Evidence of a large scale positive rotation-metallicity correlation in the Galactic thick disk

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 disk 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 disk 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 disk and on cosmological models of Galaxy formation.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

New Signatures of the Milky Way Formation in the Local Halo and Inner Halo Streamers in the Era of Gaia

We explore the vicinity of the Milky Way through the use of spectro-photometric data from the Sloan Digital Sky Survey and high-quality proper motions derived from multi-epoch positions extracted from the Guide Star Catalogue II database. In order to identify and characterise streams as relics of the Milky Way formation, we start with classifying, select, and study $2417$ subdwarfs with $\rm{[Fe/H] < -1.5}$ up to $3$ kpc away from the Sun as tracers of the local halo system. Then, through phase-space analysis, we find statistical evidence of five discrete kinematic overdensities among $67$ of the fastest-moving stars, and compare them to high-resolution N-body simulations of the interaction between a Milky-Way like galaxy and orbiting dwarf galaxies with four representative cases of merging histories. The observed overdensities can be interpreted as fossil substructures consisting of streamers torn from their progenitors, such progenitors appear to be satellites on prograde and retrograde orbits on different inclinations. In particular, of the five detected overdensities, two appear to be associated, yelding twenty-one additional main-sequence members, with the stream of Helmi et al. (1999) that our analysis confirms on a high inclination prograde orbit. The three newly identified kinematic groups could be associated with the retrograde streams detected by Dinescu (2002) and Kepley et al. (2007), whatever their origin, the progenitor(s) would be on retrograde orbit(s) and inclination(s) within the range $10^{\circ} \div 60^{\circ}$. Finally, we use our simulations to investigate the impact of observational errors and compare the current picture to the promising prospect of highly improved data expected from the Gaia mission.Comment: 16 pages, 10 figures, 6 Tables. Accepted for publication in The Astronomical Journa

Icarus: A Flat and Fast Prograde Stellar Stream in the Milky Way Disk

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 ($langle {Z}_{max } angle lesssim 0.5,mathrm{kpc}$, 〈V + V LSR〉 sime 231 km s−1). Its peculiar chemical (〈[Fe/H]〉 sime −1.45, 〈[Mg/Fe]〉 sime −0.02) and dynamical (mean eccentricity sime 0.11) properties are consistent with the accretion of debris from a dwarf galaxy progenitor with a stellar mass of ~109 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

Relics of the Formation of the Galactic Halo from Gaia and APOGEE

The solar neighborhood potentially contains a very large number of kinematic groups which are related to the various building blocks of the stellar halo. We explore the vicinity of the Milky Way through the use of high quality astrometric and spectroscopic data from the most recent releases by Gaia and APOGEE. We chemically select 663 halo stars and analyse their kinematics and orbital properties in order to investigate and characterise the possibly detectable signatures that remain in phase-space. We find evidence of statistically significant substructures among 177 stars, with velocity difference less than 20 km/s, that are classified in 15-25 kinematic groups and compared to the high velocity streamers by Re Fiorentin et al. 2015. The signal is even stronger among the stars with [Mg/Fe] < 0.2 dex, that more likely have been accreted; preliminary results are shown. The chemical properties of the kinematically selected moving groups are going to be analysed to reconstruct the accretion history of the stellar halo

The Overdensity in Virgo, Sagittarius Debris, and the Asymmetric Spheroid

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 the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (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. It also is not 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 (g ~ 20.3) turnoff stars in S297+63-20.5 have line-of-sight, Galactic standard of rest velocities V(GSR)= 130 +/- 10 km/s, 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 additionally show that the number densities of brighter (g ~ 19.8) 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.Comment: 10 figures, ApJ in pres

The Gaia mission: the dawn of Astrometric Cosmology? Status and prospects after 14 months of science operations

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

The thick disk rotation-metallicity correlation, comparison with Galactic cosmological simulations

Although the existence of a thick disc in the Milky Way was revealed 35 years ago and its spatial, kinematic, and chemical properties are today better defined, its origin is still matter of debate. Proposed scenarios include the heating of a pre-existing thin disc through a minor merger, accretion of dwarf galaxies stars from disrupted satellites, or stars formed in situ from gas-rich mergers at high redshift.In order to better understand these processes, we have investigated the chemo-dynamical evolution of a Milky Way-like disk galaxy, as produced by the recent cosmological simulations, integrating a sub-resolution ISM model, published by Murante et al. (2015). In particular, we evidence a global inside-out and top-down disk evolution.Recently, Re Fiorentin, Lattanzi & Spagna (2019) analysed a new chemo-kinematic catalogue based on Gaia DR2 and APOGEE DR14 and showed evidence that the thick disk rotation-metallicity correlation is persistently positive from R=5 kpc to 13 kpc, in spite of a quasi-flat metallicity gradient.Our simulation at redshift z=0 shows very similar properties when we look at the "thick disk" stellar particles at 1 kpc 2) with a negative rotation-metallicity correlation associated with a negative radial metallicity gradient (cfr. also Schoenrich & McMillan 2017; Kawata et al. 2018)

Chemo-dynamical signatures in simulated Milky Way-like galaxies

We have investigated the chemo-dynamical evolution of a Milky Way-like disk galaxy, AqC4, produced by a cosmological simulation integrating a sub-resolution ISM model. We evidence a global inside-out and upside-down disk evolution, that is consistent with a scenario where the ``thin disk'' stars are formed from the accreted gas close to the galactic plane, while the older ``thick disk'' stars are originated in situ at higher heights. Also, the bar appears the most effective heating mechanism in the inner disk. Finally, no significant metallicity-rotation correlation has been observed, in spite of the presence of a negative [Fe/H] radial gradient

Euclid FGS Input Star Catalogue Verification Test Report

This document is applicable to the FGS Star Catalogue in the frame of the EUCLID Program in the Pre-CD phase and CD phases.The objective of the verification testing is to assess to what degree the delivered third version of the ISC (ISC.R3) satisfy the requirements defined by the Requirement Specifications (EUCL-TAST-RS-2-056). This document presents the results of the executed verification test cases, described in the ISC Verification Test Specifications (VTS), as part of the requirement verification phase described in the DDVP

Euclid FGS Input Star Catalogue - Performance Assessment

This document is applicable to the FGS Star Catalogue in the frame of the EUCLID Program in the Pre-CD phase and CD phases.The objective of this performance assessment is to evaluate the ISC performance with respect to some of the key requirements defined in EUCL-TAST-RS-2-056 that may impact the performance of the FGS. This document presents a broader analysis of the ISC performance, taking into consideration the results of the executed verification test cases described in the VTR