Context. We present the early installment of the third Gaia data release, Gaia EDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities from Gaia DR2. Aims. A summary of the contents of Gaia EDR3 is presented, accompanied by a discussion on the differences with respect to Gaia DR2 and an overview of the main limitations which are present in the survey. Recommendations are made on the responsible use of Gaia EDR3 results. Methods. The raw data collected with the Gaia instruments during the first 34 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium and turned into this early third data release, which represents a major advance with respect to Gaia DR2 in terms of astrometric and photometric precision, accuracy, and homogeneity. Results. Gaia EDR3 contains celestial positions and the apparent brightness in G for approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the (GBP − GRP) colour are also available. The passbands for G, GBP, and GRP are provided as part of the release. For ease of use, the 7 million radial velocities from Gaia DR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, the Gaia-CRF3, which is based solely on extragalactic sources. The creation of the source list for Gaia EDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that for Gaia DR2, but it does feature new sources and there are some notable changes. The source list will not change for Gaia DR3. Conclusions. Gaia EDR3 represents a significant advance over Gaia DR2, with parallax precisions increased by 30 per cent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30–40% for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband for G, GBP, and GRP is valid over the entire magnitude and colour range, with no systematics above the 1% level

In this paper we present an improved fitting formula for the dark matter bispectrum motivated by the previous phenomenological approach of Ref [1]. We use a set of LCDM simulations to calibrate the fitting parameters in the k-range of 0.03 h/Mpc ≤ k ≤ 0.4 h/Mpc and in the redshift range of 0 ≤ z ≤ 1.5. This new proposed fit describes well the BAO-features although it was not designed to. The deviation between the simulations output and our analytic prediction is typically less than 5% and in the worst case is never above 10%. We envision that this new analytic fitting formula will be very useful in providing reliable predictions for the non-linear dark matter bispectrum for LCDM models.

We report evidence from APOGEE for the presence of a new metal-poor stellar structure located within ∼4 kpc of the Galactic Centre. Characterized by a chemical composition resembling those of low-mass satellites of the Milky Way, this new inner Galaxy structure (IGS) seems to be chemically and dynamically detached from more metal-rich populations in the inner Galaxy. We conjecture that this structure is associated with an accretion event that likely occurred in the early life of the Milky Way. Comparing the mean elemental abundances of this structure with predictions from cosmological numerical simulations, we estimate that the progenitor system had a stellar mass of ∼5 × 108 M⊙, or approximately twice the mass of the recently discovered Gaia-Enceladus/Sausage system. We find that the accreted:in situ ratio within our metal-poor ([Fe/H] < –0.8) bulge sample is somewhere between 1:3 and 1:2, confirming predictions of cosmological numerical simulations by various groups.

We analyse a set of cosmological magneto-hydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called “Gaia Sausage” found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50$\%$ of gas to a merger-induced centrally concentrated starburst that results in the rapid formation of a compact, rotationally supported thick disc that occupies the typical chemical thick disc region of chemical abundance space. We find evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering stars onto less-circular orbits such that their rotation velocity and metallicity positively correlate, thus contributing an additional component that connects the Galactic thick disc to the inner stellar halo. We demonstrate that the level of kinematic heating of the proto-galaxy correlates with the kinematic state of the population before the merger, the progenitor mass and orbital eccentricity of the merger. Furthermore, we show that the mass and time of the merger can be accurately inferred from local stars on counter-rotating orbits.

We explore the chemodynamical properties of a sample of barred galaxies in the Auriga magnetohydrodynamical cosmological zoom-in simulations, which form boxy/peanut (b/p) bulges, and compare these to the Milky Way (MW). We show that the Auriga galaxies which best reproduce the chemodynamical properties of stellar populations in the MW bulge have quiescent merger histories since redshift z ∼ 3.5: their last major merger occurs at $t_{\rm lookback}\gt 12\, \rm Gyr$, while subsequent mergers have a stellar mass ratio of ≤1:20, suggesting an upper limit of a few per cent for the mass ratio of the recently proposed Gaia Sausage/Enceladus merger. These Auriga MW-analogues have a negligible fraction of ex-situ stars in the b/p region ($\lt 1{{\ \rm per\ cent}}$), with flattened, thick disc-like metal-poor stellar populations. The average fraction of ex-situ stars in the central regions of all Auriga galaxies with b/p’s is 3 per cent – significantly lower than in those which do not host a b/p or a bar. While the central regions of these barred galaxies contain the oldest populations, they also have stars younger than 5 Gyr (>30 per cent) and exhibit X-shaped age and abundance distributions. Examining the discs in our sample, we find that in some cases a star-forming ring forms around the bar, which alters the metallicity of the inner regions of the galaxy. Further out in the disc, bar-induced resonances lead to metal-rich ridges in the Vϕ − r plane – the longest of which is due to the Outer Lindblad Resonance. Our results suggest the Milky Way has an uncommonly quiet merger history, which leads to an essentially in-situ bulge, and highlight the significant effects the bar can have on the surrounding disc.

We explore the origin of phase-space substructures revealed by the second Gaia data release in the disc of the Milky Way, such as the ridges in the V φ -r plane, the undulations in the V φ -r-V r space and the streams in the V φ -V r plane. We use a collisionless N-body simulation with co-spatial thin and thick discs, along with orbit integration, to study the orbital structure close to the Outer Lindblad Resonance (OLR) of the bar. We find that a prominent, long-lived ridge is formed in the V φ -r plane due to the OLR which translates to streams in the V φ -V r plane and examine which closed periodic and trapped librating orbits are responsible for these features. We find that orbits which carry out small librations around the x 1 (1) family are preferentially found at negative V r , giving rise to a 'horn'-like feature, while orbits with larger libration amplitudes, trapped around the x 1 (2) and x 1 (1) families, constitute the positive V r substructure, i.e. the Hercules-like feature. This changing libration amplitude of orbits will translate to a changing ratio of thin/thick disc stars, which could have implications on the metallicity distribution in this plane. We find that a scenario in which the Sun is placed close to the OLR gives rise to a strong asymmetry in V r in the V φ -V r plane (i.e. Hercules vs. 'the horn') and subsequently to undulations in the V φ -r-V r space. We also explore a scenario in which the Sun is placed closer to the bar corotation and find that the bar perturbation alone cannot give rise to the these features.

We show that the bulge and the disk of the Milky Way (MW) at R 7 kpc are well described by a unique chemical evolution and a two phase star-formation history (SFH). We argue that the populations within this inner disk, not the entire disk, are the same, and that the outer Lindblad resonance (OLR) of the bar plays a key role in explaining this uniformity. In our model of a two phase star formation history, the metallicity, [α/Fe] and [α/H] distributions, and age-metallicity relation are all compatible with the observations of both the inner disk and bulge. The dip at [Fe/H]∼0 dex seen in the metallicity distributions of the bulge and inner disk reflects the quenching episode in the SFH of the inner MW at age ∼8 Gyr, and the common evolution of the bulge and inner disk stars. Our results for the inner region, R 7 kpc, of the MW, are consistent with a rapid build-up of a large fraction of its total baryonic mass within a few Gyr. We show that at z≤1.5, when the MW was starting to quench, transitioning between the end of the α-enhanced thick disk formation to the start of the thin disk, and yet was still gas rich, the gas accretion rate could not have been significant. The [α/Fe] abundance ratio before and after this quenching phase would be different, which is not observed. The decrease in the accretion rate and gas fraction at z≤2 was necessary to stabilize the disk allowing the transition from thick to thin disks, and for beginning the secular phase of the MW's evolution. This possibly permitted a stellar bar to develop which we hypothesize is responsible for quenching the star formation. The present analysis suggests that Milky Way history, and in particular at the transition from the thick to the thin disk -the epoch of the quenching -must have been driven by a decrease of the star formation efficiency. We argue that the decline in the intensity of gas accretion, the formation of the bar, and the quenching of the SFR at the same epoch may be causally connected thus explaining their temporal coincidence. Assuming that about 20% of the gas reservoir in which metals are diluted is molecular, we show that our model is well positioned on the Schmidt-Kennicutt relation at all times.

We explore trends in the morphology and line-of-sight (los) velocity of stellar populations in the inner regions of disc galaxies using N-body simulations with a thin (kinematically cold) and a thick (kinematically hot) disc which form a bar and a boxy/peanut (b/p) bulge. The bar in the thin disc component is ∼50% stronger than the thick disc bar and is more elongated, with an axis ratio almost half that of the thick disc bar. The thin disc b/p bulge has a pronounced X-shape, while the thick disc b/p is weaker with a rather boxy shape. This leads to the signature of the b/p bulge in the thick disc being weaker and further away from the plane than in the thin disc. Regarding the kinematics, we find that the los velocity of thick disc stars in the outer parts of the b/p bulge can be higher than that of thin disc stars, by up to 40% and 20% for side-on and Milky Way-like orientations of the bar, respectively. This is due to the different orbits followed by thin and thick disc stars in the bar-b/p region, which are affected by two factors. First, thin disc stars are trapped more efficiently in the bar-b/p instability and thus lose more angular momentum than their thick disc counterparts and second, thick disc stars have large radial excursions and therefore stars from large radii with high angular momenta can be found in the bar region. We also find that the difference between the los velocities of the thin and thick disc in the b/p bulge (∆v los ) correlates with the initial difference between the radial velocity dispersions of the two discs (∆σ). We therefore conclude that stars in the bar-b/p bulge will have considerably different morphologies and kinematics depending on the kinematic properties of the disc population they originate from.