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Register a new userChronologically dating the early assembly of the Milky Way
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Added by
Josefina Montalb\\'an
Publication date
2020
fields
Physics
and research's language is
English
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The standard cosmological model ($Lambda$-CDM) predicts that galaxies are built through hierarchical assembly on cosmological timescales$^{1,2}$. The Milky Way, like other disc galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Thanks to Gaia-DR2$^3$ and spectroscopic surveys$^4$, the stellar remnants of such mergers have been identified$^{5-7}$. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging owing to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology -- the study of stellar oscillations -- with kinematics and chemical abundances, to estimate precise stellar ages ($sim$ 11%) for a sample of stars observed by the $mathit{Kepler}$ space mission$^8$. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy, but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disc) before the in-fall of the satellite galaxy Gaia-Enceladus/Sausage$^{5,6}$ around 10 billions years ago
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Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. While these accreted stellar populations can be forensically identified as kinematically distinct structures within the Galaxy, it is difficult in general to precisely date the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called textit{Gaia}-Enceladus, leading to a substantial pollution of the chemical and dynamical properties of the Milky Way. Here, we identify the very bright, naked-eye star $ u$,Indi as a probe of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric, and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be $11.0 pm 0.7$ (stat) $pm 0.8$ (sys)$,rm Gyr$. The star bears hallmarks consistent with it having been kinematically heated by the textit{Gaia}-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95% confidence, respectively. Input from computations based on hierarchical cosmological models tightens (i.e. reduces) slightly the above limits.
How the Milky Way has accumulated its mass over the Hubble time, whether significant amounts of gas and stars were accreted from satellite galaxies, or whether the Milky Way has experienced an initial gas assembly and then evolved more-or-less in isolation is one of the burning questions in modern astronomy, because it has consequences for our understanding of galaxy formation in the cosmological context. Here we present the evolutionary model of a Milky Way-type satellite system zoomed into a cosmological large-scale simulation. Embedded into Dark Matter halos and allowing for baryonic processes these chemo-dynamical simulations aim at studying the gas and stellar loss from the satellites to feed the Milky Way halo and the stellar chemical abundances in the halo and the satellite galaxies.
The Gaia Sausage is the major accretion event that built the stellar halo of the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage. The Sequoia Event provided the bulk of the high energy retrograde stars in the stellar halo, as well as the recently discovered globular cluster FSR 1758. There are up to 6 further globular clusters, including $omega$~Centauri, as well as many of the retrograde substructures in Myeong et al. (2018), associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is $sim 5 times 10^{7} M_odot$, whilst the total mass is $sim 10^{10} M_odot$, as judged from abundance matching or from the total sum of the globular cluster mass. Although clearly less massive than the Sausage, the Sequoia has a distinct chemo-dynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of $sim0.6$, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by $sim 0.3$ dex and higher abundance ratios as compared to the Sausage. We conjecture that the Sausage and the Sequoia galaxies may have been associated and accreted at a comparable epoch.
Star formation takes place in the dense gas phase, and therefore a simple dense gas and star formation rate relation has been proposed. With the advent of multi-beam receivers, new observations show that the deviation from linear relations is possible. In addition, different dense gas tracers might also change significantly the measurement of dense gas mass and subsequently the relation between star formation rate and dense gas mass. We report the preliminary results the DEnse GAs in MAssive star-forming regions in the Milky Way (DEGAMA) survey that observed the dense gas toward a suit of well-characterized massive star forming regions in the Milky Way. Using the resulting maps of HCO$^{+}$ 1--0, HCN 1--0, CS 2--1, we discuss the current understanding of the dense gas phase where star formation takes place.
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 clearly bimodal at sub-solar [Fe/H], with the low-$alpha$ and high-$alpha$ peaks separated by a valley that is $sim 3$ times lower. 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 disks gas accretion, star formation, or outflow history in addition to radial mixing of stellar populations.
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