Shaping Disk Galaxy Stellar Populations via Internal and External Processes

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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The Gaia-ESO Survey: Churning through the Milky Way

Astronomy and Astrophysics ◽

10.1051/0004-6361/201730412 ◽

2018 ◽

Vol 609 ◽

pp. A79 ◽

Cited By ~ 14

Author(s):

M. R. Hayden ◽

A. Recio-Blanco ◽

P. de Laverny ◽

S. Mikolaitis ◽

G. Guiglion ◽

...

Keyword(s):

Milky Way ◽

Past History ◽

Stellar Populations ◽

Solar Neighborhood ◽

The Sun ◽

Radial Migration ◽

Orbital Parameters ◽

The Past ◽

Major Merger ◽

History Of

Context. There have been conflicting results with respect to the extent that radial migration has played in the evolution of the Galaxy. Additionally, observations of the solar neighborhood have shown evidence of a merger in the past history of the Milky Way that drives enhanced radial migration. Aims. We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H] > 0.1) stars within 2 kpc of the Sun. We also aim to investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the Sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. Methods. We used a sample of more than 3000 stars selected from the fourth internal data release of the Gaia-ESO Survey. We used the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Results. Analyzing the kinematics of the most metal-rich stars ([Fe/H] > 0.1), we find that more than half have small eccentricities (e< 0.2) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons Rp> 7 kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by ~10 km s-1. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s-1 slower than stars with lower [Mg/Fe] ratios. Conclusions. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.

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Building Blocks of the Milky Way's Stellar Halo

Proceedings of the International Astronomical Union ◽

10.1017/s1743921315007061 ◽

2015 ◽

Vol 11 (S317) ◽

pp. 373-374

Author(s):

Pim van Oirschot ◽

Else Starkenburg ◽

Amina Helmi ◽

Gijs Nelemans

Keyword(s):

Galaxy Formation ◽

Stellar Population ◽

Milky Way ◽

Building Blocks ◽

Formation Model ◽

Halo Stars ◽

Stellar Halo ◽

History Of ◽

Dynamical Properties ◽

Star Formation Histories

AbstractWe study the assembly history of the stellar halo of Milky Way-like galaxies using the six high-resolution Aquarius dark matter simulations combined with the Munich-Groningen semi-analytic galaxy formation model. Our goal is to understand the stellar population contents of the building blocks of the Milky Way halo, including their star formation histories and chemical evolution, as well as their internal dynamical properties. We are also interested in how they relate or are different from the surviving satellite population. Finally, we will use our models to compare to observations of halo stars in an attempt to reconstruct the assembly history of the Milky Way's stellar halo itself.

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Chemodynamical History of the Galactic Bulge

Annual Review of Astronomy and Astrophysics ◽

10.1146/annurev-astro-081817-051826 ◽

2018 ◽

Vol 56 (1) ◽

pp. 223-276 ◽

Cited By ~ 67

Author(s):

Beatriz Barbuy ◽

Cristina Chiappini ◽

Ortwin Gerhard

Keyword(s):

Stellar Population ◽

Chemical Properties ◽

Stellar Populations ◽

Observational Evidence ◽

Galactic Bulge ◽

Large Samples ◽

Self Consistent ◽

History Of ◽

Bulge Formation ◽

And Chemical Properties

The Galactic Bulge can uniquely be studied from large samples of individual stars and is therefore of prime importance for understanding the stellar population structure of bulges in general. Here the observational evidence on the kinematics, chemical composition, and ages of Bulge stellar populations based on photometric and spectroscopic data is reviewed. The bulk of Bulge stars are old and span a metallicity range of −1.5≲[Fe/H]≲+0.5. Stellar populations and chemical properties suggest a star-formation timescale below ∼2 Gyr. The overall Bulge is barred and follows cylindrical rotation, and the more metal-rich stars trace a box/peanut (B/P) structure. Dyna-mical models demonstrate the different spatial and orbital distributions of metal-rich and metal-poor stars. We discuss current Bulge-formation scenarios based on dynamical, chemical, chemodynamical, and cosmological models. Despite impressive progress, we do not yet have a successful fully self-consistent chemodynamical Bulge model in the cosmological framework, and we will also need a more extensive chrono-chemical-kinematic 3D map of stars to better constrain such models.

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Star Clusters in Local Group Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900108551 ◽

1995 ◽

Vol 164 ◽

pp. 181-192

Author(s):

Edward W. Olszewski

Keyword(s):

Stellar Population ◽

Local Group ◽

Star Clusters ◽

Stellar Populations ◽

Cluster Systems ◽

Star Forming ◽

Long Time ◽

History Of ◽

Individual Objects

In this paper I will try to give the flavor of two different concepts of stellar population, namely, the population of cluster systems within galaxies, and the stellar populations within individual clusters. I hope to show, in concert with other papers in this volume, that a generic vision of Population I and II has been outdated for a long time. I believe that it is still proper to talk about populations within individual objects; in the case of galaxies, one might make a correspondence of populations with the important star forming events in the history of a galaxy.

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Stellar Population Models of Distant Radio Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900081948 ◽

1996 ◽

Vol 175 ◽

pp. 588-590

Author(s):

D. Villani ◽

S. Di Serego Alighieri

Keyword(s):

Energy Distribution ◽

Galaxy Formation ◽

Stellar Population ◽

Spectral Energy Distribution ◽

Formation Rate ◽

Stellar Populations ◽

Radio Galaxies ◽

Initial Mass Function ◽

Spectral Energy ◽

The Galaxy

Stellar populations of high redshift radio galaxies (HzRG) (z up to 4.2) are the oldest stellar systems known, that is the ones formed at the earliest cosmological epochs. Therefore they are the best objects for providing us with information about the epoch of galaxy formation. The information on the stellar populations in HzRG are obtained from the study of their Integrated Spectral Energy Distribution (ISED) which are gathered both from spectra and integrated magnitudes. The most common approach for the interpretation of colors and spectral features of the energy distribution of galaxies is the Evolutionary Population Synthesis (EPS), which has been introduced for the first time by Tinsley in 1972. EPS models have often been used in the past to interpret the ISED of HzRG (Chambers & Charlot 1990; Lilly & Longair 1984; di Serego Alighieri et al. 1994) in order to draw conclusions on the age of the stellar populations and therefore on the epoch of galaxy formation. The results are sometimes conflicting and a number of very recent EPS models have become available (Bressan et al. 1995; Bruzual & Charlot 1993; Buzzoni 1989; Guiderdoni & Rocca-Volmerange 1987): we are therefore analysing the differences between the various EPS models with the aim of assessing their suitability to study the stellar population at early epochs. The EPS models assume for stars a given Initial Mass Function(IMF) as well as a Star Formation Rate (SFR). Then one can compute the number of stars with given mass present in the galaxy as a function of time. The position of each star in the HR diagram is determined by means of the isochrones, which are calculated from stellar evolutionary models. The ISED of a galaxy is obtained from the superposition of the spectra of single stars obtained from a stellar spectral library. Thus these models describe the galaxy ISED as a function of the time, giving a complete evolutionary picture.

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Resolved stellar populations: the outlook for JWST and ELT

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318007032 ◽

2018 ◽

Vol 14 (S343) ◽

pp. 49-58

Author(s):

Eline Tolstoy

Keyword(s):

Stellar System ◽

Stellar Populations ◽

Large Telescope ◽

James Webb Space Telescope ◽

History Of ◽

Entire History ◽

Nearby Galaxies ◽

Star Formation Histories ◽

Insight Into

AbstractThe study of the resolved stellar populations in nearby galaxies and star clusters through the analysis of colour-magnitude diagrams provides the most detailed and quantitative determination of the star formation histories of these systems. The properties of different age populations provide an insight into distinct physical processes taking place during the entire history of the stellar system. The detection of the oldest main sequence turn-offs is currently restricted to stellar systems within the Local Group due to the limitations in spatial resolution and flux sensitivity of available telescopes. Individual stars need to be detected and accurately distinguished from their neighbours. To improve this situation we need to build new telescopes with larger primary mirrors that can deliver a very stable image quality at the diffraction limit. Over the next decade we can look forward to new larger telescope in space: the James Webb Space Telescope, currently scheduled to be launched in 2021; and several large telescope projects, the largest of which is the 39m ESO extremely large telescope on Cerro Armazones in Chile, currently scheduled to start operations in 2024.

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The HR Diagram for Luminous Stars in Nearby Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900145620 ◽

1978 ◽

Vol 80 ◽

pp. 263-267

Author(s):

Roberta M. Humphreys

Keyword(s):

Chemical Composition ◽

Stellar Evolution ◽

Milky Way ◽

Stellar Populations ◽

Nearby Galaxies ◽

Hr Diagram

HR diagrams for the stellar populations in other galaxies play a fundamental role in our understanding of the progress of stellar evolution and the effects of possible variations in chemical composition. It is important to compare what little information we have about stars in other galaxies with the same types of stars in our own Milky Way. Basically we are asking - are they the same, and how universal are the processes we observe in our Galaxy?

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Stellar population gradients in GASS

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313005103 ◽

2012 ◽

Vol 8 (S295) ◽

pp. 304-307

Author(s):

Jonas Johansson ◽

Guinevere Kauffmann ◽

Sean Moran

Keyword(s):

Star Formation ◽

Galaxy Formation ◽

Stellar Population ◽

Neutral Hydrogen ◽

Stellar Populations ◽

Element Abundance ◽

Molecular Gas ◽

Massive Galaxies ◽

Additional Formation ◽

Inside Out

AbstractWe study relationships between the stellar populations and interstellar medium in massive galaxies using the Galex Arecibo SDSS Survey (GASS). The sample consists of HI-observations (~1000 galaxies) and complementary H2-observations (330 galaxies) and long-slit spectroscopy (230 galaxies). Luminosity-weighted stellar population ages, metallicitites and element abundance ratios, are derived by fitting stellar population models of absorption line indices. We find that the ages correlate more strongly with molecular gas fraction (MH2/M*) than with neutral Hydrogen fraction (MHI/M*). This result strengthens the theory that H2 is a better tracer of star-formation than HI. The sample is dominated by negative metallicity-gradients and flat Mg/Fe-gradients. Galaxies with high MH2/M*-ratios show in general flat or weakly negative age-gradients. For low MH2/M*-ratios the age-gradients are overall negative. These results are in agreement with the inside-out galaxy formation scenario. For galaxies with high r90/r50-ratios, a sub-population show positive age-gradients indicating additional formation channels. Furthermore, for galaxies with high MH2/M*-ratios more massive systems have older stellar populations in their centers, suggesting downsizing within the inside-out formation scenario.

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Stellar population synthesis: Application To Galactic Bulges

Symposium - International Astronomical Union ◽

10.1017/s0074180900123162 ◽

1993 ◽

Vol 153 ◽

pp. 133-150

Author(s):

N. Arimoto

Keyword(s):

Observational Data ◽

Stellar Population ◽

Stellar Populations ◽

Population Synthesis ◽

Firm Conclusion ◽

Formation History ◽

Stellar Population Synthesis ◽

Central Regions ◽

History Of ◽

Metallicity Distribution

The stellar populations give traces of the formation history of the bulges. The metallicity distribution of K-giants in the Galactic bulge resembles to that of the giant ellipticals. There seems to be no conspicuous colour-magnitude relation intrinsic to the bulges. This can be explained if the bulges formed by the dissipative collapse of central regions of proto-galaxies followed by the supernova-driven bulge wind which was induced later than the dwarf ellipticals of the similar mass (the biased wind). Unfortunately, the observational data available at present of stellar populations of the bulges are not yet sufficient to get a firm conclusion on the origin of the bulges.

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Characterisation of the Galactic thick disk

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313006169 ◽

2013 ◽

Vol 9 (S298) ◽

pp. 17-27

Author(s):

Thomas Bensby

Keyword(s):

Galaxy Formation ◽

Milky Way ◽

Spiral Galaxies ◽

Thin Disk ◽

Thick Disk ◽

Host Galaxy ◽

Galaxy Formation And Evolution ◽

Formation History ◽

History Of ◽

Formation And Evolution

AbstractThick disks appear to be common in external large spiral galaxies and our own Milky Way also hosts one. The existence of a thick disk is possibly directly linked to the formation history of the host galaxy and if its properties is known it can constrain models of galaxy formation and help us to better understand galaxy formation and evolution. This brief review attempts to highlight some of the characteristics of the Galactic thick disk and how it relates to other stellar populations such as the thin disk and the Galactic bulge. Focus has been put on results from high-resolution spectroscopic data obtained during the last 10 to 15 years.

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