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We re-examine the extremely metal-poor dwarf galaxy AGC 198691 using a high quality spectrum obtained by the LBT's MODS instrument. Previous spectral observations obtained from KOSMOS on the Mayall 4-m and the Blue channel spectrograph on the MMT 6.5-m telescope did not allow for the determination of sulfur, argon, or helium abundances. We report an updated and full chemical abundance analysis for AGC 198691, including confirmation of the extremely low "direct" oxygen abundance with a value of 12 + log (O/H) = 7.06 ± 0.03. AGC 198691's low metallicity potentially makes it a high value target for helping determine the primordial helium abundance (Y p). Though complicated by a Na  i night sky line partially overlaying the He  i λ5876 emission line, the LBT/MODS spectrum proved adequate for determining AGC 198691's helium abundance. We employ the recently expanded and improved model of Aver et al. incorporating higher Balmer and Paschen lines, augmented by the observation of the infrared helium emission line He  i λ10830 obtained by Hsyu et al. Applying our full model produced a reliable helium abundance determination, consistent with the expectation for its metallicity. Although this is the lowest metallicity object with a detailed helium abundance, unfortunately, due to its faintness [EW(Hβ) < 100 Å] and the compromised He  i λ5876, the resultant uncertainty on the helium abundance is too large to allow a significant improvement on the measurement of Y p. [ABSTRACT FROM AUTHOR]

The abundance of magnesium in the interstellar medium is a powerful probe of star formation processes over cosmological time-scales. Magnesium has three stable isotopes, 24Mg, 25Mg, and 26Mg, which can be produced in both massive and intermediate-mass (IM) stars with masses between 2 and 8 M⊙. In this work, we use constraints on the cosmic star formation rate density (SFRD) and explore the role and mass range of IM stars using the observed isotopic ratios. We compare several models of stellar nucleosynthesis with metallicity-dependent yields and also consider the effect of rotation on the yields of massive stars and its consequences on the evolution of the Mg isotopes. We use a cosmic evolution model updated with new observational SFRD data and new reionization constraints coming from 2018 Planck Collaboration determinations. We find that the main contribution of 24Mg comes from massive stars whereas 25Mg and 26Mg come from IM stars. To fit the observational data on magnesium isotopic ratios, an additional IM SFRD component is preferred. Moreover, the agreement between model and data is further improved when the range of IM masses is narrowed towards higher masses (5–8 M⊙). While some rotation also improves the fit to data, we can exclude the case where all stars have high rotational velocities due to an overproduction of 26Mg. [ABSTRACT FROM AUTHOR]

The abundance of nitrogen in the interstellar medium is a powerful probe of star formation processes over cosmological time-scales. Since nitrogen can be produced both in massive and intermediate-mass stars with metallicity-dependent yields, its evolution is challenging to model, as evidenced by the differences between theoretical predictions and observations. In this work, we attempt to identify the sources of these discrepancies using a cosmic evolution model. To further complicate matters, there is considerable dispersion in the abundances from observations of damped Lya absorbers (DLAs) at z ~ 2-3. We study the evolution of nitrogen with a detailed cosmic chemical evolution model and find good agreement with these observations, including the relative abundances of (N/O) and (N/Si). We find that the principal contribution of nitrogen comes from intermediate-mass stars, with the exception of systems with the lowest N/H, where nitrogen productionmight possibly be dominated by massive stars. This last result could be strengthened if stellar rotationwhich is important at lowmetallicity can produce significant amounts of nitrogen. Moreover, these systems likely reside in host galaxies with stellar masses below 108.5M☉. We also study the origin of the observed dispersion in nitrogen abundances using the cosmological hydrodynamical simulations Horizon-AGN. We conclude that this dispersion can originate from two effects: difference in the masses of the DLA host galaxies, and difference in their position inside the galaxy. [ABSTRACT FROM AUTHOR]

The recent detection of the binary black hole merger GW150914 demonstrates the existence of black holes more massive than previously observed in X-ray binaries in our Galaxy. This article explores different scenarios of black hole formation in the context of self-consistent cosmic chemical evolution models that simultaneously match observations of the cosmic star formation rate, optical depth to reionization and metallicity of the interstellar medium. This framework is used to calculate the mass distribution of merging black hole binaries and its evolution with redshift. We also study the implications of the black hole mass distribution for the stochastic gravitational wave background from mergers and from core-collapse events. [ABSTRACT FROM AUTHOR]

Recent observations in the total luminosity density have led to significant progress in establishing the star formation rate (SFR) at high redshift. Concurrently observed gamma-ray burst rates have also been used to extract the SFR at high redshift. The SFR in turn can be used to make a host of predictions concerning the ionization history of the Universe, the chemical abundances, and supernova rates. We compare the predictions made using a hierarchical model of cosmic chemical evolution based on three recently proposed SFRs: two based on extracting the SFR from the observed gamma-ray burst rate at high redshift, and one based on the observed galaxy luminosity function at high redshift. Using the WMAP/Planck data on the optical depth and epoch of reionization, we find that only the SFR inferred from gamma-ray burst data at high redshift suffices to allow a single mode (in the initial mass function – IMF) of star formation which extends from z = 0 to redshifts >10. For the case of the SFR based on the observed galaxy luminosity function, the reionization history of the Universe requires a bimodal IMF which includes at least a coeval high- (or intermediate-) mass mode of star formation at high redshift (z > 10). Therefore, we also consider here a more general bimodal case which includes an early-forming high-mass mode as a fourth model to test the chemical history of the Universe. We conclude that observational constraints on the global metallicity and optical depth at high redshift favour unseen faint but active star-forming galaxies as pointed out in many recent studies. [ABSTRACT FROM AUTHOR]

ABSTRACT Standard big bang nucleosynthesis (BBN) at the baryon density determined by the microwave anisotropy spectrum predicts an excess of 7Li compared to observations by a factor of 4-5. In contrast, BBN predictions for D/H are somewhat below (but within 2σ of) the weighted mean of observationally determined values from quasar absorption systems. Solutions to the 7Li problem which alter the nuclear processes during or subsequent to BBN often lead to a significant increase in the deuterium abundance consistent with the highest values of D/H seen in absorption systems. Furthermore, the observed D/H abundances show considerable dispersion. Here, we argue that those systems with D/H ≃ 4 × 10−5 may be more representative of the primordial abundance and as a consequence, those systems with lower D/H would necessarily have been subject to local processes of deuterium destruction. This can be accounted for by models of cosmic chemical evolution able to destroy in situ deuterium due to the fragility of this isotope. [ABSTRACT FROM AUTHOR]

We explore the effect of an early population of intermediate-mass stars in the 2-8 M range on the cosmic chemical evolution. We discuss the implications of this population as it pertains to several cosmological and astrophysical observables. For example, some very metal poor Galactic stars show large enhancements of carbon, typical of the C-rich ejecta of intermediate-mass stars; moreover, halo star carbon and oxygen abundances show wide scatter, which imply a wide range of star formation and nucleosynthetic histories contributed to the first generations of stars. Also, recent analyses of the abundance in metal-poor extragalactic H regions suggest an elevated abundance by mass, higher than the predicted result from the big bang nucleosynthesis, assuming the baryon density determined by the WMAP, . Although there are large uncertainties in the observational determination of , this offset may suggest a prompt initial enrichment of in early metal-poor structures. We also discuss the effect of intermediate-mass stars on the global cosmic evolution, the reionization of the Universe, the density of white dwarfs, as well as Type II supernovae (SN II) and SN Ia rates at high redshift. We also comment on the early astration of D and . We conclude that if intermediate-mass stars are to be associated with Population III stars, their relevance is limited (primarily from observed abundance patterns) to low-mass structures involving a limited fraction of the total baryon content of the Universe. [ABSTRACT FROM AUTHOR]