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The COMBS Survey - II. Distinguishing the metal-poor bulge from the halo interlopers

Lucey, Madeline ; Hawkins, Keith ; Ness, Melissa ; Debattista, Victor P. ; Luna, Alice ; Asplund, Martin ; Bensby, Thomas LU orcid ; Casagrande, Luca ; Feltzing, Sofia LU orcid and Freeman, Kenneth C. , et al. (2021) In Monthly Notices of the Royal Astronomical Society 501(4). p.5981-5996
Abstract

The metal-poor stars in the bulge are important relics of the Milky Way's formation history, as simulations predict that they are some of the oldest stars in the Galaxy. In order to determine if they are truly ancient stars, we must understand their origins. Currently, it is unclear if the metal-poor stars in the bulge ([Fe/H] < -1 dex) are merely halo interlopers, a unique accreted population, part of the boxy/peanut-shaped bulge, or a classical bulge population. In this work, we use spectra from the VLT/FLAMES spectrograph to obtain metallicity estimates using the Ca-II triplet of 473 bulge stars (187 of which have [Fe/H] < -1 dex), targeted using SkyMapper photometry. We also use Gaia DR2 data to infer the Galactic positions... (More)

The metal-poor stars in the bulge are important relics of the Milky Way's formation history, as simulations predict that they are some of the oldest stars in the Galaxy. In order to determine if they are truly ancient stars, we must understand their origins. Currently, it is unclear if the metal-poor stars in the bulge ([Fe/H] < -1 dex) are merely halo interlopers, a unique accreted population, part of the boxy/peanut-shaped bulge, or a classical bulge population. In this work, we use spectra from the VLT/FLAMES spectrograph to obtain metallicity estimates using the Ca-II triplet of 473 bulge stars (187 of which have [Fe/H] < -1 dex), targeted using SkyMapper photometry. We also use Gaia DR2 data to infer the Galactic positions and velocities along with orbital properties for 523 stars. We employ a probabilistic orbit analysis and find that about half of our sample has a >50 per cent probability of being bound to the bulge, and half are halo interlopers. We also see that the occurrence rate of halo interlopers increases steadily with decreasing metallicity across the full range of our sample (-3 < [Fe/H] < 0.5). Our examination of the kinematics of the confined compared to the unbound stars indicates the metal-poor bulge comprises at least two populations; those confined to the boxy/peanut bulge and halo stars passing through the inner galaxy. We conclude that an orbital analysis approach, as we have employed, is important to understand the composite nature of the metal-poor stars in the inner region.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Galaxy: bulge, Galaxy: evolution, stars: kinematics and dynamics, stars: Population II
in
Monthly Notices of the Royal Astronomical Society
volume
501
issue
4
pages
16 pages
publisher
Oxford University Press
external identifiers
  • scopus:85100802365
ISSN
0035-8711
DOI
10.1093/mnras/stab003
language
English
LU publication?
yes
id
6aefff74-8362-4a73-b484-12bd964eb0b2
date added to LUP
2021-03-01 09:45:21
date last changed
2022-06-29 13:38:35
@article{6aefff74-8362-4a73-b484-12bd964eb0b2,
  abstract     = {{<p>The metal-poor stars in the bulge are important relics of the Milky Way's formation history, as simulations predict that they are some of the oldest stars in the Galaxy. In order to determine if they are truly ancient stars, we must understand their origins. Currently, it is unclear if the metal-poor stars in the bulge ([Fe/H] &lt; -1 dex) are merely halo interlopers, a unique accreted population, part of the boxy/peanut-shaped bulge, or a classical bulge population. In this work, we use spectra from the VLT/FLAMES spectrograph to obtain metallicity estimates using the Ca-II triplet of 473 bulge stars (187 of which have [Fe/H] &lt; -1 dex), targeted using SkyMapper photometry. We also use Gaia DR2 data to infer the Galactic positions and velocities along with orbital properties for 523 stars. We employ a probabilistic orbit analysis and find that about half of our sample has a &gt;50 per cent probability of being bound to the bulge, and half are halo interlopers. We also see that the occurrence rate of halo interlopers increases steadily with decreasing metallicity across the full range of our sample (-3 &lt; [Fe/H] &lt; 0.5). Our examination of the kinematics of the confined compared to the unbound stars indicates the metal-poor bulge comprises at least two populations; those confined to the boxy/peanut bulge and halo stars passing through the inner galaxy. We conclude that an orbital analysis approach, as we have employed, is important to understand the composite nature of the metal-poor stars in the inner region. </p>}},
  author       = {{Lucey, Madeline and Hawkins, Keith and Ness, Melissa and Debattista, Victor P. and Luna, Alice and Asplund, Martin and Bensby, Thomas and Casagrande, Luca and Feltzing, Sofia and Freeman, Kenneth C. and Kobayashi, Chiaki and Marino, Anna F.}},
  issn         = {{0035-8711}},
  keywords     = {{Galaxy: bulge; Galaxy: evolution; stars: kinematics and dynamics; stars: Population II}},
  language     = {{eng}},
  number       = {{4}},
  pages        = {{5981--5996}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{The COMBS Survey - II. Distinguishing the metal-poor bulge from the halo interlopers}},
  url          = {{http://dx.doi.org/10.1093/mnras/stab003}},
  doi          = {{10.1093/mnras/stab003}},
  volume       = {{501}},
  year         = {{2021}},
}