The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the MW

Marcel S. Pawlowski (mpawlow@astro.uni-bonn.de)

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.