| 1. |
- Ahumada, Romina, et al.
(författare)
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The 16th Data Release of the Sloan Digital Sky Surveys : First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra
- 2020
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Ingår i: Astrophysical Journal Supplement Series. - : Institute of Physics (IOP). - 0067-0049 .- 1538-4365. ; 249:1
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Tidskriftsartikel (refereegranskat)abstract
- This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).
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| 2. |
- Feuillet, Diane, et al.
(författare)
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The SkyMapper-Gaia RVS view of the Gaia–Enceladus–Sausage – an investigation of the metallicity and mass of the Milky Way’s last major merger
- 2020
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Ingår i: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 1365-2966 .- 0035-8711. ; 497:1, s. 109-124
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Tidskriftsartikel (refereegranskat)abstract
- We characterize the Gaia–Enceladus–Sausage kinematic structure recently discovered in the Galactic halo using photometric metallicities from the SkyMapper survey, and kinematics from Gaia radial velocities measurements. By examining the metallicity distribution functions (MDFs) of stars binned in kinematic/action spaces, we find that the JR‾‾‾√ versus Lz space allows for the cleanest selection of Gaia–Enceladus–Sausage stars with minimal contamination from disc or halo stars formed in situ or in other past mergers. Stars with 30≤JR‾‾‾√≤50 (kpc km s−1)1/2 and −500 ≤ Lz ≤ 500 kpc km s−1 have a narrow MDF centred at [Fe/H] = −1.17 dex with a dispersion of 0.34 dex. This [Fe/H] estimate is more metal-rich than literature estimates by 0.1−0.3 dex. Based on the MDFs, we find that selection of Gaia–Enceladus–Sausage stars in other kinematic/action spaces without additional population information leads to contaminated samples. The clean Gaia–Enceladus–Sausage sample selected according to our criteria is slightly retrograde and lies along the blue sequence of the high VT halo colour magnitude diagram dual sequence. Using a galaxy mass–metallicity relation derived from cosmological simulations and assuming a mean stellar age of 10 Gyr, we estimate the mass of the Gaia–Enceladus–Sausage progenitor satellite to be 108.85–9.85 M⊙, which is consistent with literature estimates based on disc dynamic and simulations. Additional information on detailed abundances and ages would be needed for a more sophisticated selection of purely Gaia–Enceladus–Sausage stars.
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| 3. |
- Hasselquist, Sten, et al.
(författare)
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Exploring the stellar age distribution of the milky way bulge using APOGEE
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 901:2
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Tidskriftsartikel (refereegranskat)abstract
- We present stellar age distributions of the Milky Way bulge region using ages for ∼6000 high-luminosity (log(g)< 2.0), metal-rich ([Fe/H] ≥ -0.5) bulge stars observed by the Apache Point Observatory Galactic Evolution Experiment. Ages are derived using The Cannon label-transfer method, trained on a sample of nearby luminous giants with precise parallaxes for which we obtain ages using a Bayesian isochrone-matching technique. We find that the metal-rich bulge is predominantly composed of old stars (>8 Gyr). We find evidence that the planar region of the bulge (ZGC| 0.25 kpc) is enriched in metallicity, Z, at a faster rate (dZ/dt ∼ 0.0034 Gyr-1) than regions farther from the plane (dZ/dt ∼ 0.0013 Gyr-1 at | ZGC| > 1.00 kpc). We identify a nonnegligible fraction of younger stars (age ∼2-5 Gyr) at metallicities of +0.2 < [Fe/H] < +0.4. These stars are preferentially found in the plane (ZGC| ≤ 0.25 kpc) and at R cy ≈ 2-3 kpc, with kinematics that are more consistent with rotation than are the kinematics of older stars at the same metallicities. We do not measure a significant age difference between stars found inside and outside the bar. These findings show that the bulge experienced an initial starburst that was more intense close to the plane than far from the plane. Then, star formation continued at supersolar metallicities in a thin disk at 2 kpc ≲ R cy ≲ 3 kpc until ∼2 Gyr ago.
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| 4. |
- Kushniruk, Iryna, et al.
(författare)
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The HR 1614 moving group is not a dissolving cluster
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 638
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Tidskriftsartikel (refereegranskat)abstract
- Context. The HR 1614 is an overdensity in velocity space and has for a long time been known as an old (2 Gyr) and metal-rich ([Fe=H] +0:2) nearby moving group that has a dissolving open cluster origin. The existence of such old and metal-rich groups in the solar vicinity is quite unexpected since the vast majority of nearby moving groups are known to be young. Aims. In the light of new and significantly larger data sets than ever before (astrometric, photometric, and spectroscopic), we aim to re-investigate the properties and origin of the HR 1614 moving group. If the HR 1614 overdensity is a dissolving cluster, its stars should represent a single-age and single-elemental abundance population. Methods. To identify and characterise the HR 1614 moving group we use astrometric data from Gaia DR2; distances, extinction, and reddening corrections from the StarHorse code; elemental abundances from the GALAH and APOGEE spectroscopic surveys; and photometric metallicities from the SkyMapper survey. Bayesian ages were estimated for the SkyMapper stars. Since the Hercules stream is the closest kinematical structure to the HR 1614 moving group in velocity space and as its origin is believed to be wellunderstood, we use the Hercules stream for comparison purposes. Stars that are likely to be members of the two groups were selected based on their space velocities. Results. The HR 1614 moving group is located mainly at negative U velocities, does not form an arch of constant energy in the U V space, and is tilted in V. We find that the HR 1614 overdensity is not chemically homogeneous, but that its stars exist at a wide range of metallicities, ages, and elemental abundance ratios. They are essentially similar to what is observed in the Galactic thin and thick discs, a younger population (around 3 Gyr) that is metal-rich (0:2 [Fe=H] 0:4) and alpha-poor. These findings are very similar to what is seen for the Hercules stream, which is believed to have a dynamical origin and consists of regular stars from the Galactic discs. Conclusions. The HR 1614 overdensity has a wide spread in metallicity, [Mg/Fe], and age distributions resembling the general properties of the Galactic disc. It should therefore not be considered a dissolving open cluster, or an accreted population. Based on the kinematic and chemical properties of the HR 1614 overdensity we suggest that it has a complex origin that could be explained by combining several different mechanisms such as resonances with the Galactic bar and spiral structure, phase mixing of dissolving spiral structure, and phase mixing due to an external perturbation.
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| 5. |
- Nidever, David L., et al.
(författare)
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The Lazy Giants : APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds
- 2020
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Ingår i: Astrophysical Journal. - : Institute of Physics (IOP). - 0004-637X .- 1538-4357. ; 895:2
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Tidskriftsartikel (refereegranskat)abstract
- We report the first APOGEE metallicities and alpha-element abundances measured for 3600 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds, the largest Milky Way (MW) dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument on the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity, from [Fe/H] = -0.2 to very metal-poor stars with [Fe/H] -2.5, the most metal-poor Magellanic Cloud (MC) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by similar to 0.1 dex at -1.0 < [Fe/H] less than or similar to -0.5. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent "starburst." At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H] and constrain the "alpha-knee" to [Fe/H] less than or similar to -2.2 in both MCs, implying a low star formation efficiency of similar to 0.01 Gyr(-1). The MC knees are more metal-poor than those of less massive MW dwarf galaxies such as Fornax, Sculptor, or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW's gravitational potential only recently.
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