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Gaia's precise distances and motions for two billion stars now give a detailed chronology of how the Milky Way formed some 12–13 Gyr ago, the review argues.

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

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T0 review · deepseek-v4-flash

2026-08-04 17:26 UTC pith:MZYPUVOW

load-bearing objection A thorough, authoritative DR3-era review of Gaia science; no new results, but a reliable and well-caveated synthesis worth having on the shelf.

arxiv 2509.10883 v1 pith:MZYPUVOW submitted 2025-09-13 astro-ph.IM astro-ph.EPastro-ph.GAastro-ph.SR

Space Astrometry with Gaia: Advances in Understanding our Galaxy

Michael Perryman This is my paper
classification astro-ph.IM astro-ph.EPastro-ph.GAastro-ph.SR
keywords Gaiaastrometryparallax zero pointMilky Way structureGalactic archaeologystellar evolutiondistance scalespace mission
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This review argues that the Gaia space mission's catalogue—accurate positions, distances, and space motions for roughly two billion stars, plus solar system objects and quasars—has transformed stellar and Galactic astrophysics. The core claim is that these measurements supply a detailed chronology of the Milky Way's assembly across cosmic time, from its oldest halo populations to ongoing dynamical processes. A sympathetic reader would care because the review shows how one homogeneous dataset now links solar system science, stellar structure and evolution, the distance scale, Galactic structure and dynamics, and even cosmology.

Core claim

The review's central claim is that Gaia's astrometric catalogue, built from microarcsecond-level parallaxes and proper motions on an extragalactic reference frame, enables a 'rather detailed chronology' of how the Galaxy came into existence 12–13 Gyr ago. The paper presents a wide survey of results—from asteroids and exoplanets to the Galaxy's bar, warp, spiral arms, halo streams, and merger remnants, and to Cepheid-based distances—arguing that these all trace back to the same precise, all-sky astrometric measurements.

What carries the argument

The enabling mechanism is the combination of two widely separated fields of view (basic angle 106.5°) for absolute parallax measurement, and the Astrometric Global Iterative Solution (AGIS), a cyclic block-iterative estimation of source parameters, spacecraft attitude, instrument calibration, and global parameters that converges to microarcsecond consistency. The review emphasizes that correcting the parallax zero point and bias (for example, a −21 microarcsecond weighted mean from a million quasars in EDR3) is essential for almost all distance-dependent conclusions.

Load-bearing premise

The review's conclusions depend on the accuracy of the Gaia parallax zero-point and bias corrections: if these systematic corrections are wrong, systematic distance errors propagate into nearly every distance-dependent result.

What would settle it

Compare Gaia EDR3/DR3 parallaxes of Cepheids or RR Lyrae stars in the Large Magellanic Cloud with independent geometric distances from eclipsing binaries or water masers; if a magnitude- or colour-dependent offset outside the quoted systematic uncertainties (e.g., 10–20 microarcseconds) emerges, the zero-point corrections underpinning the review's distance-scale results would need revision.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

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If this is right

  • If the Gaia-derived distances and motions are accurate, stellar luminosities, radii, and masses become calibrated for billions of stars, transforming stellar evolution studies.
  • The Milky Way's merger and accretion history can be reconstructed from phase-space fossils such as the Enceladus remnant, halo streams, and the phase-space spiral.
  • Cepheid and RR Lyrae distance scales can be anchored geometrically, directly feeding determinations of the Hubble constant.
  • Gaia provides a dense, accurate reference frame used for pointing and calibrating other observatories, from JWST to the New Horizons flyby.
  • Future data releases extending to 5.5 and 10.5 years of observations will yield more precise parallaxes, proper motions, and binary orbit solutions for close to three billion sources.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Editorial inference: the review's survey implies that Gaia effectively converts astrometry into a discovery engine, not just a measurement service; the same data underlie fields as disparate as asteroid mass determination and cosmological distance-ladder calibration.
  • Editorial inference: the reliance on quasar-based zero-point corrections suggests that any future revision of the parallax bias will propagate through distance-dependent results, potentially shifting the inferred Hubble constant or the age of the Galaxy.
  • Editorial inference: the review's emphasis on homogeneous, all-sky catalogues hints that future space astrometry missions could push similar techniques to fainter magnitudes or other wavelengths, extending this style of science beyond the optical.
  • Editorial inference: tests of the parallax zero point using independent geometric distances (e.g., eclipsing binaries, water masers, or pulsar parallaxes) would directly validate or correct the review's underlying data quality assumptions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

0 major / 4 minor

Summary. This manuscript is a broad, authoritative review of the Gaia space astrometry mission and its scientific impact, written by a central figure in the mission. It begins with the historical and technical context of astrometry, describes the Gaia satellite and the data-processing pipeline (including AGIS, the astrometric solution, photometry, radial velocities, and the Apsis classification modules), and then surveys results across solar-system science, stellar structure and evolution, Galactic structure and dynamics, and Local Group/cosmology. The central claim is that Gaia's catalogue—roughly two billion sources with astrometry, photometry, and spectroscopy—has transformed stellar and Galactic astrophysics and provided a detailed chronology of the Milky Way's assembly over the past 12–13 Gyr. The paper is explicitly framed as an update of earlier reviews, with the aim of synthesizing the roughly 7000 refereed papers that have used Gaia data before the upcoming DR4 release.

Significance. If the synthesis is accurate, this will be a valuable reference for the community. The manuscript is particularly strong in its transparency: it explicitly acknowledges the parallax zero-point and bias problem (§4.3.4), the model-dependence of inferred stellar properties (§4.7), the effects of unresolved binaries on astrometric quality (§4.3.2), and the limitations of machine-learning-based classifications. The author's first-hand knowledge of the mission gives the technical sections unusual authority, and the extensive citation base makes the review a useful entry point for non-specialists and specialists alike. The paper does not present new measurements, but the relevant standard is whether the reviewed results actually support the claim that Gaia has transformed Galactic astrophysics. On that standard, the manuscript succeeds: the headline discoveries (halo streams, cluster kinematics, disk structure, phase-space spirals, and the like) are qualitative and corroborated across many independent analyses, and do not hinge on a single systematic correction. The explicit caveats about residual astrometric systematics and model-dependent quantities are appropriate and not hidden.

minor comments (4)
  1. [Table 2] In the DR1 row, the listing of both '5-parameter solutions' and '6-parameter solutions' as 2,057,050 is inconsistent with the body text and with the source-count arithmetic: DR1 contained the Tycho–Gaia Astrometric Solution (5-parameter) for ~2 million stars and 2-parameter solutions for the rest; there were no Gaia-only 6-parameter solutions in DR1. The row should be corrected (likely with the 6-parameter entry left blank), otherwise readers may be misled about the DR1 data content.
  2. [§4.3.4] The discussion of the Z5/Z6 parallax-bias correction functions is clear, but for a review that will be used as a reference it would help to give the exact applicability conditions (magnitude/colour/position ranges, 5-parameter vs 6-parameter solutions) in a compact table or boxed summary rather than only referring to the Python implementations. This is a readability suggestion, not a correctness issue.
  3. [§4.7] The caution about physics-driven versus data-driven spectral inference is welcome, but the sentence 'at least some algorithms which estimate [α/Fe] from the XP spectra do so by exploiting known correlations between [α/Fe] and other elements' is important enough that it could be elevated to a more prominent warning in the summary of the Apsis outputs. As written, it appears somewhat buried in the machine-learning subsection.
  4. [§11] The final summary is useful but mostly lists achievements. A short paragraph collecting the main systematics and model-dependencies (parallax zero point, unresolved binaries, synthetic photometry assumptions, model-dependent masses/ages) would help readers who skim the review and would mirror the transparency shown elsewhere in the text.

Circularity Check

0 steps flagged

No significant circularity: the review's synthesis rests on independent, externally benchmarked Gaia results, not on self-citation or fitted inputs.

full rationale

This is a broad review article rather than a new derivation or prediction. Its central claim—that Gaia has transformed stellar and Galactic astrophysics—is supported by the publicly released Gaia catalogues and the thousands of independent refereed papers that use them. The paper explicitly frames the astrophysical parameters as model-dependent and discusses known systematics, including the parallax zero point. Self-citations (e.g., Perryman 2010, 2012; Lindegren & Perryman 1996; Perryman et al. 2001, 2014a) appear only as historical or mission-design documentation, not as load-bearing evidence for scientific results. The parallax-bias correction is presented transparently as a calibration estimated from quasars and cross-checked against open clusters, VLBI, and independent analyses, so it is not a fitted input renamed as a prediction. No uniqueness theorem, ansatz, or known result is smuggled in via the author's prior work. The review's conclusions are corroborated by external, independent analyses and are predominantly qualitative in nature, making the synthesis self-contained with respect to circularity concerns.

Axiom & Free-Parameter Ledger

0 free parameters · 3 axioms · 0 invented entities

The paper is a review and introduces no new free parameters or entities. It relies on the accuracy of the Gaia catalogue and the correctness of the cited literature.

axioms (3)
  • domain assumption The Gaia astrometric solution (AGIS) provides unbiased parallaxes after applying the published zero-point corrections.
    The review's survey of distance-dependent results relies on this; Section 4.3.4 discusses the corrections.
  • domain assumption The extragalactic reference frame is non-rotating.
    Used for absolute proper motions, Section 4.4.
  • domain assumption The reviewed literature's results are correctly summarized.
    The review is a synthesis; if the sources were misread, the conclusions would be wrong.

pith-pipeline@v1.3.0-alltime-deepseek · 53236 in / 6551 out tokens · 72791 ms · 2026-08-04T17:26:05.604372+00:00 · methodology

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Cite this review

Pith. "Pith review of Space Astrometry with Gaia: Advances in Understanding our Galaxy." pith.science (2026). https://pith.science/paper/MZYPUVOW

@misc{pith2026250910883,
  author       = {Pith},
  title        = {Pith review of: Space Astrometry with Gaia: Advances in Understanding our Galaxy},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MZYPUVOW}},
  note         = {Machine review of arXiv:2509.10883}
}
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read the original abstract

Gaia is a satellite mission of the European Space Agency which is creating a catalogue of extremely accurate positions, distances and space motions of two billion stars in our Galaxy, along with more than one hundred thousand solar system asteroids, and several million distant quasars, all on the same extragalactic reference system. Complementary information on each object's multi-epoch photometry and spectra provides a vast and unprecedented data base of (model-dependent) fundamental physical quantities, such as each star's mass, age, and chemical composition. I outline the field's historical context, and explain the key principles involved in these space measurements. This is followed by a broad review of the many areas of solar system science, stellar structure and evolution, and topics in Galactic structure, evolution, and dynamics, that are being derived from these data.

Figures

Figures reproduced from arXiv: 2509.10883 by Michael Perryman.

Figure 1
Figure 1. Figure 1: The accuracy of star positions (left ordinate, log scale) through history, with some examples of these accuracies at the right. Only a [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Left (a): the principle of parallax measurement. Earth’s annual orbit around the Sun provides a baseline of [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Schematic of the Gaia focal plane. The upper figure shows the focal plane viewed from above. Star images pass from left to right as the [PITH_FULL_IMAGE:figures/full_fig_p014_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Integration of the Gaia satellite: (a) the focal plane assembly (106 large format CCDs occupying an area of nearly 0 [PITH_FULL_IMAGE:figures/full_fig_p015_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Left (a): rigidity of simulated ‘great circle solutions’ versus basic angle between the two viewing directions; higher peaks correspond to [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: The astrometric global iterative solution, AGIS. Left (a): schematic showing three scans across a small region of sky; depending on the [PITH_FULL_IMAGE:figures/full_fig_p017_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Positional accuracy of the Gaia catalogue over time, due to the uncertainties in the proper motions (Hobbs et al. (2016), Figure 4b). [PITH_FULL_IMAGE:figures/full_fig_p022_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Left (a): G, GBP, and GRP passbands for the Gaia EDR3 photometric system (grey curves are the nominal pre-launch systems). Right (b): uncertainties on the weighted means as a function of magnitude. Only sources with 20 or more transits (200 CCD observations in G) are included. The Gaia DR1 and DR2 uncertainties are shown for comparison. From Riello et al. (2021), Figures 24 and 14. reproduced by the XP syn… view at source ↗
Figure 9
Figure 9. Figure 9: Left (a): RVS grating demonstrator model (Airbus Defence and Space). Right (b): summary of the RVS results in Gaia Data Release 3. [PITH_FULL_IMAGE:figures/full_fig_p026_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Examples of the Gaia DR3 radial velocity results. Left (a): median values as a function of Galactic longitude and latitude. Middle (b): [PITH_FULL_IMAGE:figures/full_fig_p028_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Left pair, top: the V = 6.7 mag K5 star HIP 86564 from a single 4.4 sec Gaia exposure; bottom: with the NARVAL spectrograph at the Observatoire du Pic du Midi, at the same spectral resolution (Cropper et al. (2018), [PITH_FULL_IMAGE:figures/full_fig_p028_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Radial velocity time-series of long-period variables. Left (a): three examples showing, top to bottom: a source with ‘mixed consistency’ [PITH_FULL_IMAGE:figures/full_fig_p030_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Object classification and parameter determination carried out for Data Release 3 by the Gaia DPAC Coordination Unit 8. The 13 Apsis [PITH_FULL_IMAGE:figures/full_fig_p031_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Examples of the all-sky characterisation of sources with a 5-parameter solution in Gaia EDR3 [PITH_FULL_IMAGE:figures/full_fig_p035_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Gaia EDR3 astrometry (also applicable to Gaia DR3). Left (a): magnitude distribution, showing all sources (grey), and 5-parameter [PITH_FULL_IMAGE:figures/full_fig_p036_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Sky projections, in Galactic coordinates, showing various properties of the Gaia DR3 data release (see text for details). [PITH_FULL_IMAGE:figures/full_fig_p039_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: Left (a): Light deflection due to Jupiter, measured by Gaia for the February 2017 grazing incidence observations (Abbas et al., 2022, [PITH_FULL_IMAGE:figures/full_fig_p041_17.png] view at source ↗
Figure 18
Figure 18. Figure 18: Left (a): a star moving through space, with some [PITH_FULL_IMAGE:figures/full_fig_p043_18.png] view at source ↗
Figure 19
Figure 19. Figure 19: Left: solar systems objects in the 66-month ‘Focused Product Release’, showing the arc length measured in orbital periods, for the [PITH_FULL_IMAGE:figures/full_fig_p047_19.png] view at source ↗
Figure 20
Figure 20. Figure 20: Left: reflectance spectrum of asteroid (4) Vesta from Gaia (red), with other data from ground-based observations and NASA’s Dawn [PITH_FULL_IMAGE:figures/full_fig_p048_20.png] view at source ↗
Figure 21
Figure 21. Figure 21: Left: predicted occultation event, based on Gaia astrometry, for the Galilean moon Io, on 2 April 2021 (Morgado et al. (2019)). Right: [PITH_FULL_IMAGE:figures/full_fig_p052_21.png] view at source ↗
Figure 22
Figure 22. Figure 22: Left: successive versions of the Catalogue of Nearby Stars (CNS) as a function of distance; pre-Gaia (CNS1–CNS4) numbers are from [PITH_FULL_IMAGE:figures/full_fig_p056_22.png] view at source ↗
Figure 23
Figure 23. Figure 23: Left: the relative density of stars with identified rotational modulation in the Gaia DR2 colour–magnitude diagram, increasing from red [PITH_FULL_IMAGE:figures/full_fig_p058_23.png] view at source ↗
Figure 24
Figure 24. Figure 24: Left: solar neighbourhood colour–magnitude diagram for the 212 728 stars with Gaia DR2 parallaxes [PITH_FULL_IMAGE:figures/full_fig_p063_24.png] view at source ↗
Figure 25
Figure 25. Figure 25: Left (a): 45 × 45 arcsec2 PanSTARRS field of the G = 13.8 mag binary companion to the Gaia black hole BH1 (El-Badry et al., 2023b). Middle (b): Gaia DR3 astrometry defines a photocentric ellipse with semi-major axis a0 = 2.98±0.22 mas (solid curve); thin blue curves are orbits drawn randomly from a posterior distribution (Chakrabarti et al., 2023). Right (c): parameters of the black hole binaries BH1, BH2… view at source ↗
Figure 26
Figure 26. Figure 26: Examples of three (out of more than 10 million) Gaia folded light curves: a Cepheid variable, a RR Lyrae variable, and an eclipsing [PITH_FULL_IMAGE:figures/full_fig_p082_26.png] view at source ↗
Figure 27
Figure 27. Figure 27: Left (a): number of field of view crossings for the variable stars identified in Gaia DR3, in the [PITH_FULL_IMAGE:figures/full_fig_p083_27.png] view at source ↗
Figure 28
Figure 28. Figure 28: Left: the period–luminosity relation for the Cepheids identified in DR2 by Clementini et al. (2019, Figure 6), as a function of apparent [PITH_FULL_IMAGE:figures/full_fig_p085_28.png] view at source ↗
Figure 29
Figure 29. Figure 29: Left: schematic location of radial (green) and non-radial pulsators (blue) in the HR diagram (adapted from wikipedia, Instability Strip). [PITH_FULL_IMAGE:figures/full_fig_p090_29.png] view at source ↗
Figure 30
Figure 30. Figure 30: Normalised histograms of the Gaia GSP–Phot values for 11 636 Gaia DR3 γ Dor stars. Asteroseismic values derived from Kepler data are shown for 37 γ Dor stars (hatched). Panels show, left to right, Teff, log g, log(L/L⊙), and derived R⋆ (from Aerts et al. (2023), Figures 1a–4a). overshooting, caused by the momentum of cool sinking material into the deeper stable radiative regions, alters the structure of t… view at source ↗
Figure 31
Figure 31. Figure 31: Gaia colour–magnitude diagrams of three clusters (members from Hunt and Re [PITH_FULL_IMAGE:figures/full_fig_p099_31.png] view at source ↗
Figure 32
Figure 32. Figure 32: Left (a): open clusters in Galactic XY coordinates, divided in age groups, with an over-density map in grey. Results also reproduce the Galactic metallicity gradient found in spectroscopic surveys (Cavallo et al. (2024), [PITH_FULL_IMAGE:figures/full_fig_p100_32.png] view at source ↗
Figure 33
Figure 33. Figure 33: Left: Hyades cluster members from Gaia EDR3, in equatorial coordinates. Grey dots denote all 3055 initial candidates. Small blue [PITH_FULL_IMAGE:figures/full_fig_p102_33.png] view at source ↗
Figure 34
Figure 34. Figure 34: Left pair (from Brandner et al. (2023b)). Top: Hyades EDR3 colour–absolute magnitude diagram, showing stars from the Gaia Catalogue [PITH_FULL_IMAGE:figures/full_fig_p105_34.png] view at source ↗
Figure 35
Figure 35. Figure 35: The 6504 (probable) Young Stellar Objects of Class II (i.e. showing both optical and infrared emission) identified from Gaia EDR3, and [PITH_FULL_IMAGE:figures/full_fig_p111_35.png] view at source ↗
Figure 36
Figure 36. Figure 36: Left: fits to the 862.1 nm and 864.8 nm di [PITH_FULL_IMAGE:figures/full_fig_p115_36.png] view at source ↗
Figure 37
Figure 37. Figure 37: From the catalogue of 1.3 million resolved binaries, this shows the magnitude di [PITH_FULL_IMAGE:figures/full_fig_p118_37.png] view at source ↗
Figure 38
Figure 38. Figure 38: Left (a): cut-off in orbital period of spectroscopic binaries versus temperature, with the best-fit linear model in red (Bashi et al., 2023). Middle (b): precision of the various mass determination methods as a function of mass: darker colours are less model-dependent, with the red regions providing the most accurate masses (Serenelli et al. (2021)). Right (c): mass–luminosity relation showing the Gaia va… view at source ↗
Figure 39
Figure 39. Figure 39: The orbit of Gaia DR3 5136025521527939072, detected as both an astrometric and single-lined spectroscopic binary [PITH_FULL_IMAGE:figures/full_fig_p127_39.png] view at source ↗
Figure 40
Figure 40. Figure 40: (a) Schematic showing the simulated motion on the sky of a star–planet system. The system barycentre movies linearly through space. [PITH_FULL_IMAGE:figures/full_fig_p129_40.png] view at source ↗
Figure 41
Figure 41. Figure 41: Schematic orbital motions of the components of a binary system (star–star, or star–planet) about the barycentre cause the photocentre [PITH_FULL_IMAGE:figures/full_fig_p131_41.png] view at source ↗
Figure 42
Figure 42. Figure 42: Two Gaia science alerts microlensing events. Left: the exoplanet system Gaia22dkv, showing follow-up photometry over 250 days, from [PITH_FULL_IMAGE:figures/full_fig_p140_42.png] view at source ↗
Figure 43
Figure 43. Figure 43: (a, left): recent estimates of R0, taken from Leung et al. (2023), with their value included at top. (b, right upper): simulated kinematic maps of the bar and disk from Leung et al. (2023, [PITH_FULL_IMAGE:figures/full_fig_p144_43.png] view at source ↗
Figure 44
Figure 44. Figure 44: Three of the Gaia determinations of the Galaxy’s rotation curve. (a, left): data from Gaia DR2 (black circles), also showing their best [PITH_FULL_IMAGE:figures/full_fig_p147_44.png] view at source ↗
Figure 45
Figure 45. Figure 45: Velocity structure of the solar neighbourhood. (a): pre-Gaia velocity distribution of F and G dwarfs in the Hipparcos-based catalogue of [PITH_FULL_IMAGE:figures/full_fig_p149_45.png] view at source ↗
Figure 46
Figure 46. Figure 46: Distribution in the vertical position–velocity plane for 930 000 stars from Gaia DR2 with parallaxes [PITH_FULL_IMAGE:figures/full_fig_p151_46.png] view at source ↗
Figure 47
Figure 47. Figure 47: The Galaxy’s spiral arm structure from Gaia DR3. Left: the distribution of young open clusters ( [PITH_FULL_IMAGE:figures/full_fig_p153_47.png] view at source ↗
Figure 48
Figure 48. Figure 48: Left: estimates of the mass of the Milky Way from Gaia DR2 and EDR3 [PITH_FULL_IMAGE:figures/full_fig_p158_48.png] view at source ↗
Figure 49
Figure 49. Figure 49: The Galactic escape speed. Left: High-speed and high-quality sample of stars in Gaia DR3 with 6d kinematics, and the derived escape [PITH_FULL_IMAGE:figures/full_fig_p160_49.png] view at source ↗
Figure 50
Figure 50. Figure 50: The globular cluster ω Cen. (a) Colour–magnitude diagram of 66 467 members, with the dashed lines dividing it into four main regions (from Soltis et al. (2021), [PITH_FULL_IMAGE:figures/full_fig_p163_50.png] view at source ↗
Figure 51
Figure 51. Figure 51: Left: Palomar 5, with RR Lyrae stars tracing the stream on the sky, coloured by membership probability. The background greyscale [PITH_FULL_IMAGE:figures/full_fig_p164_51.png] view at source ↗
Figure 52
Figure 52. Figure 52: Left: stellar streams in the E − Lz plane identified by Naidu et al. (2020). Right: contributions versus distance from the Galactic plane. The high-α disk is taken as e < 0.5, and the in situ halo as e > 0.5, although they form a continuous distribution. GSE is the dominant component within Z ∼10 − 20 kpc (Rgal ∼15 − 25 kpc), while most stars at larger distances belong to Sgr (Naidu et al. (2020), Figures… view at source ↗
Figure 53
Figure 53. Figure 53: Left: LMS–1 stream members detected in Gaia EDR3 using [PITH_FULL_IMAGE:figures/full_fig_p171_53.png] view at source ↗
Figure 54
Figure 54. Figure 54: Left: the observed Gaia colour–magnitude diagrams for the halo and thick disk sub-populations, using a log normalisation of the number [PITH_FULL_IMAGE:figures/full_fig_p177_54.png] view at source ↗
Figure 55
Figure 55. Figure 55: Left: Gaia view of the Large and Small Magellanic Clouds, with colours denoting di [PITH_FULL_IMAGE:figures/full_fig_p178_55.png] view at source ↗
Figure 56
Figure 56. Figure 56: Three dwarf spheroidals with members selected by Gaia DR2 astrometry/ [PITH_FULL_IMAGE:figures/full_fig_p181_56.png] view at source ↗
Figure 57
Figure 57. Figure 57: The first 12 confirmed quadruply-imaged Gaia quasars, presented as false-colour images from PanSTARRS (PS) or the Dark Energy [PITH_FULL_IMAGE:figures/full_fig_p187_57.png] view at source ↗
Figure 58
Figure 58. Figure 58: High-resolution Ks band Large Binocular Telescope images (2 [PITH_FULL_IMAGE:figures/full_fig_p188_58.png] view at source ↗
Figure 59
Figure 59. Figure 59: Evolution of TNG50 halo #519311 (from the IllustrisTNG cosmological simulations) over cosmic time, covering a region 30 [PITH_FULL_IMAGE:figures/full_fig_p193_59.png] view at source ↗

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