Disk origin of the Milky Way bulge: the necessity of the thick disk

In the Milky Way bulge, metal-rich stars form a strong bar and are more peanut-shaped than metal-poor stars. It has recently been claimed that this behavior is driven by the initial (i.e., before bar formation) in-plane radial velocity dispersion of these populations, rather than by their initial vertical random motions. This has led to the suggestion that a thick disk is not necessary to explain the characteristics of the Milky Way bulge. We discuss this issue again by analyzing two dissipationless N-body simulations of boxy or peanut-shaped bulges formed from composite stellar disks that consist of kinematically cold and hot stellar populations. These two models represent two extreme cases: one where all three components of the disk have a fixed vertical velocity dispersion and different in-plane radial dispersion, and another where they all have a fixed radial dispersion and different vertical random motions (thickness). This is intended to quantify the drivers of the main features that are observed in composite boxy or peanut-shaped bulges and their origin. We quantify the mapping into a boxy or peanut-shaped bulge of disk populations in these two cases, and we conclude that initial vertical random motions are as important as in-plane random motions in determining the relative contribution of cold- and hot-disk populations with height above the plane, the metallicity and age trends. Previous statements emphasizing the dominant role of in-plane motions in determining these trends are not confirmed. However, significant differences exist in the morphology and strength of the resulting boxy or peanut-shaped bulges. In particular, the model where disk populations initially have only different in-plane random motions, but similar thickness, results in a boxy or peanut-shaped bulge where all populations have a similar peanut shape, independent of their initial kinematics or metallicity. This is at odds with the trends observed in the Milky Way bulge. We discuss the reasons behind these differences, and also predict the signatures that these two extreme initial conditions would leave on the vertical age and metallicity gradients of disk stars outside the bulge region. As a consequence of this analysis, we conclude that given our current knowledge of the Milky Way bulge and of the properties of its main stellar components, a metal-poor, kinematically (radial and vertical) hot component, that is, a thick disk, is necessary in the Milky Way before bar formation. This supports the scenario that has been traced in previous works. Boxy or peanut-shaped bulges and their surrounding regions are fossil records of the conditions present at early times in disk galaxies, and by dissecting their stellar components by chemical compositions and/or age, it may be possible to reconstruct their early state.

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Streams, Substructures, and the Early History of the Milky Way

Annual Review of Astronomy and Astrophysics ◽

10.1146/annurev-astro-032620-021917 ◽

2020 ◽

Vol 58 (1) ◽

pp. 205-256 ◽

Cited By ~ 10

Author(s):

Amina Helmi

Keyword(s):

Star Formation ◽

Phase Space ◽

Milky Way ◽

Current Knowledge ◽

Thick Disk ◽

Age Dating ◽

Wide Field ◽

Chemical Labeling ◽

The Galaxy ◽

Hydrodynamical Simulations

The advent of the second data release of the Gaia mission, in combination with data from large spectroscopic surveys, is revolutionizing our understanding of the Galaxy. Thanks to these transformational data sets and the knowledge accumulated thus far, a new, more mature picture of the evolution of the early Milky Way is currently emerging. ▪ Two of the traditional Galactic components, namely, the stellar halo and the thick disk, appear to be intimately linked: Stars with halo-like kinematics originate in similar proportions from a heated (thick) disk and from debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and was completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ∼ 1.8. This is likely related to the previously known metal-weak thick disk and may be traceable to metallicities [Fe/H] [Formula: see text] −4. As importantly, there is evidence that the merger with Gaia-Enceladus triggered star formation in the early Milky Way, plausibly leading to the appearance of the thick disk as we know it. ▪ Other merger events have been characterized better, and new ones have been uncovered. These include, for example, the Helmi streams, Sequoia, and Thamnos, which add to the list of those discovered in wide-field photometric surveys, such as the Sagittarius streams. Current knowledge of their progenitors’ properties, star formation, and chemical evolutionary histories is still incomplete. ▪ Debris from different objects shows different degrees of overlap in phase-space. This sometimes confusing situation can be improved by determining membership probabilities via quantitative statistical methods. A task for the next few years will be to use ongoing and planned spectroscopic surveys for chemical labeling and to disentangle events from one another using dimensions other than phase-space, metallicity, or [α/Fe]. ▪ These large surveys will also provide line-of-sight velocities missing for faint stars in Gaia releases and more accurate distance determinations for distant objects, which in combination with other surveys could also lead to more accurate age dating. The resulting samples of stars will cover a much wider volume of the Galaxy, allowing, for example, the linking of kinematic substructures found in the inner halo to spatial overdensities in the outer halo. ▪ All the results obtained so far are in line with the expectations of current cosmological models. Nonetheless, tailored hydrodynamical simulations to reproduce in detail the properties of the merger debris, as well as constrained cosmological simulations of the Milky Way, are needed. Such simulations will undoubtedly unravel more connections between the different Galactic components and their substructures, and will aid in pushing our knowledge of the assembly of the Milky Way to the earliest times.

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Chemo-Kinematic Properties of the Galactic Disk with SEGUE G and K Dwarfs: Constraints on Formation

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317007463 ◽

2017 ◽

Vol 13 (S334) ◽

pp. 306-307

Author(s):

Doori Han ◽

Young Sun Lee ◽

Young Kwang Kim ◽

Timothy C. Beers

Keyword(s):

Milky Way ◽

Velocity Dispersion ◽

Galactic Disk ◽

Thin Disk ◽

Thick Disk ◽

Formation Mechanisms ◽

Orbital Eccentricity ◽

Disk Formation ◽

Velocity Gradients ◽

Kinematic Properties

AbstractWe present the derived kinematic characteristics of low-α thin-disk and high-α thick-disk stars in the Milky Way, investigated with a sample of about 32,000 G- and K-type dwarfs from the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Based on the level of α-element enhancement as a function of [Fe/H], we separate our sample into thin- and thick-disk stars and then derive mean velocities, velocity dispersions, and velocity gradients for the U, V, and W velocity components, respectively, as well as the orbital eccentricity distribution. There are notable gradients in the V velocity over [Fe/H] in both populations: −23 km s−1 dex−1 for the thin disk and +44 km s−1 dex−1 for the thick disk. The velocity dispersion of the thick disk decreases with increasing [Fe/H], while the velocity dispersion gradient over [Fe/H] for the thin disk is almost flat for all velocity components, except for the W velocity dispersion of the metal-poor thin-disk stars. The eccentricity distribution exhibits a peak at a higher value, and is more symmetric as [α/Fe] increases, implying that complex formation mechanisms may be involved. Our results can be used to constrain several proposed disk-formation scenarios of the Milky Way and other large spirals.

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On the origin of the asymmetric dwarf galaxy distribution around andromeda

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3477 ◽

2019 ◽

Vol 492 (1) ◽

pp. 456-467

Author(s):

Zhen Wan ◽

William H Oliver ◽

Geraint F Lewis ◽

Justin I Read ◽

Michelle L M Collins

Keyword(s):

Milky Way ◽

Dwarf Galaxies ◽

Initial Conditions ◽

Dwarf Galaxy ◽

High Energy ◽

Initial Position ◽

Asymmetric Structures ◽

Galaxy Distribution ◽

Very High Energy ◽

Radial Dispersion

ABSTRACT The dwarf galaxy distribution surrounding M31 is significantly anisotropic in nature. Of the 30 dwarf galaxies in this distribution, 15 form a disc-like structure and 23 are contained within the hemisphere facing the Milky Way. Using a realistic local potential, we analyse the conditions required to produce and maintain these asymmetries. We find that some dwarf galaxies are required to have highly eccentric orbits in order to preserve the presence of the hemispherical asymmetry with an appropriately large radial dispersion. Under the assumption that the dwarf galaxies originate from a single association or accretion event, we find that the initial size and specific energy of that association must both be relatively large in order to produce the observed hemispherical asymmetry. However if the association was large in physical size, the very high-energy required would enable several dwarf galaxies to escape from the M31 and be captured by the Milky Way. Furthermore, we find that associations that result in this structure have total specific energies concentrated around $E = V_{\rm esc}^{2} - V_{\rm init}^{2} \sim 200^2$ – $300^2\ \rm {km^2\ s^{-2}}$, implying that the initial velocity and initial position needed to produce the structure are strongly correlated. The overlap of initial conditions required to produce the radial dispersion, angular dispersion, and the planar structure is small and suggests that either they did not originate from a single accretion event, or that these asymmetric structures are short-lived.

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Probing non-spherical dark halos in the Galactic dwarf satellites

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313006789 ◽

2013 ◽

Vol 9 (S298) ◽

pp. 411-411

Author(s):

Kohei Hayashi ◽

Masashi Chiba

Keyword(s):

Dark Matter ◽

Cold Dark Matter ◽

Milky Way ◽

Velocity Dispersion ◽

Line Of Sight ◽

Spherical Structure ◽

Spherical Models ◽

Galactic Satellites ◽

Best Fitting

AbstractWe construct axisymmetric mass models for dwarf spheroidal (dSph) galaxies in the Milky Way to obtain realistic limits on the non-spherical structure of their dark halos. This is motivated by the fact that the observed luminous parts of the dSphs are actually non-spherical and cold dark matter models predict non-spherical virialized dark halos on sub-galactic scales. Applying these models to line-of-sight velocity dispersion profiles along three position angles in six Galactic satellites, we find that the best fitting cases for most of the dSphs yield not spherical but oblate and flattened dark halos. We also find that the mass of the dSphs enclosed within inner 300 pc varies depending on their total luminosities, contrary to the conclusion of previous spherical models. This suggests the importance of considering non-spherical shapes of dark halos in dSph mass models.

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STELLAR VELOCITY DISPERSION AND ANISOTROPY OF THE MILKY WAY INNER HALO

The Astrophysical Journal ◽

10.1088/0004-637x/813/2/89 ◽

2015 ◽

Vol 813 (2) ◽

pp. 89 ◽

Cited By ~ 16

Author(s):

Charles King III ◽

Warren R. Brown ◽

Margaret J. Geller ◽

Scott J. Kenyon

Keyword(s):

Milky Way ◽

Velocity Dispersion ◽

Stellar Velocity

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The AMBRE project: The thick thin disk and thin thick disk of the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731494 ◽

2017 ◽

Vol 608 ◽

pp. L1 ◽

Cited By ~ 23

Author(s):

M. R. Hayden ◽

A. Recio-Blanco ◽

P. de Laverny ◽

S. Mikolaitis ◽

C. C. Worley

Keyword(s):

Vertical Velocity ◽

Velocity Dispersion ◽

Stellar Populations ◽

Thin Disk ◽

Thick Disk ◽

Solar Neighborhood ◽

Chemical Enrichment ◽

History Of ◽

External Galaxies ◽

Subgiant Stars

We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from Gaia DR1, providing reliable age estimates with relative uncertainties of ±1 or 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 high-[Mg/Fe] sequence, which are often associated with thick disk stellar populations. We find that the high-[Mg/Fe] chemical sequence has extended star formation for several Gyr and is coeval with the oldest stars of the low-[Mg/Fe] chemical sequence: both the low- and high-[Mg/Fe] sequences were forming stars at the same time. We find that the high-[Mg/Fe] stellar populations are only vertically extended for the oldest, most-metal poor and highest [Mg/Fe] stars. When comparing vertical velocity dispersion for the low- and high-[Mg/Fe] sequences, the high-[Mg/Fe] sequence has lower vertical velocity dispersion than the low-[Mg/Fe] sequence for stars of similar age. This means that identifying either group as thin or thick disk based on chemistry is misleading. The stars belonging to the high-[Mg/Fe] sequence have perigalacticons that originate in the inner disk, while the perigalacticons of stars on the low-[Mg/Fe] sequence are generally around the solar neighborhood. From the orbital properties of the stars, the high-[Mg/Fe] and low-[Mg/Fe] sequences are most likely a reflection of the chemical enrichment history of the inner and outer disk populations, respectively; radial mixing causes both populations to be observed in situ at the solar position. Based on these results, we emphasize that it is important to be clear in defining what populations are being referenced when using the terms thin and thick disk, and that ideally the term thick disk should be reserved for purely geometric definitions to avoid confusion and be consistent with definitions in external galaxies.

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Search for streams in thick disk and halo of the Milky Way

Journal of Physics Conference Series ◽

10.1088/1742-6596/771/1/012032 ◽

2016 ◽

Vol 771 ◽

pp. 012032

Author(s):

Dian Puspita Triani ◽

M Ikbal Arifyanto

Keyword(s):

Milky Way ◽

Thick Disk

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Chemical characterization and synergetic source apportionment of PM<sub>2.5</sub> at multiple sites in the Beijing–Tianjin–Hebei region, China

10.5194/acp-2017-446 ◽

2017 ◽

Cited By ~ 2

Author(s):

Xiaojuan Huang ◽

Zirui Liu ◽

Jingyun Liu ◽

Bo Hu ◽

Tianxue Wen ◽

...

Keyword(s):

Motor Vehicle ◽

Dominant Role ◽

Chemical Compositions ◽

Backward Trajectory ◽

Entire Study ◽

Background Site ◽

Haze Pollution ◽

Secondary Aerosols ◽

Haze Formation ◽

Urban Sites

Abstract. High frequencies of haze in China, especially in the Beijing–Tianjin–Hebei region, have received widespread attention in recent years. In this study, samples of filtered atmospheric fine particulate matter (PM2.5) were collected synchronously at three urban sites (Beijing, Tianjin, and Shijiazhuang) and at a regional background site (Xinglong) for one month during each season from June 2014 to April 2015. Chemical composition determination/analysis, chemical mass closure, positive matrix factorization (PMF) and backward trajectory clustering were employed to investigate the chemical speciation, haze formation mechanism, emission sources, and influences of regional transport in North China. Our results reported that the aerosol chemical compositions were very similar at the urban sites and the background site and mainly comprised organic matter (16.0 %–25.0 %), sulfate (14.4 %–20.5 %), nitrate (15.1 %–19.5 %), ammonium (11.6 %–13.1 %) and mineral dust (14.7 %–20.8 %). Sources apportionment of PM2.5 by PMF model revealed that secondary aerosols (background) and secondary inorganic aerosols (urban) were the dominant sources, which accounted for 29.2 %–45.1 % of PM2.5 throughout the entire study and played a vital role in the formation and development of haze pollution. Emissions of motor vehicle exhaust exerted a significant impact on haze formation at urban sites, particularly at Beijing; and coal combustion also played an dominant role in winter, especially at Shijiazhuang. Backward trajectory analysis revealed that haze pollution has remarkable regional characteristics and usually occurs when air masses originated from polluted industrial regions of the south prevailed, which accompanied by high PM2.5 loadings with high contributions of secondary aerosols. This study suggests that the control strategies to mitigate the haze formation in BTH region should be focused on the emission reduction of gaseous precursors from fossil fuel combustion, particularly from motor vehicles by improving the quality of oil products.

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Autonomous Gaussian decomposition of the Galactic Ring Survey

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038479 ◽

2020 ◽

Vol 640 ◽

pp. A72

Author(s):

M. Riener ◽

J. Kainulainen ◽

J. D. Henshaw ◽

H. Beuther

Keyword(s):

Milky Way ◽

Velocity Dispersion ◽

Galactic Plane ◽

Gas Emission ◽

Spiral Arms ◽

Precise Knowledge ◽

Significant Difference ◽

The Impact ◽

Co Emission ◽

Spiral Arm

Knowledge about the distribution of CO emission in the Milky Way is essential to understanding the impact of the Galactic environment on the formation and evolution of structures in the interstellar medium. However, our current insight as to the fraction of CO in the spiral arm and interarm regions is still limited by large uncertainties in assumed rotation curve models or distance determination techniques. In this work we use the Bayesian approach from Reid et al. (2016, ApJ, 823, 77; 2019, ApJ, 885, 131), which is based on our most precise knowledge at present about the structure and kinematics of the Milky Way, to obtain the current best assessment of the Galactic distribution of 13CO from the Galactic Ring Survey. We performed two different distance estimates that either included (Run A) or excluded (Run B) a model for Galactic features, such as spiral arms or spurs. We also included a prior for the solution of the kinematic distance ambiguity that was determined from a compilation of literature distances and an assumed size-linewidth relationship. Even though the two distance runs show strong differences due to the prior for Galactic features for Run A and larger uncertainties due to kinematic distances in Run B, the majority of their distance results are consistent with each other within the uncertainties. We find that the fraction of 13CO emission associated with spiral arm features ranges from 76 to 84% between the two distance runs. The vertical distribution of the gas is concentrated around the Galactic midplane, showing full-width at half-maximum values of ~75 pc. We do not find any significant difference between gas emission properties associated with spiral arm and interarm features. In particular, the distribution of velocity dispersion values of gas emission in spurs and spiral arms is very similar. We detect a trend of higher velocity dispersion values with increasing heliocentric distance, which we, however, attribute to beam averaging effects caused by differences in spatial resolution. We argue that the true distribution of the gas emission is likely more similar to a combination of the two distance results discussed, and we highlight the importance of using complementary distance estimations to safeguard against the pitfalls of any single approach. We conclude that the methodology presented in this work is a promising way to determine distances to gas emission features in Galactic plane surveys.

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