Impacts of Radial Mixing on the Galactic Thick and Thin Disks
Daisuke Kawata
备注：
4 pages, 1 figure, to appear in the proceedings of the IAU Symposium 334 "Rediscovering our Galaxy", Potsdam, 10-14 July 2017, eds. C. Chiappini, I. Minchev, E. Starkenburg, M. Valentini
Using N-body simulations of the Galactic disks, we qualitatively study how the metallicity distributions of the thick and thin disk stars are modified by radial mixing induced by the bar and spiral arms. We show that radial mixing drives a positive vertical metallicity gradient in the mono-age disk population whose initial scale-height is constant and initial radial metallicity gradient is tight and negative. On the other hand, if the initial disk is flaring, with scale-height increasing with galactocentric radius, radial mixing leads to a negative vertical metallicity gradient, which is consistent with the current observed trend. We also discuss impacts of radial mixing on the metallicity distribution of the thick disk stars. By matching the metallicity distribution of N-body models to the SDSS/APOGEE data, we argue that the progenitor of the Milky Way's thick disk should not have a steep negative metallicity gradient.
In the Milky Way, the thick disk can be defined using individual stellar abundances, kinematics, or age; or geometrically, as stars high above the mid-plane. In nearby galaxies, where only a geometric definition can be used, thick disks appear to have large radial scale-lengths, and their red colors suggest that they are uniformly old. The Milky Way's geometrically thick disk is also radially extended, but it is far from chemically uniform: alpha-enhanced stars are confined within the inner Galaxy. In simulated galaxies, where old stars are centrally concentrated, geometrically thick disks are…
Based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan telescopes we present detailed elemental abundances for 20 red giant stars in the outer Galactic disk, located at Galactocentric distances between 9 and 13 kpc. The outer disk sample is complemented with samples of red giants from the inner Galactic disk and the solar neighbourhood, analysed using identical methods. For Galactocentric distances beyond 10 kpc, we only find chemical patterns associated with the local thin disk, even for stars far above the Galactic plane. Our results show that the relative densi…
Since thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6-D coordinates from Gaia-DR2 and age estimates from Sanders & Das (2018), we select $\sim 24000$ old stars (${τ> 10}$ Gyr, with uncertainties $\lesssim 15\%$) within 2 kpc from the Sun (full sample). A cross-match with APOGEE-DR16 ($\sim 1000$ stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the f…
We present a study of the extent to which the Sellwood & Binney radial migration of stars is affected by their vertical motion about the midplane. We use both controlled simulations in which only a single spiral mode is excited, as well as slightly more realistic cases with multiple spiral patterns and a bar. We find that rms angular momentum changes are reduced by vertical motion, but rather gradually, and the maximum changes are almost as large for thick disc stars as for those in a thin disc. We find that particles in simulations in which a bar forms suffer slightly larger angular momentum …
We present evidence that isolated growing discs, subject to internal spiral perturbations, thicken due to both heating \emph{and} radial migration. We show this by demonstrating that the thickness and vertical velocity dispersions of coeval stars depend on their age as well as the change in their radii. While the disc thickens due to internal processes, we find that this induces only a minor amount of flaring. We further demonstrate the consequences of such thickening on the structural properties of stellar populations and find that they qualitatively agree with recent studies of the Milky Way…
We study the evolution of Milky Way thick and thin discs in the light of the most recent observational data. In particular, we analyze abundance gradients of O, N, Fe and Mg along the thin disc as well as the [Mg/Fe] vs. [Fe/H] relations and the metallicity distribution functions at different Galactocentric distances. We run several models starting from the two-infall paradigm, assuming that the thick and thin discs formed by means of two different infall episodes, and we explore several physical parameters, such as radial gas flows, variable efficiency of star formation, different times for t…
We study the chemical evolution of the thick and thin discs of the Galaxy by comparing detailed chemical evolution models with recent data from the AMBRE Project. The data suggest that the stars in the thick and thin discs form two distinct sequences with the thick disc stars showing higher [α/Fe] ratios. We adopt two different approaches to model the evolution of thick and thin discs. In particular, we adopt: i) a two-infall approach where the thick disc forms fast and before the thin disc and by means of a fast gas accretion episode, whereas the thin disc forms by means of a second accretion…
In this letter we examine the evolution of the radial metallicity gradient induced by secular processes, in the disk of an $N$-body Milky Way-like galaxy. We assign a [Fe/H] value to each particle of the simulation according to an initial, cosmologically motivated, radial chemical distribution and let the disk dynamically evolve for 6 Gyr. This direct approach allows us to take into account only the effects of dynamical evolution and to gauge how and to what extent they affect the initial chemical conditions. The initial [Fe/H] distribution increases with R in the inner disk up to R ~ 10 kpc a…
We have developed a novel Markov Chain Mote Carlo (MCMC) chemical "painting" technique to explore possible radial and vertical metallicity gradients for the thick disc progenitor. In our analysis we match an N-body simulation to the data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We assume that the thick disc has a constant scale-height and has completed its formation at an early epoch, after which time radial mixing of its stars has taken place. Under these assumptions, we find that the initial radial metallicity gradient of the thick disc progenitor shou…
We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies, using N-body simulations with both a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and boxy/peanut (b/p) bulge. The bar in the thin disc component is $\sim$50\% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in …
We highlight some results from our detailed abundance analysis study of 703 kinematically selected F and G dwarf stars in the solar neighbourhood. The analysis is based on spectra of high-resolution (R=45000 to 110000) and high signal-to-noise (S/N=150 to 300). The main findings include: (1) at a given metallicity, the thick disk abundance trends are more $α$-enhanced than those of the thin disk; (2) the metal-rich limit of the thick disk reaches at least solar metallicities; (3) the metal-poor limit of the thin disk is around [Fe/H]=-0.8; (4) the thick disk shows an age-metallicity gradient; …
Revisiting the Radial Metallicity Gradient-Age Relation in the Milky Way's Thin and Thick Disks
Shanghai Jiao Tong University（上海交通大学）；University of Chinese Academy of Sciences（中国科学院大学）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Tianjin Normal University（天津师范大学）
We study the role of radial migration of stars on the chemical evolution of the Milky Way disk. In particular, we are interested in the impact of that process on the local properties of the disk (age-metallicity relation and its dispersion, metallicity distribution, evolution of abundance ratios) and on the morphological properties of the resulting thick and thin disks.We use a model with several new or up-dated ingredients: atomic and molecular gas phases, star formation depending on molecular gas, yields from the recent homogeneous grid provided by Nomoto et al. (2013), observationally infer…
We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from \textit{Gaia}/DR1, providing reliable age estimates with relative uncertainties of $\pm1-2$ Gyr and allowing precise orbital determinations. The sample is split based on chemistry into a low-[Mg/Fe] sequence, which are often identified as thin disk stellar populations, and a high-[Mg/Fe] sequence, which are often associated with the thick disk. We find that the high-[Mg/Fe] chemical sequence has extended star formation for several Gyr and is coeval with th…
The ongoing large spectroscopic surveys of the Milky Way such as SEGUE and RAVE have enabled us to take a fresh look at the structure of the Galactic thin and thick disks, and how their structure fits within the framework of structure formation via hierarchical clustering. In this article I will summarize some recent results mainly based in the RAVE survey with respect to the structure of the Galactic disks, their possible origin as well as indications of substructure and asymmetries.
ESO 243-49 is a high-mass (circular velocity $v_{\rm c}\approx200\,{\rm km\,s^{-1}}$) edge-on S0 galaxy in the Abell 2877 cluster at a distance of $\sim95\,{\rm Mpc}$. To elucidate the origin of its thick disc, we use MUSE science verification data to study its kinematics and stellar populations. The thick disc emits $\sim80\%$ of the light at heights in excess of $3.5^{\prime\prime}$ ($1.6\,{\rm kpc}$). The rotation velocities of its stars lag by $30-40\,{\rm km\,s^{-1}}$ compared to those in the thin disc, which is compatible with the asymmetric drift. The thick disc is found to be more meta…
We analyze the radial and vertical metallicity and [alpha/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemodynamical galaxy evolution code, GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity ([alpha/Fe]) gradient becomes more positive (negative) with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity…
As heating processes can broaden the distributions of radial actions and the vertical distributions of the Galactic disks, we investigate the vertical distribution of the Galactic disks as a function of radial action based on Apache Point Observatory Galactic Evolution Experiment(APOGEE) and Gaia data in order to deepen our understanding of the formation and heating history of the Galactic disks. We find that the distributions of radial action for both the thin and thick disks can be approximately described by pseudo-isothermal distributions, which give a statistical measurement for the temper…
We analyse disc heating and radial migration in N-body models of growing disc galaxies with thick and thin discs. Similar to thin-disc-only models, galaxies with appropriate non-axisymmetric structures reproduce observational constraints on radial disc heating in and migration to the Solar Neighbourhood (Snhd). The presence of thick discs can suppress non-axisymmetries and thus higher baryonic-to-dark matter fractions are required than in models that only have a thin disc. Models that are baryon-dominated to roughly the Solar radius R_0 are favoured, in agreement with data for the Milky Way. F…
This article is based on our discussion session on Milky Way models at the 592 WE-Heraeus Seminar, Reconstructing the Milky Way's History: Spectroscopic Surveys, Asteroseismology and Chemodynamical models. The discussion focused on the following question: "Are there distinct thick and thin disks?". The answer to this question depends on the definition one adopts for thin and thick disks. The participants of this discussion converged to the idea that there are at least two different types of disks in the Milky Way. However, there are still important open questions on how to best define these tw…
Disc non-axisymmetrc components, such as spirals and central bars, are nowadays known to play an important role in shaping galactic discs. Here we use Tree-SPH N-body simulations to examine the effect of these perturbers on two aspects: the occurrence of multiple patterns in discs and the effects of radial migration on disc thickening. We find that, in addition to a central bar, multiple spiral patterns and lopsided modes develop in all models. Interaction among these asymmetric features results in a large scale stellar migration. However, we show that, despite the strong radial mixing, discs …
Divergent Evolution of Radial Metallicity Gradients in the Thin and Thick Disks of the Milky Way
CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）；ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D)（澳大利亚天体物理学三维全天空卓越研究中心）；Department of Astronomy, Tsinghua University（清华大学天文系）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；School of Physics, University of New South Wales（新南威尔士大学物理学院）；South-Western Institute for Astronomy Research, Yunnan University（云南大学西南天文研究所）
We investigate the evolution of the abundance of fluorine in the Milky Way thick and thin discs by means of detailed chemical evolution models compared with recent observational data. The chemical evolution models adopted here have already been shown to fit the observed abundance patterns of CNO and $α$-elements as well as the metallicity distribution functions for the Galactic thick and thin disc stars. We apply them here to the study of the origin and evolution of fluorine, which is still a matter of debate. First, we study the importance of the various sites proposed for the production of f…
We investigate thin and thick stellar disc formation in Milky-Way-mass galaxies using twelve FIRE-2 cosmological zoom-in simulations. All simulated galaxies experience an early period of bursty star formation that transitions to a late-time steady phase of near-constant star formation. Stars formed during the late-time steady phase have more circular orbits and thin-disc-like morphology at $z=0$, whilst stars born during the bursty phase have more radial orbits and thick-disc structure. The median age of thick-disc stars at $z=0$ correlates strongly with this transition time. We also find that…
We present a list of 72 radial velocity member stars in the metal-rich globular cluster NGC5927. The radial velocities are based on multi-epoch, multi-fibre spectra. We identify 46 RGB/HB stars and 26 turn-off stars that are radial velocity members in the cluster. This cluster is situated quite close to the disk and hence fore- and/or background contamination, especially in the outskirts of the cluster, can be quite severe. Fortunately, the cluster has a radial velocity (we determine it to v_rad=--104.03 +/- 5.03 km/s) that sets it clearly apart from the bulk velocities of the surrounding, bac…
We perform collisionless N-body simulations to investigate the evolution of the structural and kinematical properties of simulated thick disks induced by the growth of an embedded thin disk. The thick disks used in the present study originate from cosmologically-common 5:1 encounters between initially-thin primary disk galaxies and infalling satellites. The growing thin disks are modeled as static gravitational potentials and we explore a variety of growing-disk parameters that are likely to influence the response of thick disks. We find that the final thick-disk properties depend strongly on …
We obtain a magnitude-limited sample of Andromeda (M 31) disc PNe with chemical abundance estimated through the direct detection of the [OIII] 4363 $\mathring{\mathrm A}$ line. This leads to $205$ and $200$ PNe with oxygen and argon abundances respectively. We find that high- and low-extinction M 31 disc PNe have statistically distinct argon and oxygen abundance distributions. In the radial range $2-30$ kpc, the older low-extinction disc PNe are metal-poorer on average with a slightly positive radial oxygen abundance gradient ($0.006 \pm 0.003$ dex/kpc) and slightly negative for argon ($-0.005…
Non-axisymmetric components, such as spirals and central bars, play a major role in shaping galactic discs. An important aspect of the disc secular evolution driven by these perturbers is the radial migration of stars. It has been suggested recently that migration can populate a thick-disc component from inner-disc stars with high vertical energies. Since this has never been demonstrated in simulations, we study in detail the effect of radial migration on the disc velocity dispersion and disc thickness, by separating simulated stars into migrators and non-migrators. We apply this method to thr…
We present a uniform study of the chemical abundances of 12 elements (Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg, and Al) derived from the spectra of 451 stars observed as part of one of the HARPS GTO planet search programs. Sixty eight of these are planet-bearing stars. The main goals of our work are: i) the investigation of possible differences between the abundances of stars with and without planets; ii) the study of the possible differences in the abundances of stars in the thin and the thick disc. We confirm that there is a systematically higher metallicity in planet host stars, when compa…
