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1. The Globular Cluster System of the Virgo Cluster Ultradiffuse Galaxy VCC 615

   https://doi.org/10.3847/1538-4357/ad99cd  

   Mihos, J Christopher; Durrell, Patrick R; Toloba, Elisa; Peng, Eric W; Lim, Sungsoon; Côté, Patrick; Guhathakurta, Puragra; Ferrarese, Laura (January 2025, The Astrophysical Journal)

   Abstract We use Hubble Space Telescope imaging to study the globular cluster system of the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615. We select globular cluster candidates through a combination of size and color, while simultaneously rejecting contamination from background galaxies that would be unresolved in ground-based imaging. Our sample of globular cluster candidates is essentially complete down to a limiting magnitude of F814W = 24.0, ≈90% down the globular cluster luminosity function (GCLF). We estimate a total globular cluster population for VCC 615 of $N_{\mathrm{GC}}=25.1_{-5.4}^{+6.5}$, resulting in a specific frequency of $S_N=55.5_{-12.0}^{+14.5}$, quite high compared to normal galaxies of similar luminosity, but consistent with the large specific frequencies found in some other UDGs. The abundant cluster population suggests the galaxy is enshrouded by a massive dark halo, consistent with previous dynamical mass estimates using globular cluster kinematics. While the peak of the GCLF appears slightly brighter than expected (by ≈0.3–0.5 mag), this difference is comparable to the 0.3 mag uncertainty in the measurement, and we see no sign of an extremely luminous population of clusters similar to those detected in the UDGs NGC1054-DF2 and -DF4. However, we do find a relatively high fraction ( $32_{-4}^{+5}$%) of large clusters with half-light radiiR_h > 9 pc. The galaxy's offset nucleus appears photometrically distinct from the globular clusters, and is more akin to ultracompact dwarfs (UCDs) in Virgo. Over time, VCC615’s already diffuse stellar body may be further stripped by cluster tides, leaving the nucleus intact to form a new Virgo UCD. 

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2. Star formation beyond galaxies: widespread in-situ formation of intra-cluster stars

   https://doi.org/10.33232/001c.127132  

   Ahvazi, Niusha; Sales, Laura V; Navarro, Julio F; Benson, Andrew; Boselli, Alessandro; D'Souza, Richard (January 2024, The Open Journal of Astrophysics)

   We study the fraction of the intra-cluster light (ICL) formed in-situ in the three most massive clusters of the TNG50 simulation, with virial masses $\sim {10}^{14}$. We find that a significant fraction of ICL stars ( $8\%$- $28\%$) are born in-situ. This amounts to a total stellar mass comparable to the central galaxy itself. Contrary to simple expectations, only a sub-dominant fraction of these in-situ ICL stars are born in the central regions and later re-distributed to more energetic orbits during mergers. Instead, many in-situ ICL stars form directly hundreds of kiloparsecs away from the central galaxy, in clouds condensing out of the circum-cluster medium. The simulations predict a present-date diffuse star formation rate of $$1 ${\mathrm{M}}_{\odot }$/yr, with higher rates at higher redshifts. The diffuse star forming component of the ICL is filamentary in nature, extends for hundreds of kiloparsecs and traces the distribution of neutral gas in the cluster host halo. We discuss briefly how numerical details of the baryonic treatment in the simulation, in particular the density threshold for star formation and the equation of state, may play a role in this result. We conclude that a sensitivity of $1.6\times {10}^{-19}-2.6\times {10}^{-18}$erg s ${}^{-1}$cm ${}^{-2}$arcsec ${}^{-2}$in H ${}_{\alpha }$flux (beyond current observational capabilities) would be necessary to detect this diffuse star-forming component in galaxy clusters. 

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3. Chemical Abundances in the Leiptr Stellar Stream: A Disrupted Ultra-faint Dwarf Galaxy?

   https://doi.org/10.33232/001c.139013  

   Atzberger, Kaia R; Usman, Sam A; Ji, Alexander P; Cullinane, Lara R; Erkal, Denis; Hansen, Terese T; Lewis, Geraint F; Li, Ting S; Limberg, Guilherme; Luna, Alice; et al (January 2025, The Open Journal of Astrophysics)

   Chemical abundances of stellar streams can be used to determine the nature of a stream’s progenitor. Here we study the progenitor of the recently discovered Leiptr stellar stream, which was previously suggested to be a tidally disrupted halo globular cluster. We obtain high-resolution spectra of five red giant branch stars selected from the Gaia DR2 $\text{𝚂𝚃𝚁𝙴𝙰𝙼𝙵𝙸𝙽𝙳𝙴𝚁}$catalog with Magellan/MIKE. One star is a clear non-member. The remaining four stars display chemical abundances consistent with those of a low-mass dwarf galaxy: they have a low mean metallicity, ; they do not all have identical metallicities; and they display low [ $\alpha$/Fe] $\sim 0$and [Sr/Fe] and [Ba/Fe] $\sim -1$. This pattern of low $\alpha$and neutron-capture element abundances is only found in intact dwarf galaxies with stellar mass $\lesssim {10}^5{M}_{\odot }$. Although more data are needed to be certain, Leiptr’s chemistry is consistent with being the lowest-mass dwarf galaxy stream without a known intact progenitor, possibly in the mass range of ultra-faint dwarf galaxies. Leiptr thus preserves a record of one of the lowest-mass early accretion events into the Milky Way. 

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4. Constraints on $f\left(R\right)$ gravity from thermal-Sunyaev-Zel’dovich-effect-selected SPT galaxy clusters and weak lensing mass calibration from DES and HST

   https://doi.org/10.1103/PhysRevD.111.043519  

   Vogt, S_M L; Bocquet, S; Davies, C T; Mohr, J J; Schmidt, F; Ruan, C-Z; Li, B; Hernández-Aguayo, C; Grandis, S; Bleem, L E; et al (February 2025, Physical Review D)

   We present constraints on the $f\left(R\right)$gravity model using a sample of 1005 galaxy clusters in the redshift range 0.25–1.78 that have been selected through the thermal Sunyaev-Zel’dovich effect from South Pole Telescope data and subjected to optical and near-infrared confirmation with the multicomponent matched filter algorithm. We employ weak gravitational lensing mass calibration from the Dark Energy Survey Year 3 data for 688 clusters at $z<0.95$and from the Hubble Space Telescope for 39 clusters with $0.6<z<1.7$. Our cluster sample is a powerful probe of $f\left(R\right)$gravity, because this model predicts a scale-dependent enhancement in the growth of structure, which impacts the halo mass function (HMF) at cluster mass scales. To account for these modified gravity effects on the HMF, our analysis employs a semianalytical approach calibrated with numerical simulations. Combining calibrated cluster counts with primary cosmic microwave background temperature and polarization anisotropy measurements from the Planck 2018 release, we derive robust constraints on the $f\left(R\right)$parameter $f_{R0}$. Our results, ${\log }_{10}\left|f_{R0}\right|<-5.32$at the 95% credible level, are the tightest current constraints on $f\left(R\right)$gravity from cosmological scales. This upper limit rules out $f\left(R\right)$-like deviations from general relativity that result in more than a $\sim 20\%$enhancement of the cluster population on mass scales $M_{200\mathrm{c}}>3\times {10}^{14}{M}_{\odot }$. Published by the American Physical Society2025 

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5. First $\beta$ -Delayed Two-Neutron Spectroscopy of the $r$ -Process Nucleus ${}^{134}\mathrm{In}$ and Observation of the $i_{13/2}$ Single-Particle Neutron State in ${}^{133}\mathrm{Sn}$

   https://doi.org/10.1103/l24v-5m31  

   Dyszel, P; Grzywacz, R; Xu, Z Y; Kitamura, N; Karny, M; Korgul, A; Madurga, M; Neupane, S; Algora, A; Andreyev, A N; et al (October 2025, Physical Review Letters)

   This manuscript reports on the direct observation of a $\beta$-delayed two-neutron emission in a study of ${}^{134}\mathrm{In}$at the ISOLDE Decay Station using neutron spectroscopy. We also report on the first measurement in ${\beta }^{-}$decay of the long-sought $13/2^{+}$excited state in ${}^{133}\mathrm{Sn}$, attributed to be the neutron single-particle $i_{13/2}$orbital. The observation of sequential neutron emission is used to extract the relative population of the $i_{13/2}$state, which was found to be much smaller than the predictions of the statistical model. The experiment was possible because of the innovative use of a neutron array with neutron discrimination and interaction tracking capabilities. This is the first study of the details of the two-neutron emission for a nucleus, which belongs to the $r$-process path. Understanding $\beta$-delayed two-neutron emission probabilities is essential to validate models used in astrophysical $r$-process nucleosynthesis calculations. Observing two-neutron emissions in ${\beta }^{-}$decay paves the way for new experiments to study energy and angular correlations for $\beta$-delayed multineutron emitters. 

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