1. Gaia – Revue des Exigences préliminaires 1 Testing dark matter with Gaia O. Bienaymé O. Bienaymé Strasbourg Observatory

2. Gaia / DM 2 Dark matter within our Galaxy Flat rotation curve => missing luminous mass =>dark matter halo

3. Gaia / DM 3 Dark matter within our Galaxy Vertical motion of stars (Kaptein, Oort 1920’) =>total mass density in the solar neigbourhood

4. Gaia / DM 4 Dark matter within our Galaxy Vertical motion of stars (Kaptein, Oort 1920’) =>total mass density in the solar neigbourhood (Hipparcos sat.) versus : seen mass (stars+gas) =>dark halo cannot be extremely flattened (Crézé et al 1998 ). =>dark disk is not massive, if any ….

5. Gaia / DM 5 Dark matter within our Galaxy Halo shape Local stellar 3D kinematics with Hipparcos satellite (about ten thousand stars) => vertical force perpendicular to the disk => (u,v,w) 3D velocity coupling => (u,w) tilt with RAVE and SDSS they give local constrain on the shape of the potential that is nearly spherical

6. Gaia / DM 6 Dark matter within our Galaxy Halo shape Local stellar kinematics with Hipparcos satellite => vertical force perpendicular to the disk => (u,v,w) 3D velocity coupling => (u,w) tilt with RAVE and SDSS they give local constrain on the shape of the potential that is nearly spherical

7. Siebert et al 2008; RAVE Tilt=6deg at z=1kpc Potential nearly spherical 500 red clump stars velocity ellpsoid tilt

8. Gaia / DM 8 Dark matter within our Galaxy Halo stars velocity distribution (radial velocities) (but no proper motions i.e. tangential motion) => flat rotation curve at large R High velocity star D.F. in the solar neigbourhood  Galactic escape velocity  total galaxy mass & flat rotation curve (model dependent) Globular clusters, dwarf galaxy satellites

9. Gaia / DM 9 Galactic escape velocity 33 stars

10. Gaia / DM 10 Halo stars: velocity distribution 552 Vrad

11. Gaia / DM 11 Dark matter within our Galaxy Halo shape Sagittarius tails precession of the orbit measures the shape of the potential Other streams

12. Gaia / DM 12 Dark matter within our Galaxy Halo shape Sagittarius tails precession of the orbit measures the shape of the potential Other streams Correnti 2010

13. Gaia / DM 13 Dark matter within our Galaxy

14. Gaia / DM 14 Dark matter within our Galaxy Need for distances (parallaxes), proper motions tangential velocities, radial velocities and very large samples, to constrain accurately the 3D shape of the galactic potential

15. Gaia / DM 15 Gaia: a unique experiment The next cornerstone of the ESA Science Programme Unique characteristics  Unprecedented astrometric accuracy (7-200  as)  Simultaneous astrophysical characterisation + radial velocity of observed objects  Survey down to V = 20  10 9 objects observed all over the sky  Launch 2012, Soyouz from Kourou

16. Gaia / DM 16 The third dimension: further and further Solar neighbourhood Up to ~ 30 pc Solar neighbourhood Up to ~ 200 pc All over the Galaxy Up to ~ 10 000 pc In the bulge of the Galaxy Up to ~ 10 000 pc In the Galaxy and the Local Group Up to ~ 30 000 pc Ground-based, Hubble Precision: 3-5 mas Hipparcos 1989-1993(-2007) Accuracy: (0.1) - 0.2-1 mas Gaia (2012-2018) Accuracy: 8-20  as Jasmine (?) Precision: 10  as SIM (?) Precision: 3  as

17. Gaia / DM 17 Performances for a G2 V star Astrometry: Photometry: accuracy on G magnitude Spectroscopy Magnitude< 101520 Accuracy [  as] 620240 V=15V=20 Per observation [mag]0.0020.03 End of mission [mag]0.00020.003 MagnitudeV=12.5V=16.5 RV accuracy [km/s]< 115

18. Gaia / DMAPC, 9 June 2010 18

19. Gaia / DM Gavitational Potential with GAIA APC, 9 June 2010 19 Measure the total mass distribution from the gravitational potential Measure the baryonic mass (mainly stars, mainly the disk) Deduce the ‘exact’ shape of the D.M. distribution Hayashi, Navarro … 2007

20. Gaia / DM disk structure Disk warp and flare, relation with Monoceros stream? At 15 kpc: disk rotation ~ 6 mas/yr For a 1 kpc high warp: ~90  as/yr in latitude ~600  as/yr in longitude  easily measurable by Gaia 20

21. Gaia / DMAPC, 9 June 2010 21 Streams Intégrale du mouvement Amina’s figures Streams in the Galactic Halo

22. Gaia / DM 22 Streams in the Galactic Halo Again here, 3D kinematics at the faint end of the Gaia survey (V>16-17) would be a plus…or would even be crucial to identify sub-structures of the phase space Simulation of the accretion of 100 satellites galaxies (A. Helmi) Position spaceVelocity space

23. Gaia / DM Streams in the Galactic Halo APC, 9 June 2010 23 Integrals of motion space

24. Gaia / DMAPC, 9 June 2010 24 Constrains on the Clumpiness of the dark matter halo

25. Gaia / DM Gaia: long-term objective Choose potential, write Hamiltonian, write closest integrable Hamiltonian, find distribution function F(J), adjust potential… On shorter term: can we answer the crucial question of the existence of galactic dark matter by confirming/excluding (or at least constraining) a modified gravity approach?

26. Gaia / DM MOND within our Galaxy Stellar Disk within MOND  a newtonian astronomer observes  a spherical dark halo  and a dark disk (Milgrom, 86)

27. Gaia / DM Testing Newtonian gravity on galactic scales Modified gravity is only one version of MOND Only the relation between the potential and the matter source is altered, so one can constrain the potential in the usual way Crucially depends on our knowledge of the baryonic distribution Depends on the exact choice for  Then, the theory makes a unique and falsifiable prediction for the galactic potential => as an example let us use  (x)=x/(1+x) and the Besançon model based on the synthesis approach

28. Gaia / DM The « dark disk » from the Besançon Galactic model in MOND  eff = 78 M  pc -2 With  (x)=x/(1+x), at the solar position one has  eff = 78 M  pc -2 within z=1.1 kpc  dyn = 74+-6 M  pc -2 (TEST 1) to compare with present constraints  dyn = 74+-6 M  pc -2 (TEST 1) scale-length enhanced by 25% The effective radial density distribution in the disk has a scale-length enhanced by 25% [deep MOND => 50%]  even exclude MOND as modified gravity (TEST 2) => measuring dynamically the disk surface density as a function of R with GAIA (but problem of extinction, maybe JASMINE too) should allow to constrain  or even exclude MOND as modified gravity (TEST 2) => quick way to exploit GAIA data Bienaymé, Famaey et al. 2009, A&A 2.5kpc  3.1kpc Counts  Kz force

29. Gaia / DM The vertical tilt of the velocity ellipsoid (Bienaymé 2009) Angle  = arctg[2  2 UW /(  2 U -  2 W ) ]/2 is linked to the disk scale-legnth and dark halo flattening (Bienaymé 2009) => compute orbits in axisymmetric Besançon model to measure the tilt as a function of z at solar position Newton+DM MOND 6° 10°<14° RAVE data Siebert et al. 2008  (z=1kpc)= 7.3°+-1.8° TEST 3

30. Gaia / DM Conclusion We presented 3 quick tests to test MOND as modified gravity in the Milky Way with GAIA-like quality data constrain  even exclude MOND as modified gravity This should allow to constrain  or even exclude MOND as modified gravity knowledge of the baryonic distribution Testing gravity crucially depends on our knowledge of the baryonic distribution (even more than when determining the DM distribution) => importance of : - star counts, stellar population synthesis - gaseous content (including molecular gas) - inhomogeneities (clusters, gas clouds) Test other alternative theory