wedges, and use Haro 11’s track as a conservative boundary to balance completeness vs contamination.

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2510.06362v1

Evidence of violation of Case B recombination in Little Red Dots

G. P. Nikopoulos, D. Watson, A. Sneppen, V. Rusakov, K. E. Heintz, J. Witstok, G. Brammer

Theme match 5/5

Digest

Measures Balmer-line ratios (Hα, Hβ, Hγ, Hδ) in a dozen LRDs with JWST/NIRSpec, decomposing broad and narrow components in the seven sources with ≥3 lines. Broad components are largely consistent with Case B plus substantial local extinction (A_V ≃ 1–8), except RUBIES EGS-49140 (z=6.6847), which deviates >5σ—showing high Hα/Hβ but suppressed Hγ/Hα and Hδ/Hα inconsistent with dust-reddened Case B, pointing to Balmer-line optical depth from very high-density gas (log ne > 9). Narrow lines show minimal attenuation overall, with two objects exhibiting flat Hα/Hβ ≈ 1.8 suggestive of density-bounded nebulae but also explainable by unresolved absorption. If Case B generally holds, the obscuration is confined to the compact LRD region while the hosts remain effectively dust-free.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Balmer-series line decompositions for representative objects (e.g., JADES-GN-68797): inspect the narrow cores versus exponential wings that set the broad/narrow fluxes used in the ratios.
Broad-component Balmer-ratio diagnostic plot: compare each source to Case B plus extinction curves and highlight RUBIES EGS-49140 as the outlier with high Hα/Hβ but low Hγ/Hα and Hδ/Hα.
Narrow-line decrement distribution: identify the two cases with Hα/Hβ ≈ 1.8 and assess whether a density-bounded interpretation or an unresolved absorption correction better explains them.
NIRSpec spectral coverage per target: verify which Balmer lines fall in-band for each redshift/grating to justify the ≥3-line subset used for ratio analysis.
Appendix SEDs (their Fig. 9): check the V-shaped continua and Balmer-break location versus the inferred A_V, supporting dust-poor hosts with locally extinguished LRD emission.

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2510.06408v1

The rise and fall of Little Red Dots could be driven by the environment

Rosa M. Mérida, Gaia Gaspar, Yoshihisa Asada, Marcin Sawicki, Kiyoaki Christopher Omori, Chris J. Willott, Nicholas S. Martis, Adam Muzzin, Gaël Noirot, Gregor Rihtaršič, Ghassan T. E. Sarrouh, Roberta Tripodi

Theme match 4/5

Digest

Presents “The Stingray,” a compact three-galaxy system at z_spec = 5.12 from CANUCS, comprising a transitional LRD (tLRD) AGN, a Balmer-break galaxy, and a star-forming satellite separated by 2.7–5.1 kpc. JWST/NIRSpec MSA G395M spectra show a broad Hα line plus a blue rest-UV slope and compact size typical of LRDs, but with a flatter rest-optical slope than canonical LRDs, marking a transition stage. The configuration resembles a Building Block System in which interactions elevate stellar and black hole growth beyond secular levels. The authors argue the environment likely triggers the emergence or fading of the LRD phase, tying LRD duty cycles to small-scale group dynamics at z > 5.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

NIRCam cutouts and emission-line maps (Fig. 1): compare the tLRD’s strong Hβ+[O III] and Hα excess with the BBG and SAT1 to see which bands drive the color contrast and how the three sources align spatially within the overdensity.
NIRSpec G395M spectra of the tLRD: inspect the broad Hα profile (width/shape) and the Hβ+[O III] complex to gauge where the object lands between normal AGN and canonical LRD line properties.
SED comparison for the three members: verify the tLRD’s blue rest-UV together with its flatter rest-optical slope versus the classic LRD “V”-shape, and check compactness/morphology metrics tied to the SED fits.
Slit layout and extraction diagnostics: review the MSA configuration, the misaligned slit requiring larger photometric scaling, and the custom background subtraction to understand flux reliability in this crowded system.
Group geometry/redshifts: look for the panel listing Hα-based z_spec for all three and their projected separations (2.7–5.1 kpc) as evidence for a physically associated, interaction-driven system.

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2510.06325v1

Preferential Accretion onto the Secondary Black Hole Strengthens Gravitational Wave Signals

Julia M. Comerford, Joseph Simon

Theme match 4/5

Digest

ApJ-accepted, this paper delivers the first observationally anchored test of how preferential accretion onto the secondary SMBH reshapes binary demographics and gravitational-wave output. Using SDSS close-pair merger rates and empirical SMBH scaling inputs, the authors show that modest differential growth raises mass ratios, converting many minor galaxy mergers into major SMBH mergers and boosting the major-merger fraction by a factor of 2–3. Only ~10% total SMBH mass growth onto the secondary brings the predicted nanohertz gravitational-wave background into agreement with PTA measurements and accelerates the first continuous-wave binary detection. The effect is even stronger for LISA-era binaries in gas-rich, higher‑z environments, offering a path toward the rapid build-up of overmassive black holes seen by JWST.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Mass-ratio evolution: before/after preferential accretion q-distributions or q vs accreted-mass fraction—verify how many systems cross the paper’s adopted major-merger threshold and quantify the shift in the median q.
GWB amplitude impact: predicted background amplitude versus level of preferential accretion—identify where the curve intersects the PTA-inferred amplitude band and check that ≈10% total SMBH growth suffices.
Major-merger fraction: comparison of fractions with and without differential accretion, possibly binned by host mass or redshift—confirm the stated 2–3× boost and where it is most pronounced.
LISA population implications: strain–frequency or SNR distribution for massive binaries with/without preferential accretion—inspect the enhancement at higher redshift where gas fractions are larger.
PTA continuous-wave prospects: timeline or detection probability versus observing time under different accretion assumptions—see how preferential accretion advances the first resolvable source.

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2510.10772v1

Black Holes in the Shadow: The Missing High-Ionization Lines in the Earliest JWST AGNs

Greta Zucchi, Xihan Ji, Piero Madau, Roberto Maiolino, Ignas Juodžbalis, Francesco D'Eugenio, Sophia Geris, Yuki Isobe

Theme match 3/5

Digest

Using JWST/NIRSpec prism+grating spectra of 34 JADES Type 1 AGNs spanning z=1.7–9, the authors assemble the first statistical broad-line diagnostic set for this population. They find that at z>5 the high-ionization lines He II, C IV, and N V are strongly suppressed compared to prominent broad Hα and narrow [O III], sharply constraining the ionizing SED and BLR conditions. Photoionization grids based on sub‑Eddington AGN SEDs fail to reproduce the observed broad-line ratios and equivalent widths across ionization parameters. The results point to ionizing continua that are intrinsically softened or externally filtered—potentially tied to super‑Eddington geometry or inner‑disk radiative inefficiency—as a key ingredient in early black-hole growth.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1 — Use the per-object disperser coverage to verify that C IV, He II, and N V fall within the observed windows, distinguishing true non-detections from wavelength gaps; note where Hα+[N II] and [O III] are securely covered for the BLR/NLR comparison.
Figure 2 — Inspect the high‑z (z>5) grating stack to see the core result: strong broad Hα and narrow [O III] contrasted with weak/undetected high‑ionization UV lines; the [O III]‑based rescaling clarifies relative prominence without affecting measurements.
Figure 3 — Compare stacked EWs to SDSS quasar medians: upper‑limit arrows on broad C IV and He II quantify the deficit relative to low‑z templates, while Hα remains strong—capturing the tension standard SEDs cannot match.
Figure 4 — Contrast the Jin (2012) vs Pezzulli (2017) SED shapes around 1–4 Ryd and soft X‑rays to see why predicted high‑ionization lines differ; relate this to the paper’s finding that sub‑Eddington templates still miss the observed EW/ratio pattern.

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2510.10376v1

JWST Catches a Strongly Gravitationally Lensed AGN In Transition from Type II to Type I

Michael K Florian, Michael D Gladders, Gourav Khullar, Keren Sharon, Aidan P Cloonan, Eirk Solhaug, Brian Welch, Matthew Bayliss, Hakon Dahle, Taylor A Hutchison, Jane R Rigby

Theme match 3/5

Digest

JWST NIRCam plus HST imaging and lens modeling of the six-image system SDSSJ2222+2745 (z=2.801) enable host–AGN decomposition at ≈20 pc scales, with GALFIT morphology and rest-UV–NIR photometry. The host shows only minor asymmetry and modest dust, sits below the star-forming main sequence, and is not fully quenched. SED fitting indicates a last uptick in star formation ~300 Myr ago, consistent with a recent merger that ignited the AGN. Together these point to an AGN actively transitioning from type II to type I, offering a rare snapshot of the evolutionary link at cosmic noon.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Fig. 1 (NIRCam RGB F444W/F150W/F115W): Inspect all six lensed images (A–F) and the deep F444W insets to see intrinsic clumpy substructure in the host and the magnification-driven resolution reaching ≈20 pc.
Fig. 2 (images A, B, C with clumps marked): Use the cross-image persistence of the two arrowed clumps—visible even in lower-magnification image C—to confirm these are real, minor host asymmetries rather than differential magnification artifacts.
Fig. 3 (image A: data/GALFIT/residuals across F115W, F150W, F444W): Check PSF–host separation quality and wavelength-dependent residuals to validate the decomposition and reveal off-nuclear clumps driving the small asymmetry.
Fig. 4 (image B: data/GALFIT/residuals): Compare to Fig. 3 to test robustness across a different parity/magnification region; consistent residual features strengthen the intrinsic-clump interpretation and color gradients informing the SED fit.

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2510.08354v1

Mephisto: Self-Improving Large Language Model-Based Agents for Automated Interpretation of Multi-band Galaxy Observations

Zechang Sun, Yuan-Sen Ting, Yaobo Liang, Nan Duan, Song Huang, Zheng Cai

Theme match 3/5

Digest

Introduces Mephisto, an LLM-driven multi‑agent framework that interfaces with CIGALE to iteratively propose and test SED models from multi‑band photometry using tree search, temporal memory, and self‑play knowledge distillation. Validated on COSMOS2020 galaxies and a JWST Little Red Dot case (JADES 90354), it recovers physical properties with transparent, step‑by‑step reasoning and a synthesized report. Against a 360‑million‑point exhaustive grid, Mephisto consistently reaches solutions within ~20% while exploring ~100× smaller grids, and sometimes outperforms the baseline fits. This positions it as a scalable, human‑aligned copilot for triaging rare sources and interpreting diverse galaxy populations.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 2 (LRD case study): Follow how iterative model refinements (dust, nebular, and potential AGN components) shift the SED to reproduce the extreme red colors of JADES 90354, and read the reasoning chain that weighs competing interpretations for Little Red Dots.
Figure 4 (performance vs exhaustive grid): Inspect the fractional‑difference scatter to confirm the ~20% agreement band, identify points below zero where Mephisto beats the exhaustive search, and gauge computational savings versus fit quality.
Figure 3 (COSMOS2020 exemplars): Use the component‑decomposed SEDs to see when an AGN contribution is required (panel c) and compare derived M*, attenuation, and SFR across dusty SF, dwarf, and massive systems.
Figure 1 (architecture): Trace the tree‑based workflow, noting prompt templates tied to CIGALE docs, the knowledge‑learning example that adjusts AGN parameters, and how temporal memory steers/prunes branches before the final summarized report.

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2510.09820v1

Resolving stellar populations, star formation, and ISM conditions with JWST in a large spiral galaxy at z $\sim$ 2

Eleonora Parlanti, Giulia Tozzi, Natascha M. Förster Schreiber, Claudia Pulsoni, Letizia Scaloni, Stavros Pastras, Pascal Oesch, Capucine Barfety, Francesco Belfiore, Jianhang Chen, Giovanni Cresci, Ric Davies, Frank Eisenhauer, Juan M. Espejo Salcedo, Reinhard Genzel, Rodrigo Herrera-Camus, Jean-Baptiste Jolly. Lilian L. Lee, Minju M. Lee, Daizhong Liu, Dieter Lutz, Filippo Mannucci, Giovanni Mazzolari, Thorsten Naab, Amit Nestor Shachar, Sedona H. Price, Alvio Renzini, T. Taro Shimizu, Amiel Sternberg, Martina Scialpi, Eckhard Sturm, Linda J. Tacconi, Hannah Übler, Stijn Wuyts

Theme match 2/5

Digest

JWST/NIRSpec MSA and NIRCam WFSS, combined with VLT/ERIS, resolve the ISM and stellar populations of the z=2.224 grand‑design spiral K20-ID7 from rest‑optical to near‑IR. They find massive clumps with M⋆=(0.67–3.5)×10^9 M⊙ and SFR≈3–24 M⊙/yr with low AV≈0.4, ne<300 cm⁻³, and log U≈−3; the compact bulge is modestly massive, heavily obscured (AV=6.43±0.55), and shows declining recent SFR (82±42 over 100 Myr to 12±8 M⊙/yr over 10 Myr). Gas abundances are fairly uniform with 12+log(O/H)≈8.54 and elevated N/O (log N/O≈−1.0) plus sub‑solar S/O (log S/O≈−1.9), consistent with dilution by radial inflows of metal‑poor gas. Paβ mapping and a radial age gradient indicate inside‑out growth in an unperturbed spiral disk.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the F435W–F444W morphology to verify the regular two‑armed spiral and the compact, red F444W‑bright bulge whose prominence aligns with the high AV and older central ages inferred from SED fitting.
Figure 2: Compare PRISM/CLEAR spectra from the seven MSA shutters to see region‑to‑region changes in continuum shape and key rest‑optical lines (e.g., Hβ, [O III], Hα, [N II], [S II]) used to derive ne, ionisation parameter, and abundances; note which shutters sample clumps versus bulge.
Figure 3: Use the spatially resolved Paβ in WFSS to locate star‑forming clumps and quantify SFR; the double‑peaked 1D profile traces spatial offsets plus a velocity gradient along the dispersion, offering a cross‑check on kpc‑scale gas motions.
Figure 4: Read the pixel‑scale maps (M⋆, SFR10 Myr/100 Myr, AV, age, χ²) to connect heavily obscured, older central stellar populations with younger, lower‑AV clumps in the arms, and to visualize the inside‑out growth signature; the χ²≈1 map supports the robustness of the SED solutions.

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2510.09753v1

BEES: Quasar lifetime measurements from extended rest-optical emission line nebulae at $z\sim6$

Dominika Ďurovčíková, Anna-Christina Eilers, Yuzo Ishikawa, Minghao Yue, Marianne Vestergaard, Frederick B. Davies, Jan-Torge Schindler, Xiaohui Fan, Fabrizio Arrigoni Battaia, Marta Volonteri, Robert A. Simcoe, Joseph F. Hennawi, Laura Blecha, Irham T. Andika, Sarah E. I. Bosman, Rebekka Bieri

Theme match 2/5

Digest

JWST/NIRSpec IFU maps of five z∼6 quasars with small proximity zones reveal extended Hα and [O III]5007 nebulae in four cases, enabling transverse lifetime estimates from their light-crossing extents. The nebular sizes imply extremely short UV-luminous phases, t_Q ≲ 10^5 yr, and Hα confirms that recombination powers the previously reported Lyα halos. Rest-optical Hα/Hβ fits also yield ∼10^9 M⊙ black holes at these epochs, reinforcing a picture of rapid, episodic accretion to assemble early SMBHs.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the Hα and Hβ zoom-ins and MCMC component fits to see the broad-line widths and continuum/Fe II decomposition used for the ∼10^9 M⊙ BH mass estimates; check residuals for systematics near Balmer lines.
Figure 2 (J0100+2802): Compare the PSF-subtracted pseudo–narrowband Hα and [O III] maps and SNR panels to verify genuine extended emission; use the annulus-averaged surface-brightness profiles to read off the maximum nebular radius that sets the transverse t_Q, noting the masked PSF core and IFU FoV limits.
Figure 2 (right panels): Contrast Hα vs [O III] radial extents for J0100+2802 to gauge multiphase CGM structure and which line controls the lifetime constraint (larger detected radius → tighter t_Q).
Figure 3 (J158–14): Repeat the PSF-subtracted map and profile reading to extract the nebular edge and corresponding t_Q; compare Hα to [O III] morphology to assess recombination dominance vs. harder-ionization zones.
Figure 3 (right panels): Examine the noise-floor shading and hatched inaccessible regions to understand how the nebular cutoff—and thus the lifetime—was determined and limited by sensitivity and FoV.

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2510.08455v1

A first look at quasar-galaxy clustering at $z\simeq7.3$

Jan-Torge Schindler, Joseph F. Hennawi, Frederick B. Davies, Sarah E. I. Bosman, Feige Wang, Jinyi Yang, Anna-Christina Eilers, Xiaohui Fan, Koki Kakiichi, Elia Pizzati, Riccardo Nanni

Theme match 2/5

Digest

JWST/NIRCam+NIRSpec around two frontier quasars, J0252−0503 (z=7.0) and J1007+2115 (z=7.5), delivers the first quasar–galaxy clustering constraint at z≈7.3. From 51 z>5 galaxies, eight fall within ΔvLOS=±1500 km s−1 of the quasars; notably J0252_8713 sits just 7 pkpc away with Δv≈360 km s−1, a compelling z>7 merger candidate. Combining both fields yields r0^QG≈7.6+1.7−1.6 h−1 cMpc (γ=2), implying log10(Mhalo,min/M⊙)=11.6+0.6−0.7 and a tiny duty cycle f_duty≈0.05%. Relative to EIGER/ASPIRE results at z≈6–6.7 (≳10^12.3 M⊙), this hints at non‑monotonic redshift evolution in quasar clustering, sharpening where the earliest SMBHs live in the dark‑matter hierarchy.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Figure 1: Inspect the NIRSpec discovery spectra near J1007+2115 for the continuum break and marked emission features to verify redshifts and velocity offsets that enter the cross‑correlation sample.
Figure 2: Do the same around J0252−0503, paying special attention to the spectrum of J0252_8713 to check the ≈360 km s−1 offset and the robustness of its line/break identification supporting the merger claim.
Figure 3: Use the NIRCam composites plus separation–redshift panels to see which galaxies fall inside the ±1500 km s−1 window, compare the two fields’ galaxy distributions, and note the serendipitous LRD marked by the black star.
Figure 4: Examine the close pair around J0252−0503; the 7 pkpc projected separation and morphology of the diffuse companion help assess interaction/merger plausibility at z≈7.

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2510.07365v1

A JWST MIRI LRS Survey of 37 Massive Star-Forming Galaxies and AGN at Cosmic Noon -- Overview and First Results

Jed McKinney, Miriam Eleazer, Alexandra Pope, Anna Sajina, Stacey Alberts, Meredith Stone, Leonid Sajkov, Virginia Vanicek, Allison Kirkpatrick, Thomas Lai, Caitlin M. Casey, Lee Armus, Tanio Diaz-Santos, Andrew Korkus, Olivia Cooper, Lindsay R. House, Hollis Akins, Erini Lambrides, Arianna Long, Lin Yan

Theme match 2/5

Digest

Overview paper for a JWST/MIRI LRS survey stitches new 5–14 μm spectra to archival Spitzer/IRS to cover 5–30 μm for 37 infrared-bright galaxies and obscured AGN at z=0.65–2.46. They detect bright 3.3–5.3 μm PAH emission plus Paschen/Brackett lines; the 3.3 μm PAH reaches up to 1% of L_IR and tightly traces the dust-obscured SFR. CO gas, CO2 and H2O ices, and aliphatic dust are common in absorption, and the joint spectra show 11.3/3.3 μm PAH ratios about three times higher than local LIRGs, pointing to systematically larger PAH grains at z~1–2. The implied grain growth in dense, cold ISM at cosmic noon suggests reduced photoelectric heating and weaker stellar feedback.

Open paper page arXiv abstract arXiv PDF Highlighted PDF

Key figures to inspect

Stitched MIRI LRS + Spitzer/IRS spectra (5–30 μm) for representative sources: verify 3.3, 6.2, 7.7, 8.6, and 11.2 μm PAHs alongside Paschen/Brackett lines, and note where absorption troughs (CO, CO2, H2O ice, aliphatics) imprint on the continuum.
Correlation plot of L(3.3 μm PAH) or L(3.3 μm)/L_IR versus dust-obscured SFR: check scatter, outliers, and any AGN/SF separation to assess the tightness and utility of 3.3 μm as an SFR tracer.
Distribution or comparison figure for PAH 11.3/3.3 μm ratios: examine the factor ~3 elevation over local LIRGs and how it trends with redshift, L_IR, or M_* as evidence for larger PAH sizes.
Zoom-in spectral panels around 3.0–3.5 μm: inspect the 3.05 μm H2O ice absorption profile and nearby aliphatic features to gauge ISM coldness and column densities.
Feature maps or stacked spectra highlighting CO gas and CO2 ice (e.g., ~4.27 μm): confirm detections and relative depths versus continuum to link dense, cold conditions with the PAH-size inference.