Gaia – Revue des Exigences préliminaires1

Testing dark matterTesting dark matterwith Gaiawith Gaia

O. BienayméO. BienayméStrasbourg ObservatoryStrasbourg Observatory

Gaia / DM 2

Dark matter within our Galaxy

Flat rotation curve

=> missing luminous mass

=>dark matter halo

Gaia / DM 3

Dark matter within our Galaxy

Vertical motion of stars (Kaptein, Oort 1920’)

=>total mass density in the solar neigbourhood

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 ….

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

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

Siebert et al 2008; RAVE

Tilt=6deg at z=1kpc

Potential nearly spherical

500 red clump stars

velocity ellpsoid tilt

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

Gaia / DM 9

Galactic escape velocity

33 stars

Gaia / DM 10

Halo stars: velocity distribution

552 Vrad

Gaia / DM 11

Dark matter within our Galaxy

Halo shape

Sagittarius tails precession of the orbit measures the shape of the potential

Other streams

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

Gaia / DM 13

Dark matter within our Galaxy

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

Gaia / DM 15

Gaia: a unique experiment

The next cornerstone of the ESA Science Programme

Unique characteristicsUnprecedented astrometric accuracy (7-200 as)Simultaneous astrophysical characterisation +

radial velocity of observed objectsSurvey down to V = 20109 objects observed all over the sky

Launch 2012, Soyouz from Kourou

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, HubblePrecision: 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

Gaia / DM 17

Performances for a G2 V star

Astrometry:

Photometry: accuracy on G magnitude

Spectroscopy

Magnitude < 10 15 20

Accuracy [as] 6 20 240

V=15 V=20

Per observation [mag] 0.002 0.03

End of mission [mag] 0.0002 0.003

Magnitude V=12.5 V=16.5

RV accuracy [km/s] < 1 15

Gaia / DM APC, 9 June 2010 18

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

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

Gaia / DM APC, 9 June 2010 21

StreamsIntégrale du mouvement

Amina’s figures

Streams in the

Galactic Halo

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 space Velocity space

Gaia / DM

Streams in the Galactic Halo

APC, 9 June 2010 23

Integrals of motion space

Gaia / DM APC, 9 June 2010 24

Constrains on the Clumpiness of the dark matter halo

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?

Gaia / DM

MOND within our Galaxy

Stellar Disk within MONDa newtonian astronomer observesa spherical dark haloand a dark disk (Milgrom, 86)

Gaia / DM

Testing Newtonian gravity on galactic scales

Modified gravity is only one version of MONDOnly 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

Gaia / DM

The « dark disk » from the Besançon Galactic model in MOND

With (x)=x/(1+x), at the solar position one has effeff = 78 = 78 MMpcpc-2-2 within z=1.1 kpc

to compare with present constraints dyndyn = = 74+-6 M 74+-6 Mpcpc-2 -2 (TEST 1)(TEST 1)The effective radial density distribution in the disk has a scale-length enhanced by 25%scale-length enhanced by 25%

[deep MOND => 50%][deep MOND => 50%]

=> 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 even exclude

MOND as modified gravity MOND as modified gravity (TEST 2)(TEST 2)=> quick way to exploit GAIA data

Bienaymé, Famaey et al. Bienaymé, Famaey et al. 2009, A&A2009, A&A

2.5kpc3.1kpcCountsKz force

Gaia / DM

The vertical tilt of the velocity ellipsoid

Angle = arctg[22UW /(2

U - 2W) ]/2 is linked to the disk scale-legnth and dark halo

flattening (Bienaymé 2009)(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°10°<14°

RAVE data

Siebert et al. 2008Siebert et al. 2008

(z=1kpc)= 7.3°+-1.8°(z=1kpc)= 7.3°+-1.8°

TEST 3

Gaia / DM

ConclusionWe presented 3 quick tests to test MOND as modified

gravity in the Milky Way with GAIA-like quality data

This should allow to constrain constrain or even exclude MOND as even exclude MOND as modified gravitymodified gravity

Testing gravity crucially depends on our knowledge of the knowledge of the baryonic distributionbaryonic 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