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1. Using gravitational waves & multi-messenger astronomy to reverse-engineer the properties of galactic nuclei

   https://doi.org/10.1088/1361-6382/ae0aad  

   Ford, K_E Saavik; McKernan, Barry (February 2026, Classical and Quantum Gravity)

   Abstract Active galactic nuclei (AGN) are powered by accretion disks onto supermassive black holes (SMBHs) in the centers of galaxies. AGN are believed to play important roles in the evolution of both SMBHs and their host galaxies over cosmic time. AGN and the nuclear star clusters (NSCs) that interact with them remain unresolved with present and planned telescopes. As a result, the properties of AGN and NSCs are highly uncertain. Here we review how binary black hole (BBH) mergers can occur in AGN disks and how both the gravitational wave and electromagnetic wave properties of such mergers allow us to reverse-engineer the properties of AGN disks and NSCs over cosmic time. We point out that the feature in the BBH mass spectrum around $\sim 35M_{\odot }$ is an excellent probe of hierarchical merger models. Likewise constraints on the spins of upper-mass gap BH ( $\gtrsim 50M_{\odot }$ ) test the AGN channel. The effective spin ( ${\chi }_{\mathrm{eff}}$ ) distribution, including asymmetry, islands of structure and magnitudes are excellent tests of AGN model predictions. We also argue, that the rate of AGN-driven BBH mergers as a function of redshift should scale slightly shallower than the AGN number density, at least out to redshifts of $\sim 2$ , and should turnover at the same redshift as the AGN number density. Finally, we emphasize a determination of an AGN fraction of observed BBH mergers ( $f_{\mathrm{BBH},\mathrm{AGN}}$ ),regardless of the actual value, allows us to infer the average properties of AGN disks and NSCs out to high redshift. 

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2. Tip of the Iceberg: Overmassive Black Holes at 4 < z < 7 Found by JWST Are Not Inconsistent with the Local $M_{\mathrm{BH}}-M_{\star }$ Relation

   https://doi.org/10.3847/1538-4357/ada603  

   Li, Junyao; Silverman, John D; Shen, Yue; Volonteri, Marta; Jahnke, Knud; Zhuang, Ming-Yang; Scoggins, Matthew T; Ding, Xuheng; Harikane, Yuichi; Onoue, Masafusa; et al (February 2025, The Astrophysical Journal)

   Abstract JWST is revealing a remarkable new population of high-redshift (z ≳ 4), low-luminosity active galactic nuclei in deep surveys and detecting the host galaxy's stellar light in the most luminous and massive quasars atz ∼ 6 for the first time. Recent findings claim that supermassive black holes (SMBHs) in these systems are significantly more massive than predicted by the local black hole (BH) mass–stellar mass ( ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ ) relation and that this is not due to sample selection effects. Through detailed statistical modeling, we demonstrate that the coupled effects of selection biases (i.e., finite detection limit and requirements for detecting broad lines) and measurement uncertainties can largely explain the reported offset and flattening in the observed ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation toward the upper envelope of the local relation, even for those at ${\mathcal{M}}_{\mathrm{BH}}<10^8{M}_{\odot }$ . We further investigate the possible evolution of the ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation atz ≳ 4 with careful treatment of observational biases and consideration of the degeneracy between intrinsic evolution and dispersion in this relation. The bias-corrected intrinsic ${\mathcal{M}}_{\mathrm{BH}}-{\mathcal{M}}_{\star }$ relation in the low-mass regime ( ${\mathcal{M}}_{\star }\lesssim 10^{10}{M}_{\odot }$ ) suggests a large population of low-mass BHs ( ${\mathcal{M}}_{\mathrm{BH}}\lesssim 10^5{M}_{\odot }$ ), possibly originating from lighter seeds, may remain undetected or unidentified. These results underscore the importance of forward modeling observational biases to better understand BH seeding and SMBH–galaxy coevolution mechanisms in the early universe, even with the deepest JWST surveys. 

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3. The Sloan Digital Sky Survey Reverberation Mapping Project: The Black Hole Mass–Stellar Mass Relations at 0.2 ≲ z ≲ 0.8

   https://doi.org/10.3847/1538-4357/acddda  

   Li, Jennifer I-Hsiu; Shen, Yue; Ho, Luis C; Brandt, W N; Grier, Catherine J; Hall, Patrick B; Homayouni, Y; Koekemoer, Anton M; Schneider, Donald P; Trump, Jonathan R (September 2023, The Astrophysical Journal)

   Abstract We measure the correlation between black hole massM_BHand host stellar massM_*for a sample of 38 broad-line quasars at 0.2 ≲z≲ 0.8 (median redshiftz_med= 0.5). The black hole masses are derived from a dedicated reverberation mapping program for distant quasars, and the stellar masses are derived from two-band optical+IR Hubble Space Telescope imaging. Most of these quasars are well centered within ≲1 kpc from the host galaxy centroid, with only a few cases in merging/disturbed systems showing larger spatial offsets. Our sample spans two orders of magnitude in stellar mass (∼10⁹–10¹¹M_⊙) and black hole mass (∼10⁷–10⁹M_⊙) and reveals a significant correlation between the two quantities. We find a best-fit intrinsic (i.e., selection effects corrected)M_BH–M_*,hostrelation of $\log \left(M_{\mathrm{BH}}/M_{\odot }\right)={7.01}_{-0.33}^{+0.23}+{1.74}_{-0.64}^{+0.64}\log \left(M_{*,\mathrm{host}}/{10}^{10}{M}_{\odot }\right)$ , with an intrinsic scatter of ${0.47}_{-0.17}^{+0.24}$ dex. Decomposing our quasar hosts into bulges and disks, there is a similarM_BH–M_*,bulgerelation with slightly larger scatter, likely caused by systematic uncertainties in the bulge–disk decomposition. TheM_BH–M_*,hostrelation atz_med= 0.5 is similar to that in local quiescent galaxies, with negligible evolution over the redshift range probed by our sample. With direct black hole masses from reverberation mapping and the large dynamical range of the sample, selection biases do not appear to affect our conclusions significantly. Our results, along with other samples in the literature, suggest that the locally measured black hole mass–host stellar mass relation is already in place atz∼ 1. 

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4. Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII

   https://doi.org/10.3847/1538-4357/ad76a2  

   Fu, Sal Wanying; Weisz, Daniel R; Starkenburg, Else; Martin, Nicolas; Collins, Michelle_L M; Savino, Alessandro; Boylan-Kolchin, Michael; Côté, Patrick; Dolphin, Andrew E; Longeard, Nicolas; et al (October 2024, The Astrophysical Journal)

   Abstract We present ∼300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (M_V= −7.5) and Andromeda XXVIII (M_V= –8.8), derived using metallicity-sensitive calcium H and K narrowband Hubble Space Telescope imaging. These are the first individual stellar metallicities in And XVI (95 stars). Our And XXVIII sample (191 stars) is a factor of ∼15 increase over literature metallicities. For And XVI, we measure $〈\mathrm{[Fe/H]}〉=-{2.17}_{-0.05}^{+0.05}$ , ${\sigma }_{\mathrm{[Fe/H]}}={0.33}_{-0.07}^{+0.07}$ , and ∇_[Fe/H]= −0.23 ± 0.15 dex $R_e^{-1}$ . We find that And XVI is more metal-rich than Milky Way ultrafaint dwarf galaxies of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure $〈\mathrm{[Fe/H]}〉=-{1.95}_{-0.04}^{+0.04}$ , ${\sigma }_{\mathrm{[Fe/H]}}={0.34}_{-0.05}^{+0.05}$ , and ∇_[Fe/H]= −0.46 ± 0.10 dex $R_e^{-1}$ , placing it on the dwarf galaxy mass–metallicity relation. Neither galaxy has a metallicity distribution function (MDF) with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint (L≲ 10⁶L_⊙) galaxies outside the Milky Way halo. Both galaxies’ gradients are consistent with predictions from the FIRE simulations, where an age–gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal-poor ([Fe/H] < –3.0) star candidates, which make up 7% of And XVI’s MDF and 6% of And XXVIII’s. 

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5. Galaxies Going Bananas: Inferring the 3D Geometry of High-redshift Galaxies with JWST-CEERS

   https://doi.org/10.3847/1538-4357/ad1a13  

   Pandya, Viraj; Zhang_张, Haowen 昊文; Huertas-Company, Marc; Iyer, Kartheik G; McGrath, Elizabeth; Barro, Guillermo; Finkelstein, Steven L; Kümmel, Martin; Hartley, William G; Ferguson, Henry C; et al (February 2024, The Astrophysical Journal)

   Abstract The 3D geometries of high-redshift galaxies remain poorly understood. We build a differentiable Bayesian model and use Hamiltonian Monte Carlo to efficiently and robustly infer the 3D shapes of star-forming galaxies in James Webb Space Telescope Cosmic Evolution Early Release Science observations with $\log M_{*}/M_{\odot }=9.0–10.5$ atz= 0.5–8.0. We reproduce previous results from the Hubble Space Telescope Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey in a fraction of the computing time and constrain the mean ellipticity, triaxiality, size, and covariances with samples as small as ∼50 galaxies. We find high 3D ellipticities for all mass–redshift bins, suggesting oblate (disky) or prolate (elongated) geometries. We break that degeneracy by constraining the mean triaxiality to be ∼1 for $\log M_{*}/M_{\odot }=9.0–9.5$ dwarfs atz> 1 (favoring the prolate scenario), with significantly lower triaxialities for higher masses and lower redshifts indicating the emergence of disks. The prolate population traces out a “banana” in the projected $b/a–\log a$ diagram with an excess of low-b/a, large- $\log a$ galaxies. The dwarf prolate fraction rises from ∼25% atz= 0.5–1.0 to ∼50%–80% atz= 3–8. Our results imply a second kind of disk settling from oval (triaxial) to more circular (axisymmetric) shapes with time. We simultaneously constrain the 3D size–mass relation and its dependence on 3D geometry. High-probability prolate and oblate candidates show remarkably similar Sérsic indices (n∼ 1), nonparametric morphological properties, and specific star formation rates. Both tend to be visually classified as disks or irregular, but edge-on oblate candidates show more dust attenuation. We discuss selection effects, follow-up prospects, and theoretical implications. 

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