Evidence for the Third Stellar Population in the Milky Way’s Disk

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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“…>90% ends at [Fe/H]≈-2 dex. This is consistent with the MDF of the metal-weak thick disk (Beers et al 2014;Carollo et al 2019).…”

Section: Discussionsupporting

confidence: 89%

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

Lucey

Hawkins

Ness

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…However, DTG-10 and DTG-15 are rotating much faster around the Galactic center ( v φ +140 km s −1 ; J φ +1100 kpc km s −1 ) than DTG-27 ( v φ ≈ +70 km s −1 ; J φ ≈ +600 kpc km s −1 ). The rotational motions of DTG-10/15 are compatible with the value suggested for the MWTD (Carollo et al 2019;. Comparison with both Carollo et al (2014) and Beers et al (2014) also point to similarities in (E, J φ ), but the Galactic gravitational-potential model used by these authors is different from ours, so this should be taken with caution.…”

Section: New Prograde Groupssupporting

confidence: 77%

“…used the powerful combination of photometric metallicities and precision astrometric surveys to produce a "blueprint" of the known stellar populations in the disk and halo systems of the Galaxy. In the process, these authors confirmed the presence of a dynamically heated disk population, named the "Splashed Disk" (SD; Bonaca et al 2017;Di Matteo et al 2019;Belokurov et al 2020;Amarante et al 2020a,b), and demonstrated that the metal-weak thick disk (MWTD) is an independent structure from the canonical thick disk (see also Carollo et al 2019).…”

Section: Introductionmentioning

confidence: 86%

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

Limberg¹,

Rossi²,

Perottoni³

et al. 2021

ApJ

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We analyze the dynamical properties of ∼1500 very metal-poor (VMP; [Fe/H] −2.0) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past thirty years, are supplemented by a number of calibration stars and other small samples, along with astrometric information from Gaia DR2. We apply a clustering algorithm to the 4-D energy-action space of the sample, and identify a set of 38 Dynamically Tagged Groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as Gaia-Sausage/Enceladus (GSE), Sequoia, the Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and ∼300 individual r-processenhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which r-process nucleosynthesis took place, and then were disrupted by the Milky Way.

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“…The evidence accumulated so far and discussed above suggests that we may have identified the reality of the proto-disk as the metal-weak (or "hot") thick disk (see also Carollo et al 2019), and that this was present more than 10 Gyr ago. It is interesting that relatively low eccentricity stars have survived as such the dynamical impact of a massive minor merger 24 , allowing this disk to be traced back to lower and lower metallicities as Figure 14 shows.…”

Section: Further Insights On the Early Disk From Chemistry And Dynamicsmentioning

confidence: 87%

Streams, Substructures, and the Early History of the Milky Way

Helmi

2020

Annu. Rev. Astron. Astrophys.

303

169

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The advent of the second data release of the Gaia mission, in combination with data from large spectroscopic surveys, is revolutionizing our understanding of the Galaxy. Thanks to these transformational data sets and the knowledge accumulated thus far, a new, more mature picture of the evolution of the early Milky Way is currently emerging. ▪ Two of the traditional Galactic components, namely, the stellar halo and the thick disk, appear to be intimately linked: Stars with halo-like kinematics originate in similar proportions from a heated (thick) disk and from a debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and was completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ∼ 1.8. This is likely related to the previously known metal-weak thick disk and may be traceable to metallicities [Fe/H] [Formula: see text] −4. As importantly, there is evidence that the merger with Gaia-Enceladus triggered star formation in the early Milky Way, plausibly leading to the appearance of the thick disk as we know it. ▪ Other merger events have been characterized better, and new ones have been uncovered. These include, for example, the Helmi streams, Sequoia, and Thamnos, which add to the list of those discovered in wide-field photometric surveys, such as the Sagittarius streams. Current knowledge of their progenitors’ properties, star formation, and chemical evolutionary histories is still incomplete. ▪ Debris from different objects shows different degrees of overlap in phase-space. This sometimes confusing situation can be improved by determining membership probabilities via quantitative statistical methods. A task for the next few years will be to use ongoing and planned spectroscopic surveys for chemical labeling and to disentangle events from one another using dimensions other than phase-space, metallicity, or [α/Fe]. ▪ These large surveys will also provide line-of-sight velocities missing for faint stars in Gaia releases and more accurate distance determinations for distant objects, which in combination with other surveys could also lead to more accurate age dating. The resulting samples of stars will cover a much wider volume of the Galaxy, allowing, for example, the linking of kinematic substructures found in the inner halo to spatial overdensities in the outer halo. ▪ All the results obtained so far are in line with the expectations of current cosmological models. Nonetheless, tailored hydrodynamical simulations to reproduce in detail the properties of the merger debris, as well as constrained cosmological simulations of the Milky Way, are needed. Such simulations will undoubtedly unravel more connections between the different Galactic components and their substructures, and will aid in pushing our knowledge of the assembly of the Milky Way to the earliest times. Expected final online publication date for the Annual Review of Astronomy, Volume 58 is August 18, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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Evidence for the Third Stellar Population in the Milky Way’s Disk

Cited by 60 publications

References 61 publications

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

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

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

Streams, Substructures, and the Early History of the Milky Way

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