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

Transcription

1 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)

2 What is known so far The Disc of Satellite Galaxies (DoS)

3 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. Cartesian Galactic Z-Axis [kpc] Milky Way satellites MW disc edge on Kroupa et al. (2010) [kpc] Diemand et al. (2008)

4 Disc of Satellites (DoS) 11 classical bright satellites (Metz et al. 2007) [kpc] DoS described by normal vector DoS (11 classical satellites) [kpc] Galactic longitude MW equator = disc plane Galactic latitude

5 Disc of Satellites (DoS) 11 classical bright satellites (Metz et al. 2007) 13 faint satellites (mostly discovered in SDSS) (Kroupa et al. 2010) [kpc] [kpc] DoS (13 faint satellites) DoS (all 24 satellites)

6 Disc of Satellites (DoS) + Orbital Poles Proper motions measured for 8 satellites Orbital poles (L = r x v) Metz et al. (2008) [kpc] Orbital Pole [kpc]

7 Disc of Satellites (DoS) + Orbital Poles Proper motions measured for 8 satellites Orbital poles (L = r x v) Metz et al. (2008) 6 satellites co-orbit in the DoS extremely unlikely if drawn from CDM simulations Sculptor: counter-orbiting (L offset by 180º) but also in the DoS Sagittarius: close to MW precession or scattering

8 First addition: globular clusters

9 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).

10 Old halo GCs: Normal to OH GC distribution far away from DoS normal Bootstrapping shows large scatter

11 Bulge/disc GCs: Normal aligns with MW pole, so BD GCs lie in MW plane as expected. Strongly clustered.

12 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.

13 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

14 Second addition: streams of stars and gas

15 stream Streams of Stars and Gas Take two anchor-points s and e from literature. e s' s Determine plane through anchorpoints and galactic center (s -e -GC plane), assuming this is the center of the orbit. MW disc e' GC n Sun 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.

16 Stream Normal Directions

17 Average direction Stream Normal Directions

18 Average direction is close to DoS and YH GC normals and average of orbital poles. 50% of streams within 1/6 th 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.

19 Putting it Together Rotating edge-on view of the MW The MW is surrounded by a Vast Polar Structure (VPOS) consisting of: Classical satellite galaxies Faint satellite galaxies Movies will be available at Young halo globular cluster Streams (3x magnified)

20 Where does the VPOS come from?

21 Suggested origins 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.

22 Tidal Dwarf Galaxies and Clusters MW satellite distribution looks like that predicted by tidal debris: Galaxy collisions can be polar. VV 340 X-ray NASA/CXC/IfA/D.Sanders et al; Optical NASA/ STScI/NRAO/A.Evans et al

23 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. Wetzstein et al. (2007) Tadpole NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA Dentist Chair Weilbacher et al. (2002)

24 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)

25 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 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.

26 Can counter-orbiting tidal debris be formed? Pawlowski et al. (2011)

27 Can counter-orbiting tidal debris be formed? Pawlowski et al. (2011)

28 Can counter-orbiting tidal debris be formed? Pawlowski et al. (2011)

29 The Setup Pawlowski et al. (2011) Nbody models with SUPERBOX++ Scaled MW (10 Gyr ago) exponential disc Mdisc = 8 x 10 9 Msun Rscale = 1.6 kpc vrot = 125 km /s N = 5 x 10 5 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)

30 Two examples Fly-by and Merger

31 Fly-By Movie Projection into the plane of the interaction = disc of tidal debris seen face-on Infalling Galaxy 2 Phases: (face-on) retrograde first tail sweeps over target then prograde Target Galaxy (edge-on) Movie available at Prograde Particles Retrograde Particles

32 Fly-By Comparison to MW satellite system Model MW Satellites Arbitrarily picked particles

33 Fly-By Comparison to MW satellite system Model MW Satellites Arbitrarily picked particles

34 Fly-By Orbital Poles Contours: orbital poles of particles in galaxy interaction model Prograde Retrograde

35 Merger Movie debris disc edge-on debris disc face-on Infalling Galaxy face on Target Galaxy (edge on) Movies available at Prograde Particles Retrograde Particles

36 Merger Orbital Poles Contours: orbital poles of particles in galaxy interaction model Φ Prograde Retrograde

37 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

38 Is it possible to reconstruct the early MW-encounter?

39 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

40 What did we learn?

41 Conclusion 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 ( 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 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 accepted Pawlowski et al. (soon): Can filamentary accretion explain the orbital poles of the MW satellites?, MNRAS subm.