The vast majority of Milky Way stellar halo stars were likely accreted from a small number (<~3) of relatively large
dwarf galaxy accretion events. However, the timing of these events is poorly constrained and predominantly relies
on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures.
Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain
the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available,
we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be within the stellar halo,
a subset of which are associated with the Gaia–Enceladus–Sausage halo substructure, and compare their ages to 3
red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined
here, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age
determination packages, we measure a weighted average age for the Gaia–Enceladus–Sausage stars in our sample
of 8+/-3 (stat.)+/-1 (sys.) Gyr. We also determine hierarchical ages using isochrones for the populations of
Gaia–Enceladus–Sausage, in situ halo and disk stars, finding a Gaia–Enceladus–Sausage population age of
8.0+2.3-3.2 Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited
data and sample of stars, this framework should allow a distinct characterization of Galactic substructures using
larger stellar samples and additional data available in the near future
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Galactic Gas Flows from Halo to Disk: Tomography and Kinematics at the Milky Way’s Disk–Halo Interface
- Award ID(s):
- 1715630
- PAR ID:
- 10253343
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 882
- Issue:
- 2
- ISSN:
- 1538-4357
- Page Range / eLocation ID:
- 76
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Phosphorus (P) is a critical element for life on Earth, yet the cosmic production sites of P are relatively uncertain. To understand how P has evolved in the solar neighborhood, we measured abundances for 163 FGK stars over a range of –1.09 < [Fe/H] < 0.47 using observations from the Habitable-zone Planet Finder instrument on the Hobby–Eberly Telescope. Atmospheric parameters were calculated by fitting a combination of astrometry, photometry, and Fe I line equivalent widths. Phosphorus abundances were measured by matching synthetic spectra to a P I feature at 10529.52 Å. Our [P/Fe] ratios show that chemical evolution models generally underpredict P over the observed metallicity range. Additionally, we find that the [P/Fe] differs by ∼0.1 dex between thin disk and thick disk stars that were identified with kinematics. The P abundances were compared with
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Abstract Stellar kinematics and metallicity are key to exploring formation scenarios for galactic disks and halos. In this work, we characterized the relationship between kinematics and photometric metallicity along the line of sight to M31's disk. We combined optical Hubble Space Telescope/Advanced Camera for Surveys photometry, from the Panchromatic Hubble Andromeda Treasury survey, with Keck/DEIMOS spectra, from the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. The resulting sample of 3512 individual red giant branch stars spans 4–19 projected kpc, making it a useful probe of both the disk and inner halo. We separated these stars into disk and halo populations, by modeling the line-of-sight velocity distributions as a function of position across the disk region, where ∼73% stars have a high likelihood of belonging to the disk and ∼14% to the halo. Although stellar halos are typically thought to be metal-poor, the kinematically identified halo contains a significant population of stars (∼29%) with disk-like metallicity ([Fe/H]phot∼ −0.10). This metal-rich halo population lags the gaseous disk to a similar extent as the rest of the halo, indicating that it does not correspond to a canonical thick disk. Its properties are inconsistent with those of tidal debris originating from the Giant Stellar Stream merger event. Moreover, the halo is chemically distinct from the phase-mixed component previously identified along the minor axis (i.e., away from the disk), implying contributions from different formation channels. These metal-rich halo stars provide direct chemodynamical evidence in favor of the previously suggested “kicked-up” disk population in M31's inner stellar halo.
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/1538-4357/ab3414
