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1. The Chemodynamics of the Stellar Populations in M31 from APOGEE Integrated-light Spectroscopy

   https://doi.org/10.3847/1538-4357/acd9a9  

   Gibson, Benjamin J. ; Zasowski, Gail ; Seth, Anil ; Ashok, Aishwarya ; Goold, Kameron ; Wainer, Tobin ; Hasselquist, Sten ; Holtzman, Jon ; Imig, Julie ; Bizyaev, Dmitry ; et al ( July 2023 , The Astrophysical Journal)

   We present an analysis of nearly 1000 near-infrared, integrated-light spectra from APOGEE in the inner ∼7 kpc of M31. We utilize full-spectrum fitting with A-LIST simple stellar population spectral templates that represent a population of stars with the same age, [M/H], and [α/M]. With this, we determine the mean kinematics, metallicities,αabundances, and ages of the stellar populations of M31's bar, bulge, and inner disk (∼4–7 kpc). We find a nonaxisymmetric velocity field in M31 resulting from the presence of a bar. The bulge of M31 is less metal-rich (mean [M/H] =$-{0.149}_{-0.081}^{+0.067}$dex) than the disk, features minima in metallicity on either side of the bar ([M/H] ∼ −0.2), and is enhanced inαabundance (mean [α/M] =${0.281}_{-0.038}^{+0.035}$). The disk of M31 within ∼7 kpc is enhanced in both metallicity ([M/H] =$-{0.023}_{-0.052}^{+0.050}$) andαabundance ([α/M] =${0.274}_{-0.025}^{+0.020}$). Both of these structural components are uniformly old at ≃12 Gyr. We find the mean metallicity increases with distance from the center of M31, with the steepest gradient along the disk major axis (0.043 ± 0.021 dex kpc⁻¹). This gradient is the result of changing light contributions from the bulge and disk. The chemodynamics of stellar populations encodes information about a galaxy’s chemical enrichment, star formation history, and merger history, allowing us to discuss new constraints on M31's formation. Our results provide a stepping stone between our understanding of the Milky Way and other external galaxies.

    

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2. Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana

   https://doi.org/10.3847/1538-4357/ad25ed  

   Fu, Sal_Wanying ; Weisz, Daniel_R ; Starkenburg, Else ; Martin, Nicolas ; Mercado, Francisco_J ; Savino, Alessandro ; Boylan-Kolchin, Michael ; Côté, Patrick ; Dolphin, Andrew_E ; Longeard, Nicolas ; et al ( April 2024 , The Astrophysical Journal)

   We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with$〈\mathrm{[Fe/H]}〉=-{1.55}_{-0.04}^{+0.04}$and${\sigma }_{\mathrm{[Fe/H]}}={0.54}_{-0.03}^{+0.03}$. (ii) A metallicity gradient of −0.54 ± 0.07 dex$R_e^{-1}$(−2.1 ± 0.3 dex kpc⁻¹) over the extent of our imaging (∼2.5R_e), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores. (iii) Tucana’s bifurcated RGB has distinct metallicities: a blue RGB with$〈\mathrm{[Fe/H]}〉=-{1.78}_{-0.06}^{+0.06}$and${\sigma }_{\mathrm{[Fe/H]}}={0.44}_{-0.06}^{+0.07}$and a red RGB with$〈\mathrm{[Fe/H]}〉=-{1.08}_{-0.07}^{+0.07}$and${\sigma }_{\mathrm{[Fe/H]}}={0.42}_{-0.06}^{+0.06}$. (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼0.4 dex, but its blue RGB is chemically comparable to the satellites. Tucana’s MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well populated. (v) About 2% of Tucana’s stars have [Fe/H] < −3% and 20% have [Fe/H] > −1. We provide a catalog for community spectroscopic follow-up.

    

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3. Spectroscopic Analysis of Milky Way Outer Halo Satellites: Aquarius II and Boötes II

   https://doi.org/10.3847/1538-4357/acc943  

   Bruce, Jordan ; Li, Ting S. ; Pace, Andrew B. ; Heiger, Mairead ; Song, Ying-Yi ; Simon, Joshua D. ( June 2023 , The Astrophysical Journal)

   In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s⁻¹and a metallicity of [Fe/H] =$-{2.57}_{-0.17}^{+0.17}$. When compared with previous measurements, these values display a ∼6 km s⁻¹difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of$-{130.4}_{-1.1}^{+1.4}$km s⁻¹, more than 10 km s⁻¹different from the literature, a metallicity almost 1 dex smaller at [Fe/H] =$-{2.71}_{-0.10}^{+0.11}$, and a velocity dispersion 3 times smaller at${\sigma }_{v_{\mathrm{hel}}}={2.9}_{-1.2}^{+1.6}$km s⁻¹. Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density.

    

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4. Signatures of Tidal Disruption of the Hercules Ultrafaint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ad2f27  

   Ou 欧, Xiaowei 筱葳 ; Chiti, Anirudh ; Shipp, Nora ; Simon, Joshua D. ; Geha, Marla ; Frebel, Anna ; Mardini, Mohammad K. ; Erkal, Denis ; Necib, Lina ( April 2024 , The Astrophysical Journal)

   The Hercules ultrafaint dwarf galaxy (UFD) has long been hypothesized to be tidally disrupting, yet no conclusive evidence has been found for tidal disruption owing partly to difficulties in identifying Hercules member stars. In this work, we present a homogeneous reanalysis of new and existing observations of Hercules, including the detection of a new potential member star located ∼1° (∼1.7 kpc) west of the center of the system. In addition to measuring the line-of-sight velocity gradient, we compare predictions from dynamical models of stream formation to these observations. We report an updated velocity dispersion measurement based on 28 stars,${1.9}_{-0.6}^{+0.6}$km s⁻¹, which is significantly lower than previous measurements. We find that the line-of-sight velocity gradient is${1.8}_{-1.8}^{+1.8}$km s⁻¹kpc along the major axis of Hercules, consistent with zero within 1σ. Our dynamical models of stream formation, on the other hand, can reproduce the morphology of the Hercules UFD, specifically the misalignment between the elongation and the orbital motion direction. Additionally, these dynamical models indicate that any radial velocity gradient from tidal disruption would be too small,${0.00}_{-0.91}^{+0.97}$km s⁻¹kpc, to be detectable with current sample sizes. Combined with our analysis of the tidal radius evolution of the system as a function of its orbital phase, we argue that it is likely that Hercules is indeed currently undergoing tidal disruption in its extended stellar halo with a line-of-sight velocity gradient too small to be detected with current observational data sets.

    

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5. The Triangulum Extended (TREX) Survey: The Stellar Disk Dynamics of M33 as a Function of Stellar Age

   https://doi.org/10.3847/1538-3881/ac5324  

   Quirk, Amanda C. N. ; Guhathakurta, Puragra ; Gilbert, Karoline M. ; Chemin, Laurent ; Dalcanton, Julianne J. ; Williams, Benjamin F. ; Seth, Anil ; Patel, Ekta ; Fung, Justin T. ; Tangirala, Pujita ; et al ( March 2022 , The Astronomical Journal)

   Triangulum (M33) is a low-mass, relatively undisturbed spiral galaxy that offers a new regime in which to test models of dynamical heating. In spite of its proximity, M33's dynamical heating history has not yet been well-constrained. In this work, we present the TREX Survey, the largest stellar spectroscopic survey across the disk of M33. We present the stellar disk kinematics as a function of age to study the past and ongoing dynamical heating of M33. We measure line-of-sight velocities for ∼4500 disk stars. Using a subset, we divide the stars into broad age bins using Hubble Space Telescope and Canada–France–Hawaii Telescope photometric catalogs: massive main-sequence stars and helium-burning stars (∼80 Myr), intermediate-mass asymptotic branch stars (∼1 Gyr), and low-mass red giant branch stars (∼4 Gyr). We compare the stellar disk dynamics to that of the gas using existing Hi, CO, and Hαkinematics. We find that the disk of M33 has relatively low-velocity dispersion (∼16 km s⁻¹), and unlike in the Milky Way and Andromeda galaxies, there is no strong trend in velocity dispersion as a function of stellar age. The youngest disk stars are as dynamically hot as the oldest disk stars and are dynamically hotter than predicted by most M33-like low-mass simulated analogs in Illustris. The velocity dispersion of the young stars is highly structured, with the large velocity dispersion fairly localized. The cause of this high-velocity dispersion is not evident from the observations and simulated analogs presented here.

    

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