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1. Neutron-capture elements in dwarf galaxies III: A homogenized analysis of 13 dwarf spheroidal and ultra-faint galaxies

   Reichert, M. ; Hansen, C. ; Hanke, M. ; Skúladóttir, A. ; Arcones, A. ; Grebel, E.K. ( September 2020 , ArXivorg)

   We present a large homogeneous set of stellar parameters and abundances across a broad range of metallicities, involving 13 classical dwarf spheroidal (dSph) and ultra-faint dSph (UFD) galaxies. In total this study includes 380 stars in Fornax, Sagittarius, Sculptor, Sextans, Carina, Ursa Minor, Draco, Reticulum II, Bootes I, Ursa Major II, Leo I, Segue I, and Triangulum II. This sample represents the largest, homogeneous, high-resolution study of dSph galaxies to date. With our homogeneously derived catalog, we are able to search for similar and deviating trends across different galaxies. We investigate the mass dependence of the individual systems on the production of α-elements, but also try to shed light on the long-standing puzzle of the dominant production site of r-process elements. We use data from the Keck observatory archive and the ESO reduced archive to reanalyze stars from these 13 dSph galaxies. We automatize the step of obtaining stellar parameters, but run a full spectrum synthesis to derive all abundances except for iron. The homogenized set of abundances yielded the unique possibility to derive a relation between the onset of type Ia supernovae and the stellar mass of the galaxy. Furthermore, we derived a formula to estimate the evolution of α-elements. Placing all abundances consistently on the same scale is crucial to answer questions about the chemical history of galaxies. By homogeneously analysing Ba and Eu in the 13 systems, we have traced the onset of the s-process and found it to increase with metallicity as a function of the galaxy's stellar mass. Moreover, the r-process material correlates with the α-elements indicating some co-production of these, which in turn would point towards rare core-collapse supernovae rather than binary neutron star mergers as host for the r-process at low [Fe/H] in the investigated dSph systems. 

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2. The Cosmic Hunt for members in the outskirts of ultra-faint dwarf galaxies: Ursa Major I, Coma Berenices, and Boötes I

   https://doi.org/10.1093/mnras/stac3563  

   Waller, Fletcher ; Venn, Kim A. ; Sestito, Federico ; Jensen, Jaclyn ; Kielty, Collin L. ; Borukhovetskaya, Asya ; Hayes, Christian ; McConnachie, Alan W. ; Navarro, Julio F. ( December 2022 , Monthly Notices of the Royal Astronomical Society)

   Gaia EDR3 data were used to identify potential members in the outskirts of three ultra-faint dwarf (UFD) galaxies: Coma Berenices (>2Rh), Ursa Major I (∼4Rh), and Boötes I (∼4Rh), as well as a new member in the central region of Ursa Major I. These targets were observed with the Gemini GRACES spectrograph, which was used to determine precision radial velocities and metallicities that confirm their associations with the UFD galaxies. The spectra were also used to measure absorption lines for 10 elements (Na, Mg, K, Ca, Sc, Ti, Cr, Fe, Ni, and Ba), which confirm that the chemical abundances of the outermost stars are in good agreement with stars in the central regions. The abundance ratios and chemical patterns of the stars in Coma Berenices are consistent with contributions from SN Ia, which is unusual for its star formation history and in conflict with previous suggestions that this system evolved chemically from a single core collapse supernova event. The chemistries for all three galaxies are consistent with the outermost stars forming in the central regions, then moving to their current locations through tidal stripping and/or supernova feedback. In Boötes I, however, the lower metallicity and lack of strong carbon enrichment of its outermost stars could also be evidence of a dwarf galaxy merger.

    

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3. Reading the CARDs: The Imprint of Accretion History in the Chemical Abundances of the Milky Way's Stellar Halo

   https://doi.org/10.3847/1538-4357/ac78ea  

   Cunningham, Emily C. ; Sanderson, Robyn E. ; Johnston, Kathryn V. ; Panithanpaisal, Nondh ; Ness, Melissa K. ; Wetzel, Andrew ; Loebman, Sarah R. ; Escala, Ivanna ; Horta, Danny ; Faucher-Giguère, Claude-André ( August 2022 , The Astrophysical Journal)

   In the era of large-scale spectroscopic surveys in the Local Group, we can explore using chemical abundances of halo stars to study the star formation and chemical enrichment histories of the dwarf galaxy progenitors of the Milky Way (MW) and M31 stellar halos. In this paper, we investigate using the chemical abundance ratio distributions (CARDs) of seven stellar halos from the Latte suite of FIRE-2 simulations. We attempt to infer galaxies’ assembly histories by modeling the CARDs of the stellar halos of the Latte galaxies as a linear combination oftemplateCARDs from disrupted dwarfs, with different stellar massesM_⋆and quenching timest₁₀₀. We present a method for constructing these templates using present-day dwarf galaxies. For four of the seven Latte halos studied in this work, we recover the mass spectrum of accreted dwarfs to a precision of <10%. For the fraction of mass accreted as a function oft₁₀₀, we find the residuals of 20%–30% for five of the seven simulations. We discuss the failure modes of this method, which arise from the diversity of star formation and chemical enrichment histories that dwarf galaxies can take. These failure cases can be robustly identified by the high model residuals. Although the CARDs modeling method does not successfully infer the assembly histories in these cases, the CARDs of these disrupted dwarfs contain signatures of their unusual formation histories. Our results are promising for using CARDs to learn more about the histories of the progenitors of the MW and M31 stellar halos.

    

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4. 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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5. Constraints on the Frequency and Mass Content of r-process Events Derived from Turbulent Mixing in Galactic Disks

   https://doi.org/10.3847/1538-4357/acca80  

   Kolborg, Anne Noer ; Ramirez-Ruiz, Enrico ; Martizzi, Davide ; Macias, Phillip ; Soares-Furtado, Melinda ( June 2023 , The Astrophysical Journal)

   Metal-poor stars in the Milky Way (MW) halo display large star-to-star dispersion in theirr-process abundance relative to lighter elements. This suggests a chemically diverse and unmixed interstellar medium (ISM) in the early universe. This study aims to help shed light on the impact of turbulent mixing, driven by core-collapse supernovae (cc-SNe), on ther-process abundance dispersal in galactic disks. To this end, we conduct a series of simulations of small-scale galaxy patches which resolve metal-mixing mechanisms at parsec scales. Our setup includes cc-SNe feedback and enrichment fromr-process sources. We find that the relative rate of ther-process events to cc-SNe is directly imprinted on the shape of ther-process distribution in the ISM with more frequent events causing more centrally peaked distributions. We consider also the fraction of metals that is lost on galactic winds and find that cc-SNe are able to efficiently launch highly enriched winds, especially in smaller galaxy models. This result suggests that smaller systems, e.g., dwarf galaxies, may require higher levels of enrichment in order to achieve similar meanr-process abundances as MW-like progenitors systems. Finally, we are able to place novel constraints on the production rate ofr-process elements in the MW,$6\times {10}^{-7}{M}_{\odot }{\mathrm{yr}}^{-1}\lesssim {\dot{m}}_{\mathrm{rp}}\ll 4.7\times {10}^{-4}{M}_{\odot }{\mathrm{yr}}^{-1}$, imposed by accurately reproducing the mean and dispersion of [Eu/Fe] in metal-poor stars. Our results are consistent with independent estimates from alternate methods and constitute a significant reduction in the permitted parameter space.

    

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