Evolution of N/O ratios in galaxies from cosmological hydrodynamical simulations

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We study the redshift evolution of the gas-phase O/H and N/O abundances, both (i) for individual interstellar medium (ISM) regions within single spatially resolved galaxies and (ii) when dealing with average abundances in the whole ISM of many unresolved galaxies. We make use of a cosmological hydrodynamical simulation including detailed chemical enrichment, which properly takes into account the variety of different stellar nucleosynthetic sources of O and N in galaxies. We identify 33 galaxies in the simulation, lying within dark matter haloes with virial mass in the range 10 11 ≤ M DM ≤ 10 13 M ⊙ and reconstruct how they evolved with redshift. For the local and global measurements, the observed increasing trend of N/O at high O/H can be explained, respectively, (i) as the consequence of metallicity gradients that have settled in the galaxy ISM, where the innermost galactic regions have the highest O/H abundances and the highest N/O ratios, and (ii) as the consequence of an underlying average mass-metallicity relation that galaxies obey as they evolve across cosmic epochs, where - at any redshift - less massive galaxies have lower average O/H and N/O ratios than the more massive ones. We do not find a strong dependence on the environment. For both local and global relations, the predicted N/O-O/H relation is due to the mostly secondary origin of N in stars. We also predict that the O/H and N/O gradients in the galaxy ISM gradually flatten as functions of redshift, with the average N/O ratios being strictly coupled with the galaxy star formation history. Because N production strongly depends on O abundances, we obtain a universal relation for the N/O-O/H abundance diagram whether we consider average abundances of many unresolved galaxies put together or many abundance measurements within a single spatially resolved galaxy.Peer reviewedFinal Accepted Versio

On the [α/Fe]-[Fe/H] relations in early-type galaxies

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We study how the predicted [α/Fe]-[Fe/H] relations in early-type galaxies vary as functions of their stellar masses, ages, and stellar velocity dispersions, by making use of cosmological chemodynamical simulations with feedback from active galactic nuclei. Our model includes a detailed treatment for the chemical enrichment from dying stars, core-collapse supernovae (both Type II and hypernovae) and Type Ia supernovae. At redshift z = 0, we create a catalogue of 526 galaxies, among which we determine 80 early-type galaxies. From the analysis of our simulations, we find [α/Fe]-[Fe/H] relations similar to the Galactic bulge. We also find that, in the oldest galaxies, Type Ia supernovae start to contribute at higher [Fe/H] than in the youngest ones. On the average, early-type galaxies with larger stellar masses (and, equivalently, higher stellar velocity dispersions) have higher [α/Fe] ratios, at fixed [Fe/H]. This is qualitatively consistent with the recent observations of Sybilska et al., but quantitatively there are mismatches, which might require stronger feedback, sub-classes of Type Ia Supernovae, or a variable initial mass function to address.Peer reviewedFinal Published versio

Zoom-in cosmological hydrodynamical simulation of a star-forming barred, spiral galaxy at redshift z=2

Accepted for publication in MNRASWe present gas and stellar kinematics of a high-resolution zoom-in cosmological chemodynamical simulation, which fortuitously captures the formation and evolution of a star-forming barred spiral galaxy, from redshift $z\sim3$ to $z\sim2$ at the peak of the cosmic star formation rate. The galaxy disc grows by accreting gas and substructures from the environment. The spiral pattern becomes fully organised when the gas settles from a thick (with vertical dispersion $\sigma_{v} >$ 50 km/s) to a thin ($\sigma_{v} \sim 25$ km/s) disc component in less than 1 Gyr. Our simulated disc galaxy also has a central X-shaped bar, the seed of which formed by the assembly of dense gas-rich clumps by $z \sim 3$. The star formation activity in the galaxy mainly happens in the bulge and in several clumps along the spiral arms at all redshifts, with the clumps increasing in number and size as the simulation approaches $z=2$. We find that stellar populations with decreasing age are concentrated towards lower galactic latitudes, being more supported by rotation, and having also lower velocity dispersion; furthermore, the stellar populations on the thin disc are the youngest and have the highest average metallicities. The pattern of the spiral arms rotates like a solid body with a constant angular velocity as a function of radius, which is much lower than the angular velocity of the stars and gas on the thin disc; moreover, the angular velocity of the spiral arms steadily increases as function of time, always keeping its radial profile constant. The origin of our spiral arms is also discussed.Peer reviewe

Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 1, July 2020, Pages 80–94 ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.In order to understand the roles of metal flows in galaxy formation and evolution, we analyse our self-consistent cosmological chemo-dynamical simulation of a Milky Way like galaxy during its thin-disc phase. Our simulated galaxy disc qualitatively reproduces the variation of the dichotomy in [$\alpha$/Fe]-[Fe/H] at different Galactocentric distances as derived by APOGEE-DR16, as well as the stellar age distribution in [$\alpha$/Fe]-[Fe/H] from APOKASC-2. The disc grows from the inside out, with a radial gradient in the star-formation rate during the entire phase. Despite the radial dependence, the outflow-to-infall ratio of metals in our simulated halo shows a universal (time-independent) profile scaling with the disc growth. The simulated disc undergoes two modes of gas inflow: (i) an infall of metal-poor and relatively low-[$\alpha$/Fe] gas, and (ii) a radial flow where already chemically-enriched gas moves inwards with an average velocity of $\sim0.7$ km/s. Moreover, we find that stellar migrations mostly happen outwards, on typical time scales of $\sim5$ Gyr. Our predicted radial metallicity gradients agree with the observations from APOGEE-DR16, and the main effect of stellar migrations is to flatten the radial metallicity profiles by 0.05 dex/kpc in the slopes. We also show that the effect of migrations can appear more important in [$\alpha$/Fe] than in the [Fe/H]-age relation of thin-disc stars.Peer reviewe

He abundances in disc galaxies. I. Predictions from cosmological chemodynamical simulations

Accepted for publication in A&AWe investigate how the stellar and gas-phase He abundances evolve as a function of time within simulated star-forming disc galaxies with different star formation histories. We make use of a cosmological chemodynamical simulation for galaxy formation and evolution, which includes star formation as well as energy and chemical enrichment feedback from asymptotic giant branch stars, core-collapse supernovae, and Type Ia supernovae. The predicted relations between the He mass fraction, Y, and the metallicity, Z, in the interstellar medium of our simulated disc galaxies depend on the galaxy star formation history. In particular, dY/dZ is not constant and evolves as a function of time, depending on the specific chemical element that we choose to trace Z; in particular, dY/dX O and dY/dX C increase as a function of time, whereas dY/dX N decreases. In the gas-phase, we find negative radial gradients of Y, due to the inside-out growth of our simulated galaxy discs as a function of time; this gives rise to longer chemical enrichment timescales in the outer galaxy regions, where we find lower average values for Y and Z. Finally, by means of chemical-evolution models, in the galactic bulge and inner disc, we predict steeper Y vs. age relations at high Z than in the outer galaxy regions. We conclude that for calibrating the assumed Y-Z relation in stellar models, C, N, and C+N are better proxies for the metallicity than O because they show steeper and less scattered relations.Peer reviewedFinal Published versio

The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage

We present the first chemical evolution model for Enceladus, alias the Gaia Sausage, to investigate the star formation history of one of the most massive satellites accreted by the Milky Way during a major merger event. Our best chemical evolution model for Enceladus nicely fits the observed stellar [$\alpha$/Fe]-[Fe/H] chemical abundance trends, and reproduces the observed stellar metallicity distribution function, by assuming low star formation efficiency, fast infall time scale, and mild outflow intensity. We predict a median age for Enceladus stars $12.33^{+0.92}_{-1.36}$ Gyr, and - at the time of the merger with our Galaxy ($\approx10$ Gyr ago from Helmi et al.) - we predict for Enceladus a total stellar mass $M_{\star} \approx 5 \times 10^{9}\,\text{M}_{\odot}$. By looking at the predictions of our best model, we discuss that merger events between the Galaxy and systems like Enceladus may have inhibited the gas accretion onto the Galaxy disc at high redshifts, heating up the gas in the halo. This scenario could explain the extended period of quenching in the star formation activity of our Galaxy about 10 Gyr ago, which is predicted by Milky Way chemical evolution models, in order to reproduce the observed bimodality in [$\alpha$/Fe]-[Fe/H] between thick- and thin-disc stars.Comment: Accepted for publication in MNRAS Letter

On the [α/Fe]–[Fe/H] relations in early-type galaxies

We study how the predicted [α/Fe]–[Fe/H] relations in early-type galaxies vary as functions of their stellar masses, ages, and stellar velocity dispersions, by making use of cosmological chemodynamical simulations with feedback from active galactic nuclei. Our model includes a detailed treatment for the chemical enrichment from dying stars, core-collapse supernovae (both Type II and hypernovae) and Type Ia supernovae. At redshift z = 0, we create a catalogue of 526 galaxies, among which we determine 80 early-type galaxies. From the analysis of our simulations, we find [α/Fe]–[Fe/H] relations similar to the Galactic bulge. We also find that, in the oldest galaxies, Type Ia supernovae start to contribute at higher [Fe/H] than in the youngest ones. On the average, early-type galaxies with larger stellar masses (and, equivalently, higher stellar velocity dispersions) have higher [α/Fe] ratios, at fixed [Fe/H]. This is qualitatively consistent with the recent observations of Sybilska et al., but quantitatively there are mismatches, which might require stronger feedback, sub-classes of Type Ia Supernovae, or a variable initial mass function to address

The IGIMF and other IMFs in dSphs: : the case of Sagittarius

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedWe have studied the effects of various initial mass functions (IMFs) on the chemical evolution of the Sagittarius dwarf galaxy (Sgr). In particular, we tested the effects of the integrated galactic initial mass function (IGIMF) on various predicted abundance patterns. The IGIMF depends on the star formation rate and metallicity and predicts less massive stars in a regime of low star formation, as it is the case in dwarf spheroidals. We adopted a detailed chemical evolution model following the evolution of α-elements, Fe and Eu, and assuming the currently best set of stellar yields. We also explored different yield prescriptions for the Eu, including production from neutron star mergers. Although the uncertainties still present in the stellar yields and data prevent us from drawing firm conclusions, our results suggest that the IGIMF applied to Sgr predicts lower [α/Fe] ratios than classical IMFs and lower [hydrostatic/explosive] α-element ratios, in qualitative agreement with observations. In our model, the observed high [Eu/O] ratios in Sgr is due to reduced O production, resulting from the IGIMF mass cut-off of the massive oxygen-producing stars, as well as to the Eu yield produced in neutron star mergers, a more promising site than core-collapse supernovae, although many uncertainties are still present in the Eu nucleosynthesis. We find that a model, similar to our previous calculations, based on the late addition of iron from the Type Ia supernova time-delay (necessary to reproduce the shape of [X/Fe] versus [Fe/H] relations) but also including the reduction of massive stars due to the IGIMF, better reproduces the observed abundance ratios in Sgr than models without the IGIMF.Peer reviewedFinal Published versio

The distribution of [α\alpha/Fe] in the Milky Way disc

Using a sample of red giant stars from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we infer the conditional distribution $p([\alpha/\text{Fe}]\,|\,[\text{Fe/H}])$ in the Milky Way disk for the $\alpha$-elements Mg, O, Si, S, and Ca. In each bin of [Fe/H] and Galactocentric radius $R$, we model $p([\alpha/\text{Fe}])$ as a sum of two Gaussians, representing "low-$\alpha$" and "high-$\alpha$" populations with scale heights $z_1=0.45\,\text{kpc}$ and $z_2=0.95\,\text{kpc}$, respectively. By accounting for age-dependent and $z$-dependent selection effects in APOGEE, we infer the [$\alpha$/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-$\alpha$ and high-$\alpha$ peaks clearly separated by a minimum at intermediate [$\alpha$/Fe]. In agreement with previous results, we find that the high-$\alpha$ 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 [$\alpha$/Fe] at fixed [Fe/H] for both the low-$\alpha$ and high-$\alpha$ populations, typically $\sim 0.04$-dex. The means, dispersions, and relative amplitudes of this two-Gaussian description, and the dependence of these parameters on $R$, [Fe/H], and $\alpha$-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.Comment: Accepted for publication in MNRA