the vpos: a vast polar structure of satellite ... -...
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The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the MW
Marcel S. Pawlowski ([email protected])
Supervisor: Pavel Kroupa (Bonn)
Collaborators: Jan Pflamm-Altenburg (Bonn)Klaas S. de Boer (Bonn)Benoit Famaey (Strasbourg & Bonn)Garry Angus (Cape Town)Gerhard Hensler (Vienna)Sylvia Plöckinger (Vienna)
What is known so farThe Disc of Satellite Galaxies (DoS)
Number:• MW-Observation:
~ 24 satellite galaxies• ΛCDM-prediction:
~1000 DM subhalos
Distribution:• Disc of Satellites (DoS)• Satellite distribution not affected by
baryonic physics or the type of dark matter if they are of cosmological origin.
Milky Way satellites
Kroupa et al. (2010)
Diemand et al. (2008)
[kpc]
Car
tesi
an G
alac
tic Z
-Axi
s [k
pc]
MW disc edge on
• 11 ‘classical’ bright satellites (Metz et al. 2007)
Disc of Satellites (DoS)
MW equator = disc plane
DoS (11 classical satellites)
[kpc]
[kp
c]
DoS describedby normal
vector
Galactic longitude
Gal
actic
latit
ude
• 11 ‘classical’ bright satellites (Metz et al. 2007)
• 13 faint satellites (mostly discovered in SDSS) (Kroupa et al. 2010)
Disc of Satellites (DoS)
[kpc]
[kp
c]
DoS (13 faint satellites) DoS
(all 24 satellites)
• Proper motions measured for 8 satellites➡ Orbital poles (L = r x v) Metz et al. (2008)
OrbitalPole
Disc of Satellites (DoS)+ Orbital Poles
[kpc]
[kp
c]
Disc of Satellites (DoS)+ Orbital Poles
• Proper motions measured for 8 satellites➡ Orbital poles (L = r x v) Metz et al. (2008)
Sagittarius:close to MW➡ precessionor scattering
Sculptor:counter-orbiting (L offset by 180º) but also in the DoS
6 satellites co-orbit in the DoS➡extremely unlikely if drawn from CDM simulations
First addition: globular clusters
Globular Cluster Distributions
GCs divided into three groups (Mackey & van den Bergh, 2005):
• 70 Old Halo GCs: thought to have formed with the early MW.
• 37 Bulge/Disc GCs: thought to have formed with the early MW and confined to MW bulge and disc.
• 30 Young Halo GCs: significant fraction is younger than OH GCs, have similarities to GCs of satellite galaxies, thought to be of accretion origin.
Discs fitted to all three groups, described by normal vector (same method as used on the satellite galaxies in Kroupa et al. 2010).
Uncertainties estimated with bootstrapping analysis (1σ contours).
Old halo GCs:• Normal to OH GC distribution far away from DoS normal• Bootstrapping shows large scatter
Bulge/disc GCs: • Normal aligns with MW pole, so BD GCs lie in MW plane as expected. • Strongly clustered.
Young Halo GCs:• very close to DoS, YH GCs define the same plane as satellite galaxies!• well clustered.• Radial cut: 10 GCs outside of 25 kpc, 20 GCs inside of 20 kpc➡both give normals close to DoS
• Probability of 0.1% to have both this close to DoS in isotropic distribution.
Disc of Globular Clusters
• Old Halo: no disc, distributed in a spheroidal
• Bulge/Disc: lie in MW bulge and disc as expected
• Young Halo: lie in the same plane as the DoS, valid independently for GCs < 20 kpc and > 25 kpc
Second addition: streams of stars and gas
Streams of Stars and Gas
• Take two anchor-points s and e from literature.
• Determine plane through anchor-points and galactic center (s’-e’-GC plane), assuming this is the center of the orbit.
• This gives ‘stream normal vector’ n
• Uncertainties estimated with Monte-Carlo method, varying anchor point positions.
Analysed 14 long streams around the MW (>25 tidal radii, most > 20º) with Galactocentric anchor-point distances of 10 to 50 kpc.
ses'
e'n SunGC
stream
MW disc
Stream Normal Directions
• Average direction
Stream Normal Directions
• Average direction is close to DoS and YH GC normals and average of orbital poles.• 50% of streams within 1/6th of the area around DoS normal
➡ Probability of 0.2% to have 7 of 14 uniformly distributed streams this close.• gaseous stream GCN has normal closest to DoS, might be tidal tail remnant.• consistent with GC results: 2 of 3 YH-cluster-stream-normals close to DoS-normal.
Putting it Together
The MW is surrounded by a Vast Polar Structure (VPOS) consisting of:
• Classical satellite galaxies
• Faint satellite galaxies
• Young halo globular cluster
• Streams (3x magnified)
Rotating edge-on view of the MW
Movies will be available athttp://www.astro.uni-bonn.de/download/
Where does the VPOS come from?
• Chance Alignment:• BUT: Too many objects in DoS; orbits correlated, too.
• Group Infall (Li & Helmi 2008, D’Onghia & Lake 2008):• BUT: DoS too thin compared to observed dwarf associations (Metz et al. 2009);
Inconsistent: to stay in DoS satellites have to have fallen in recently (Deason et al. 2011), but to be gas-free they have to orbit MW for a long time (Nichols & Bland-Hawthorn 2011).
• Filamentary Accretion (e.g. Lovell et al. 2011):• BUT: Model gives no DoS of subhalos, only preferred orbital direction (most likely
aligned with galaxy disc spin perpendicular to DoS) (Pawlowski et al. submitted).
• Magellanic Satellites (Nichols et al. 2011):• BUT: “extended DoS cannot be explained by the dwarfs being bound to the LMC
within the last two apogalacticons”. Also too many objects in DoS.
• Tidal Dwarf Galaxies (e.g. Lynden-Bell 1976, Kroupa et al. 2010, Pawlowski et al. 2011):• Naturally accounts for phase-space structure of satellite galaxies.
Suggested origins
Tidal Dwarf Galaxies and Clusters
MW satellite distribution looks like that predicted by tidal debris:
• Galaxy collisions can be polar.
VV 340X-ray NASA/CXC/IfA/D.Sanders et al; Optical NASA/
STScI/NRAO/A.Evans et al
Tidal Dwarf Galaxies and Clusters
MW satellite distribution looks like that predicted by tidal debris:
• Galaxy collisions can be polar.
• TDGs and GCs form in galaxy interactions.
TadpoleNASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA
Dentist ChairWeilbacher et al. (2002)
Wetzstein et al. (2007)
Tidal Dwarf Galaxies and Clusters
MW satellite distribution looks like that predicted by tidal debris:
• Galaxy collisions can be polar.
• TDGs and GCs form in galaxy interactions.
• TDGs are long lived and stay within the plane.
Duc et al. (2011)
Tidal Dwarf Galaxies and Clusters
MW satellite distribution looks like that predicted by tidal debris:
• Galaxy collisions can be polar.
• TDGs and GCs form in galaxy interactions.
• TDGs are long lived and stay within the plane.
Suggested scenario:
• About 10-12 Gyr ago, the early MW had a near-polar interaction with another galaxy.
• In the encounter, a polar structure of tidal debris was formed.
• The young disc-galaxies were gas-rich, supporting the formation of TDGs in the debris.
Can counter-orbiting tidal debris be formed?Pawlowski et al. (2011)
Can counter-orbiting tidal debris be formed?Pawlowski et al. (2011)
Can counter-orbiting tidal debris be formed?Pawlowski et al. (2011)
The Setup
• Nbody models with SUPERBOX++
• Scaled MW (10 Gyr ago)• exponential disc• Mdisc = 8 x 109 Msun
• Rscale = 1.6 kpc• vrot = 125 km/s
• N = 5 x 105 particles• Hernquist halo, 10 x Mdisc
• Similar to M33 today
• Parameter study• Mass ratios Target to Infalling
1:1 and 4:1• 74 models (>200 CPU-days)
Pawlowski et al. (2011)
Two examplesFly-by and Merger
Target Galaxy(edge-on)
Infalling Galaxy(face-on)
Fly-ByMovie
Prograde Particles
Retrograde Particles
2 Phases:retrograde first
tail sweeps over targetthen prograde
Projection into the plane of the interaction= disc of tidal debris seen face-on
Movie available athttp://www.astro.uni-bonn.de/download/
Fly-ByComparison to MW satellite system
Model
Arbitrarily picked particles
MW Satellites
Fly-ByComparison to MW satellite system
Model MW Satellites
Arbitrarily picked particles
Prograde Retrograde
Fly-ByOrbital Poles
Contours: orbital poles of particles in galaxy interaction model
MergerMovie
Prograde Particles
Retrograde Particles
Target Galaxy(edge on)
Infalling Galaxyface on
debris disc face-on
debris disc edge-on
Movies available athttp://www.astro.uni-bonn.de/download/
Prograde Retrograde
MergerOrbital Poles
Φ
Contours: orbital poles of particles in galaxy interaction model
Radial Distribution
Retrograde fraction
Merger• Prograde and retrograde out to 150 kpc• Pretro higher in central and outside
region• Pretro ~ 1:7 on average
Fly-by:• Retrograde material has a maximum
distance (2-phase origin)• Prograde material spreads out to large
distances• Pretro high in central region, drops to
zero for large distances• Pretro ~ 1:7 on average
r in kpc
Is it possible to reconstruct the early MW-encounter?
Possible interaction partners for the MW
Merger
• Formation of the MW bulge, disc re-formed afterwards.
Fly-by
• Magellanic Clouds progenitor (Lynden-Bell 1976):
• Positions, orbits and stream are in VPOS, very unlikely if unrelated (3%)
• LMC/SMC orbit in the same direction as majority of MW satellites
• Wide but bound orbit preferred by fly-by scenario
• Counter-orbiting satellite Sculptor consistent with retrograde material in model being more centrally concentrated.
• Andromeda Galaxy / M31:
• M31 satellites have similar spacial distribution
• M31 disc inclined to MW disc, polar orbit possible
• MW-M31 collision would have been a major interaction
What did we learn?
• The MW is surrounded by a Vast Polar Structure (VPOS).
• Satellite galaxies, young halo globular clusters, satellite orbits and streams are correlated over a wide range in distance (10-250 kpc).
• Cosmological accretion does not form such strongly correlated structures.
• Tidal debris of galaxy-galaxy interactions ...
... are correlated in phase-space
... form Tidal Dwarf Galaxies and star clusters
... show both co- and counter-orbiting material
... can explain many other features of the VPOS
Conclusion
MW satellites might be ancient tidal dwarf galaxies.
More details in Pawlowski et al. (2011): “Making Counter-Orbiting Tidal Debris”, A&A 532, A118
Pawlowski et al. (2012): “The VPOS: a vast polar structure of ... ”, MNRAS acceptedPawlowski et al. (soon): “Can filamentary accretion explain the orbital poles of the MW satellites?”, MNRAS subm.