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1. Resolved SPLASH Chemodynamics in Andromeda’s PHAT Stellar Halo and Disk: On the Nature of the Inner Halo along the Major Axis

   https://doi.org/10.3847/1538-3881/aca9cd  

   Escala, Ivanna ; Quirk, Amanda C. N. ; Guhathakurta, Puragra ; Gilbert, Karoline M. ; Wojno, J. Leigh ; Cullinane, Lara ; Williams, Benjamin F. ; Dalcanton, Julianne ( January 2023 , The Astronomical Journal)

   Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line of sight to M31's disk. We combined optical Hubble Space Telescope/Advanced Camera for Surveys photometry, from the Panchromatic Hubble Andromeda Treasury survey, with Keck/DEIMOS spectra, from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. The resulting sample of 3512 individual red giant branch stars spans 4–19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations, by modeling the line-of-sight velocity distributions as a function of position across the disk region, where ∼73% stars have a high likelihood of belonging to the disk and ∼14% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars (∼29%) with disk-like metallicity ([Fe/H]_phot∼ −0.10). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from themore »Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested “kicked-up” disk population in M31's inner stellar halo.

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2. The Atari Disk, a Metal-poor Stellar Population in the Disk System of the Milky Way

   https://doi.org/10.3847/1538-4357/ac8102  

   Mardini, Mohammad K. ; Frebel, Anna ; Chiti, Anirudh ; Meiron, Yohai ; Brauer, Kaley V. ; Ou, Xiaowei ( September 2022 , The Astrophysical Journal)

   Abstract We have developed a chemodynamical approach to assign 36,010 metal-poor SkyMapper stars to various Galactic stellar populations. Using two independent techniques (velocity and action space behavior), Gaia EDR3 astrometry, and photometric metallicities, we selected stars with the characteristics of the “metal-weak” thick-disk population by minimizing contamination by the canonical thick disk or other Galactic structures. This sample comprises 7127 stars, spans a metallicity range of −3.50 < [Fe/H] < −0.8, and has a systematic rotational velocity of 〈 V ϕ 〉 = 154 km s −1 that lags that of the thick disk. Orbital eccentricities have intermediate values between typical thick-disk and halo values. The scale length is h R = 2.48 − 0.05 + 0.05 kpc, and the scale height is h Z = 1.68 − 0.15 + 0.19 kpc. The metallicity distribution function is well fit by an exponential with a slope of Δ log N / Δ [ Fe / H ] = 1.13 ± 0.06 . Overall, we find a significant metal-poor component consisting of 261 SkyMapper stars with [Fe/H] < −2.0. While our sample contains only 11 stars with [Fe/H] ≲ −3.0, investigating the JINAbase compilation of metal-poor stars reveals another 18 such starsmore »(five have [Fe/H] < −4.0) that kinematically belong to our sample. These distinct spatial, kinematic, and chemical characteristics strongly suggest that this metal-poor, phase-mixed kinematic sample represents an independent disk component with an accretion origin in which a massive dwarf galaxy radially plunged into the early Galactic disk. Going forward, we propose to call the metal-weak thick-disk population the Atari disk, given its likely accretion origin, and in reference to it sharing space with the Galactic thin and thick disks.« less
3. Stellar kinematics of dwarf galaxies from multi-epoch spectroscopy: application to Triangulum II

   https://doi.org/10.1093/mnras/stac1441  

   Buttry, Rachel ; Pace, Andrew B. ; Koposov, Sergey E. ; Walker, Matthew G. ; Caldwell, Nelson ; Kirby, Evan N. ; Martin, Nicolas F. ; Mateo, Mario ; Olszewski, Edward W. ; Starkenburg, Else ; et al ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   We present new MMT/Hectochelle spectroscopic measurements for 257 stars observed along the line of sight to the ultrafaint dwarf galaxy Triangulum II (Tri II). Combining results from previous Keck/DEIMOS spectroscopy, we obtain a sample that includes 16 likely members of Tri II, with up to 10 independent redshift measurements per star. To this multi-epoch kinematic data set, we apply methodology that we develop in order to infer binary orbital parameters from sparsely sampled radial velocity curves with as few as two epochs. For a previously identified (spatially unresolved) binary system in Tri II, we infer an orbital solution with period $296.0_{-3.3}^{+3.8} \rm ~ d$, semimajor axis $1.12^{+0.41}_{-0.24}\rm ~au$, and systemic velocity $-380.0 \pm 1.7 \rm ~km ~s^{-1}$ that we then use in the analysis of Tri II’s internal kinematics. Despite this improvement in the modelling of binary star systems, the current data remain insufficient to resolve the velocity dispersion of Tri II. We instead find a 95 per cent confidence upper limit of $\sigma _{v} \lesssim 3.4 \rm ~km~s^{-1}$.
4. Spectroscopic analysis of VVV CL001 cluster with MUSE

   https://doi.org/10.1093/mnras/stac934  

   Olivares Carvajal, J. ; Zoccali, M. ; Rojas-Arriagada, A. ; Contreras Ramos, R. ; Gran, F. ; Valenti, E. ; Minniti, J. H. ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   Like most spiral galaxies, the Milky Way contains a population of blue, metal-poor globular clusters and another of red, metal-rich ones. Most of the latter belong to the bulge, and therefore they are poorly studied compared to the blue (halo) ones because they suffer higher extinction and larger contamination from field stars. These intrinsic difficulties, together with a lack of low-mass bulge globular clusters, are reasons to believe that their census is not complete yet. Indeed, a few new clusters have been confirmed in the last few years. One of them is VVV CL001, the subject of the present study. We present a new spectroscopic analysis of the recently confirmed globular cluster VVV CL001, made by means of MUSE@VLT integral field data. Individual spectra were extracted for stars in the VVV CL001 field. Radial velocities were derived by cross-correlation with synthetic templates. Coupled with proper motions from the VVV (VISTA Variables in the Vía Láctea) survey, these data allow us to select 55 potential cluster members, for which we derive metallicities using the public code The Cannon. The mean radial velocity of the cluster is Vhelio = −324.9 ± 0.8 km s−1, as estimated from 55 cluster members. This high velocity, together with amore »low metallicity [Fe/H] = −2.04 ± 0.02 dex, suggests that VVV CL001 could be a very old cluster. The estimated distance is d = 8.23 ± 0.46 kpc, placing the cluster in the Galactic bulge. Furthermore, both its current position and the orbital parameters suggest that VVV CL001 is most probably a bulge globular cluster.

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5. SHARP – VIII. J0924+0219 lens mass distribution and time-delay prediction through adaptive-optics imaging

   https://doi.org/10.1093/mnras/stac1081  

   Chen, Geoff C-F ; Fassnacht, Christopher D. ; Suyu, Sherry H. ; Koopmans, Léon V. E. ; Lagattuta, David J. ; McKean, John P. ; Auger, Matt W. ; Vegetti, Simona ; Treu, Tommaso ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   Strongly lensed quasars can provide measurements of the Hubble constant (H0) independent of any other methods. One of the key ingredients is exquisite high-resolution imaging data, such as Hubble Space Telescope (HST) imaging and adaptive-optics (AO) imaging from ground-based telescopes, which provide strong constraints on the mass distribution of the lensing galaxy. In this work, we expand on the previous analysis of three time-delay lenses with AO imaging (RX J1131−1231, HE 0435−1223, and PG 1115+080), and perform a joint analysis of J0924+0219 by using AO imaging from the Keck telescope, obtained as part of the Strong lensing at High Angular Resolution Program (SHARP) AO effort, with HST imaging to constrain the mass distribution of the lensing galaxy. Under the assumption of a flat Λ cold dark matter (ΛCDM) model with fixed Ωm = 0.3, we show that by marginalizing over two different kinds of mass models (power-law and composite models) and their transformed mass profiles via a mass-sheet transformation, we obtain $\Delta t_{\rm BA}=6.89\substack{+0.8\\-0.7}\, h^{-1}\hat{\sigma }_{v}^{2}$ d, $\Delta t_{\rm CA}=10.7\substack{+1.6\\-1.2}\, h^{-1}\hat{\sigma }_{v}^{2}$ d, and $\Delta t_{\rm DA}=7.70\substack{+1.0\\-0.9}\, h^{-1}\hat{\sigma }_{v}^{2}$ d, where $h=H_{0}/100\,\rm km\, s^{-1}\, Mpc^{-1}$ is the dimensionless Hubble constant and $\hat{\sigma }_{v}=\sigma ^{\rm ob}_{v}/(280\,\rm km\, s^{-1})$ is the scaled dimensionless velocity dispersion. Future measurements of timemore »delays with 10 per cent uncertainty and velocity dispersion with 5 per cent uncertainty would yield a H0 constraint of ∼15 per cent precision.

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