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1. Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ac96ed  

   Chiti, Anirudh ; Simon, Joshua D. ; Frebel, Anna ; Pace, Andrew B. ; Ji, Alexander P. ; Li, Ting S. ( November 2022 , The Astrophysical Journal)

   We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of$M_{1/2}(r_h)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V=${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in line with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

    

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2. 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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3. Structural Parameters and Possible Association of the Ultra-faint Dwarfs Pegasus III and Pisces II from Deep Hubble Space Telescope Photometry

   https://doi.org/10.3847/1538-4357/ac7226  

   Richstein, Hannah ; Patel, Ekta ; Kallivayalil, Nitya ; Simon, Joshua D. ; Zivick, Paul ; Tollerud, Erik ; Fritz, Tobias ; Warfield, Jack T. ; Besla, Gurtina ; van der Marel, Roeland P. ; et al ( July 2022 , The Astrophysical Journal)

   We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of$a_h=1.\prime {88}_{-0.33}^{+0.42}$(${118}_{-30}^{+31}$pc) and$M_V=-{4.17}_{-0.22}^{+0.19};$for Psc II, we measure$a_h=1.\prime {31}_{-0.09}^{+0.10}$(69 ± 8 pc) and$M_V=-{4.28}_{-0.16}^{+0.19}$. We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only ∼40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in an MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close (∼10–20 kpc) passage about each other just over ∼1 Gyr ago, followed by a collective passage around the LMC (∼30–60 kpc) just under ∼1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group.

    

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4. Pegasus V/Andromeda XXXIV–a newly discovered ultrafaint dwarf galaxy on the outskirts of Andromeda

   https://doi.org/10.1093/mnrasl/slac063  

   Collins, Michelle L. M. ; Charles, Emily J. E. ; Martínez-Delgado, David ; Monelli, Matteo ; Karim, Noushin ; Donatiello, Giuseppe ; Tollerud, Erik J. ; Boschin, Walter ( July 2022 , Monthly Notices of the Royal Astronomical Society: Letters)

   We report the discovery of an ultrafaint dwarf in the constellation of Pegasus. Pegasus V (Peg V)/Andromeda XXXIV was initially identified in the public imaging data release of the DESI Legacy Imaging Surveys and confirmed with deep imaging from Gemini/GMOS-N. The colour–magnitude diagram shows a sparse red giant branch (RGB) population and a strong overdensity of blue horizontal branch stars. We measure a distance to Peg V of $D=692^{+33}_{-31}$ kpc, making it a distant satellite of Andromeda with MV = −6.3 ± 0.2 and a half-light radius of rhalf = 89 ± 41 pc. It is located ∼260 kpc from Andromeda in the outskirts of its halo. The RGB is well fitted by a metal-poor isochrone with [Fe/H] = −3.2, suggesting it is very metal poor. This, combined with its blue horizontal branch, could imply that it is a reionization fossil. This is the first detection of an ultrafaint dwarf outside the deep Pan-Andromeda Archaeological Survey area, and points to a rich, faint satellite population in the outskirts of our nearest neighbour.

    

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5. Abundance Analysis of Stars at Large Radius in the Sextans Dwarf Spheroidal Galaxy*

   https://doi.org/10.3847/1538-4357/ace3c1  

   Roederer, Ian U. ; Pace, Andrew B. ; Placco, Vinicius M. ; Caldwell, Nelson ; Koposov, Sergey E. ; Mateo, Mario ; Olszewski, Edward W. ; Walker, Matthew G. ( August 2023 , The Astrophysical Journal)

   We present the stellar parameters and chemical abundances of 30 elements for five stars located at large radii (3.5–10.7 times the half-light radius) in the Sextans dwarf spheroidal galaxy. We selected these stars using proper motions, radial velocities, and metallicities, and we confirm them as metal-poor members of Sextans with −3.34 ≤ [Fe/H] ≤ −2.64 using high-resolution optical spectra collected with the Magellan Inamori Kyocera Echelle spectrograph. Four of the five stars exhibit normal abundances of C (−0.34 ≤ [C/Fe] ≤ + 0.36), mild enhancement of theαelements Mg, Si, Ca, and Ti ([α/Fe] = +0.12 ± 0.03), and unremarkable abundances of Na, Al, K, Sc, V, Cr, Mn, Co, Ni, and Zn. We identify three chemical signatures previously unknown among stars in Sextans. One star exhibits large overabundances ([X/Fe] > +1.2) of C, N, O, Na, Mg, Si, and K, and large deficiencies of heavy elements ([Sr/Fe] = −2.37 ± 0.25, [Ba/Fe] = −1.45 ± 0.20, [Eu/Fe] < + 0.05), establishing it as a member of the class of carbon-enhanced metal-poor stars with no enhancement of neutron-capture elements. Three stars exhibit moderate enhancements of Eu (+0.17 ≤ [Eu/Fe] ≤ + 0.70), and the abundance ratios among 12 neutron-capture elements are indicative ofr-process nucleosynthesis. Another star is highly enhanced in Sr relative to heavier elements ([Sr/Ba] = +1.21 ± 0.25). These chemical signatures can all be attributed to massive, low-metallicity stars or their end states. Our results, the first for stars at large radius inSextans, demonstrate that these stars were formed in chemically inhomogeneous regions, such as those found in ultra-faint dwarf galaxies.

    

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