Identifying Contributions to the Stellar Halo From Accreted, Kicked-Out, and in Situ Populations

Sheffield, Allyson A.; Majewski, Steven R.; Johnston, Kathryn V.; Cunha, Kátia; Smith, Verne V.; Cheung, Andrew M.; Hampton, Christina M.; David, Trevor J.; Wagner-Kaiser, R.; Johnson, Marshall C.; Kaplan, Evan; Miller, Judith E.; Patterson, Richard J.

doi:10.1088/0004-637x/761/2/161

Cited by 50 publications

(70 citation statements)

References 97 publications

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“…Acknowledging the much smaller net samples, we still find that even the more metal-rich stars of the LMg exhibit halo-like motions, which further justifies that the LMg population is a coherent and distinct population from the dynamically colder HMg population. (Nissen & Schuster 2010Navarro et al 2011;Ramírez et al 2012;Schuster et al 2012;Sheffield et al 2012;Jackson-Jones et al 2014;Hawkins et al 2015). These groups appear generally to correspond well with our HMg and LMg populations, as we now demonstrate.…”

Section: Kinematical Nature Of the Lmg And Hmg Populationssupporting

confidence: 90%

“…From the α-element abundances and kinematics of the two populations, these past studies have suggested that the low-α population has been accreted through the mergers of dwarf spheroidal-like galaxies (an origin also suggested for "young halo" globular clusters; see Zinn 1993), whereas the high-α stars were likely formed in situ or have been kicked out from the disk (Sheffield et al 2012;Johnston 2016). Recent studies have also revealed low-α bulge stars, most of which are thought to be chemically associated with the thin disk (Recio-Blanco et al 2017).…”

Section: Introductionmentioning

confidence: 98%

“…Among differences in Mg and other α-elements (distinguishable as "high-and low-α" halo star groupings), these two metal-poor populations were shown to have different abundances in many of the odd-Z and heavier elements listed above (Nissen & Schuster 2010Navarro et al 2011;Ramírez et al 2012;Sheffield et al 2012;Jackson-Jones et al 2014;Hawkins et al 2015).…”

Section: Introductionmentioning

confidence: 99%

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

Hayes

Majewski

Shetrone

et al. 2018

ApJ

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158

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We find two chemically distinct populations separated relatively cleanly in the [Fe/H]-[Mg/Fe] plane, but also distinguished in other chemical planes, among metal-poor stars (primarily with metallicities < -[ ] Fe H 0.9) observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and analyzed for Data Release 13 (DR13) of the Sloan Digital Sky Survey. These two stellar populations show the most significant differences in their [X/Fe] ratios for the α-elements, C+N, Al, and Ni. In addition to these populations having differing chemistry, the low metallicity high-Mg population (which we denote "the HMg population") exhibits a significant net Galactic rotation, whereas the low-Mg population (or "the LMg population") has halo-like kinematics with little to no net rotation. Based on its properties, the origin of the LMg population is likely an accreted population of stars. The HMg population shows chemistry (and to an extent kinematics) similar to the thick disk, and is likely associated with in situ formation. The distinction between the LMg and HMg populations mimics the differences between the populations of low-and high-α halo stars found in previous studies, suggesting that these are samples of the same two populations.

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Section: Kinematical Nature Of the Lmg And Hmg Populationssupporting

confidence: 90%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

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

Hayes

Majewski

Shetrone

et al. 2018

ApJ

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158

122

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“…These results are due to the fact that in situ stars form in a deeper potential well than the accreted population. Several studies have used abundances in an effort to disentangle these populations locally (e.g., Nissen & Schuster (2010), with F and G main sequence stars within 335 pc; Sheffield et al (2012) with M Giants out to 10 kpc). However, no such studies exist using main sequence stars outside the solar neighborhood.…”

Section: Chemistrymentioning

confidence: 99%

Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

Cunningham

Deason

Guhathakurta

et al. 2016

ApJ

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We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions (PMs). The PMs were measured using long baseline (5-7 years) multi-epoch Hubble Space Telescope/Advanced Camera for Surveys photometry, and the LOS velocities were extracted from deep (5-6 hr integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l, b, LOS) coordinate system are v 138 We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find 0.3 0.9 0.4 b = --+ , consistent with isotropy and lower than solar neighborhood β measurements by 2σ (β SN ∼0.5-0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to 0.1 1.0which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r∼25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.

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“…accreted stars). These two populations are commonly referred as "α-rich" and "α-poor" (Nissen & Schuster 2010), which have been subject of interest among other studies (Ramírez, Meléndez & Chanamé 2012;Sheffield et al 2012). These two populations have been extensively studied in a series of papers by Schuster & Nissen. They have used a sample of high resolution and high signal-to-noise ratio (SNR) spectra of halo stars to show that these two populations are distinct in kinematics and abundances of α-elements (N10), but indistinguishable from other chemical abundances such as Li and Mn (Nissen & Schuster 2011.…”

Section: Introductionmentioning

confidence: 99%

On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

Hawkins

Jofré

Gilmore

et al. 2014

Monthly Notices of the Royal Astronomical Society

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We study the ages of α-rich and α-poor stars in the halo using a sample of F and G dwarfs from the Sloan Digital Sky Survey (SDSS). To separate stars based on [α/Fe], we have developed a new semi-empirical spectral-index based method and applied it to the low-resolution, moderate signal-to-noise SDSS spectra. The method can be used to estimate the [α/Fe] directly providing a new and widely applicable way to estimate [α/Fe] from low-resolution spectra. We measured the main-sequence turnoff temperature and combined it with the metallicities and a set of isochrones to estimate the age of the α-rich and α-poor populations in our sample. We found all stars appear to be older than 8 Gyr confirming the idea that the Galactic halo was formed very early on. A bifurcation appears in the age-metallicity relation such that in the low metallicity regime the α-rich and α-poor populations are coeval while in the high metallicity regime the α-rich population is older than the α-poor population. Our results indicate the α-rich halo population, which has shallow age-metallicity relation, was formed in a rapid event with high star formation, while the α-poor stars were formed in an environment with a slower chemical evolution timescale.

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Identifying Contributions to the Stellar Halo From Accreted, Kicked-Out, and in Situ Populations

Cited by 50 publications

References 97 publications

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

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

Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

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