Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae35fb
Andrew J. Fox, Sapna Mishra, Frances H. Cashman, David M. French, Philipp Richter, Rongmon Bordoloi, Nicolas Lehner, Jason Tumlinson and Sanchayeeta Borthakur
We present a UV absorption-line analysis of the circumgalactic medium (CGM) of Sextans B, a dwarf irregular galaxy at 1.3 Mpc distance on the outer frontier of the Local Group (LG). Using Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy of two active galactic nucleus sight lines passing through the Sextans B CGM at small impact parameters of 4 and 8 kpc (≈0.04–0.08R200), we detect the CGM in Si ii, Si iii, Si iv, and C ii absorption. All four ions show a column-density profile that declines with radius. The profiles fall below the average CGM profiles of other nearby dwarfs (by ≈0.3–0.6 dex, depending on the ion), likely due to the low halo mass and low metallicity of Sextans B. Using Cloudy photoionization models and interferometric measurements of the H i column density, we find gas-phase silicon and carbon abundances in the Sextans B CGM of [Si/H] = −1.7 ± 0.2 and [C/H] = −2.1 ± 0.2, respectively, among the lowest gas-phase abundances anywhere in the LG. We calculate a cool CGM gas mass within 8 kpc of ≈4 × 107M⊙, comparable to the H i mass and the stellar mass of Sextans B.
{"title":"Low-metallicity Gas on the Outskirts of the Local Group: The Circumgalactic Medium of Sextans B*","authors":"Andrew J. Fox, Sapna Mishra, Frances H. Cashman, David M. French, Philipp Richter, Rongmon Bordoloi, Nicolas Lehner, Jason Tumlinson and Sanchayeeta Borthakur","doi":"10.3847/1538-4357/ae35fb","DOIUrl":"https://doi.org/10.3847/1538-4357/ae35fb","url":null,"abstract":"We present a UV absorption-line analysis of the circumgalactic medium (CGM) of Sextans B, a dwarf irregular galaxy at 1.3 Mpc distance on the outer frontier of the Local Group (LG). Using Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy of two active galactic nucleus sight lines passing through the Sextans B CGM at small impact parameters of 4 and 8 kpc (≈0.04–0.08R200), we detect the CGM in Si ii, Si iii, Si iv, and C ii absorption. All four ions show a column-density profile that declines with radius. The profiles fall below the average CGM profiles of other nearby dwarfs (by ≈0.3–0.6 dex, depending on the ion), likely due to the low halo mass and low metallicity of Sextans B. Using Cloudy photoionization models and interferometric measurements of the H i column density, we find gas-phase silicon and carbon abundances in the Sextans B CGM of [Si/H] = −1.7 ± 0.2 and [C/H] = −2.1 ± 0.2, respectively, among the lowest gas-phase abundances anywhere in the LG. We calculate a cool CGM gas mass within 8 kpc of ≈4 × 107M⊙, comparable to the H i mass and the stellar mass of Sextans B.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"6 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287536","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae3a91
Jennifer N. Stafford, Sebastian Heinz, Mateusz Ruszkowski, Torsten Enßlin and Yi-Hao Chen
Conduction as a mechanism for explaining the disrupted cooling-flow in galaxy clusters has been mostly discounted, as the process is inefficient at transporting heat all the way from the cluster into the core. However, thermal conduction can be strongly enhanced when materials of significantly different temperature are brought into proximity, and thus into close thermal contact. Jets of active galactic nuclei (AGNs) may act as heat pumps by bringing low-entropy gas from the cluster core into thermal contact with the hot outer atmosphere of the cluster, significantly increasing the feedback efficiency of AGNs. We test this hypothesis by running a suite of 3D magnetohydrodynamic simulations of AGNs jets in a Perseus-like cluster, including anisotropic conduction. We find that the heat pump efficiency η can reach up to 50% of the maximum possible efficiency if conduction operates near the Spitzer–Braginskii limit, while if conduction along the field lines is substantially suppressed below the Spitzer–Braginskii value by a factor fsp by kinetic effects, as recently suggested. We further find that jet-induced thermal conduction is self-limiting. Magnetic draping during the uplift results in a magnetic field orientation close to perpendicular to the induced temperature gradients, significantly reducing conduction along the ideal conductive pathways. Thus, for conservative assumptions about thermal conduction suppression by fsp ≲ 0.1, the heat pump effect leads to only marginal heat transfer and, correspondingly, to immaterial changes in the overall thermal evolution of cool core clusters beyond the isolated effects of conduction and jet-induced heating alone.
{"title":"The Self-limiting Nature of Jet-modulated Thermal Conduction in Cool Core Clusters","authors":"Jennifer N. Stafford, Sebastian Heinz, Mateusz Ruszkowski, Torsten Enßlin and Yi-Hao Chen","doi":"10.3847/1538-4357/ae3a91","DOIUrl":"https://doi.org/10.3847/1538-4357/ae3a91","url":null,"abstract":"Conduction as a mechanism for explaining the disrupted cooling-flow in galaxy clusters has been mostly discounted, as the process is inefficient at transporting heat all the way from the cluster into the core. However, thermal conduction can be strongly enhanced when materials of significantly different temperature are brought into proximity, and thus into close thermal contact. Jets of active galactic nuclei (AGNs) may act as heat pumps by bringing low-entropy gas from the cluster core into thermal contact with the hot outer atmosphere of the cluster, significantly increasing the feedback efficiency of AGNs. We test this hypothesis by running a suite of 3D magnetohydrodynamic simulations of AGNs jets in a Perseus-like cluster, including anisotropic conduction. We find that the heat pump efficiency η can reach up to 50% of the maximum possible efficiency if conduction operates near the Spitzer–Braginskii limit, while if conduction along the field lines is substantially suppressed below the Spitzer–Braginskii value by a factor fsp by kinetic effects, as recently suggested. We further find that jet-induced thermal conduction is self-limiting. Magnetic draping during the uplift results in a magnetic field orientation close to perpendicular to the induced temperature gradients, significantly reducing conduction along the ideal conductive pathways. Thus, for conservative assumptions about thermal conduction suppression by fsp ≲ 0.1, the heat pump effect leads to only marginal heat transfer and, correspondingly, to immaterial changes in the overall thermal evolution of cool core clusters beyond the isolated effects of conduction and jet-induced heating alone.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"228 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287538","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae40bb
Devesh Nandal and Sunmyon Chon
Supermassive stars (SMSs) are candidate progenitors of massive black hole seeds and may contribute to anomalous abundance patterns in high-redshift galaxies and globular clusters. Recent radiation-hydrodynamic simulations indicate that SMSs can form at finite metallicity, not only in metal-free direct-collapse conditions. We model SMS growth with GENEC over Z/Z⊙ = 10−5 to 10−2 using simulation-motivated accretion histories. The final masses reach ∼7.2 × 104M⊙ at 10−5Z⊙ and ∼2.3 × 103M⊙ at 10−2Z⊙. Models are evolved through the pre-main-sequence and core H-burning phases, terminating at the onset of general relativistic instability for Z ≲ 10−4Z⊙ or at core He exhaustion for Z ≳ 10−3Z⊙. The dominant mass growth channel transitions from collision driven to accretion driven between Z = 10−4 and 10−3. With stellar lifetimes remaining nearly constant at 1.8–2.0 Myr, collisions do not significantly rejuvenate the star, implying that collision-driven runaway collapse cannot proceed in isolation and must be supplemented and is likely dominated by gas accretion. We further compute the critical inflow rate required to keep the stellar envelope inflated, , which decreases with increasing Z and decreasing central mass fraction of hydrogen (Xc). The critical rate falls below 10−5M⊙ yr−1 at Xc = 0.60 for 10−2 Z⊙. This indicates that SMSs with 0.01 Z⊙ are cool supergiants during most of their lifetimes, where UV photon emissivity and radiative feedback are strongly suppressed. Overall, SMS evolution remains viable up to Z ≃ 0.01 Z⊙, supporting SMS formation in proto–globular clusters and other metal-enriched dense environments.
{"title":"Growth of Metal-enriched Supermassive Stars by Accretion and Collisions","authors":"Devesh Nandal and Sunmyon Chon","doi":"10.3847/1538-4357/ae40bb","DOIUrl":"https://doi.org/10.3847/1538-4357/ae40bb","url":null,"abstract":"Supermassive stars (SMSs) are candidate progenitors of massive black hole seeds and may contribute to anomalous abundance patterns in high-redshift galaxies and globular clusters. Recent radiation-hydrodynamic simulations indicate that SMSs can form at finite metallicity, not only in metal-free direct-collapse conditions. We model SMS growth with GENEC over Z/Z⊙ = 10−5 to 10−2 using simulation-motivated accretion histories. The final masses reach ∼7.2 × 104M⊙ at 10−5Z⊙ and ∼2.3 × 103M⊙ at 10−2Z⊙. Models are evolved through the pre-main-sequence and core H-burning phases, terminating at the onset of general relativistic instability for Z ≲ 10−4Z⊙ or at core He exhaustion for Z ≳ 10−3Z⊙. The dominant mass growth channel transitions from collision driven to accretion driven between Z = 10−4 and 10−3. With stellar lifetimes remaining nearly constant at 1.8–2.0 Myr, collisions do not significantly rejuvenate the star, implying that collision-driven runaway collapse cannot proceed in isolation and must be supplemented and is likely dominated by gas accretion. We further compute the critical inflow rate required to keep the stellar envelope inflated, , which decreases with increasing Z and decreasing central mass fraction of hydrogen (Xc). The critical rate falls below 10−5M⊙ yr−1 at Xc = 0.60 for 10−2 Z⊙. This indicates that SMSs with 0.01 Z⊙ are cool supergiants during most of their lifetimes, where UV photon emissivity and radiative feedback are strongly suppressed. Overall, SMS evolution remains viable up to Z ≃ 0.01 Z⊙, supporting SMS formation in proto–globular clusters and other metal-enriched dense environments.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"417 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287505","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae41c2
Saroj Khanal, Sanjay Kumar Sah, Kiran Khanal and Sapana Khanal
The cusp–core problem remains a challenge to the ΛCDM model, since dwarf galaxies often exhibit flat central density cores rather than the steep cusps (ρ ∝ r−1) predicted by collisionless N-body simulations. We model the dark-matter-dominated dwarf irregular galaxy DDO 168 within the Bose–Einstein condensate (BEC) or fuzzy dark matter (FDM) framework, in which ultralight bosons form a solitonic core governed by the Gross–Pitaevskii–Poisson (GPP) equations, with the soliton mass–radius relation enforced. We numerically validate the ground-state solution of the GPP system as a consistency check and fit the inner rotation curve of DDO 168 using SPARC data. Within this framework, the data are consistent with an axion mass , and yield a solitonic core with characteristic radius , enclosing a mass M(<2.47 kpc) ≃ (1.5 ± 0.2) × 109M⊙. The observed flat inner rotation curve is reproduced and the presence of a weak H i bar is compatible with multigigayear survival timescales, consistent with reduced Chandrasekhar dynamical friction in a shallow central potential. These results demonstrate that the BEC/FDM framework provides an internally consistent description of DDO 168, simultaneously reproducing the observed rotation curve, alleviating the cusp–core tension, and allowing long-lived weak bars under conservative dynamical assumptions.
{"title":"Modeling Solitonic Cores, Stabilization of Bar, and Suppression of Bar Dissolution in DDO 168 via GPP Formalism: A Detailed Analysis of Bose–Einstein Condensate/Fuzzy Dark Matter Halo Structure and Bar Dynamics in the Dwarf Galaxy DDO 168","authors":"Saroj Khanal, Sanjay Kumar Sah, Kiran Khanal and Sapana Khanal","doi":"10.3847/1538-4357/ae41c2","DOIUrl":"https://doi.org/10.3847/1538-4357/ae41c2","url":null,"abstract":"The cusp–core problem remains a challenge to the ΛCDM model, since dwarf galaxies often exhibit flat central density cores rather than the steep cusps (ρ ∝ r−1) predicted by collisionless N-body simulations. We model the dark-matter-dominated dwarf irregular galaxy DDO 168 within the Bose–Einstein condensate (BEC) or fuzzy dark matter (FDM) framework, in which ultralight bosons form a solitonic core governed by the Gross–Pitaevskii–Poisson (GPP) equations, with the soliton mass–radius relation enforced. We numerically validate the ground-state solution of the GPP system as a consistency check and fit the inner rotation curve of DDO 168 using SPARC data. Within this framework, the data are consistent with an axion mass , and yield a solitonic core with characteristic radius , enclosing a mass M(<2.47 kpc) ≃ (1.5 ± 0.2) × 109M⊙. The observed flat inner rotation curve is reproduced and the presence of a weak H i bar is compatible with multigigayear survival timescales, consistent with reduced Chandrasekhar dynamical friction in a shallow central potential. These results demonstrate that the BEC/FDM framework provides an internally consistent description of DDO 168, simultaneously reproducing the observed rotation curve, alleviating the cusp–core tension, and allowing long-lived weak bars under conservative dynamical assumptions.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287508","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae42bc
D. L. Moutard, L. R. Corrales, I. Psaradaki, E. Temple and M. Shi
The abundance of elements in the interstellar medium is a key facet for many fields of astrophysical study. In the soft X-ray spectra, absorption by interstellar gas can result in deep absorption features that affect continuum measurements. In this paper, we focus on measuring the abundance of interstellar iron and neon from the column densities observed in soft spectra from XMM-Newton and Chandra for various low-mass X-ray binaries, which allows for a direct probe of elemental abundances. As a noble gas, neon will not deplete into solid form, thus providing a benchmark with abundances determined via UV spectroscopy. We find that, when assuming Fe is 90% depleted into grains, [Fe/Ne] = −0.523 ± 0.025, [Fe/H]+12 = 7.482 ± 0.016, and [Ne/H]+12 = 8.012 ± 0.022, which are the tightest observational constraints on these abundances to date, while being consistent with the literature, which uses protosolar abundances. We also test how depletion into solid grains and scattering affect the results. The choice of depletion fraction can affect the abundance measurement by roughly 5%, and the inclusion of a scattering component can affect abundance measurements by ∼1%–7%.
{"title":"Ne and Fe Abundances in the Interstellar Medium: Archival Study of Fe-L and Ne-K Edges in Chandra and XMM-Newton","authors":"D. L. Moutard, L. R. Corrales, I. Psaradaki, E. Temple and M. Shi","doi":"10.3847/1538-4357/ae42bc","DOIUrl":"https://doi.org/10.3847/1538-4357/ae42bc","url":null,"abstract":"The abundance of elements in the interstellar medium is a key facet for many fields of astrophysical study. In the soft X-ray spectra, absorption by interstellar gas can result in deep absorption features that affect continuum measurements. In this paper, we focus on measuring the abundance of interstellar iron and neon from the column densities observed in soft spectra from XMM-Newton and Chandra for various low-mass X-ray binaries, which allows for a direct probe of elemental abundances. As a noble gas, neon will not deplete into solid form, thus providing a benchmark with abundances determined via UV spectroscopy. We find that, when assuming Fe is 90% depleted into grains, [Fe/Ne] = −0.523 ± 0.025, [Fe/H]+12 = 7.482 ± 0.016, and [Ne/H]+12 = 8.012 ± 0.022, which are the tightest observational constraints on these abundances to date, while being consistent with the literature, which uses protosolar abundances. We also test how depletion into solid grains and scattering affect the results. The choice of depletion fraction can affect the abundance measurement by roughly 5%, and the inclusion of a scattering component can affect abundance measurements by ∼1%–7%.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"55 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287541","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae4005
Sourabh Magare, Anupreeta More and Shasvath J. Kapadia
Gravitational lensing of gravitational waves can be leveraged to provide early-warning times of to before the merger of binary neutron stars (BNSs) and neutron star black holes (NSBHs). This in turn could enable electromagnetic (EM) telescopes to capture emissions surrounding the time of the merger. In this work, we assess the practicability of lensing-driven early warning by analysing optical images of the lensed host galaxy to predict the arrival time of subsequent BNS/NSBH signals following the observation of the first signal. We produce mock lenses with image quality and resolution similar to images taken with the Hubble Space Telescope (HST) and the ground-based Hyper Suprime-Cam (HSC) on the Subaru telescope. We compare the time delay uncertainties between these two cases for typical lensed image configurations and multiplicity. These include doubles and quads and, among quads, the fold, cusp, and cross-image configurations. We find that time delay uncertainties for doubles are comparable for both HST and HSC mocks. On the other hand, quads tend to provide accurate time-delay predictions (typical relative error ∼0.1) with HST. Analysis of a real lens led to a difference in time-delay estimates of between the predictions derived from HST and HSC data. Our work therefore strongly advocates the need for high-resolution EM observations of lensed host galaxies to feasibly enable lensing-driven early-warning.
{"title":"Early Warning for Lensed Gravitational Wave Counterparts from Time Delays of Their Host Galaxies Observed in the Optical","authors":"Sourabh Magare, Anupreeta More and Shasvath J. Kapadia","doi":"10.3847/1538-4357/ae4005","DOIUrl":"https://doi.org/10.3847/1538-4357/ae4005","url":null,"abstract":"Gravitational lensing of gravitational waves can be leveraged to provide early-warning times of to before the merger of binary neutron stars (BNSs) and neutron star black holes (NSBHs). This in turn could enable electromagnetic (EM) telescopes to capture emissions surrounding the time of the merger. In this work, we assess the practicability of lensing-driven early warning by analysing optical images of the lensed host galaxy to predict the arrival time of subsequent BNS/NSBH signals following the observation of the first signal. We produce mock lenses with image quality and resolution similar to images taken with the Hubble Space Telescope (HST) and the ground-based Hyper Suprime-Cam (HSC) on the Subaru telescope. We compare the time delay uncertainties between these two cases for typical lensed image configurations and multiplicity. These include doubles and quads and, among quads, the fold, cusp, and cross-image configurations. We find that time delay uncertainties for doubles are comparable for both HST and HSC mocks. On the other hand, quads tend to provide accurate time-delay predictions (typical relative error ∼0.1) with HST. Analysis of a real lens led to a difference in time-delay estimates of between the predictions derived from HST and HSC data. Our work therefore strongly advocates the need for high-resolution EM observations of lensed host galaxies to feasibly enable lensing-driven early-warning.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"38 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287503","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae40f6
Po-Sheng Ou, 柏 昇 歐, Ke-Jung Chen and 科 榮 陳
This study investigates the physical origin of the critical metallicity required for the formation of cool supergiants, as revealed by stellar evolution models. Using grids of stellar models, we show that the terminal-age main-sequence (TAMS) radius, RTAMS, defines a threshold that determines whether a star of a given mass can evolve into the red supergiant (RSG) phase. Metallicity influences the supergiant outcome because it modifies RTAMS through its effects on opacity and nuclear energy generation, as demonstrated by our stellar models and dimensional analysis based on homology relations. The value of RTAMS sets the initial radius for post-main-sequence expansion and therefore controls the envelope radius reached at subsequent core-evolution stages. Higher-metallicity stars develop larger RTAMS and rapidly expand into the stable RSG regime during core helium burning. In contrast, lower-metallicity stars have smaller RTAMS and advance to more evolved core helium- or carbon-burning stages while retaining compact envelopes, thereby preventing expansion into the RSG regime during core helium burning. Our results explain the origin of the critical metallicity and offer insight into the evolution of metal-poor massive stars in the early Universe.
{"title":"Critical Metallicity of Cool Supergiant Formation. II. Physical Origin","authors":"Po-Sheng Ou, 柏 昇 歐, Ke-Jung Chen and 科 榮 陳","doi":"10.3847/1538-4357/ae40f6","DOIUrl":"https://doi.org/10.3847/1538-4357/ae40f6","url":null,"abstract":"This study investigates the physical origin of the critical metallicity required for the formation of cool supergiants, as revealed by stellar evolution models. Using grids of stellar models, we show that the terminal-age main-sequence (TAMS) radius, RTAMS, defines a threshold that determines whether a star of a given mass can evolve into the red supergiant (RSG) phase. Metallicity influences the supergiant outcome because it modifies RTAMS through its effects on opacity and nuclear energy generation, as demonstrated by our stellar models and dimensional analysis based on homology relations. The value of RTAMS sets the initial radius for post-main-sequence expansion and therefore controls the envelope radius reached at subsequent core-evolution stages. Higher-metallicity stars develop larger RTAMS and rapidly expand into the stable RSG regime during core helium burning. In contrast, lower-metallicity stars have smaller RTAMS and advance to more evolved core helium- or carbon-burning stages while retaining compact envelopes, thereby preventing expansion into the RSG regime during core helium burning. Our results explain the origin of the critical metallicity and offer insight into the evolution of metal-poor massive stars in the early Universe.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"53 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287506","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae41b9
Songting Li, Wenting Wang, Sergey E. Koposov, João A. S. Amarante, Alis J. Deason, Nathan R. Sandford, Ting S. Li, Gustavo E. Medina, Jiaxin Han, Monica Valluri, Oleg Y. Gnedin, Namitha Kizhuprakkat, Andrew P. Cooper, Leandro Beraldo e Silva, Carlos Frenk, Raymond G. Carlberg, Mika Lambert, Tian Qiu, Jessica Nicole Aguilar, Steven Ahlen, Davide Bianchi, David Brooks, Todd Claybaugh, Axel de la Macorra, Peter Doel, Jaime E. Forero-Romero, Enrique Gaztañaga, Satya Gontcho A Gontcho, Gaston Gutierrez, Dick Joyce, Robert Kehoe, Anthony Kremin, Claire Lamman, Martin Landriau, Laurent Le Guillou, Ramon Miquel, Will Percival, Francisco Prada, Ignasi Pérez-Ràfols, Graziano Rossi, Eusebio Sanchez, David Schlegel, Ray Sharples, Joseph Harry Silber, David Sprayberry, Gregory Tarlé, Benjamin Alan Weaver and Hu Zou
Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey, we measure the shape, orientation, radial profile, and density anisotropies of the Milky Way (MW) stellar halo over 8 kpc < rGC < 200 kpc. We identify a triaxial stellar halo (axis ratio 10:8:7), 43° tilted from the disk, showing two break radii at ∼16 and ∼76 kpc, likely associated with Gaia-Sausage/Enceladus and the Large Magellanic Cloud (LMC), respectively. The inner stellar halo (<30 kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the vast polar structure of MW satellites. The twisted halo may arise from the disk−halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful reidentification of the Hercules-Aquila Cloud North/South (HAC-N/S) overdensity and the Virgo overdensity (VOD). Break radii are found at 15 and 30 kpc for VOD and HAC-N/S, respectively. We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at ∼100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.
{"title":"The Milky Way Stellar Halo Is Twisted and Doubly Broken: Insights from DESI DR2 Milky Way Survey Observation","authors":"Songting Li, Wenting Wang, Sergey E. Koposov, João A. S. Amarante, Alis J. Deason, Nathan R. Sandford, Ting S. Li, Gustavo E. Medina, Jiaxin Han, Monica Valluri, Oleg Y. Gnedin, Namitha Kizhuprakkat, Andrew P. Cooper, Leandro Beraldo e Silva, Carlos Frenk, Raymond G. Carlberg, Mika Lambert, Tian Qiu, Jessica Nicole Aguilar, Steven Ahlen, Davide Bianchi, David Brooks, Todd Claybaugh, Axel de la Macorra, Peter Doel, Jaime E. Forero-Romero, Enrique Gaztañaga, Satya Gontcho A Gontcho, Gaston Gutierrez, Dick Joyce, Robert Kehoe, Anthony Kremin, Claire Lamman, Martin Landriau, Laurent Le Guillou, Ramon Miquel, Will Percival, Francisco Prada, Ignasi Pérez-Ràfols, Graziano Rossi, Eusebio Sanchez, David Schlegel, Ray Sharples, Joseph Harry Silber, David Sprayberry, Gregory Tarlé, Benjamin Alan Weaver and Hu Zou","doi":"10.3847/1538-4357/ae41b9","DOIUrl":"https://doi.org/10.3847/1538-4357/ae41b9","url":null,"abstract":"Using K giants from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) Milky Way Survey, we measure the shape, orientation, radial profile, and density anisotropies of the Milky Way (MW) stellar halo over 8 kpc < rGC < 200 kpc. We identify a triaxial stellar halo (axis ratio 10:8:7), 43° tilted from the disk, showing two break radii at ∼16 and ∼76 kpc, likely associated with Gaia-Sausage/Enceladus and the Large Magellanic Cloud (LMC), respectively. The inner stellar halo (<30 kpc) is oblate and aligned with the disk, whereas the outer stellar halo becomes prolate and perpendicular to the disk, consistent with the vast polar structure of MW satellites. The twisted halo may arise from the disk−halo angular momentum shift triggered by the infall of a massive satellite. The anisotropic density distribution of the stellar halo is also measured, with successful reidentification of the Hercules-Aquila Cloud North/South (HAC-N/S) overdensity and the Virgo overdensity (VOD). Break radii are found at 15 and 30 kpc for VOD and HAC-N/S, respectively. We identify the LMC transient density wake with a break radius at 60 kpc in the Pisces overdensity region. We also find new observational evidence of the LMC collective density wake, by showing a break radius at ∼100 kpc in the northern Galactic cap with a clear density peak at 90 kpc. In the end, we found that more metal-poor halo stars are more radially extended. Our results provide important clues to the assembly and evolution of the MW stellar halo under the standard cosmic structure formation framework.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"48 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287539","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae3d95
H. Thankful Cromartie, Matthew Kerr, Scott M. Ransom, Paul S. Ray, Lucas Guillemot, Ismaël Cognard, Emmanuel Fonseca and Gilles Theureau
We present 15 yr of Nançay and Green Bank radio telescope timing observations for PSR J1231−1411. This millisecond pulsar is a primary science target for the Neutron Star Interior Composition Explorer telescope (NICER, which discovered its X-ray pulsations), has accumulated near-continuous γ-ray data since the Fermi-Large Area Telescope’s launch, and has been studied extensively with the Green Bank and Nançay radio telescopes. We have undertaken a campaign with the Green Bank Telescope targeting specific orbital phases designed to improve our constraint on the pulsar’s mass through the detection of a relativistic Shapiro delay. Both frequentist and Bayesian techniques—the latter incorporating priors from white dwarf binary evolution models—are applied to 15 yr of radio observations, yielding relatively weak constraints on the companion and pulsar masses of M⊙ and M⊙, respectively (68.3% CI from Bayesian fits); however, the orbital inclination is measured to better relative precision ( °). Restricting the maximum allowed pulsar mass to 3 M⊙ improves the constraint and lowers the measured mass to M⊙. A fully generalized Bayesian fit that simultaneously samples the noise and timing models yields a pulsar mass in close agreement with this value. While our radio-derived inclination result has informed recent NICER X-ray studies of J1231−1411, the lessons learned from this troublesome pulsar will also bolster future high-precision mass measurement campaigns and resulting constraints on the neutron star interior equation of state.
{"title":"Shapiro Delay Measurements from Fifteen Years of PSR J1231−1411 Radio Observations","authors":"H. Thankful Cromartie, Matthew Kerr, Scott M. Ransom, Paul S. Ray, Lucas Guillemot, Ismaël Cognard, Emmanuel Fonseca and Gilles Theureau","doi":"10.3847/1538-4357/ae3d95","DOIUrl":"https://doi.org/10.3847/1538-4357/ae3d95","url":null,"abstract":"We present 15 yr of Nançay and Green Bank radio telescope timing observations for PSR J1231−1411. This millisecond pulsar is a primary science target for the Neutron Star Interior Composition Explorer telescope (NICER, which discovered its X-ray pulsations), has accumulated near-continuous γ-ray data since the Fermi-Large Area Telescope’s launch, and has been studied extensively with the Green Bank and Nançay radio telescopes. We have undertaken a campaign with the Green Bank Telescope targeting specific orbital phases designed to improve our constraint on the pulsar’s mass through the detection of a relativistic Shapiro delay. Both frequentist and Bayesian techniques—the latter incorporating priors from white dwarf binary evolution models—are applied to 15 yr of radio observations, yielding relatively weak constraints on the companion and pulsar masses of M⊙ and M⊙, respectively (68.3% CI from Bayesian fits); however, the orbital inclination is measured to better relative precision ( °). Restricting the maximum allowed pulsar mass to 3 M⊙ improves the constraint and lowers the measured mass to M⊙. A fully generalized Bayesian fit that simultaneously samples the noise and timing models yields a pulsar mass in close agreement with this value. While our radio-derived inclination result has informed recent NICER X-ray studies of J1231−1411, the lessons learned from this troublesome pulsar will also bolster future high-precision mass measurement campaigns and resulting constraints on the neutron star interior equation of state.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"190 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287500","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2026-02-26DOI: 10.3847/1538-4357/ae3e82
Robert Minchin, Rhys Taylor, Emmanuel Momjian, Boris Deshev and Vojtěch Partík
We have observed six “dark” neutral hydrogen (H i) clouds discovered in the Virgo Cluster by the Arecibo Galaxy Environment Survey (AGES) with the Karl G. Jansky Very Large Array (VLA), giving higher angular and velocity resolution than the original AGES observations. We detected compact H i emission in two of the sources, AGESVC1 231 and AGESVC1 274, allowing us to firmly associate them with faint (mg > 18.5), blue (g − i < 0.1) optical counterparts with high MH I/Lg ratios. In a further two sources, we detected low-column-density extended H i emission, consistent with these being dispersing clouds from ram pressure stripping or tidal interactions. The final two sources were not detected with the VLA, allowing us to set low column-density limits on the H i detected by AGES that are consistent with these clouds also being formed from H i that is dispersing into the intracluster medium. The four H i sources not associated with optical counterparts thus appear likely to be relatively short-lived objects. No evidence was found for either pressure-supported turbulent spheres or stable dark galaxies.
{"title":"High-resolution Observations of “Dark” Neutral Hydrogen Clouds in the Virgo Cluster with the Very Large Array","authors":"Robert Minchin, Rhys Taylor, Emmanuel Momjian, Boris Deshev and Vojtěch Partík","doi":"10.3847/1538-4357/ae3e82","DOIUrl":"https://doi.org/10.3847/1538-4357/ae3e82","url":null,"abstract":"We have observed six “dark” neutral hydrogen (H i) clouds discovered in the Virgo Cluster by the Arecibo Galaxy Environment Survey (AGES) with the Karl G. Jansky Very Large Array (VLA), giving higher angular and velocity resolution than the original AGES observations. We detected compact H i emission in two of the sources, AGESVC1 231 and AGESVC1 274, allowing us to firmly associate them with faint (mg > 18.5), blue (g − i < 0.1) optical counterparts with high MH I/Lg ratios. In a further two sources, we detected low-column-density extended H i emission, consistent with these being dispersing clouds from ram pressure stripping or tidal interactions. The final two sources were not detected with the VLA, allowing us to set low column-density limits on the H i detected by AGES that are consistent with these clouds also being formed from H i that is dispersing into the intracluster medium. The four H i sources not associated with optical counterparts thus appear likely to be relatively short-lived objects. No evidence was found for either pressure-supported turbulent spheres or stable dark galaxies.","PeriodicalId":501813,"journal":{"name":"The Astrophysical Journal","volume":"94 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2026-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"147287540","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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