On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster

AbstractIn recent years, effects such as the radial migration of stars in disks have been recognized as important drivers of the properties of stellar populations. Radial migration arises due to perturbative effects of disk structures such as bars and spiral arms, and can deposit stars formed in disks to regions far from their birthplaces. Migrant stars can significantly affect the demographics of their new locales, especially in low-density regions such as in the outer disks. However, in the cosmological environment, other effects such as mergers and filamentary gas accretion also influence the disk formation process. Understanding the relative importance of these processes on the detailed evolution of stellar population signatures is crucial for reconstructing the history of the Milky Way and other nearby galaxies. In the Milky Way disk in particular, the formation of the thickened component has recently attracted much attention due to its potential to serve as a diagnostic of the galaxy's early history. Some recent work suggests, however, that the vertical structure of Milky Way stellar populations is consistent with models that build up the thickened component through migration. I discuss these developments in the context of cosmological galaxy formation.

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Effects of the selection function on metallicity trends in spectroscopic surveys of the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731099 ◽

2017 ◽

Vol 606 ◽

pp. A97 ◽

Cited By ~ 15

Author(s):

G. Nandakumar ◽

M. Schultheis ◽

M. Hayden ◽

A. Rojas-Arriagada ◽

G. Kordopatis ◽

...

Keyword(s):

Stellar Population ◽

Milky Way ◽

Target Selection ◽

Selection Function ◽

Population Synthesis ◽

Sample Distribution ◽

Stellar Population Synthesis ◽

Source Sample ◽

Correction Terms ◽

Metallicity Distribution

Context. Large spectroscopic Galactic surveys imply a selection function in the way they performed their target selection. Aims. We investigate here the effect of the selection function on the metallicity distribution function (MDF) and on the vertical metallicity gradient by studying similar lines of sight using four different spectroscopic surveys (APOGEE, LAMOST, RAVE, and Gaia-ESO), which have different targeting strategies and therefore different selection functions. Methods. We use common fields between the spectroscopic surveys of APOGEE, LAMOST, RAVE (ALR) and APOGEE, RAVE, Gaia-ESO (AGR) and use two stellar population synthesis models, GALAXIA and TRILEGAL, to create mock fields for each survey. We apply the selection function in the form of colour and magnitude cuts of the respective survey to the mock fields to replicate the observed source sample. We make a basic comparison between the models to check which best reproduces the observed sample distribution. We carry out a quantitative comparison between the synthetic MDF from the mock catalogues using both models to understand the effect of the selection function on the MDF and on the vertical metallicity gradient. Results. Using both models, we find a negligible effect of the selection function on the MDF for APOGEE, LAMOST, and RAVE. We find a negligible selection function effect on the vertical metallicity gradients as well, though GALAXIA and TRILEGAL have steeper and shallower slopes, respectively, than the observed gradient. After applying correction terms on the metallicities of RAVE and LAMOST with respect to our reference APOGEE sample, our observed vertical metallicity gradients between the four surveys are consistent within 1σ. We also find consistent gradient for the combined sample of all surveys in ALR and AGR. We estimated a mean vertical metallicity gradient of − 0.241 ± 0.028 dex kpc-1. There is a significant scatter in the estimated gradients in the literature, but our estimates are within their ranges. Conclusions. We have shown that there is a negligible selection function effect on the MDF and the vertical metallicity gradients for APOGEE, RAVE, and LAMOST using two stellar population synthesis models. Therefore, it is indeed possible to combine common fields of different surveys in studies using MDF and metallicity gradients provided their metallicities are brought to the same scale.

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The WAGGS project-III. Discrepant mass-to-light ratios of Galactic globular clusters at high metallicity

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa091 ◽

2020 ◽

Vol 492 (3) ◽

pp. 3859-3871 ◽

Cited By ~ 2

Author(s):

H Dalgleish ◽

S Kamann ◽

C Usher ◽

H Baumgardt ◽

N Bastian ◽

...

Keyword(s):

Globular Clusters ◽

Stellar Population ◽

Milky Way ◽

Population Models ◽

Theoretical Predictions ◽

Field Unit ◽

Stellar Masses ◽

Kinematic Information ◽

High Metallicity ◽

Integral Field

ABSTRACT Observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1622 stars located in the centres of 59 Milky Way GCs – 12 of which have no previous kinematic information – using integral-field unit data from the WAGGS project. Using N-body models, we determine dynamical masses and M/LV for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ∼ −0.1 dex. We find that metal-rich clusters have M/LV more than two times lower than what is predicted by simple stellar population models. This confirms that the discrepant M/L–[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects.

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Was the Milky Way Bulge Formed from the Buckling Disk Instability, Hierarchical Collapse, Accretion of Clumps, or All of the Above?

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2017.32 ◽

2017 ◽

Vol 34 ◽

Cited By ~ 12

Author(s):

David M. Nataf

Keyword(s):

Stellar Population ◽

Milky Way ◽

Velocity Dispersion ◽

High Redshift ◽

Rapid Development ◽

High Redshift Galaxies ◽

Star Forming ◽

Thin And Thick Disks ◽

Bulge Formation ◽

Redshift Galaxies

AbstractThe assembly of the Milky Way bulge is an old topic in astronomy, one now in a period of renewed and rapid development. That is due to tremendous advances in observations of bulge stars, motivating observations of both local and high-redshift galaxies, and increasingly sophisticated simulations. The dominant scenario for bulge formation is that of the Milky Way as a nearly pure disk galaxy, with the inner disk having formed a bar and buckled. This can potentially explain virtually all bulge stars with [Fe/H] ≳ −1.0, which comprise 95% of the stellar population. The evidence is the incredible success in N-body models of this type in making non-trivial, non-generic predictions, such as the rotation curve and velocity dispersion measured from radial velocities, and the spatial morphologies of the peanut/X-shape and the long bar. The classical bulge scenario, whereby the bulge formed from early dissipative collapse and mergers, remains viable for stars with [Fe/H] ≲ −1.0 and potentially a minority of the other stars. A classical bulge is expected from Λ-CDM cosmological simulations, can accentuate the properties of an existing bar in a hybrid system, and is most consistent with the bulge abundance trends such as [Mg/Fe], which are elevated relative to both the thin and thick disks. Finally, the clumpy-galaxy scenario is considered, as it is the correct description of most Milky Way precursors given observations of high-redshift galaxies. Simulations predict that these star-forming clumps will sometimes migrate to the centres of galaxies where they may form a bulge, and galaxies often include a bulge clump as well. They will possibly form a bar with properties consistent with those of the Milky Way, such as the exponential profile and metallicity gradient. Given the relative successes of these scenarios, the Milky Way bulge is plausibly of composite origin, with a classical bulge and/or inner halo numerically dominant for stars with [Fe/H] ≲ −1.0, a buckling thick disk for stars with − 1.0 ≲ [Fe/H]] ≲ -0.50 perhaps descended from the clumpy-galaxy phase, and a buckling thin disk for stars with [Fe/H] ≳ −0.50. Overlaps from these scenarios are uncertain throughout.

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The Bulge-disc connection in the Milky Way

Proceedings of the International Astronomical Union ◽

10.1017/s174392130802752x ◽

2008 ◽

Vol 4 (S254) ◽

pp. 145-152

Author(s):

James Binney

Keyword(s):

Angular Momentum ◽

Stellar Population ◽

Milky Way ◽

Dark Halo ◽

Principal Characteristics ◽

Corotation Radius ◽

The Galaxy

AbstractBulges come in two flavours – classical and pseudo. The principal characteristics of each flavour are summarised and their impact on discs is considered. Classical bulges probably inhibit the formation of stellar discs. Pseudobulges exchange angular momentum with stars and gas in their companion discs, and also with its embedding dark halo. Since the structure of a pseudobulge depends critically on its angular momentum, these exchanges are expected to modify the bulge. The consequences of this modification are not yet satisfactorily understood. The Galaxy has a pseudobulge. I review the manifestations of its interaction with the disc. More work is needed on the dynamics of gas near the bulge's corotation radius, and on tracing the stellar population in the inner few hundred parsecs of the Galaxy.

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Substructure in Dwarf Galaxies

Publications of the Astronomical Society of Australia ◽

10.1071/as04040 ◽

2004 ◽

Vol 21 (4) ◽

pp. 379-381

Author(s):

Matthew Coleman

Keyword(s):

Dark Matter ◽

Star Formation ◽

Large Scale ◽

Stellar Population ◽

Milky Way ◽

Dwarf Galaxies ◽

Tidal Interaction ◽

Kinematic Evolution ◽

Dark Material ◽

Star Formation Histories

AbstractRecent years have seen a series of large-scale photometric surveys with the aim of detecting substructure in nearby dwarf galaxies. Some of these objects display a varying distribution of each stellar population, reflecting their star formation histories. Also, dwarf galaxies are dominated by dark matter, therefore luminous substructure may represent a perturbation in the underlying dark material. Substructure can also be the effect of tidal interaction, such as the disruption of the Sagittarius dSph by the Milky Way. Therefore, substructure in dwarf galaxies manifests the stellar, structural, and kinematic evolution of these objects.

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The nuclear cluster of the Milky Way: our primary testbed for the interaction of a dense star cluster with a massive black hole

Classical and Quantum Gravity ◽

10.1088/0264-9381/31/24/244007 ◽

2014 ◽

Vol 31 (24) ◽

pp. 244007 ◽

Cited By ~ 40

Author(s):

R Schödel ◽

A Feldmeier ◽

N Neumayer ◽

L Meyer ◽

S Yelda

Keyword(s):

Black Hole ◽

Milky Way ◽

Star Cluster ◽

Massive Black Hole ◽

Nuclear Cluster

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The Nuclear Cluster of the Milky Way: Star Formation and Velocity Dispersion in the Central 0.5 Parsec

The Astrophysical Journal ◽

10.1086/309579 ◽

1995 ◽

Vol 447 (2) ◽

Cited By ~ 248

Author(s):

A. Krabbe, ◽

R. Genzel, ◽

A. Eckart, ◽

F. Najarro, ◽

D. Lutz, ◽

...

Keyword(s):

Star Formation ◽

Milky Way ◽

Velocity Dispersion ◽

Nuclear Cluster

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Extended stellar systems in the solar neighborhood

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834950 ◽

2019 ◽

Vol 622 ◽

pp. L13 ◽

Cited By ~ 29

Author(s):

Stefan Meingast ◽

João Alves ◽

Verena Fürnkranz

Keyword(s):

Wavelet Decomposition ◽

Stellar Population ◽

Main Sequence ◽

Milky Way ◽

Velocity Dispersion ◽

Galactic Disk ◽

Mass Function ◽

Solar Neighborhood ◽

Giant Stars ◽

Stellar Streams

We report the discovery of a large, dynamically cold, coeval stellar stream that is currently traversing the immediate solar neighborhood at a distance of only 100 pc. The structure was identified in a wavelet decomposition of the 3D velocity space of all stars within 300 pc of the Sun. Its members form a highly elongated structure with a length of at least 400 pc, while its vertical extent measures only about 50 pc. Stars in the stream are not isotropically distributed but instead form two parallel lanes with individual local overdensities, that may correspond to a remnant core of a tidally disrupted cluster or OB association. Its members follow a very well-defined main sequence in the observational Hertzsprung–Russel diagram and also show a remarkably low 3D velocity dispersion of only 1.3 km s−1. These findings strongly suggest a common origin as a single coeval stellar population. An extrapolation of the present-day mass function indicates a total mass of at least 2000 M⊙, making it larger than most currently known clusters or associations in the solar neighborhood. We estimated the age of the stream to be around 1 Gyr based on a comparison with a set of isochrones and giant stars in our member selection and find a mean metallicity of [Fe/H] = −0.04. This structure may very well represent the Galactic disk counterpart to the prominent stellar streams observed in the Milky Way halo. As such, it constitutes a new valuable probe to constrain the Galaxy’s mass distribution.

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The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038305 ◽

2020 ◽

Vol 641 ◽

pp. A96 ◽

Cited By ~ 4

Author(s):

A. Savino ◽

A. Koch ◽

Z. Prudil ◽

A. Kunder ◽

R. Smolec

Keyword(s):

Stellar Population ◽

Milky Way ◽

Population Synthesis ◽

Milky Way Galaxy ◽

First Stars ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

The Galaxy ◽

Stellar Halo ◽

Lyrae Stars

The central kiloparsecs of the Milky Way are known to host an old, spheroidal stellar population, whose spatial and kinematical properties set it apart from the boxy-peanut structure that constitutes most of the central stellar mass. The nature of this spheroidal population, whether it is a small classical bulge, the innermost stellar halo, or a population of disk stars with large initial velocity dispersion, remains unclear. This structure is also a promising candidate to play host to some of the oldest stars in the Galaxy. Here we address the topic of the inner stellar spheroid age, using spectroscopic and photometric metallicities for a sample of 935 RR Lyrae stars that are constituents of this component. By means of stellar population synthesis, we derive an age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae stars in the bulge spheroid, an extremely ancient age of 13.41 ± 0.54 Gyr and conclude they were among the first stars to form in what is now the Milky Way galaxy. Our age estimate for the central spheroid shows a remarkable agreement with the age profile that has been inferred for the Milky Way stellar halo, suggesting a connection between the two structures. However, we find mild evidence for a transition in the halo properties at rGC ∼ 5 kpc. We also investigate formation scenarios for metal-rich RR Lyrae stars, such as binarity and helium variations, and consider whether they can provide alternative explanations for the properties of our sample. We conclude that within our framework, the only viable alternative is to have younger, slightly helium-rich, RR Lyrae stars. This is a hypothesis that would open intriguing questions for the formation of the inner stellar spheroid.

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