scholarly journals Chemical abundance analysis of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

2020 ◽  
Vol 636 ◽  
pp. A111 ◽  
Author(s):  
M. Aoki ◽  
W. Aoki ◽  
P. François
Keyword(s):  
Milky Way ◽  
Galactic Halo ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Building Blocks ◽  
High Resolution Spectroscopy ◽  
High Dispersion ◽  
Chemical Abundance ◽  
Halo Structure ◽  
Halo Stars

Context. Metal-poor components of dwarf galaxies around the Milky Way could be remnants of the building blocks of the Galactic halo structure. Low-mass stars that are currently observed as metal-poor stars are expected to have formed in chemically homogeneous clusters in the early phases of galaxy formation. They should have already disintegrated and should exhibit large scatter in abundance ratios of some sets of elements (e.g., Sr/Ba) in the Milky Way field stars. However, chemical abundance ratios are expected to cluster in very metal-poor stars in dwarf galaxies because the number of clusters formed in individual galaxies in the very early phase is expected to be quite limited. Aims. We examine the possible clustering of abundance ratios of Sr and Ba in the Sextans dwarf galaxy to test for the clustering star formation scenario. Methods. We investigate a total of 11 elements (C, Mg, Ca, Sc, Ti, Cr, Mn, Ni, Zn, Sr, Ba) in five stars in the Sextans dwarf galaxy. Previous studies suggest that these have similar abundance ratios. In this study, we focus on the abundance ratio of Sr to Ba. The observations are based on high-resolution spectroscopy (R = 40 000) using the Subaru Telescope High Dispersion Spectrograph. Results. The distribution of α/Fe abundance ratios of the Sextans dwarf galaxy stars is slightly lower than the average of the values of stars in the Galactic halo. The Sr/Ba abundance ratios for the five metal-poor stars are in good agreement, and this clumping is distinctive compared to the [Sr/Ba] spread seen in the metal-poor halo stars. We find that the probability of such clumping is very small if the Sextans stars have distributions of Sr and Ba abundances similar to halo stars. Conclusions. In the Sextans dwarf galaxy, five out of six of the extremely metal-poor stars for which abundance ratios are well studied so far show clear clustering in abundance ratios including Sr/Ba. These observations tend to support the hypothesis that these stars were formed from a cloud of homogeneous chemical composition.

Stellar Populations of the Outer Milky-Way Halo

2017 ◽  
Vol 13 (S334) ◽  
pp. 29-33
Author(s):  
Baslio Santiago ◽  
Elmer Luque ◽  
Adriano Pieres ◽  
Anna Bárbara Queiroz
Keyword(s):  
Galaxy Evolution ◽  
Galaxy Formation ◽  
Milky Way ◽  
Galactic Halo ◽  
Dwarf Galaxy ◽  
Building Blocks ◽  
Early Stages ◽  
Dark Energy Survey ◽  
Stellar Streams ◽  
The Galaxy

AbstractThe stellar spheroidal components of the Milky-Way contain the oldest and most metal poor of its stars. Inevitably the processes governing the early stages of Galaxy evolution are imprinted upon them. According to the currently favoured hierarchical bottom-up scenario of galaxy formation, these components, specially the Galactic halo, are the repository of most of the mass built up from accretion events in those early stages. These events are still going on today, as attested by the long stellar streams associated to the Sagittarius dwarf galaxy and several other observed tidal substructure, whose geometry, extent, and kinematics are important constraints to reconstruct the MW gravitational potential and infer its total (visible + dark) mass. In addition, the remaining system of MW satellites is expected to be a fossil record of the much larger population of Galactic building blocks that once existed and got accreted. For all these reasons, it is crucial to unravel as much of this remaining population as possible, as well as the current stellar streams that orbit within the halo. The best bet to achieve this task is to carry out wide, deep, and multi-band photometric surveys that provide homogeneous stellar samples. In this contribution, we summarize the results of several years of work towards detecting and characterizing distant MW stellar systems, star clusters and dwarf spheroidals alike, with an emphasis on the analysis of data from the Dark Energy Survey (DES). We argue that most of the volume in distance, size and luminosity space, both in the Galaxy and in the Clouds, is still unprobed. We then discuss the perspectives of exploring this outer MW volume using the current surveys, as well as other current and future surveys, such as the Large Synoptic Survey Telescope (LSST).

scholarly journals Kinematics of highly r-process-enhanced halo stars: Evidence for origins in now-destroyed ultra-faint dwarf galaxies

2019 ◽  
Vol 14 (S353) ◽  
pp. 71-74
Author(s):  
Kaley Brauer ◽  
Alexander P. Ji ◽  
Kohei Hattori ◽  
Sergio Escobar ◽  
Anna Frebel
Keyword(s):  
Milky Way ◽  
Dwarf Galaxies ◽  
Building Blocks ◽  
Specific Chemical ◽  
Halo Stars ◽  
Novel Approach ◽  
Chemical Signatures ◽  
Formation History ◽  
Stellar Halo ◽  
R Process

AbstractThe Milky Way’s stellar halo preserves a fossil record of smaller dwarf galaxies that merged with the Milky Way throughout its formation history. Currently, though, we lack reliable ways to identify which halo stars originated in which dwarf galaxies or even which stars were definitively accreted. Selecting stars with specific chemical signatures may provide a way forward. We investigate this theoretically and observationally for stars with r-process nucleosynthesis signatures. Theoretically, we combine high-resolution cosmological simulations with an empirically-motivated treatment of r-process enhancement. We find that around half of highly r-process-enhanced metal-poor halo stars may have originated in early ultra-faint dwarf galaxies that merged into the Milky Way during its formation. Observationally, we use Gaia DR2 to compare the kinematics of highly r-process-enhanced halo stars with those of normal halo stars. R-process-enhanced stars have higher galactocentric velocities than normal halo stars, suggesting an accretion origin. If r-process-enhanced stars largely originated in accreted ultra-faint dwarf galaxies, halo stars we observe today could play a key role in understanding the smallest building blocks of the Milky Way via this novel approach of chemical tagging

scholarly journals Chemical abundances of field halo stars - Implications for the building blocks of the Milky Way

2019 ◽  
Vol 14 (S351) ◽  
pp. 24-33
Author(s):  
Miho N. Ishigaki
Keyword(s):  
Milky Way ◽  
Building Blocks ◽  
Chemical Information ◽  
Wide Field ◽  
Chemical Abundance ◽  
Chemical Abundances ◽  
Halo Stars ◽  
Dimensional Phase Space ◽  
Stellar Halo ◽  
Detailed Chemical

AbstractI would like to review recent efforts of detailed chemical abundance measurements for field Milky Way halo stars. Thanks to the advent of wide-field spectroscopic surveys up to a several kpc from the Sun, large samples of field halo stars with detailed chemical measurements are continuously expanding. Combination of the chemical information and full six dimensional phase-space information is now recognized as a powerful tool to identify cosmological accretion events that have built a sizable fraction of the present-day stellar halo. Future observational prospects with wide-field spectroscopic surveys and theoretical prospects with supernova nucleosynthetic yields are also discussed.

scholarly journals Contributions to the Galactic halo from in-situ, kicked-out, and accreted stars

2015 ◽  
Vol 11 (S317) ◽  
pp. 241-246
Author(s):  
Allyson A. Sheffield ◽  
Kathryn V. Johnston ◽  
Katia Cunha ◽  
Verne V. Smith ◽  
Steven R. Majewski
Keyword(s):  
Neutron Capture ◽  
Milky Way ◽  
Galactic Halo ◽  
Galactic Disk ◽  
High Resolution Spectroscopy ◽  
Chemical Abundances ◽  
Giant Stars ◽  
Halo Stars ◽  
Tentative Evidence

AbstractWe report chemical abundances for a sample of 66 M giants with high S/N high-resolution spectroscopy in the inner halo of the Milky Way. The program giant stars have radial velocities that vary significantly from those expected for stars moving on uniform circular orbits in the Galactic disk. Thus, based on kinematics, we expect a sample dominated by halo stars. Abundances are derived for α-elements and neutron capture elements. By analyzing the multi-dimensional abundance space, the formation site of the halo giants – in-situ or accreted – can be assessed. Of particular interest are a class of stars that form in-situ, deep in the Milky Way's gravitational potential well, but are “kicked out” of the disk into the halo due to a perturbation event. We find: (1) our sample is dominated by accreted stars and (2) tentative evidence of a small kicked-out population in our Milky Way halo sample.

scholarly journals The building blocks of the Milky Way halo using APOGEE and Gaia or Is the Galaxy a typical galaxy?

2019 ◽  
Vol 14 (S351) ◽  
pp. 170-173 ◽  
Author(s):  
Ricardo P. Schiavon ◽  
J. Ted Mackereth ◽  
Joel Pfeffer ◽  
Rob A. Crain ◽  
Jo Bovy
Keyword(s):  
Numerical Simulations ◽  
Globular Clusters ◽  
Milky Way ◽  
Galactic Halo ◽  
Dwarf Galaxies ◽  
Stellar Populations ◽  
Building Blocks ◽  
Stellar Content ◽  
The Galaxy ◽  
Milky Way Halo

AbstractWe summarise recent results from analysis of APOGEE/Gaia data for stellar populations in the Galactic halo, disk, and bulge, leading to constraints on the contribution of dwarf galaxies and globular clusters to the stellar content of the Milky Way halo. Intepretation of the extant data in light of cosmological numerical simulations suggests that the Milky Way has been subject to an unusually intense accretion history at z ≳ 1.5.

scholarly journals Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history

2019 ◽  
Vol 631 ◽  
pp. L9 ◽  
Author(s):  
Helmer H. Koppelman ◽  
Amina Helmi ◽  
Davide Massari ◽  
Adrian M. Price-Whelan ◽  
Tjitske K. Starkenburg
Keyword(s):  
High Resolution ◽  
Clustering Algorithm ◽  
Galactic Halo ◽  
High Energy ◽  
Galaxy Mergers ◽  
Chemical Abundance ◽  
Distinct Structure ◽  
Halo Stars ◽  
Energy Part ◽  
Hydrodynamical Simulations

Aims. Several kinematic and chemical substructures have been recently found amongst Milky Way halo stars with retrograde motions. It is currently unclear how these various structures are related to each other. This Letter aims to shed light on this issue. Methods. We explore the retrograde halo with an augmented version of the Gaia DR2 RVS sample, extended with data from three large spectroscopic surveys, namely RAVE, APOGEE, and LAMOST. In this dataset, we identify several structures using the HDBSCAN clustering algorithm. We discuss their properties and possible links using all the available chemical and dynamical information. Results. In concordance with previous work, we find that stars with [Fe/H] < −1 have more retrograde motions than those with [Fe/H] > −1. The retrograde halo contains a mixture of debris from objects like Gaia-Enceladus, Sequoia, and even the chemically defined thick disc. We find that the Sequoia has a smaller range in orbital energies than previously suggested and is confined to high energy. Sequoia could be a small galaxy in itself, but since it overlaps both in integrals-of-motion space and chemical abundance space with the less bound debris of Gaia-Enceladus, its nature cannot yet be fully settled. In the low-energy part of the halo, we find evidence for at least one more distinct structure: Thamnos. Stars in Thamnos are on low-inclination, mildly eccentric retrograde orbits, moving at vϕ ≈ −150 km s−1, and are chemically distinct from the other structures. Conclusions. Even with the excellent Gaia DR2 data, piecing together all the fragments found in the retrograde halo remains challenging. At this point, we are very much in need of large datasets with high-quality high-resolution spectra and tailored high-resolution hydrodynamical simulations of galaxy mergers.

scholarly journals Distance to the nearby dwarf galaxy [TT2009] 25 in the NGC 891 group using the tip of the red giant branch

2019 ◽  
Vol 629 ◽  
pp. L2 ◽  
Author(s):  
Oliver Müller ◽  
Rodrigo Ibata ◽  
Marina Rejkuba ◽  
Lorenzo Posti
Keyword(s):  
Milky Way ◽  
Local Group ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Small Scale ◽  
Accurate Information ◽  
Standard Candle ◽  
Stellar Halo ◽  
Red Giant ◽  
Small Field Of View

Dwarf galaxies are key objects for small-scale cosmological tests like the abundance problems or the planes-of-satellites problem. A crucial task is therefore to get accurate information for as many nearby dwarf galaxies as possible. Using extremely deep, ground-based V and i-band Subaru Suprime Cam photometry with a completeness of i = 27 mag, we measure the distance of the dwarf galaxy [TT2009] 25 using the tip of the red giant branch as a standard candle. This dwarf resides in the field around the Milky Way-analog NGC 891. Using a Bayesian approach, we measure a distance of 10.28−1.73+1.17 Mpc, which is consistent with the distance of NGC 891, and thus confirm it as a member of NGC 891. The dwarf galaxy follows the scaling relations defined by the Local Group dwarfs. We do not find an extended stellar halo around [TT2009] 25. In the small field of view of 100 kpc covered by the survey, only one bright dwarf galaxy and the giant stream are apparent. This is comparable to the Milky Way, where one bright dwarf resides in the same volume, as well as the Sagittarius stream – excluding satellites which are farther away but would be projected in the line-of-sight. It is thus imperative to survey for additional dwarf galaxies in a larger area around NGC 891 to test the abundance of dwarf galaxies and compare this to the number of satellites around the Milky Way.

A deep view into the nucleus of the Sagittarius dwarf spheroidal galaxy: M54

2019 ◽  
Vol 14 (S351) ◽  
pp. 47-50
Author(s):  
M. Alfaro-Cuello ◽  
N. Kacharov ◽  
N. Neumayer ◽  
A. Mastrobuono-Battisti ◽  
N. Lützgendorf ◽  
...  
Keyword(s):  
Metal Complex ◽  
Detailed Analysis ◽  
Globular Clusters ◽  
Milky Way ◽  
Dwarf Galaxies ◽  
Dwarf Galaxy ◽  
Star Clusters ◽  
High Mass ◽  
Data Set

AbstractNuclear star clusters hosted by dwarf galaxies exhibit similar characteristics to high-mass, metal complex globular clusters. This type of globular clusters could, therefore, be former nuclei from accreted galaxies. M54 resides in the photometric center of the Sagittarius dwarf galaxy, at a distance where resolving stars is possible. M54 offers the opportunity to study a nucleus before the stripping of their host by the tidal field effects of the Milky Way. We use a MUSE data set to perform a detailed analysis of over 6600 stars. We characterize the stars by metallicity, age, and kinematics, identifying the presence of three stellar populations: a young metal-rich (YMR), an intermediate-age metal-rich (IMR), and an old metal-poor (OMP). The evidence suggests that the OMP population is the result of accretion of globular clusters in the center of the host, while the YMR population was born in-situ in the center of the OMP population.

scholarly journals IC 4499 revised: Spectro-photometric evidence of small light-element variations

2018 ◽  
Vol 618 ◽  
pp. A131 ◽  
Author(s):  
E. Dalessandro ◽  
C. Lardo ◽  
M. Cadelano ◽  
S. Saracino ◽  
N. Bastian ◽  
...  
Keyword(s):  
Globular Clusters ◽  
Milky Way ◽  
Dwarf Galaxy ◽  
Light Element ◽  
High Resolution Spectroscopy ◽  
Multiple Populations ◽  
Number Ratio ◽  
Red Giant ◽  
Good Agreement

It has been suggested that IC 4499 is one of the very few old globulars to not host multiple populations with light-element variations. To follow-up on this very interesting result, here we have made use of accurate HST photometry and FLAMES at VLT high-resolution spectroscopy to investigate in more detail the stellar population properties of this system. We find that the red giant branch of the cluster is clearly bimodal in near-UV-optical colour-magnitude diagrams, thus suggesting that IC 4499 is actually composed by two sub-populations of stars with different nitrogen abundances. This represents the first detection of multiple populations in IC 4499. Consistently, we also find that one star out of six is Na-rich to some extent, while we do not detect any evidence of intrinsic spread in both Mg and O. The number ratio between stars with normal and enriched nitrogen is in good agreement with the number ratio – mass trend observed in Galactic globular clusters. Also, as typically found in other systems, nitrogen rich stars are more centrally concentrated than normal stars, although this result cannot be considered conclusive because of the limited field of view covered by our observations (∼1rh). On the contrary, we observe that both the RGB UV colour spread, which is a proxy of N variations, and Na abundance variations, are significantly smaller than those observed in Milky Way globular clusters with mass and metallicity comparable to IC 4499. The modest N and Na spreads observed in this system can be tentatively connected to the fact that IC 4499 likely formed in a disrupted dwarf galaxy orbiting the Milky Way, as previously proposed based on its orbit.

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