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1. An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing

   https://doi.org/10.3847/2041-8213/ac7442  

   Lam, Casey Y. ; Lu, Jessica R. ; Udalski, Andrzej ; Bond, Ian ; Bennett, David P. ; Skowron, Jan ; Mróz, Przemek ; Poleski, Radek ; Sumi, Takahiro ; Szymański, Michał K. ; et al ( July 2022 , The Astrophysical Journal Letters)

   We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled light curves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant >1 mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6–4.4M_⊙. This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact-object lens is relatively nearby (0.70–1.92 kpc) and has a slow transverse motion of <30 km s⁻¹. OB110462 shows significant tension between models well fit to photometry versus astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources, are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are <2M_⊙, and they are unlikely to be black holes; two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (∼10⁸) and find reasonable agreement given the small sample size.

    

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2. DESI Observations of the Andromeda Galaxy: Revealing the Immigration History of Our Nearest Neighbor

   https://doi.org/10.3847/1538-4357/aca5f8  

   Dey, Arjun ; Najita, Joan R. ; Koposov, Sergey E. ; Josephy-Zack, J. ; Maxemin, Gabriel ; Bell, Eric F. ; Poppett, C. ; Patel, E. ; Beraldo e Silva, L. ; Raichoor, A. ; et al ( February 2023 , The Astrophysical Journal)

   We present Dark Energy Spectroscopic Instrument (DESI) observations of the inner halo of M31, which reveal the kinematics of a recent merger—a galactic immigration event—in exquisite detail. Of the 11,416 sources studied in 3.75 hr of on-sky exposure time, 7438 are M31 sources with well-measured radial velocities. The observations reveal intricate coherent kinematic structure in the positions and velocities of individual stars: streams, wedges, and chevrons. While hints of coherent structures have been previously detected in M31, this is the first time they have been seen with such detail and clarity in a galaxy beyond the Milky Way. We find clear kinematic evidence for shell structures in the Giant Stellar Stream, the Northeast Shelf, and Western Shelf regions. The kinematics are remarkably similar to the predictions of dynamical models constructed to explain the spatial morphology of the inner halo. The results are consistent with the interpretation that much of the substructure in the inner halo of M31 is produced by a single galactic immigration event 1–2 Gyr ago. Significant numbers of metal-rich stars ([Fe/H] > − 0.5) are present in all of the detected substructures, suggesting that the immigrating galaxy had an extended star formation history. We also investigate the ability of the shells and Giant Stellar Stream to constrain the gravitational potential of M31, and estimate the mass within a projected radius of 125 kpc to be${\log }_{10}{M}_{\mathrm{NFW}}(<125\mathrm{kpc})/M_{\odot }={11.80}_{-0.10}^{+0.12}$. The results herald a new era in our ability to study stars on a galactic scale and the immigration histories of galaxies.

    

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3. S 5: Probing the Milky Way and Magellanic Clouds potentials with the 6D map of the Orphan–Chenab stream

   https://doi.org/10.1093/mnras/stad551  

   Koposov, Sergey E. ; Erkal, Denis ; Li, Ting S. ; Da Costa, Gary S. ; Cullinane, Lara R. ; Ji, Alexander P. ; Kuehn, Kyler ; Lewis, Geraint F. ; Pace, Andrew B. ; Shipp, Nora ; et al ( February 2023 , Monthly Notices of the Royal Astronomical Society)

   We present a 6D map of the Orphan–Chenab (OC) stream by combining the data from Southern Stellar Stream Spectroscopic Survey (S5) and Gaia. We reconstruct the proper motion, radial velocity, distance, on-sky track, and stellar density along the stream with spline models. The stream has a total luminosity of MV = −8.2 and metallicity of [Fe/H] = −1.9, similar to classical Milky Way (MW) satellites like Draco. The stream shows drastic changes in its physical width varying from 200 pc to 1 kpc, but a constant line-of-sight velocity dispersion of 5 $\mathrm{km\, s^{-1}}$. Despite the large apparent variation in the stellar number density along the stream, the flow rate of stars along the stream is remarkably constant. We model the 6D stream track by a Lagrange-point stripping method with a flexible MW potential in the presence of a moving extended Large Magellanic Cloud (LMC). This allows us to constrain the mass profile of the MW within the distance range 15.6 < r < 55.5 kpc, with the best measured enclosed mass of $(2.85\pm 0.1)\times 10^{11}\, \mathrm{\, M_\odot }$ within 32.4 kpc. Our stream measurements are highly sensitive to the LMC mass profile with the most precise measurement of its enclosed mass made at 32.8 kpc, $(7.02\pm 0.9)\times 10^{10}\, {\rm M}_\odot$. We also detect that the LMC dark matter halo extends to at least 53 kpc. The fitting of the OC stream allows us to constrain the past LMC trajectory and the degree of dynamical friction it experienced. We demonstrate that the stars in the OC stream show large energy and angular momentum spreads caused by LMC perturbation.

    

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4. Estimate of the Mass and Radial Profile of the Orphan–Chenab Stream's Dwarf-galaxy Progenitor Using MilkyWay@home

   https://doi.org/10.3847/1538-4357/ac498a  

   Mendelsohn, Eric J. ; Newberg, Heidi Jo ; Shelton, Siddhartha ; Widrow, Lawrence M. ; Thompson, Jeffery M. ; Grillmair, Carl J. ( February 2022 , The Astrophysical Journal)

   We fit the mass and radial profile of the Orphan–Chenab Stream’s (OCS) dwarf-galaxy progenitor by using turnoff stars in the Sloan Digital Sky Survey and the Dark Energy Camera to constrainN-body simulations of the OCS progenitor falling into the Milky Way on the 1.5 PetaFLOPS MilkyWay@home distributed supercomputer. We infer the internal structure of the OCS’s progenitor under the assumption that it was a spherically symmetric dwarf galaxy composed of a stellar system embedded in an extended dark matter halo. We optimize the evolution time, the baryonic and dark matter scale radii, and the baryonic and dark matter masses of the progenitor using a differential evolution algorithm. The likelihood score for each set of parameters is determined by comparing the simulated tidal stream to the angular distribution of OCS stars observed in the sky. We fit the total mass of the OCS’s progenitor to (2.0 ± 0.3) × 10⁷M_⊙with a mass-to-light ratio ofγ= 73.5 ± 10.6 and (1.1 ± 0.2) × 10⁶M_⊙within 300 pc of its center. Within the progenitor’s half-light radius, we estimate a total mass of (4.0 ± 1.0) × 10⁵M_⊙. We also fit the current sky position of the progenitor’s remnant to be (α,δ) = ((166.0 ± 0.9)°, (−11.1 ± 2.5)°) and show that it is gravitationally unbound at the present time. The measured progenitor mass is on the low end of previous measurements and, if confirmed, lowers the mass range of ultrafaint dwarf galaxies. Our optimization assumes a fixed Milky Way potential, OCS orbit, and radial profile for the progenitor, ignoring the impact of the Large Magellanic Cloud.

    

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5. RR Lyrae stars as probes of the outer Galactic halo: chemical and kinematic analysis of a pilot sample

   https://doi.org/10.1093/mnras/stac3800  

   Medina, Gustavo E. ; Hansen, Camilla J. ; Muñoz, Ricardo R. ; Grebel, Eva K. ; Vivas, A. Katherina ; Carlin, Jeffrey L. ; Martínez-Vázquez, Clara E. ( December 2022 , Monthly Notices of the Royal Astronomical Society)

   We report the spectroscopic analysis of 20 halo ab-type RR Lyrae stars with heliocentric distances between 15 and 165 kpc, conducted using medium-resolution spectra from the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We obtain the systemic line-of-sight velocities of our targets with typical uncertainties of 5–10 km s−1 and compute orbital parameters for a subsample out to 50 kpc from the Galactic centre, including proper motion data from Gaia DR3. The orientation of our stars’ orbits, determined for an isolated Milky Way and for a model perturbed by the Large Magellanic Cloud, appears to suggest an accreted origin for at least half of the sample. In addition, we derive atmospheric parameters and chemical abundance ratios for seven stars beyond 20 kpc. The derived α-abundances of five of these stars follow a Milky Way halo-like trend, while the other two display an underabundance of α-elements for their [Fe/H], indicating an association with accretion events. Furthermore, based on the [Sr/Ba] ratio, we can speculate about the conditions for the formation of a potential chemically peculiar carbon-enhanced metal-poor (CEMP) RR Lyrae star. By analysing the stars’ orbital parameters and abundance ratios, we find hints of association of two of our stars with two massive satellites, namely the Large Magellanic Cloud and Sagittarius. Overall, our results are in line with the suggestion that the accretion of sub-haloes largely contributes to the outer halo stellar populations.

    

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