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We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z ≳ 7 for the low-mass (M* ∼ 105 M⊙) Local Group ultrafaint dwarf galaxy (UFD) Eridanus ii (Eri ii). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri ii’s CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri ii is well characterized by a short, exponentially declining star formation history ($\tau _\text{SFH}=0.39\pm _{0.13}^{0.18}$ Gyr), a low star formation efficiency ($\tau _\text{SFE}=27.56\pm _{12.92}^{25.14}$ Gyr), and a large mass-loading factor ($\eta =194.53\pm _{42.67}^{33.37}$). Our results are consistent with Eri ii forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri ii and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri ii. We make predictions for the distribution of [Mg/Fe]–[Fe/H] in Eri ii as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri ii ([Fe/H] > −2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era.

 

The element abundance pattern found in Milky Way disk stars is close to two-dimensional, dominated by production from one prompt process and one delayed process. This simplicity is remarkable, since the elements are produced by a multitude of nucleosynthesis mechanisms operating in stars with a wide range of progenitor masses. We fit the abundances of 14 elements for 48,659 red-giant stars from APOGEE Data Release 17 using a flexible, data-drivenK-process model—dubbedKPM. In our fiducial model, withK= 2, each abundance in each star is described as the sum of a prompt and a delayed process contribution. We find thatKPMwithK= 2 is able to explain the abundances well, recover the observed abundance bimodality, and detect the bimodality over a greater range in metallicity than has previously been possible. We compare to prior work by Weinberg et al., finding thatKPMproduces similar results, but thatKPMbetter predicts stellar abundances, especially for the elements C+N and Mn and for stars at supersolar metallicities. The model fixes the relative contribution of the prompt and delayed processes to two elements to break degeneracies and improve interpretability; we find that some of the nucleosynthetic implications are dependent upon these detailed choices. We find that moving to four processes adds flexibility and improves the model’s ability to predict the stellar abundances, but does not qualitatively change the story. The results ofKPMwill help us to interpret and constrain the formation of the Galaxy disk, the relationship between abundances and ages, and the physics of nucleosynthesis.

 

We present the Cardinal mock galaxy catalogs, a new version of the Buzzard simulation that has been updated to support ongoing and future cosmological surveys, including the Dark Energy Survey (DES), DESI, and LSST. These catalogs are based on a one-quarter sky simulation populated with galaxies out to a redshift ofz= 2.35 to a depth ofm_r= 27. Compared to the Buzzard mocks, the Cardinal mocks include an updated subhalo abundance matching model that considers orphan galaxies and includes mass-dependent scatter between galaxy luminosity and halo properties. This model can simultaneously fit galaxy clustering and group–galaxy cross-correlations measured in three different luminosity threshold samples. The Cardinal mocks also feature a new color assignment model that can simultaneously fit color-dependent galaxy clustering in three different luminosity bins. We have developed an algorithm that uses photometric data to further improve the color assignment model and have also developed a novel method to improve small-scale lensing below the ray-tracing resolution. These improvements enable the Cardinal mocks to accurately reproduce the abundance of galaxy clusters and the properties of lens galaxies in the DES data. As such, these simulations will be a valuable tool for future cosmological analyses based on large sky surveys.

 

We derive empirical constraints on the nucleosynthetic yields of nitrogen by incorporating N enrichment into our previously developed and empirically tuned multizone galactic chemical evolution model. We adopt a metallicity-independent (‘primary’) N yield from massive stars and a metallicity-dependent (‘secondary’) N yield from AGB stars. In our model, galactic radial zones do not evolve along the observed [N/O]–[O/H] relation, but first increase in [O/H] at roughly constant [N/O], then move upward in [N/O] via secondary N production. By t ≈ 5 Gyr, the model approaches an equilibrium [N/O]–[O/H] relation, which traces the radial oxygen gradient. Reproducing the [N/O]–[O/H] trend observed in extragalactic systems constrains the ratio of IMF-averaged N yields to the IMF-averaged O yield of core-collapse supernovae. We find good agreement if we adopt $y_\text{N}^\text{CC}/y_\text{O}^\text{CC}=0.024$ and $y_\text{N}^\text{AGB}/y_\text{O}^\text{CC} = 0.062(Z/Z_\odot)$. For the theoretical AGB yields we consider, simple stellar populations release half their N after only ∼250 Myr. Our model reproduces the [N/O]–[O/H] relation found for Milky Way stars in the APOGEE survey, and it reproduces (though imperfectly) the trends of stellar [N/O] with age and [O/Fe]. The metallicity-dependent yield plays the dominant role in shaping the gas-phase [N/O]–[O/H] relation, but the AGB time-delay is required to match the stellar age and [O/Fe] trends. If we add ∼40 per cent oscillations to the star formation rate, the model reproduces the scatter in the gas phase [N/O]–[O/H] relation observed in external galaxies by MaNGA. We discuss implications of our results for theoretical models of N production by massive stars and AGB stars.

 

We measure abundances of 12 elements (Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni) in a sample of 86 metal-poor (−2 ≲ [Fe/H] ≲ −1) subgiant stars in the solar neighborhood. Abundances are derived from high-resolution spectra taken with the Potsdam Echelle Polarimetric and Spectroscopic Instrument on the Large Binocular Telescope, modeled using iSpec and MOOG. By carefully quantifying the impact of photon-noise (<0.05 dex for all elements), we robustly measure theintrinsicscatter of abundance ratios. At fixed [Fe/H], the rms intrinsic scatter in [X/Fe] ranges from 0.04 (Cr) to 0.16 dex (Na), with a median of 0.08 dex. Scatter in [X/Mg] is similar, and accounting for [α/Fe] only reduces the overall scatter moderately. We consider several possible origins of the intrinsic scatter with particular attention to fluctuations in the relative enrichment by core-collapse supernovae (CCSN) and Type Ia supernovae and stochastic sampling of the CCSN progenitor mass distribution. The stochastic sampling scenario provides a good quantitative explanation of our data if the effective number of CCSN contributing to the enrichment of a typical sample star isN∼ 50. At the median metallicity of our sample, this interpretation implies that the CCSN ejecta are mixed over a gas mass ∼6 × 10⁴M_⊙before forming stars. The scatter of elemental abundance ratios is a powerful diagnostic test for simulations of star formation, feedback, and gas mixing in the early phases of the Galaxy.

 

Abstract Some studies of stars’ multielement abundance distributions suggest at least 5–7 significant dimensions, but others show that many elemental abundances can be predicted to high accuracy from [Fe/H] and [Mg/Fe] (or [Fe/H] and age) alone. We show that both propositions can be, and are, simultaneously true. We adopt a machine-learning technique known as normalizing flow to reconstruct the probability distribution of Milky Way disk stars in the space of 15 elemental abundances measured by APOGEE. Conditioning on T eff and log g minimizes the differential systematics. After further conditioning on [Fe/H] and [Mg/Fe], the residual scatter for most abundances is σ [ X /H] ≲ 0.02 dex, consistent with APOGEE’s reported statistical uncertainties of ∼0.01–0.015 dex and intrinsic scatter of 0.01–0.02 dex. Despite the small scatter, residual abundances display clear correlations between elements, which we show are too large to be explained by measurement uncertainties or by the finite sampling noise. We must condition on at least seven elements to reduce the correlations to a level consistent with the observational uncertainties. Our results demonstrate that cross-element correlations are a much more sensitive probe of a hidden structure than dispersion, and they can be measured precisely in a large sample even if the star-by-star measurement noise is comparable to the intrinsic scatter. We conclude that many elements have an independent story to tell, even for the mundane disk stars and elements produced by the core-collapse and Type Ia supernovae. The only way to learn these lessons is to measure the abundances directly, and not merely infer them. 

Abstract We investigate the [X/Mg] abundances of 16 elements for 82,910 Galactic disk stars from GALAH+ DR3. We fit the median trends of low-Ia and high-Ia populations with a two-process model, which describes stellar abundances in terms of a prompt core-collapse and delayed Type-Ia supernova component. For each sample star, we fit the amplitudes of these two components and compute the residual Δ[X/H] abundances from this two-parameter fit. We find rms residuals ≲0.07 dex for well-measured elements and correlated residuals among some elements (such as Ba, Y, and Zn) that indicate common enrichment sources. From a detailed investigation of stars with large residuals, we infer that roughly 40% of the large deviations are physical and 60% are caused by problematic data such as unflagged binarity, poor wavelength solutions, and poor telluric subtraction. As one example of a population with distinctive abundance patterns, we identify 15 stars that have 0.3–0.6 dex enhancements of Na but normal abundances of other elements from O to Ni and positive average residuals of Cu, Zn, Y, and Ba. We measure the median elemental residuals of 14 open clusters, finding systematic ∼0.1–0.4 dex enhancements of O, Ca, K, Y, and Ba and ∼0.2 dex depletion of Cu in young clusters. Finally, we present a restricted three-process model where we add an asymptotic giant branch star (AGB) component to better fit Ba and Y. With the addition of the third process, we identify a population of stars, preferentially young, that have much higher AGB enrichment than expected from their SNIa enrichment. 

Abstract We report the first direct measurement of the helium isotope ratio, 3 He/ 4 He, outside of the Local Interstellar Cloud, as part of science-verification observations with the upgraded CRyogenic InfraRed Echelle Spectrograph. Our determination of 3 He/ 4 He is based on metastable He i * absorption along the line of sight toward Θ 2 A Ori in the Orion Nebula. We measure a value 3 He/ 4 He = (1.77 ± 0.13) × 10 −4 , which is just ∼40% above the primordial relative abundance of these isotopes, assuming the Standard Model of particle physics and cosmology, ( 3 He/ 4 He) p = (1.257 ± 0.017) × 10 −4 . We calculate a suite of galactic chemical evolution simulations to study the Galactic build up of these isotopes, using the yields from Limongi & Chieffi for stars in the mass range M = 8–100 M ⊙ and Lagarde et al. for M = 0.8–8 M ⊙ . We find that these simulations simultaneously reproduce the Orion and protosolar 3 He/ 4 He values if the calculations are initialized with a primordial ratio 3 He / 4 He p = ( 1.043 ± 0.089 ) × 10 − 4 . Even though the quoted error does not include the model uncertainty, this determination agrees with the Standard Model value to within ∼2 σ . We also use the present-day Galactic abundance of deuterium (D/H), helium (He/H), and 3 He/ 4 He to infer an empirical limit on the primordial 3 He abundance, 3 He / H p ≤ ( 1.09 ± 0.18 ) × 10 − 5 , which also agrees with the Standard Model value. We point out that it is becoming increasingly difficult to explain the discrepant primordial 7 Li/H abundance with nonstandard physics, without breaking the remarkable simultaneous agreement of three primordial element ratios (D/H, 4 He/H, and 3 He/ 4 He) with the Standard Model values. 

ABSTRACT Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $p([\alpha /{\rm Fe}]\, |\, [{\rm Fe}/{\rm H}])$ in the Milky Way disk for the α-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius R, we model p([α/Fe]) as a sum of two Gaussians, representing ‘low-α’ and ‘high-α’ populations with scale heights $z_1=0.45\, {\rm kpc}$ and $z_2=0.95\, {\rm kpc}$, respectively. By accounting for age-dependent and z-dependent selection effects in APOGEE, we infer the [α/Fe] distributions that would be found for a fair sample of long-lived stars covering all z. Near the Solar circle, this distribution is bimodal at sub-solar [Fe/H], with the low-α and high-α peaks clearly separated by a minimum at intermediate [α/Fe]. In agreement with previous results, we find that the high-α population is more prominent at smaller R, lower [Fe/H], and larger |z|, and that the sequence separation is smaller for Si and Ca than for Mg, O, and S. We find significant intrinsic scatter in [α/Fe] at fixed [Fe/H] for both the low-α and high-α populations, typically ∼0.04-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on R, [Fe/H], and α-element, provide a quantitative target for chemical evolution models and a test for hydrodynamic simulations of disk galaxy formation. We argue that explaining the observed bimodality will probably require one or more sharp transitions in the disk’s gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations.