By means of N-body/SPH simulations we investigate the morphological properties of thick disks formed through minor mergers. We show that the vertical surface density profile of the post-merger thick disk follows a sech function and has an excess in the regions far from the disk mid-plane (z>2kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component, and it is usually dominated by stars from the primary galaxy. Stars in the excess have a rotational velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo. The thick disk scale height increases with radius and the rate of its increase is smaller for more gas rich primary galaxies. On the contrary, the scale height of the stellar excess is independent of both radius and gas fraction. We also find that the post-merger thick disk has a radial scale length which is 10-50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks. These disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Our simulation results are consistent with observations of the ratio of thick to thin disk scale lengths of the Milky Way and nearby galaxies, and with the Toomre diagram of the Milky Way. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the minor merger history of galaxies.
We analyse the phase-space structure of simulated thick discs that are the result of a significant merger between a disc galaxy and a satellite. Our main goal is to establish what would be the characteristic imprints of a merger origin for the Galactic thick disc. We find that the spatial distribution predicted for thick disc stars is asymmetric, seemingly in agreement with recent observations of the Milky Way thick disc. Near the Sun, the accreted stars are expected to rotate more slowly, to have broad velocity distributions, and to occupy preferentially the wings of the line-of-sight velocit…
We adopt a scenario in which the Galactic thick disk was formed by minor merging between the first generation of the Galactic thin disk (FGTD) and a dwarf galaxy about 9 Gyr ago and thereby investigate chemical and dynamical properties of the Galactic thick disk. In this scenario, the dynamical properties of the thick disk have long been influenced both by the mass growth of the second generation of the Galactic thin disk (i.e., the present thin disk) and by its non-axisymmetric structures. On the other hand, the early star formation history and chemical evolution of the thin disk was influenc…
We present the results of a survey of young intermediate mass stars (age $<$~5 Myr, 1.5 $<M_{\star} \leq $ 15 $M_{\odot}$) in the W5 massive star forming region. We use combined optical, near-infrared and {\it Spitzer} Space Telescope photometry and optical spectroscopy to define a sample of stars of spectral type A and B and examine their infrared excess properties. We find objects with infrared excesses characteristic of optically thick disks, i.e. Herbig AeBe stars. These stars are rare: $<$1.5% of the entire spectroscopic sample of A and B stars, and absent among stars more massive than 2.…
The formation of thick stellar disks in spiral galaxies is studied. Simulations of gas-rich young galaxies show formation of internal clumps by gravitational instabilities, clump coalescence into a bulge, and disk thickening by strong stellar scattering. The bulge and thick disks of modern galaxies may form this way. Simulations of minor mergers make thick disks too, but there is an important difference. Thick disks made by internal processes have a constant scale height with galactocentric radius, but thick disks made by mergers flare. The difference arises because in the first case, perpendi…
We study the orbital properties of stars in four (published) simulations of thick disks formed by: i) accretion from disrupted satellites, ii) heating of a pre-existing thin disk by a minor merger, iii) radial migration and iv) gas rich mergers. We find that the distribution of orbital eccentricities are predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas-rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back t…
As part of the "Dust, Ice, and Gas In Time (DIGIT)" Herschel Open Time Key Program, we present Herschel photometry (at 70, 160, 250, 350 and 500 micron) of 31 Weak-Line T Tauri star (WTTS) candidates in order to investigate the evolutionary status of their circumstellar disks. Thirteen stars in our sample had circumstellar disks previously known from infrared observations at shorter wavelengths, while eighteen of them had no previous evidence for a disk. We detect a total of 15 disks as all previously known disks are detected at one or more Herschel wavelengths and two additional disks are ide…
We analysed collisionless N-body simulations of intermediate and minor dry mergers onto S0s to test whether these mergers can generate S0 galaxies with kinematics intermediate between fast and slow rotators. We find that minor mergers induce a lower decrease of the global rotational support than encounters of lower mass ratios, which results in S0s with properties intermediate between fast and slow rotators. The resulting remnants are intrinsically more triaxial, less flattened, and span the whole range of apparent ellipticities up to $ε_\mathrm{e} \sim 0.8$. They do not show lower apparent el…
Context. Recent observations of the Bulge, e.g., its X-shape, cylindrical stellar motions, and a potential fraction of young stars propose that it formed through secular evolution of the disk and not through gas dissipation and/or mergers, as thought previously. Aims. We measure abundances of six iron-peak elements (Sc, V, Cr, Mn, Co and Ni) in the local thin and thick disks as well as the Bulge to provide additional observational constraints for Galaxy formation and chemical evolution models. Methods. We use high-resolution optical spectra of 291 K giants in the local disk mostly obtained by …
The numerous streams in the M31 halo are currently assumed to be due to multiple minor mergers. Here we use the GADGET2 simulation code to test whether M31 could have experienced a major merger in its past history. It results that a 3+/-0.5:1 gaseous rich merger with r(per)=25+/-5 kpc and a polar orbit can explain many properties of M31 and of its halo. The interaction and the fusion may have begun 8.75+/-0.35 Gyr and 5.5 +/-0.5 Gyr ago, respectively. With an almost quiescent star formation history before the fusion we retrieve fractions of bulge, thin and thick disks as well as relative fract…
It has been proposed that mergers induce starbursts and lead to important morphological changes in galaxies. Most studies so far have focused on large galaxies, but dwarfs might also experience such events, since the halo mass function is scale-free in the concordance cosmological model. Notably, because of their low mass, most of their interactions will be with dark satellites. In this paper we follow the evolution of gas-rich disky dwarf galaxies as they experience a minor merger with a dark satellite. We aim to characterize the effects of such an interaction on the dwarf's star formation, m…
By means of N-body simulations we study the response of a galactic disc to a minor merger event. We find that non-self-gravitating, spiral-like features are induced in the thick disc. As we have shown in a previous work, this "ringing" also leaves an imprint in velocity space (the u-v plane) in small spatial regions, such as the solar neighbourhood. As the disc relaxes after the event, clumps in the u-v plane get closer with time, allowing us to estimate the time of impact. In addition to confirming the possibility of this diagnostic, here we show that in a more realistic scenario, the in-fall…
One of the suggested thick disc formation mechanisms is that they were born quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs formed slowly within them from leftovers of the turbulent phase and from material accreted through cold flows and minor mergers. In this letter, I propose an observational test to verify this hypothesis. By combining thick disc and total stellar masses of edge-on galaxies with galaxy stellar mass functions calculated in the redshift range of $z\leq3.0$, I derived a positive correlation between the age of the youngest stars in thick discs and the…
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…
In the context of the current $Λ$CDM cosmological model small dark matter haloes are abundant and satellites of dwarf galaxies are expected to be predominantly dark. Since low mass galaxies have smaller baryon fractions interactions with these satellites may leave particularly dramatic imprints. We uncover the influence of the most massive of these dark satellites on disky dwarf galaxies and the possible dynamical and morphological transformations that result from these interactions. We use a suite of carefully set-up, controlled simulations of isolated dwarf galaxies. The primary dwarf galaxi…
We analyze the 3D kinematics of a sample of $\sim 4400$ red clump stars ranging between 5 and 10 kpc from the Galactic center and up to 3 kpc from the Galactic plane. This sample is representative for the metal-rich ([Fe/H] = -0.6 to 0.5) thick disk. Absolute proper motions are from the fourth release of the Southern Proper Motion Program, and radial velocities from the second release of the Radial Velocity Experiment. The derived kinematical properties of the thick disk include: the rotational velocity gradient $\partial V_θ / \partial z = -25.2 \pm 2.1$ km s$^{-1}$ kpc$^{-1}$, velocity dispe…
Manganese is mainly produced in type II SNe during explosive silicon burning, in incomplete Si-burning regions, and depends on several nucleosynthesis environment conditions, such as mass cut beween the matter ejected and falling back onto the remnant, electron and neutron excesses, mixing fallback, and explosion energy. Manganese is also produced in type Ia SNe. The aim of this work is the study of abundances of the iron-peak element Mn in 56 bulge giants, among which 13 are red clump stars. Four bulge fields along the minor axis are inspected. The study of abundances of Mn-over-Fe as a funct…
We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F$_{\rm …
We study the kinematics of the Galactic thin and thick disk populations using stars from the RAVE survey's second data release together with distance estimates from Breddels et al. (2009). The velocity distribution exhibits the expected moving groups present in the solar neighborhood. We separate thick and thin disk stars by applying the X (stellar-population) criterion of Schuster et al. (1993), which takes into account both kinematic and metallicity information. For 1906 thin disk and 110 thick disk stars classified in this way, we find a vertical velocity dispersion, mean rotational velocit…
We have undertaken the study of the elemental abundances and kinematic properties of a metal-poor sample of candidate thick-disk stars selected from the RAVE spectroscopic survey of bright stars to differentiate among the present scenarios of the formation of the thick disk. In this paper, we report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 main-sequence/sub-giant branch metal-poor stars, which serves to augment our previous sample of only giant stars. We find that the thick disk [alpha/Fe] ratios are enhanced, and have little variation (<0.1 dex), in agreemen…
We used deep MUSE observations to perform a stellar-kinematic and population analysis of FCC 153 and FCC 177, two edge-on S0 galaxies in the Fornax cluster. The geometrical definition of the different structural components of these two galaxies allows us to describe the nature of their thick disks. These are both old, relatively metal poor and [Mg/Fe]-enhanced, and their star formation history (SFH) reveals a minor younger component whose chemical properties suggest its later accretion. Moreover, the outer regions of these geometrically defined thick disks show higher values of metallicity and…
We examine the α-element abundance ratio, [α/Fe], of 5620 stars, observed by the Sloan Extension for Galactic Understanding and Exploration survey in the region 6 kpc < R < 16 kpc, 0.15 kpc < |Z| < 1.5 kpc, as a function of Galactocentric radius R and distance from the Galactic plane |Z|. Our results show that the high-α thick disk population has a short scale length (L_thick ~ 1.8 kpc) compared to the low-αpopulation, which is typically associated with the thin disk. We find that the fraction of high-α stars in the inner disk increases at large |Z|, and that high-α stars lag in rotation compa…
Kinematic scaling of thin and thick discs from SAMI to NewHorizon
Yonsei University（延世大学）；CSIRO Space & Astronomy（澳大利亚联邦科学与工业研究组织空间与天文部）；Australian National University（澳大利亚国立大学）；The University of Sydney（悉尼大学）；Institut d’Astrophysique de Paris, CNRS and Sorbonne Université（巴黎天体物理研究所，法国国家科学研究中心和索邦大学）；Kyung Hee University（庆熙大学）
该研究利用 SAMI 星系和 NewHorizon 模拟数据，通过施瓦兹席尔德轨道叠加等方法，重新审视盘状恒星质量与速度弥散关系并扩展到薄厚盘，揭示了相关结果及系统误差，为未来研究盘状子结构提供方向。
The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar
Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦莱布尼茨天体物理研究所）；Universität Potsdam（波茨坦大学）；Universidad Nacional Autónoma de México（墨西哥国立自治大学）；Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文研究所）；Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna（拉古纳大学）；Max Planck Institute for Astronomy（马克斯·普朗克天体物理研究所）；Universitat de Barcelona（巴塞罗那大学）；Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona（巴塞罗那大学宇宙科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）
Since chemical abundances are inherited between generations of stars, we use them to trace the evolutionary history of our Galaxy. We present a robust methodology for creating a phylogenetic tree, a biological tool used for centuries to study heritability. Combining our phylogeny with information on stellar ages and dynamical properties, we reconstruct the shared history of 78 stars in the Solar Neighbourhood. The branching pattern in our tree supports a scenario in which the thick disk is an ancestral population of the thin disk. The transition from thick to thin disk shows an anomaly, which …
Through Thick and Thin: The Cosmic Evolution of Disk Scale Height
Xiamen University（厦门大学）；Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo（东京大学宇宙物理与数学研究所（WPI））；The Kavli Institute for Astronomy and Astrophysics, Peking University（北京大学柯利天文与天体物理研究所）；Department of Astronomy, Peking University（北京大学天文学系）；Department of Astronomy, School of Science, The University of Tokyo（东京大学理学部天文学系）；Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo（东京大学UTIAS数据驱动发现中心）；Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University（约翰霍普金斯大学物理与天文学系天体科学中心）
We address the study of the \Ha\ vertical velocity field in a sample of four nearly face-on galaxies using long slit spectroscopy taken with the ISIS spectrograph attached to the WHT at the Roque de los Muchachos Observatory (Spain). The spatial structure of the velocity vertical component shows a radial corrugated pattern with spatial scales higher or within the order of { one} kiloparsec. The gas is mainly ionized by high-energy photons: only in some locations of NGC~278 and NGC~1058 is there some evidence of ionization by low-velocity shocks, which, in the case of NGC~278, could be due to m…
We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in the Galactic disk. Based on the $α$-abundance ratio, [$α$/Fe], we separated our sample into low-$α$ thin-disk and high-$α$ thick-disk stars. We find a $V_{\rm ϕ}$ gradient of $-$28.2 km s$^{-1}$ dex$^{-1}$ over [Fe/H] for the thin disk, and an almost flat trend of the velocity dispersions of $V_{\rm R}$, $V_{\rm ϕ}$, and $V_{\rm Z}$ components with [Fe/H]. The metal-poor (MP; [Fe/H] $<$ $-$0.3) thin-disk stars with low-$V_{\rm ϕ}$ velocities have high eccentricities ($e$) and small perigalacticon distances ($r_{\rm p…
We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other three fields along the major axis. Our results confirm, with large number statistics, the chemical similarity between the Galactic bu…
Dissecting the 3D chemo-dynamical structures of NGC~1381: A galaxy hosting an ancient slow bar with an accretion-related bulge and thick disc
Westlake University（西湖大学）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；Astrophysics Research Institute, Liverpool John Moores University（利物浦约翰摩尔斯大学天体物理研究所）；Instituto de Astrofísica de Canarias（加那利天体物理学研究所）；Departamento de Astrofísica, Universidad de La Laguna（拉帕尔马大学天体物理学系）；Department of Astrophysics, University of Vienna（维也纳大学天体物理学系）
