1. C. Cacciari INAF - Osservatorio Astronomico, Bologna The primary objective of Gaia is the Galaxy: to observe the physical characteristics, kinematics and distribution of stars over a large fraction of its volume, with the goal of achieving a full understanding of the MW dynamics and structure, and consequently its formation and history. (Concept and Technology Study Report, ESA-SCI(2000)4

2. Gaia in a nutshell  all sky (i.e. ~ 40,000 deg 2 ) survey complete to V lim = 20  ~ one billion sources  high (μas) accuracy astrometry (parallaxes, positions, proper motions)  optical spectrophotometry (luminosities, astrophysical parameters)  spectroscopy (radial velocities, rotation, chemistry) to V = 16

3. Satellite and System Figures courtesy EADS-Astrium ESA-only mission (Airbus DS contractor) Lifetime: 5 yr (+ 1yr possible extension) Launched on 19 December 2013 from Kourou (French Guiana) Launcher: Soyuz–Fregat L2 (gravitational equilibrium 1.5 million km from Earth away from Sun ) Figure courtesy A. Buzzoni Lissajous orbit around L2

4. Satellite and System Figures courtesy EADS-Astrium

5. Payload and Telescope Two SiC primary mirrors 1.45  0.50 m 2 at 106.5° SiC toroidal structure (optical bench) Basic angle monitoring system Combined focal plane (CCDs) Rotation axis (6 h) Superposition of two Fields of View (FoV) FoV 1.7° x 0.6°

6. Sky Scanning Principle Spin axis: 45 o to Sun Scan rate: 60 arcsec/s Spin period: 6 hr FoV-1: t 0 t 0 + 6hr FoV-2: t 0 + 106.5m t 0 + 106.5m + 6hr repeated 10-30 days later 29 revolutions of spin axis around solar direction in 5 yr :

7. Transit maps Ecliptic coordinates Galactic coordinates End of mission (5 yr) sky-average number of transits: ~ 70 (max  200 at  = 45   10  )

8. Scanning the entire sky

9. MOVIE

10. The instruments Slitless spectroscopy on Ca triplet (847–870 nm) Resolution 11,500 R ~ 80 – 20 R ~ 90 – 70

11. Focal Plane CCD X time = 4.4 s Total field: - active area: 0.75 deg 2 - CCDs: 14 + 62 + 14 + 12 - each CCD: 4500x1966 px (TDI) - pixel size = 10 µm x 30 µm = 59 mas x 177 mas Astrometric Field CCDs – Total FOV ~ 40x40 arcmin ~ 4.4 ‘ ~ 4.4 ' Blue Photometer CCDs Sky Mapper CCDs 104.26cm Red Photometer CCDs Radial-Velocity Spectrometer CCDs Basic Angle Monitor Wave Front Sensor Basic Angle Monitor Wave Front Sensor Sky mapper: - detects all objects to 20 mag - rejects cosmic-ray events - FoV discrimination Astrometry: - total detection noise: 6 e - Photometry: - spectro-photometer - blue and red CCDs Spectroscopy: - high-resolution spectra - red CCDs 42.35cm along-scan

12. Gaia spectro-photometric system Figure courtesy A. Brown Internal calibration External calibration ● same principle as for classical spectrophotometry ● much more complicated instrument model ● ~ 200 calibrators needed to model instrument response ● mmag internal accuracy, a few % external accuracy

13. BP/RP first results Star HIP 86564 K5, V=6.64 ● BP and RP low resolution internally calibrated spectral energy distributions ● single transit ESA/Gaia/DPAC/Airbus

14. Radial-Velocity Measurement Concept Field of view RVS spectrograph CCD detectors Courtesy David Katz Spectra of the star HIP 86564 Top: Gaia-RVS on FoV 0, CCD row 4 strip 15 Bottom: Narval on 2-m Bernard Lyot Telescope (Pic du Midi), convolved to the nominal resolving power of the RVS: R=11500

15. Parallax – Parsec Arcsecond = 1/60 arcmin = 1/3600 deg = 1/206265 radian = 1/1,296,000 circle 1˝ ≈ the angle subtended by a US dime coin (17.9 mm diameter) at a distance of 4 km parallax (˝) = 1/distance (pc) Parsec (pc): distance at which the average Earth-Sun distance (1 AU=150 10 6 km) is seen under a parallactic angle (parallax) of 1˝ 1pc = 3.26ly

16. Astrometry: a historical perspective 150 BC 1500 1600 1700 1800 19002000 2100 1 degree 1 arcminute 1 arcsecond 1 milli-arcsecond 1 micro- arcsecond Hipparchos Tycho Brahe Flamsteed Bessel Hipparcos Gaia ?? Year meridian circles Large-angle astrometry Small-angle astrometry Figure: Lennart Lindegren 1/60 1/1000 1/100

17. Astrometry: data reduction principles Sky scans (highest accuracy along scan) Scan width: 0.7° 1. Object matching in successive scans 2. Attitude and calibrations are updated 3. Objects positions etc. are solved 4. Higher terms are solved 5. More scans are added 6. System is iterated (Global Iterative Solution) Figure courtesy Michael Perryman

18. From Hipparcos to Gaia Hipparcos (1997)Gaia (2020-22) Magnitude limit Completeness Bright limit Number of objects Effective distance (Mv=-3) Quasars Galaxies Accuracy Broad band photometry Spectrophotometry Spectroscopy (CaT) Observing programme 12 7.3-9.0 ~ 0 1.2 10 5 10 kpc None 1 ~ mas 2 (B and V) None Pre-selected targets 20 ~ 20 ~ 3-5 1.0 10 9 1 Mpc ~5 10 5 ~ 10 6 - 10 7 ~ 5-14 μas at V  12 9-26 μas at V=15 130-600 μas at V=20 3 (to V=20) + 1 (to V=16) 2 bands (B/R) 1-15 km/s to V=16 Complete and unbiased

19. Astrometric accuracy: the Hyades d= 46 pc = 151 ly 1960 1990 2020 Figure courtesy ESA

20. One billion stars in 5D (6D … 9D) will provide: in the Galaxy … distances, velocity distributions and astrophysical parameters (APs) of all stellar populations in the MW to unprecedented accuracy allowing to:  map the spatial and dynamical structure of bulge, disk(s) and halo(s)  derive formation and chemical history (e.g. accretion and/or interaction events) of the MW & star formation history throughout  obtain the trigonometric calibration of primary distance indicators (RR Lyraes, Cepheids)  accurate and robust definition of the cosmic distance scale and beyond the Galaxy …  QSO detection and definition of rest frame  structure and stellar population studies in nearby (LG) galaxies

21. Stellar populations of the MW: The Hertzprung-Russell diagram BRIGHT intrinsic luminosities need distances FAINT HOT Temperatures from colours (spectrophotometry) COLD MOVIE

22. Trigonometric parallaxes: RR Lyr HST (Benedict et al. 2002; 2011) : RR Lyr π = 3.77 ± 0.13 mas (~265 pc)  mod = 7.13 ± 3.4% mag at [Fe/H] = -1.39, V=7.8, Av=0.13 mag,  V 0 =7.67 mag M V = 0.54  0.07 mag, M K = -0.65  0.07 mag Hipparcos (van Leeuwen 2007) : RR Lyr ( =7.8): the only RRL star with good Hipparcos parallax π = 3.46 ± 0.64 mas (~289 pc)  mod = 7.30 ± 18% mag Multiwavelength photometry & models (Catelan & Cortes 2008) : V=7.76, Av=0.048 mag, V 0 =7.71  0.06 mag, mod = 7.11 mag (weighted mean of all π)  M V = 0.60  0.13 mag evolved  overluminous by ~ 0.06  0.01 mag Non-evolved star of that metallicity should then have M V ~ 0.66 mag RR Lyr is not a `good’ candle … Δmod = 0.17 mag ΔV, ΔA V, evol

23. Gaia: MW field RR Lyraes GEOS database photometry: = – 0.15 = – 0.30  V mid IS = – 0.08 Mv = 0.52 for all (~ [Fe/H]=-1.5 dex) = (V max + V min )/2 Zero reddening  Individual end-of-mission   /  estimates ~ 5  (~ 340 RRL) to  1   1.5 kpc ~ 15  (~ 1000 RRL) to  5   4 kpc ~ 26  (~ 1750 RRL) to  10   5.7 kpc ~ 43  (~ 2900 RRL) to  20   8 kpc ~ 60  (~ 4000 RRL) to  30   10 kpc Expected from Gaia ~ 50-40 10 3 bulge ~ 70 10 3 halo (Eyer & Cuypers, 2000)

24. The distance scale: MW Cepheids Only ~ 800 Galactic Cepeheids are known – most are located in the Solar neighbourhood 400 Galactic Cepheids from David Dunlap DB distance and magnitude  Gaia predicted accuracy for parallax 15 @ d < 0.5 kpc 65 @ D < 1 kpc 165 @ d < 2 kpc Presently, ~ 250 Cepheids with parallax & photometry (10 with HST parallax) only ~ 100 with σ π ≤ 1 mas (Hipparcos) Figure courtesy A. Brown Most (~ 3/4) Galactic Cepheids will have Gaia individual parallaxes to < 3%

25. The distance scale: MC Cepheids The bulk of the distribution for fundamental LMC pulsators lies at Period = 3 – 5 days Mv ~ -3  V ~ 15.8 – 16  individual distances to ~ 150%  mean of 400 to ~ 7-8 % Cepheids with P ≥ 10 d Mv ≤ ~ - 4.2  V ~ 14.5 individual distances to ~ 80% Ultra-long period (> 100 d) Cepheids Mv ≤ ~ - 7  V ~ 12 4 in LMC, 3 in SMC (Bird et al. 2009)  individual distances to ~ 45% (LMC) - 55% (SMC) 600 Cepheids in the LMC (OGLE, Udalski et al. 1999)  direct (trigonometric) calibration of the cosmological distance scale ~ 2000/1000 Cepheids are known (and observable by Gaia) in the LMC/SMC

26. Gravitational light bending This is the dominating relativistic effect in Gaia astrometric measurements Accurate measures of γ of Parametrized Post-Newtonian (PPN) formulation of gravitational theories is of key importance in fundamental physics Gaia will measure  to ~ 5x10 -7 (10 -4 - 10 -5 present)

27. Data processing & distribution  Data Processing and Analysis Consortium (DPAC): more than 500 people from 20 European countries and ESA  Final catalogue ~ 2020–22  Intermediate data releases  Science alerts (e.g. SNe) data released immediately  No proprietary data rights

28. Thank you !

29. Parallax – Parsec Arcsecond = 1/60 arcmin = 1/3600 deg = 1/206265 radian = 1/1,296,000 circle 1˝ ≈ the angle subtended by a US dime coin (17.9 mm diameter) at a distance of 4 km parallax (˝) = 1/distance (pc) Parsec (pc): distance at which the average Earth-Sun distance (1 AU=150 10 6 km) is seen under a parallactic angle (parallax) of 1˝ 1pc = 3.26ly

30. GAIA science products: census of … Stellar pops in the Galaxy (based on the Besançon Galaxy model - Robin et al. 2003, 2004)  Disk: 9.0 x 10 8  Thick disk: 4.3 x 10 8  Spheroid: 2.1 x 10 7  Bulge: 1.7 x 10 8 Special objects in the Galaxy  Solar System bodies (~ 10 5 )  extra-solar planets (~ 10 4 )  binaries & rare stellar types (fast evolutionary phases)  WDs (~ 2 x 10 5 ), BDs (~ 5 x 10 4 ) Outside the Galaxy  brightest stars in nearby (LG) galaxies  supernovae and burst sources (~2x10 4 )  galaxies (~ 10 7 )  QSOs (~ 10 6 )  gravitational lensing events: < 10 2 photometric; a few 10 2 astrometric Fundamental Physics   to ~ 5x10 -7 (10 -4 - 10 -5 present)

31. One billion stars in 5D (6D … 9D) will provide: in the Galaxy …  detailed structure, dynamics, formation and chemical history of the Milky Way, e.g. accretion/interaction events, star formation history etc.  a rigorous framework for stellar structure and evolution modelling  dating of all spectral types and Galactic populations  microlensing distribution and rate  map dark matter distribution … and beyond  definitive and robust definition (zero-point) of the cosmic distance scale (local calibrators, SNe)  QSO detection and definition of rest frame  structure and stellar population studies in nearby (LG) galaxies

32. Summary – Conclusions  Gaia will provide the distances, velocity distributions and APs of all stellar populations in the MW to unprecedented accuracy allowing to:  map the spatial and dynamical structure of bulge, disk(s) and halo(s)  derive formation and chemical history (e.g. accretion and/or interaction events) of the MW & star formation history throughout  obtain the trigonometric calibration of primary distance indicators  accurate and robust definition of the cosmic distance scale  Future surveys (especially the spectroscopic ones): none will provide all-sky coverage, but synergy will be extremely fruitful

33. Sky-averaged end-of-mission parallax standard error, in units of μas, as a function of Johnson V magnitude for 2 reference stellar types Estimates include a 20% margin for unmodelled errors (e.g. radiation damage effect on CCDs not fully taken into account) Astrometric performance (J. De Bruijne, ESA) σ ω ≤ 300 μas V lim = 20-22 σ pos = 0.74 σ ω σ μ = 0.53 σ ω 6 < V < 12: bright-star regime (calibration errors, CCD saturation) V › 12: photon-noise regime

34. Photometry Measurement Concept Figures courtesy Anthony Brown RP spectrum of M dwarf (V = 17.3 mag) Red box: data sent to ground White contour: sky-background level Colour coding: signal intensity

35. Spectro-photometry (V < 20-22) Integrated:  white-light (G-band, 330-1050 nm) from the Astrometric Field  BP-band & RP-band  BP- RP colour Dispersed:  Blue (BP, 330-680 nm) & Red (RP, 640-1050 nm) low resolution spectra Main sequence stars from O5 to M6: G=15, A V =0.0 (Straizys et al. 2006) dispersion 4-32 nm/px dispersion 7-15 nm/px

36. Photometric bandpasses: G, BP, RP & RVS Figure courtesy ESA

37. Radial-Velocity Measurement Concept RVS spectra of F3 giant (V = 15 mag) S/N = 7 (single measurement) S/N = 130 (summed over mission) Field of view RVS spectrograph CCD detectors Figures courtesy David Katz

38. RVS first results ESA/Gaia/DPAC/CNES/Yves Viala and Francoise Crifo RVS spectra ● calibrated in wavelength and flux ● on the three along scan CCDs for a single transit The internal apparent Grvs magnitude, computed from the flux of the spectra, is about 5.5 The star is HIP 117279 and is a K5 giant

39. Parallax – Parsec Arcsecond = 1/60 arcmin = 1/3600 deg = 1/206265 radian = 1/1,296,000 circle 1˝ ≈ the angle subtended by a US dime coin (17.9 mm diameter) at a distance of 4 km parallax (˝) = 1/distance (pc) Parsec (pc): distance at which the average Earth-Sun distance (1 AU=150 10 6 km) is seen under a parallactic angle (parallax) of 1˝ 1pc = 3.26ly