scholarly journals The Milky Way has no in-situ halo other than the heated thick disc

2019 ◽  
Vol 632 ◽  
pp. A4 ◽  
Author(s):  
P. Di Matteo ◽  
M. Haywood ◽  
M. D. Lehnert ◽  
D. Katz ◽  
S. Khoperskov ◽  
...  
Keyword(s):  
Galactic Halo ◽  
The Sun ◽  
Galactic Disc ◽  
Elemental Abundances ◽  
Thick Disc ◽  
Halo Stars ◽  
Stellar Halo ◽  
Halo Population ◽  
Astrometric Data

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at [Fe/H] <  −1 within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterise the kinematics and chemistry of in-situ and accreted populations up to [Fe/H] ∼ −2. Accreted stars appear to significantly impact the galactic chemo–kinematic relations, not only at [Fe/H] <  −1, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at [Fe/H] <  −1, the remaining 40% being made of (metal-weak) thick-disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event, which heated the old galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between nine and 11 Gyr ago, and that it led to the last significant heating of the galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the galactic halo. Indeed, about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick-disc stars. Moreover, we suggest that this metal-rich thick-disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard, non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

scholarly journals The tale of the tail – disentangling the high transverse velocity stars in Gaia DR2

2020 ◽  
Vol 492 (3) ◽  
pp. 3816-3828 ◽  
Author(s):  
João A S Amarante ◽  
Martin C Smith ◽  
Corrado Boeche
Keyword(s):  
Transverse Velocity ◽  
Kinematic Model ◽  
Double Sequence ◽  
Good Match ◽  
Galactic Disc ◽  
Thick Disc ◽  
Resonant Orbits ◽  
Halo Stars ◽  
Stellar Halo ◽  
Gaia Dr2

ABSTRACT Although the stellar halo accounts for just ∼1 per cent of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung–Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick disc from Schönrich & Binney (2012). We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the Galactic disc and stellar halo. In particular, we show that the kinematically defined thick-disc contribution to this high velocity tail is $\approx 13{{\ \rm per\ cent}}$. We look in greater detail using chemistry from LAMOST DR5, identifying a population of retrograde stars with thick-disc chemistry. Our thick-disc kinematic model cannot account for this population and so we conclude there is likely to be a contribution from heated or accreted stars in the Solar Neighbourhood. We also investigate proposed dynamical substructures in this sample, concluding that they are probably due to resonant orbits rather than accreted populations. Finally, we provide new insights on the nature of the two sequences and their relation with past accretion events and the primordial Galactic disc.

scholarly journals Gaia DR1 completeness within 250 pc & star formation history of the Solar neighbourhood

2017 ◽  
Vol 12 (S330) ◽  
pp. 148-151 ◽  
Author(s):  
Edouard J. Bernard
Keyword(s):  
Star Formation ◽  
Star Formation History ◽  
Solar Neighbourhood ◽  
The Sun ◽  
Thick Disc ◽  
Formation History ◽  
Magnitude Diagram ◽  
History Of

AbstractWe took advantage of the Gaia DR1 to combine TGAS parallaxes with Tycho-2 and APASS photometry to calculate the star formation history (SFH) of the solar neighbourhood within 250 pc using the colour-magnitude diagram fitting technique. We present the determination of the completeness within this volume, and compare the resulting SFH with that calculated from the Hipparcos catalogue within 80 pc of the Sun. We also show how this technique will be applied out to ~5 kpc thanks to the next Gaia data releases, which will allow us to quantify the SFH of the thin disc, thick disc and halo in situ, rather than extrapolating based on the stars from these components that are today in the solar neighbourhood.

scholarly journals Origins of Stellar Halos

2015 ◽  
Vol 11 (S317) ◽  
pp. 1-8
Author(s):  
Kathryn V. Johnston
Keyword(s):  
Dark Matter ◽  
Milky Way ◽  
Stellar Populations ◽  
Observational Evidence ◽  
Dark Matter Halo ◽  
Theoretical Understanding ◽  
Stellar Halo ◽  
The One ◽  
Halo Population

AbstractThis contribution reviews ideas about the origins of stellar halos. It includes discussion of the theoretical understanding of and observational evidence for stellar populations formed “in situ” (meaning formed in orbits close to their current ones), “kicked-out” (meaning formed in the inner galaxy in orbits unlike their current ones) and “accreted” (meaning formed in a dark matter halo other than the one they currently occupy). At this point there is general agreement that a significant fraction of any stellar halo population is likely “accreted”. There is modest evidence for the presence of a “kicked-out” population around both the Milky Way and M31. Our theoretical understanding of and the observational evidence for an “in situ” population are less clear.

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 mass fraction of halo stars contributed by the disruption of globular clusters in the E-MOSAICS simulations

2020 ◽  
Vol 493 (3) ◽  
pp. 3422-3428 ◽  
Author(s):  
Marta Reina-Campos ◽  
Meghan E Hughes ◽  
J M Diederik Kruijssen ◽  
Joel L Pfeffer ◽  
Nate Bastian ◽  
...  
Keyword(s):  
Mass Fraction ◽  
Globular Clusters ◽  
Milky Way ◽  
Galactic Halo ◽  
Light Element ◽  
Element Abundance ◽  
Host Galaxies ◽  
Stellar Cluster ◽  
Halo Stars ◽  
Stellar Halo

ABSTRACT Globular clusters (GCs) have been posited, alongside dwarf galaxies, as significant contributors to the field stellar population of the Galactic halo. In order to quantify their contribution, we examine the fraction of halo stars formed in stellar clusters in the suite of 25 present-day Milky Way-mass cosmological zoom simulations from the E-MOSAICS project. We find that a median of 2.3 and 0.3 per cent of the mass in halo field stars formed in clusters and GCs, defined as clusters more massive than 5 × 103 and 105 M⊙, respectively, with the 25–75th percentiles spanning 1.9–3.0 and 0.2–0.5 per cent being caused by differences in the assembly histories of the host galaxies. Under the extreme assumption that no stellar cluster survives to the present day, the mass fractions increase to a median of 5.9 and 1.8 per cent. These small fractions indicate that the disruption of GCs plays a subdominant role in the build-up of the stellar halo. We also determine the contributed halo mass fraction that would present signatures of light-element abundance variations considered to be unique to GCs, and find that clusters and GCs would contribute a median of 1.1 and 0.2 per cent, respectively. We estimate the contributed fraction of GC stars to the Milky Way halo, based on recent surveys, and find upper limits of 2–5 per cent (significantly lower than previous estimates), suggesting that models other than those invoking strong mass loss are required to describe the formation of chemically enriched stellar populations in GCs.

scholarly journals Objective Prism Survey of the Outer Galactic Halo

1984 ◽  
Vol 78 ◽  
pp. 261-263
Author(s):  
Kavan Ratnatunga ◽  
K.C. Freeman
Keyword(s):  
Galactic Halo ◽  
The Sun ◽  
Abundance Distribution ◽  
Halo Stars ◽  
Metal Abundance ◽  
Objective Prism ◽  
Distant Field ◽  
K Giants

The aim of this survey is to locate samples of very distant field halo stars, to study the kinematics and metal abundance distribution in the outer regions of the galactic halo. Field halo K giants were chosen for the study, as they are intrinsically bright stars whose evolution is well understood. Halo K giants, near the tip of the giant branch and between 10 and 40 kpc from the sun, will have apparent magnitudes in the range 13 < V < 18, and colours (B-V) > 0.9.

scholarly journals Origin of the Galactic Halo: accretion vs. in situ formation

2017 ◽  
Vol 13 (S334) ◽  
pp. 34-37
Author(s):  
Emanuele Spitoni ◽  
Fiorenzo Vincenzo ◽  
Francesca Matteucci ◽  
Donatella Romano
Keyword(s):  
Star Formation ◽  
Neutron Capture ◽  
Time Scale ◽  
Galactic Halo ◽  
Building Blocks ◽  
The Other ◽  
Chemical Abundances ◽  
Halo Stars ◽  
In Situ Formation

AbstractWe test the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the our Galaxy have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] vs. [Fe/H] patterns with the ones observed in Galactic halo stars. The [O/Fe] ratio deviates substantially from the observed abundance ratios in the Galactic halo stars for [Fe/H] &gt; -2 dex, while they overlap for lower metallicities. On the other hand, for the neutron capture elements, the discrepancy is extended at all the metallicities, suggesting that the majority of stars in the halo are likely to have been formed in situ. We present the results for a model considering the effects of an enriched gas stripped from dwarf satellites on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the adopted infall time-scale, and the presence of a threshold in the gas for star formation.

The high transverse velocity stars in Gaia-LAMOST

2019 ◽  
Vol 14 (S353) ◽  
pp. 59-60
Author(s):  
João A. S. Amarante ◽  
Martin C. Smith ◽  
Corrado Boeche
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
Transverse Velocity ◽  
Double Sequence ◽  
Good Match ◽  
Thick Disc ◽  
Halo Stars ◽  
Stellar Halo ◽  
Gaia Dr2 ◽  
Hr Diagram

AbstractAlthough the stellar halo accounts for just ∼1% of the total stellar mass of the Milky Way, the kinematics of halo stars can tell us a lot about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a duality in the local halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo. We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the disc and halo. In particular, we show that the thick disc contribution to this high velocity tail is small, but not negligible, and likely has an influence on the red sequence of the HR diagram.

scholarly journals Rediscovering the origins of the stellar halo with chemical tagging

2017 ◽  
Vol 13 (S334) ◽  
pp. 38-42 ◽  
Author(s):  
Sarah L Martell
Keyword(s):  
Globular Cluster ◽  
Globular Clusters ◽  
Galactic Halo ◽  
Light Element ◽  
Element Abundance ◽  
Tidal Interactions ◽  
Halo Stars ◽  
The Galaxy ◽  
Stellar Halo ◽  
Light Element Abundance

AbstractThe Galactic halo has a complex assembly history, which can be seen in its wealth of kinematic and chemical substructure. Globular clusters lose stars through tidal interactions with the Galaxy and cluster evaporation processes, meaning that they are inevitably a source of halo stars. These “migrants” from globular clusters can be recognized in the halo field by the characteristic light element abundance anticorrelations that are commonly observed only in globular cluster stars, and the number of halo stars that can be chemically tagged to globular clusters can be used to place limits on the formation pathways of those clusters.

scholarly journals Stars made in outflows may populate the stellar halo of the Milky Way

2020 ◽  
Vol 494 (2) ◽  
pp. 1539-1559 ◽  
Author(s):  
Sijie Yu ◽  
James S Bullock ◽  
Andrew Wetzel ◽  
Robyn E Sanderson ◽  
Andrew S Graus ◽  
...  
Keyword(s):  
Milky Way ◽  
Near Field ◽  
Halo Formation ◽  
Molecular Outflows ◽  
Halo Stars ◽  
Distant Galaxies ◽  
Stellar Halo ◽  
External Galaxies ◽  
In Fire

ABSTRACT We study stellar-halo formation using six Milky-Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that $5{-}40{{\ \rm per\ cent}}$ of the outer (50–300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by superbubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0. In the outer halo, outflow stars are more spatially homogeneous, metal-rich, and alpha-element-enhanced than the accreted stellar halo. At the solar location, up to $\sim \!10 {{\ \rm per\ cent}}$ of our kinematically identified halo stars were born in outflows; the fraction rises to as high as $\sim \!40{{\ \rm per\ cent}}$ for the most metal-rich local halo stars ([Fe/H] &gt;−0.5). Such stars can be retrograde and create features similar to the recently discovered Milky Way ‘Splash’ in phase space. We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies. Searches for such a population may provide a new, near-field approach to constraining feedback and outflow physics. A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen, Lynden-Bell & Sandange, who suggested that halo stars formed in rapidly infalling gas clouds. Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich, extreme-velocity stars in the Milky Way.

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