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1. Deciphering the Milky Way’s star formation at cosmic noon with high proper-motion stars: A precursor to the merger-driven starburst

   https://doi.org/10.1051/0004-6361/202555510  

   An, Deokkeun; Lee, Young Sun; Hirai, Yutaka; Beers, Timothy C (August 2025, Astronomy & Astrophysics)

   Context. Evidence suggests that the Milky Way (MW) underwent a major collision with the Gaia–Sausage/Enceladus (GSE) dwarf galaxy around cosmic noon. While GSE has since been fully disrupted, it brought in ex situ stars and dynamically heated in situ stars into the halo. In addition, the gas-rich merger may have triggered a burst of in situ star formation, potentially giving rise to a chemically distinct stellar component. Aims. We investigated the region of phase space where stars formed during the GSE merger likely reside, and retain distinct chemical and dynamical signatures. Methods. Building on our previous investigation of metallicity ([Fe/H]) and vertical angular momentum (L_Z) distributions, we analysed spectroscopic samples from GALAH, APOGEE, SDSS, and LAMOST, combined withGaiakinematics. We focused on high proper-motion stars as effective tracers of the phase-space volume likely influenced by the GSE merger. To correct for selection effects, we incorporated metallicity estimates derived from SDSS and SMSS photometry. Results. Our analysis reveals that low-αstars with GSE-like kinematics exhibit bimodality in [Na/Fe] and [Al/Fe] at −1.0 ≲[Fe/H] ≲ −0.4. One group follows the low light-element abundances of GSE stars, while another exhibits enhanced values. These low-α, high-Na stars have eccentric orbits but are more confined to the inner MW. Eos overlaps with a high-eccentricity subset of these stars, implying that it constitutes a smaller structure nested within the broader population. After correcting for sampling biases, we estimated a population ratio of approximately 1:10 between the low-α, high-Na stars and the GSE debris. Conclusions. These results suggest that the low-α, high-Na stars formed in a compact region, likely fuelled by gas from the GSE progenitor, analogous to clumpy star-forming clouds seen in high-redshift galaxies. Such stars may trace the first sparks of more extensive merger-driven starburst activity. 

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2. Spectral stacking of radio-interferometric data

   https://doi.org/10.1051/0004-6361/202346129  

   Neumann, Lukas; den Brok, Jakob S.; Bigiel, Frank; Leroy, Adam; Usero, Antonio; Barnes, Ashley T.; Bešlić, Ivana; Eibensteiner, Cosima; Held, Malena; Jiménez-Donaire, María J.; et al (July 2023, Astronomy & Astrophysics)

   Context.Mapping molecular line emission beyond the bright low-JCO transitions is still challenging in extragalactic studies, even with the latest generation of (sub-)millimetre interferometers, such as ALMA and NOEMA. Aims.We summarise and test a spectral stacking method that has been used in the literature to recover low-intensity molecular line emission, such as HCN(1−0), HCO⁺(1−0), and even fainter lines in external galaxies. The goal is to study the capabilities and limitations of the stacking technique when applied to imaged interferometric observations. Methods.The core idea of spectral stacking is to align spectra of the low S/N spectral lines to a known velocity field calculated from a higher S/N line expected to share the kinematics of the fainter line (e.g. CO(1−0) or 21 cm emission). Then these aligned spectra can be coherently averaged to produce potentially high S/N spectral stacks. Here we used imaged simulated interferometric and total power observations at different S/N levels, based on real CO observations. Results.For the combined interferometric and total power data, we find that the spectral stacking technique is capable of recovering the integrated intensities even at low S/N levels across most of the region where the high S/N prior is detected. However, when stacking interferometer-only data for low S/N emission, the stacks can miss up to 50% of the emission from the fainter line. Conclusions.A key result of this analysis is that the spectral stacking method is able to recover the true mean line intensities in low S/N cubes and to accurately measure the statistical significance of the recovered lines. To facilitate the application of this technique we provide a public Python package, called PYSTACKER. 

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3. The 230 GHz Variability of Numerical Models of Sagittarius A*. II. The Physical Origins of the Variability

   https://doi.org/10.3847/1538-4357/adc99f  

   Chan, Ho-Sang; Chan, Chi-kwan (May 2025, The Astrophysical Journal)

   Abstract We continue our previous work, H.-S. Chan et al., to investigate how variations in the electron temperature prescription parameter,R_Low, influence the 3 hr variability at 230 GHz,M_ΔT, in magnetically arrested disk (MAD) models of Sagittarius A* (Sgr A*), through analyzing a series of general-relativistic magnetohydrodynamics and ray-tracing simulations. For models with a black hole spina > 0, we discovered that increasingR_Lowrenders the photon ring more optically thick, obscuring the varying accretion flows that contribute to the variability. However, asR_Lowincreases further, MAD flux eruptions become more pronounced, compensating for the decrease inM_ΔT. For models with spina < 0, although a higherR_Lowalso increases the optical thickness of the fluid, voids within the optically thick gas fail to cover the entire photon ring. Similarly, flux eruptions become more prominent asR_Lowincreases further, contributing to the observed rise inM_ΔTrelative toR_Low. For black holes with spina= 0, although the effect of increasing optical depth is still present, their 230 GHz light curves, and henceM_ΔT, are insensitive to changes inR_Low. Furthermore, we found that the variability of the 230 GHz light curves atR_Low = 1 might correlate with fluctuations in the internal energy of the gas near the black hole, and we listed potential causes and solutions to the over-variability problem. Our findings highlight potential approaches for refiningM_ΔTto better align with observations when modeling Sgr A*. 

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4. Origins of Very Low Helium Abundance Streams Detected in the Solar Wind Plasma

   https://doi.org/10.3847/1538-4357/ad84d6  

   Yogesh; Gopalswamy, N.; Chakrabarty, D.; Mostafavi, Parisa; Yashiro, Seiji; Srivastava, Nandita; Ofman, Leon (December 2024, The Astrophysical Journal)

   Abstract The abundance of helium (A_He) in the solar wind exhibits variations typically in the range from 2% to 5% with respect to solar cycle activity and solar wind velocity. However, there are instances where the observedA_Heis exceptionally low (<1%). These low-A_Heoccurrences are detected both near the Sun and at 1 au. The low-A_Heevents are generally observed near the heliospheric current sheet. We analyzed 28 low-A_Heevents observed by the Wind spacecraft and 4 by Parker Solar Probe to understand their origin. In this work, we make use of the ADAPT-WSA model to derive the sources of our events at the base of the solar corona. The modeling suggests that the low-A_Heevents originated from the boundaries of coronal holes, primarily from large quiescent helmet streamers. We argue that the cusp above the core of the streamer can produce such very low helium abundance events. The streamer core serves as an ideal location for gravitational settling to occur as demonstrated by previous models, leading to the release of this plasma through reconnection near the cusp, resulting in low-A_Heevents. Furthermore, observations from Ulysses provide direct evidence that these events originated from coronal streamers. 

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5. A Physically Motivated Framework to Compare Pair Fractions of Isolated Low- and High-mass Galaxies across Cosmic Time

   https://doi.org/10.3847/1538-4357/ad19d0  

   Chamberlain, Katie; Besla, Gurtina; Patel, Ekta; Rodriguez-Gomez, Vicente; Torrey, Paul; Martin, Garreth; Johnson, Kelsey; Kallivayalil, Nitya; Patton, David; Pearson, Sarah; et al (February 2024, The Astrophysical Journal)

   Abstract Low-mass galaxy pair fractions are understudied, and it is unclear whether low-mass pair fractions evolve in the same way as more massive systems over cosmic time. In the era of JWST, Roman, and Rubin, selecting galaxy pairs in a self-consistent way will be critical to connect observed pair fractions to cosmological merger rates across all mass scales and redshifts. Utilizing the Illustris TNG100 simulation, we create a sample of physically associated low-mass (10⁸<M_*< 5 × 10⁹M_⊙) and high-mass (5 × 10⁹<M_*< 10¹¹M_⊙) pairs betweenz= 0 and 4.2. The low-mass pair fraction increases fromz= 0 to 2.5, while the high-mass pair fraction peaks atz= 0 and is constant or slightly decreasing atz> 1. Atz= 0 the low-mass major (1:4 mass ratio) pair fraction is 4× lower than high-mass pairs, consistent with findings for cosmological merger rates. We show that separation limits that vary with the mass and redshift of the system, such as scaling by the virial radius of the host halo (r_sep< 1R_vir), are critical for recovering pair fraction differences between low-mass and high-mass systems. Alternatively, static physical separation limits applied equivalently to all galaxy pairs do not recover the differences between low- and high-mass pair fractions, even up to separations of 300 kpc. Finally, we place isolated mass analogs of Local Group galaxy pairs, i.e., Milky Way (MW)–M31, MW–LMC, LMC–SMC, in a cosmological context, showing that isolated analogs of LMC–SMC-mass pairs and low-separation (<50 kpc) MW–LMC-mass pairs are 2–3× more common atz≳ 2–3. 

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