ABSTRACT
Thanks to Gaia DR2, we proved for the first time that a general relativistic Milky Way rotation curve is statistically indistinguishable from its state-of-the-art dark matter analogue. Those ...results supported the ansatz that gravitational dragging can explain the observed flatness of the Milky Way rotation curve with a consistent radial matter density profile. To challenge again such a scenario, we select 719 143 young disc stars within |z| < 1 kpc and up to R ≃ 19 kpc from Gaia DR3 providing a much larger sample of high-quality astrometric and spectrophotometric data of unprecedented homogeneity. This sample comprises 241 918 OBA stars, 475 520 RGB giants, and 1705 Cepheides that we use to fit, as with DR2, both a classical velocity profile model, i.e. with a dark matter halo, and a general relativistic analogue derived from a dust disc-scale metric. Once more, further corroborating our earlier findings, both models are found to explain, with similar statistical quality, the new observed rotational velocities derived from different combinations of the selected sets of stars belonging to the disc of our Galaxy. The geometrical effect is found to drive the velocity profile from 10 kpc outwards, while being responsible for ∼30–37 per cent of this profile already at the Sun distance, similarly to the halo contribution in the classical model. This confirms our previous results on the contribution of Einstein’s geometry and pushes to further investigate the role of General Relativity in tracing the Milky Way rotation curve; notably, the origin of this gravitational dragging remains undetermined, necessitating a dedicated in-depth exploration.
ABSTRACT
We analyse from an observational perspective the formation history and kinematics of a Milky Way-like galaxy from a high-resolution zoom-in cosmological simulation that we compare to those ...of our Galaxy as seen by Gaia DR2 to better understand the origin and evolution of the Galactic thin and thick discs. The cosmological simulation was carried out with the gadget-3 TreePM+SPH code using the MUlti-Phase Particle Integrator (muppi) model. We disentangle the complex overlapping of stellar generations that rises from the top-down and inside-out formation of the galactic disc. We investigate cosmological signatures in the phase-space of mono-age populations and highlight features stemming from past and recent dynamical perturbations. In the simulation, we identify a satellite with a stellar mass of $1.2 \times 10^9~\rm {M}_\odot$, i.e. stellar mass ratio Δ ∼ 5.5 per cent at the time, accreted at z ∼ 1.6, which resembles the major merger Gaia–Sausage–Enceladus that produced the Galactic thick disc, i.e. Δ ∼ 6 per cent. We found at z ∼ 0.5–0.4 two merging satellites with a stellar mass of $8.8 \times 10^8~\rm {M}_\odot$ and $5.1 \times 10^8~\rm {M}_\odot$ that are associated to a strong starburst in the star formation history, which appears fairly similar to that recently found in the solar neighbourhood. Our findings highlight that detailed studies of coeval stellar populations kinematics, which are made available by current and future Gaia data releases and in synergy with simulations, are fundamental to unravel the formation and evolution of the Milky Way discs.
Abstract
We explore the local volume of the Milky Way via chemical and kinematical measurements from high-quality astrometric and spectroscopic data recently released by the Gaia, APOGEE, and GALAH ...programs. We chemically select 1137 stars up to 2.5 kpc of the Sun and Fe/H ≤ −1.0 dex, and find evidence of statistically significant substructures. Clustering analysis in velocity space classifies 163 objects into eight kinematical groups, whose origin is further investigated with high-resolution
N
-body numerical simulations of single merging events. The two retrograde groups appear associated with Gaia-Sausage-Enceladus (GSE), while the slightly prograde group could be connected to GSE or possibly Wukong. We find evidence of a new 44-member-strong prograde stream that we name Icarus; to our knowledge, Icarus is the fast-rotating stream closest to the Galactic disk to date (
, 〈
V
+
V
LSR
〉 ≃ 231 km s
−1
). Its peculiar chemical (〈Fe/H〉 ≃ −1.45, 〈Mg/Fe〉 ≃ −0.02) and dynamical (mean eccentricity ≃ 0.11) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of ∼10
9
M
☉
on an initial prograde low-inclination orbit, ∼10°. The remaining prograde groups are either streams previously released by the same progenitor of Icarus (or Nyx), or remnants from different satellites accreted on initial orbits at higher inclination.
THE CASE FOR THE DUAL HALO OF THE MILKY WAY BEERS, Timothy C; CAROLLO, Daniela; SIVARANI, Thirupathi ...
The Astrophysical journal,
02/2012, Letnik:
746, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Carollo et al. have recently resolved the stellar population of the Milky Way halo into at least two distinct components, an inner halo and an outer halo. This result has been criticized by Schonrich ...et al., who claim that the retrograde signature associated with the outer halo is due to the adoption of faulty distances. We refute this claim, and demonstrate that the Schonrich et al. photometric distances are themselves flawed because they adopted an incorrect main-sequence absolute magnitude relationship from the work of Ivezic et al. When compared to the recommended relation from Ivezic et al., which is tied to a Milky Way globular cluster distance scale and accounts for age and metallicity effects, the relation adopted by Schonrich et al. yields up to 18% shorter distances for stars near the main-sequence turnoff (TO). Use of the correct relationship yields agreement between the distances assigned by Carollo et al. and Ivezic et al. for low-metallicity dwarfs to within 6%-10%. Schonrich et al. also point out that intermediate-gravity stars (3.5 < or =, slant log g < 4.0) with colors redder than the TO region are likely misclassified, with which we concur. We implement a new procedure to reassign luminosity classifications for the TO stars that require it. New derivations of the rotational behavior demonstrate that the retrograde signature and high velocity dispersion of the outer-halo population remain. We summarize additional lines of evidence for a dual halo, including a test of the retrograde signature based on proper motions alone, and conclude that the preponderance of evidence strongly rejects the single-halo interpretation.
ABSTRACT
This study is based on high quality astrometric and spectroscopic data from the most recent releases by Gaia and APOGEE. We select $58\, 882$ thin and thick disc red giants in the ...Galactocentric (cylindrical) distance range 5 < R < 13 kpc and within |$z$| < 3 kpc, for which full chemo-kinematical information is available. Radial chemical gradients, $\partial \rm {M/H} / \partial \rm {R}$, and rotational velocity–metallicity correlations, $\partial V_\phi / \partial \rm {M/H}$, are re-derived firmly uncovering that the thick disc velocity–metallicity correlation maintains its positiveness over the 8 kpc range explored. This observational result is important as it sets experimental constraints on recent theoretical studies on the formation and evolution of the Milky Way disc and on cosmological models of Galaxy formation.
The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of ...galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components--an inner and an outer halo--that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Using effective temperature and metallicity derived from SDSS spectra for image60,000 F- and G-type main-sequence stars, we develop polynomial models for estimating these parameters from the SDSS ...image and image colors. These photometric estimates have similar error properties as those determined from SDSS spectra. We apply this method to SDSS photometric data for over 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, which correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from -0.6 at 500 pc to -0.8 beyond several kiloparsecs. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics-metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of image, with an rms scatter of only image0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper-motion measurements accurate to image0.5 mas yr super(-1), for about 200 million F/G dwarf stars within a distance limit of image100 kpc.
The concept of precisely gauging a gravity-dominated Universe like ours through the individual observations of its fundamental constituents, the stars, immediately calls astrometry, the oldest ...quantitative specialty of astronomy, into play. Today, thanks to the launch of the Gaia satellite, astrometry has reached such levels to become a key player in the field of local cosmology and experimental gravitation. Updates on the status of the mission, orbiting in L2 since January 2014 and in nominal observation mode since July 2014, are presented. We also discuss how the astrometric observations from within the gravitational fields of the Solar System can uniquely probe possible deviations from General Relativity and how accurate absolute kinematics at the scale of the Milky Way can, for the first time in situ, account for the predictions of the CDM model for the formation of the Galactic halo.
We investigate the relationship between several previously identified Galactic halo stellar structures in the direction of Virgo using imaging and spectroscopic observations of F turnoff stars and ...blue horizontal-branch stars from SDSS and SEGUE. We show that the Sagittarius dwarf leading tidal tail does not pass through the solar neighborhood; it misses the Sun by more than 15 kpc, passing through the Galactic plane outside the solar circle. Nor is it spatially coincident with the large stellar overdensity S297+63-20.5 in the Virgo constellation. S297+63-20.5 has a distinct turnoff color and kinematics. Faint ( unk similar to 20.3) turnoff stars in S297+63-20.5 have line-of-sight, Galactic standard of rest velocities unk = 130 plus or minus 10 km s super(-1), opposite In sign to infalling Sgr tail stars. The path of the Sgr leading tidal tail is also inconsistent with the positions of some of the nearer stars with which it has been associated and whose velocities have favored models with prolate Milky Way potentials. We also show that the number densities of brighter ( unk similar to 10.8) F turnoff stars are not symmetric about the Galactic center and that this discrepancy is not primarily due to the S297+63-20.5 moving group. Either the spheroid is asymmetric about the Galactic center or there are additional substructures that conspire to be on the same side of the Galaxy as S297+63-20.5. The S297+63-20.5 overdensity in Virgo is likely associated with two other previously identified Virgo substructures: the Virgo stellar stream (VSS) and the Virgo overdensity (VOD). However, the velocity difference between the VSS and S297+63-20.5 and the difference in distance estimates between the VOD and S297+63-20.5 must be reconciled.
We study Milky Way kinematics using a sample of 18.8 million main-sequence stars with r < 20 and proper-motion measurements derived from Sloan Digital Sky Survey (SDSS) and POSS astrometry, including ...{approx}170,000 stars with radial-velocity measurements from the SDSS spectroscopic survey. Distances to stars are determined using a photometric-parallax relation, covering a distance range from {approx}100 pc to 10 kpc over a quarter of the sky at high Galactic latitudes (|b|>20{sup 0}). We find that in the region defined by 1 kpc <Z< 5 kpc and 3 kpc <R< 13 kpc, the rotational velocity for disk stars smoothly decreases, and all three components of the velocity dispersion increase, with distance from the Galactic plane. In contrast, the velocity ellipsoid for halo stars is aligned with a spherical coordinate system and appears to be spatially invariant within the probed volume. The velocity distribution of nearby (Z < 1 kpc) K/M stars is complex, and cannot be described by a standard Schwarzschild ellipsoid. For stars in a distance-limited subsample of stars (<100 pc), we detect a multi-modal velocity distribution consistent with that seen by HIPPARCOS. This strong non-Gaussianity significantly affects the measurements of the velocity-ellipsoid tilt and vertex deviation when using the Schwarzschild approximation. We develop and test a simple descriptive model for the overall kinematic behavior that captures these features over most of the probed volume, and can be used to search for substructure in kinematic and metallicity space. We use this model to predict further improvements in kinematic mapping of the Galaxy expected from Gaia and the Large Synoptic Survey Telescope.
PDF
