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1. Simultaneous Constraints on the Star Formation History and Nucleosynthesis of Sculptor dSph

   https://doi.org/10.3847/1538-4357/ac332b  

   de los Reyes, Mithi A. C. ; Kirby, Evan N. ; Ji, Alexander P. ; Nuñez, Evan H. ( January 2022 , The Astrophysical Journal)

   We demonstrate that using up to seven stellar abundance ratios can place observational constraints on the star formation histories (SFHs) of Local Group dSphs, using Sculptor dSph as a test case. We use a one-zone chemical evolution model to fit the overall abundance patterns ofαelements (which probe the core-collapse supernovae that occur shortly after star formation),s-process elements (which probe AGB nucleosynthesis at intermediate delay times), and iron-peak elements (which probe delayed Type Ia supernovae). Our best-fit model indicates that Sculptor dSph has an ancient SFH, consistent with previous estimates from deep photometry. However, we derive a total star formation duration of ∼0.9 Gyr, which is shorter than photometrically derived SFHs. We explore the effect of various model assumptions on our measurement and find that modifications to these assumptions still produce relatively short SFHs of duration ≲1.4 Gyr. Our model is also able to compare sets of predicted nucleosynthetic yields for supernovae and AGB stars, and can provide insight into the nucleosynthesis of individual elements in Sculptor dSph. We find that observed [Mn/Fe] and [Ni/Fe] trends are most consistent with sub-M_ChType Ia supernova models, and that a combination of “prompt” (delay times similar to core-collapse supernovae) and “delayed” (minimum delaymore »times ≳50 Myr)r-process events may be required to reproduce observed [Ba/Mg] and [Eu/Mg] trends.

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2. Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II

   https://doi.org/10.3847/1538-4357/aca9d1  

   Simon, Joshua D. ; Brown, Thomas M. ; Mutlu-Pakdil, Burçin ; Ji, Alexander P. ; Drlica-Wagner, Alex ; Avila, Roberto J. ; Martínez-Vázquez, Clara E. ; Li, Ting S. ; Balbinot, Eduardo ; Bechtol, Keith ; et al ( February 2023 , The Astrophysical Journal)

   The ultra-faint dwarf galaxy Reticulum II (Ret II) exhibits a unique chemical evolution history, with${72}_{-12}^{+10}$% of its stars strongly enhanced inr-process elements. We present deep Hubble Space Telescope photometry of Ret II and analyze its star formation history. As in other ultra-faint dwarfs, the color–magnitude diagram is best fit by a model consisting of two bursts of star formation. If we assume that the bursts were instantaneous, then the older burst occurred around the epoch of reionization, forming ∼80% of the stars in the galaxy, while the remainder of the stars formed ∼3 Gyr later. When the bursts are allowed to have nonzero durations, we obtain slightly better fits. The best-fitting model in this case consists of two bursts beginning before reionization, with approximately half the stars formed in a short (100 Myr) burst and the other half in a more extended period lasting 2.6 Gyr. Considering the full set of viable star formation history models, we find that 28% of the stars formed within 500 ± 200 Myr of the onset of star formation. The combination of the star formation history and the prevalence ofr-process-enhanced stars demonstrates that ther-process elements in Ret II must havemore »been synthesized early in its initial star-forming phase. We therefore constrain the delay time between the formation of the first stars in Ret II and ther-process nucleosynthesis to be less than 500 Myr. This measurement rules out anr-process source with a delay time of several Gyr or more, such as GW170817.

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3. The detailed chemical abundance patterns of accreted halo stars from the optical to infrared

   https://doi.org/10.1093/mnras/stac518  

   Carrillo, Andreia ; Hawkins, Keith ; Jofré, Paula ; de Brito Silva, Danielle ; Das, Payel ; Lucey, Madeline ( April 2022 , Monthly Notices of the Royal Astronomical Society)

   Understanding the assembly of our Galaxy requires us to also characterize the systems that helped build it. In this work, we accomplish this by exploring the chemistry of accreted halo stars from Gaia-Enceladus/Gaia-Sausage (GES) selected in the infrared from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16. We use high resolution optical spectra for 62 GES stars to measure abundances in 20 elements spanning the α, Fe-peak, light, odd-Z, and notably, the neutron-capture groups of elements to understand their trends in the context of and in contrast to the Milky Way and other stellar populations. Using these derived abundances we find that the optical and the infrared abundances agree to within 0.15 dex except for O, Co, Na, Cu, and Ce. These stars have enhanced neutron-capture abundance trends compared to the Milky Way, and their [Eu/Mg] and neutron-capture abundance ratios (e.g. [Y/Eu], [Ba/Eu], [Zr/Ba], [La/Ba], and [Nd/Ba]) point to r-process enhancement and a delay in s-process enrichment. Their [α/Fe] trend is lower than the Milky Way trend for [Fe/H] > −1.5 dex, similar to previous studies of GES stars and consistent with the picture that these stars formed in a system with a lower rate ofmore »star formation. This is further supported by their depleted abundances in Ni, Na, and Cu abundances, again, similar to previous studies of low-α stars with accreted origins.

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4. Predictions for complex distributions of stellar elemental abundances in low-mass galaxies

   https://doi.org/10.1093/mnras/stac834  

   Patel, Preet B. ; Loebman, Sarah R. ; Wetzel, Andrew ; Faucher-Giguère, Claude-André ; El-Badry, Kareem ; Bailin, Jeremy ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   We investigate stellar elemental abundance patterns at $z$ = 0 in eight low-mass ($M_{*}=10^{6}{-}10^{9}\ \text{M}_{\odot }$) galaxies in the Feedback in Realistic Environments cosmological simulations. Using magnesium (Mg) as a representative α-element, we explore stellar abundance patterns in magnesium-to-iron ([Mg/Fe]) versus iron-to-hydrogen ([Fe/H]), which follow an overall monotonic trend that evolved slowly over time. Additionally, we explore three notable secondary features in enrichment (in three different case-study galaxies) that arise from a galaxy merger or bursty star formation. First, we observe a secondary track with a lower [Mg/Fe] than the main trend. At $z$ = 0, stars from this track are predominantly found within 2–6 kpc of the centre; they were accreted in a 1:3 total-mass-ratio merger ∼0.4 Gyr ago. Second, we find a distinct elemental bimodality that forms following a strong burst in star formation in a galaxy at $t_{\text{lookback}}\, \sim 10$ Gyr. This burst quenched star formation for ∼0.66 Gyr, allowing Type Ia supernovae to enrich the system with iron (Fe) before star formation resumed. Third, we examine stripes in enrichment that run roughly orthogonal to the dominant [Mg/Fe] versus [Fe/H] trend; these stripes correspond to short bursts of star formation during which core-collapse supernovae enrich the surrounding medium with Mg (andmore »Fe) on short time-scales. If observed, these features would substantiate the utility of elemental abundances in revealing the assembly and star-formation histories of dwarf galaxies. We explore the observability of these features for upcoming spectroscopic studies. Our results show that precise measurements of elemental abundance patterns can reveal critical events in the formation histories of low-mass galaxies.

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5. A Glimpse of the Stellar Populations and Elemental Abundances of Gravitationally Lensed, Quiescent Galaxies at z ≳ 1 with Keck Deep Spectroscopy

   https://doi.org/10.3847/1538-4357/acc79b  

   Zhuang, Zhuyun ; Leethochawalit, Nicha ; Kirby, Evan N. ; Nightingale, J. W. ; Steidel, Charles C. ; Glazebrook, Karl ; Barone, Tania M. ; Skobe, Hannah ; Sweet, Sarah M. ; Nanayakkara, Themiya ; et al ( May 2023 , The Astrophysical Journal)

   Gravitational lenses can magnify distant galaxies, allowing us to discover and characterize the stellar populations of intrinsically faint, quiescent galaxies that are otherwise extremely difficult to directly observe at high redshift from ground-based telescopes. Here, we present the spectral analysis of two lensed, quiescent galaxies atz≳ 1 discovered by theASTRO 3D Galaxy Evolution with Lensessurvey:AGEL1323 (M_*∼ 10^11.1M_⊙,z= 1.016,μ∼ 14.6) andAGEL0014 (M_*∼ 10^11.5M_⊙,z= 1.374,μ∼ 4.3). We measured the age, [Fe/H], and [Mg/Fe] of the two lensed galaxies using deep, rest-frame-optical spectra (S/N ≳40 Å⁻¹) obtained on the Keck I telescope. The ages ofAGEL1323 andAGEL0014 are${5.6}_{-0.8}^{+0.8}$Gyr and${3.1}_{-0.3}^{+0.8}$Gyr, respectively, indicating that most of the stars in the galaxies were formed less than 2 Gyr after the Big Bang. Compared to nearby quiescent galaxies of similar masses, the lensed galaxies have lower [Fe/H] and [Mg/H]. Surprisingly, the two galaxies have comparable [Mg/Fe] to similar-mass galaxies at lower redshifts, despite their old ages. Using a simple analytic chemical evolution model connecting the instantaneously recycled element Mg with the mass-loading factors of outflows averaged over the entire star formation history, we found that the lensed galaxies may have experienced enhanced outflows during their star formation compared to lower-redshift galaxies,more »which may explain why they quenched early.

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