Disentangling the Galactic Halo with APOGEE. I. Chemical and Kinematical Investigation of Distinct Metal-poor Populations

Hayes, Christian R.; Majewski, Steven R.; Shetrone, Matthew; Fernández-Alvar, Emma; Prieto, C. Allende; Schuster, W. J.; Carigi, Leticia; Cunha, Kátia; Smith, Verne V.; Sobeck, Jennifer; Almeida, Andrés; Beers, Timothy C.; Carrera, R.; Fernández-Trincado, J. G.; García-Hernández, D. A.; Geisler, Doug; Lane, Richard R.; Lucatello, S.; Matthews, A. M.; Minniti, D.; Nitschelm, Christian; Tang, Baitian; Tissera, Patricia B.; Zamora, Olga

doi:10.3847/1538-4357/aa9cec

Cited by 158 publications

(166 citation statements)

References 87 publications

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“…fers from that of the Milky Way (e.g., Tolstoy et al 2009). The accreted origin of the GCs that are kinematically associated with GE, H99 and Seq is further confirmed by the fact that their position in the [Si/Fe] plane mimics that of field populations linked with past accretion events (Hayes et al 2018;Mackereth et al 2019). We calculate the mean [Si/Fe] abundace given by GCs in the -1.5 < [Fe/H] < -1 regime for both our accreted and in situ populations, and find that the accreted groups display on average [Si/Fe] = +0.17±0.05, whereas the in situ subgroups display a higher average abundance [Si/Fe] = +0.25±0.03.…”

Section: Accreted Subgroupsmentioning

confidence: 73%

The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

Horta

Schiavon

Mackereth

et al. 2020

Monthly Notices of the Royal Astronomical Society

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Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3,090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only α (mostly Si and Mg) elements and Fe. Our results show that: (i) The loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) The chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low metallicity end, the distinction between accreted and in situ populations is blurred; (iv) Regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si-Fe plane suggests an in situ origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ.

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Section: Accreted Subgroupsmentioning

confidence: 73%

The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

Horta

Schiavon

Mackereth

et al. 2020

Monthly Notices of the Royal Astronomical Society

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“…What about other potentially major accretion sources? Studies of metal-poor stars have uncovered two chemically distinct populations in the MW (e.g., Nissen & Schuster 2010;Hawkins et al 2015;Hayes et al 2018). These two populations are (using the definitions from Fernández-Alvar et al 2018; Hayes et al 2018) a "low-magnesium" halo population, thought to be accreted satellite galaxy debris, and a "highmagnesium" population, which continues the chemical trends of the thick disk and has the chemistry expected of the classical halo.…”

Section: Discussionmentioning

confidence: 99%

“…Studies of metal-poor stars have uncovered two chemically distinct populations in the MW (e.g., Nissen & Schuster 2010;Hawkins et al 2015;Hayes et al 2018). These two populations are (using the definitions from Fernández-Alvar et al 2018; Hayes et al 2018) a "low-magnesium" halo population, thought to be accreted satellite galaxy debris, and a "highmagnesium" population, which continues the chemical trends of the thick disk and has the chemistry expected of the classical halo. The [X/Fe] ratios of TriAnd stars in (C+N), K, Mn, and Ni are 0.2-0.4 dex higher than the population of MW field stars thought to be accreted halo stars, and are also differentiated from the more classical halo population (and thick disk), which have high α-element abundances.…”

Section: Discussionmentioning

confidence: 99%

Disk-like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE

Hayes

Majewski

Hasselquist

et al. 2018

ApJL

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The nature of the Triangulum-Andromeda (TriAnd) system has been debated since the discovery of this distant, low-latitude Milky Way (MW) overdensity more than a decade ago. Explanations for its origin are either as a halo substructure from the disruption of a dwarf galaxy, or a distant extension of the Galactic disk. We test these hypotheses using the chemical abundances of a dozen TriAnd members from the Sloan Digital Sky Survey-IV's (SDSS-IV's) 14th Data Release (DR14) of Apache Point Observatory Galactic Evolution Experiment (APOGEE) data to compare to APOGEE abundances of stars with similar metallicity from both the Sagittarius (Sgr) dSph and the outer MW disk. We find that TriAnd stars are chemically distinct from Sgr across a variety of elements, (C+N), Mg, K, Ca, Mn, and Ni, with a separation in [X/Fe] of about 0.1 to 0.4 dex depending on the element. Instead, the TriAnd stars, with a median metallicity of about −0.8, exhibit chemical abundance ratios similar to those of the lowest metallicity ([Fe/H] ∼ −0.7) stars in the outer Galactic disk, and are consistent with expectations of extrapolated chemical gradients in the outer disk of the MW. These results suggest that TriAnd is associated with the MW disk, and, therefore, that the disk extends to this overdensity-i.e., past a Galactocentric radius of 24 kpc -albeit vertically perturbed about 7 kpc below the nominal disk midplane in this region of the Galaxy.

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“…Nevertheless, α peak elements, especially magnesium, can help distinguishing both populations. For example, using data from APOGEE Hawkins et al (2015) and Hayes et al (2018) have shown that for [Fe/H] −0.7 there are two distinct populations: an [α/Fe] poor sequence with zero net rotation, which is consistent with stars being mainly accreted from small dwarf satellites whose stellar populations were formed in the first few Gyr of the universe; and an [α/Fe] rich sequence, which has some net rotation and is kinematically colder than the [α/Fe] poor sequence. Hawkins et al (2015) point out that these [α/Fe] rich stars could come from either the thick disc or from some form of in-situ (in their words "canonical") halo population.…”

Section: Introductionmentioning

confidence: 99%

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

Amarante

Smith

Boeche

2020

Monthly Notices of the Royal Astronomical Society

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Although the stellar halo accounts for just ∼ 1% 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-disk 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 ≈ 13%. 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.

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Disentangling the Galactic Halo with APOGEE. I. Chemical and Kinematical Investigation of Distinct Metal-poor Populations

Cited by 158 publications

References 87 publications

The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

Disk-like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE

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

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