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1. A little red dot at z = 7.3 within a large galaxy overdensity

   https://doi.org/10.1038/s41550-025-02660-1  

   Schindler, Jan-Torge; Hennawi, Joseph F; Davies, Frederick B; Bosman, Sarah_E I; Endsley, Ryan; Wang, Feige; Yang, Jinyi; Barth, Aaron J; Eilers, Anna-Christina; Fan, Xiaohui; et al (November 2025, Nature Astronomy)

   Abstract The nature of ‘little red dots’ and their relation to other forms of accreting supermassive black holes remain an open question. Here we report the discovery of a little red dot atz = 7.3. It is attenuated by moderate amounts of dust,A_V = 2.79 mag, and has an intrinsic bolometric luminosity of 10^46.6 erg s⁻¹and a supermassive black hole mass of 5 × 10⁸ M_⊙. Most notably, this object is embedded in an overdensity of eight nearby galaxies, allowing us to calculate a spectroscopic estimate of the clustering of galaxies around little red dots. We find a little red dot versus galaxy cross-correlation length ofr₀ = 8 ± 2 h⁻¹ cMpc, comparable to that ofz ≈ 6 ultraviolet-luminous quasars. The resulting estimate of their minimum dark matter halo mass$$\log_{10}({M}_{{\rm{halo}},\min }/{{{M}}}_{\odot })=12.{0}_{-1.0}^{+0.8}$$ ${\log }_{10}\left(M_{\mathrm{halo},\min }/M_{\odot }\right)=12.0_{-1.0}^{+0.8}$ indicates that nearly all haloes above this mass must host actively accreting supermassive black holes atz ≈ 7, in strong contrast with the far smaller duty cycle of luminous quasars (<1%). Our results, taken at face value, motivate a picture in which supermassive black holes in little red dot phases could serve as the obscured precursors of ultraviolet-luminous quasars, which provides a natural explanation for the short ultraviolet-luminous lifetimes inferred from both quasar clustering and quasar proximity zones. 

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2. Resolving an Asteroseismic Catastrophe: Structural Diagnostics from p -mode Phase Functions off the Main Sequence

   https://doi.org/10.3847/1538-4357/ada949  

   Ong_王, J_M_Joel 加冕; Lindsay, Christopher J; Reyes, Claudia; Stello, Dennis; Roxburgh, Ian W (February 2025, The Astrophysical Journal)

   Abstract On the main sequence, the asteroseismic small frequency separationδν₀₂between radial and quadrupolep-modes is customarily interpreted to be a direct diagnostic of internal structure. Such an interpretation is based on a well-known integral estimator relatingδν₀₂to a radially averaged sound-speed gradient. However, this estimator fails, catastrophically, when evaluated on structural models of red giants: their small separations must therefore be interpreted differently. We derive a single expression that both reduces to the classical estimator when applied to main-sequence stellar models and reproduces the qualitative features of the small separation for stellar models of very evolved red giants. This expression indicates that the small separations of red giants scale primarily with their global seismic properties as $\delta {\nu }_{02}\propto \Delta {\nu }^2/{\nu }_{\max }$ , rather than being in any way sensitive to their internal structure. Departures from this asymptotic behavior, during the transition from the main-sequence to red giant regimes, have been recently reported in open-cluster Christensen–Dalsgaard (C-D) diagrams from K2 mission data. Investigating them in detail, we demonstrate that they occur when the convective envelope boundary passes a specific acoustic distance—roughly one-third of a wavelength at ${\nu }_{\max }$ —from the center of the star, at which point radial modes become maximally sensitive to the position of the boundary. The shape of the corresponding features onϵ_pand C-D (orr₀₂) diagrams may be useful in constraining the nature of convective boundary mixing in the context of undershooting beneath a convective envelope. 

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3. A Bimodal Metallicity Distribution Function in the Ultra-Faint Dwarf Galaxy Reticulum II

   https://doi.org/10.33232/001c.147696  

   Luna, Alice M; Ji, Alexander P; Chiti, Anirudh; Simon, Joshua D; Kelson, Daniel D; Go, Minsung; Limberg, Guilherme; Li, Ting S; Frebel, Anna (January 2025, The Open Journal of Astrophysics)

   Star formation in ultra-faint dwarf galaxies (UFDs, $M_{*}<{10}^5{M}_{\odot }$ ) is suppressed by reionization, but may not be completely quenched. The metallicity distribution function (MDF) of stars in ultra-faint dwarf galaxies could show these signatures of reionization. However, past studies of UFD MDFs have been limited, because there are only a few dozen red giant branch (RGB) stars in such low-mass galaxies. We present low-resolution Magellan/IMACS spectroscopy of 167 stars in the UFD Reticulum II ( $M_{*}\approx 3000M_{\odot }$ ), increasing the number of stellar metallicities by 6.5 times and resulting in the most populated spectroscopic metallicity distribution function of any UFD. This is possible because we determined the first spectroscopic metallicities of main sequence turn-off stars in any UFD. The MDF of Reticulum II is clearly a bimodal distribution, displaying two peaks with about $80\%$ of the stars in the metal-poor peak at and $20\%$ of the stars in the more metal-rich peak at . Such a large metallicity gap can be explained by Type Ia supernova enrichment during a long quiescent period. This supports the currently-favored two-burst star formation history for Reticulum II and shows that such low-mass galaxies clearly can form stars after reionization. 

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4. The Milky Way Radial Metallicity Gradient as an Equilibrium Phenomenon: Why Old Stars Are Metal Rich

   https://doi.org/10.3847/1538-4357/addbe5  

   Johnson, James W; Weinberg, David H; Blanc, Guillermo A; Bonaca, Ana; Rudie, Gwen C; Lu, Yuxi Lucy; Reichardt_Chu, Bronwyn; Griffith, Emily J; Sit, Tawny; Johnson, Jennifer A; et al (July 2025, The Astrophysical Journal)

   Abstract Metallicities of both gas and stars decline toward large radii in spiral galaxies, a trend known as the radial metallicity gradient. We quantify the evolution of the metallicity gradient in the Milky Way as traced by APOGEE red giants with age estimates from machine learning algorithms. Stars up to ages of ∼9 Gyr follow a similar relation between metallicity and Galactocentric radius. This constancy challenges current models of Galactic chemical evolution, which typically predict lower metallicities for older stellar populations. Our results favor anequilibrium scenario, in which the gas-phase gradient reaches a nearly constant normalization early in the disk lifetime. Using a fiducial choice of parameters, we demonstrate that one possible origin of this behavior is an outflow that more readily ejects gas from the interstellar medium (ISM) with increasing Galactocentric radius. A direct effect of the outflow is that baryons do not remain in the ISM for long, which causes the ratio of star formation to accretion, ${\dot{\Sigma }}_{\star }/{\dot{\Sigma }}_{\text{in}}$ , to quickly become constant. This ratio is closely related to the local equilibrium metallicity, since its numerator and denominator set the rates of metal production by stars and hydrogen gained through accretion, respectively. Building in a merger event results in a perturbation that evolves back toward the equilibrium state on ∼Gyr timescales. Under the equilibrium scenario, the radial metallicity gradient is not a consequence of the inside-out growth of the disk but instead reflects a trend of declining ${\dot{\Sigma }}_{\star }/{\dot{\Sigma }}_{\text{in}}$ with increasing Galactocentric radius. 

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5. A Galactic Self-portrait: Density Structure and Integrated Properties of the Milky Way Disk

   https://doi.org/10.3847/1538-4357/adf723  

   Imig, Julie; Holtzman, Jon A; Zasowski, Gail; Lian, Jianhui; Boardman, Nicholas F; Stone-Martinez, Alexander; Mackereth, J Ted; Prescott, Moire_K M; Beaton, Rachael L; Beers, Timothy C; et al (September 2025, The Astrophysical Journal)

   Abstract The evolutionary history of the Milky Way disk is imprinted in the ages, positions, and chemical compositions of individual stars. In this study, we derive the intrinsic density distribution of different stellar populations using the final data release of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. A total of 203,197 red giant branch stars are used to sort the stellar disk (R≤ 20 kpc) into subpopulations of metallicity (Δ[M/H]  = 0.1 dex), age ( $\mathrm{\Delta }\log \left(\frac{\mathrm{age}}{\mathrm{yr}}\right)=0.1$ ), andα-element abundances ([α/M]). We fit the present-day structural parameters and density distribution of each stellar subpopulation after correcting for the survey selection function. The low-αdisk is characterized by longer scale lengths and shorter scale heights, and is best fit by a broken exponential radial profile for each population. The high-αdisk is characterized by shorter scale lengths and larger scale heights, and is generally well-approximated by a single exponential radial profile. These results are applied to produce new estimates of the integrated properties of the Milky Way from early times to the present day. We measure the total stellar mass of the disk to be $5.27_{-1.5}^{+0.2}\times 10^{10}$ M_⊙, and the average mass-weighted scale length isR_d = 2.37 ± 0.2 kpc. The Milky Way’s present-day color of (g − r) = 0.72 ± 0.02 is consistent with the classification of a red spiral galaxy, although it has only been in the “green valley” region of the galaxy color–mass diagram for the last ∼3 Gyr. 

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