Galactic dynamics in the era of Gaia Amina Helmi Credits: ESA/Gaia0CC BY0SA 3.0 IGO
Galactic dynamics in the era of Gaia
Amina Helmi
Credits:)ESA
/Gaia0CC
)BY0SA
)3.0)IG
O)
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?
Cre
dit:
V. S
prin
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?
Cre
dit:
V. S
prin
gel
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)