Galactic dynamics in the era of Gaia

Amina Helmi

Credits:)ESA

/Gaia0CC

)BY0SA

)3.0)IG

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How did the Galaxy come to be like this?

•  We can observe individual stars and measure their properties •  Decipher sequence of events of formation of a typical galaxy

•  Distributed in various Galactic components, each with specific characteristics •  e.g. halo stars are as nearly as old as the Universe

•  The Milky Way is a Rosetta stone

t = 1 Gyr t = 2 Gyr

t = 3 Gyr t = 4.5 Gyr

t = 8 Gyr t = Tnow today

Galactic Archaeology

•  Key ingredient of galaxy formation: mergers

•  Were mergers important for galaxies like the Milky Way?

•  How often and when did they happen? •  What were the building blocks?

•  Stars are “fossils” –  Motions, ages, chemical composition

trace origin –  Substructures pinpoint to debris from

accretion events –  Probe force field ! mass (gravity)

snapshots:)J.)Gardner)

Testing the cold dark matter paradigm Is this “picture” correct?

•  Are galaxies like the Milky Way and its nearest neighbours embedded in dark matter halos like those predicted by the cosmological model?

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dit:

V. S

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gel

Testing the cold dark matter paradigm Is this “picture” correct?

•  Are galaxies like the Milky Way and its nearest neighbours embedded in dark matter halos like those predicted by the cosmological model?

Testing the cold dark matter paradigm Is this “picture” correct?

•  Are galaxies like the Milky Way and its nearest neighbours embedded in dark matter halos like those predicted by the cosmological model?

•  How much dark matter is there? –  how is it distributed? –  what is the dark matter?

•  Is Gravity correct?

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dit:

V. S

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The Gaia revolution: starting now!

(1 km/s proper motions)

Gaia will measure positions and motions of stars for 109 stars (10,000x larger than predecessor); over a volume 100,000 larger; up to 1,000x more precisely ! transformational

up to 21 million objects/deg2

By Jan. 2015, 16 billion photom/astrom transits, 1.6 billion spectroscopic

Courtesy of Anthony Brown & Gaia/DPAC

• Simultaneous astrometry, photometry, spectroscopy • Complete to G = 20.7 (V = 20-22), G = 16 for spectroscopy

• Whole sky already observed! • First data release in mid-2016 (positions + G magnitude) • DR2 in Jan 2017: full phase-space

Courtesy)A.)Brown))

Phot

omet

ry

RR

Lyr

ae in

the

LM

C

Spectroscopy: RVS

Spectro-photometry astrophysical parameters

“Image of the week” http://www.cosmos.esa.int/web/gaia/

)Gaia)astrometric)data)reduc9on)

Disentangling)parallax)and)proper)mo9on)needs)more)than)1)yr)of)data)

Courtesy)U.)Lam

mers,)T.)PrusA,)A

.)Brown)an

d)DPAC)

)Gaia)astrometric)data)reduc9on)

Disentangling)parallax)and)proper)mo9on)needs)more)than)1)yr)of)data)

Gaia/AGIS)team)wanted)to)experiment)with)full)5Dparameter)solu9ons)as)soon)as)possible)

Courtesy)U.)Lam

mers,)T.)PrusA,)A

.)Brown)an

d)DPAC)

)Gaia)astrometric)data)reduc9on)

Disentangling)parallax)and)proper)mo9on)needs)more)than)1)yr)of)data)

Gaia/AGIS)team)wanted)to)experiment)with)full)5Dparameter)solu9ons)as)soon)as)possible)

Combine)with)posi9ons)of)stars)in)Tycho)catalog:)pm)and)par)for)2.5)million)stars)

Courtesy)U.)Lam

mers,)T.)PrusA,)A

.)Brown)an

d)DPAC)

(Very))preliminary)results)on)TGAS)parallaxes)

Courtesy)U.)Lam

ers,)T.)PrusA,)A

.)Brown)an

d)DPAC)

What)do)we)know)now)about)the)Galaxy?)

What)are)some)of)the)open)ques9ons?)

Assembly)history)of)the)Milky)Way)The)Galac9c)dark)halo))

Belokurov)et)al.)2006)+)

Outer)halo:)• Clear)evidence)of)substructure)• Limited)to)highDsurface)brightness)features)(progenitors/9me)of)events))• Qualita9vely)consistent)with)expecta9ons)from)LCDM)(Helmi)et)al.)2011;)Deason)et)al.)2014))

North)Galac9c)Cap)

Galac9c)An9centre)Slater)et)al.)2014+)

The)accre9on)history)unveiled)so)far:))The)Galac9c)halo)from)SDSS/PanStarrs)

Kinema9cs)for)large)numbers)of)halo)stars:)crucial)

angular momentum en

ergy

conserved)quan99es)100s of streams predicted near the Sun and possibly hiding…

How to find these? Gaia! •  Clustering in conserved quantities •  Follow-up: SFH and chemical evolution of building blocks " FLAMES, 4MOST & WEAVE

Kinema9cs)for)large)numbers)of)halo)stars:)crucial)

Helmi)&

)de)Zeeu

w)200

0)

Streams, orbits and gravitational potentials As satellite galaxy disrupts, stars delineate nearly an orbit

Present-day spatial distribution (density, shape, total mass): consistent with �CDM? Granularity (critical test of �CDM) e.g. Johnston et al. 2002; Yoon et al. 2011; Carlberg 2012/2013 Time-evolution e.g. Peñarrubia et al. 2005; Gomez & Helmi 2010, Buist & Helmi (2015)

The)dark)halo)•  Total)mass:))7)x)1011)–)2)x)1012)Msun))(factor)3)uncertainty!))•  Rota9on)curve,)density)profile)…)poorly)characterized)

•  Shape)constraints:))–  CloseDin:)Pal)5)streams)not)too)flajened)towards)disk)(q)~)0.95,)Kupper)et)al.)2015))

–  At)large)distance:)possibly)triaxial)at)large)distances)but)based)on)just)1)stream:)Sagijarius;)very)debated)

Kupper)et)al.)(2015))

The Milky Way's dark halo shape and Sgr stream

•  If halo is spherical, motion is in a plane –  Deviations, if any, tell us shape

•  in CDM halos are triaxial at large radii

•  PUZZLES from models of Sgr stream in axisymmetric potentials –  Precession signal (non-spherical halo):

clearly favour OBLATE (Johnston et al. 2005)

–  Radial velocities: clearly favour PROLATE (Helmi 2004)

Law

et

al. 2

005

Solution: A triaxial halo? •  Model by Law & Majewski (2010)

#  Positions on the sky (hence precession) and radial velocities

•  However, it is odd…nearly oblate shape –  Symmetry plane: yz, i.e. perpendicular to the disk " not a stable configuration/nor intuitive physically

•  Constraints on inner shape weak: can accommodate an oblate flattened inner halo (Vera-Ciro & Helmi 2013)

–  Why long axis along y? •  LMC provides additional significant torque " effective potential is sum of contribution of triaxial

MW halo + LMC " indistinguishable so far

Vera-Ciro &

Helm

i, 2013

Is this “picture” correct?

Granularity: Hundreds of thousands dark clumps if dark matter particle is cold

Springel)et)al.)2008)Koposov)et)al)2009)

Summary / Next steps •  Milky Way: A revolution can be expected with Gaia satellite mission

–  Many streams (soon to be) discovered –  Modeling of the dark halo shape from Sgr streams accommodates inner oblate and outer triaxial –  Next: halo’s density profile, mass and granularity (missing satellites) as well as time evolution –  History of formation of the Milky Way

•  Milky Way unique probes of cosmology –  Provided some of the most stringent tests and flaws/problems of the CDM model –  There is much more to come!

ESADESO)Galaxy)WG )))))ESADESO)mee9ng,)ESTEC,)10)October)ESADESO)2008) 25)

Thank)you)for)your)ajen9on)