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Are the Magellanic about

Clouds on the Milky

their First Passage Way?

Gurtina Besla
Harvard CfA

Collaborators: Nitya Kallivayalil Lars Hernquist Brant Robertson T.J. Cox Roeland P. van der Marel Charles Alcock

EFW MC workshop


Outline
Motivation for this Orbital Re-analysis:
New Proper Motions vs Old
Kallivayalil et al (2006a,b)

Milky Way (MW) model
Isothermal vs NFW

Model Dependences:
MW Mass Role of the SMC
S L

Implications for the formation of the Stream


LMC Proper Motions:
Van der Gardiner & Kallivayalil Marel et al Noguchi et al 2006a 2002 1996 (K1) (vdM02) N is not consistent with 0 (GN96) -2.03 (±0.08) -1.68 (±0.16) -1.72 0.44 (± 0.05) 0.34 (±0.16) 0.12 378 (±18) 89 (±4) 367 (±18) 293 (± 39) 84 (±7) 281 (±41) 297 82 287

W N

(mas/yr) (mas/yr)

Total Vel. (km/s) Radial Vel. (km/s) Tangential Vel. (km/s)

New velocity ~1.3 times higher


Implications of the K1 results for the Classical Picture
Isothermal sphere model
GN96, vdM02 : Apo = 110-120 kpc T = 1.5 Gyr

K1 mean: Apo = 220 kpc T = 3 Gyr
An isothermal sphere model is likely inaccurate at large distances.


4-component MW model
Mvir = 1012 M Consistent with Klypin et al (2002) Consistent with known obs. constraints
Knapp et al 1985


New MW model:(static) model how do + new PM: the old orbits change?
Fiducial
378 km/s 450 km/s

T~6-10 Gyr Apo~400 kpc Only one previous 310 km/s passage

The LMC is on its FIRST passage about the MW


Assumptions by GN96:
1) The LMC's past orbit does NOT planethe current 1) Orbit traces the MS on the trace of the sky location of the requires LMC's ofNthe 0) (uniquely MS on the plane = sky 2) Orbital VLSR VLSR of the MS (specifies W) 3) Clouds form a binary system (VtanSMC ~ VtanLMC) (W, N) K1; vdM02 LMC E SMC Orbit deviates from the MS by ~ 10є
Putman et al (2003)

N Magellanic Stream GN96 W

S


Velocity Gradient
2) Vlsr orbit Vlsr MS

Putman et al (2003; P03)


Model Dependences:
T~6 Gyr
High Mass Model: Mvir = 2 x 1012 M STATIC MODEL 3) L/SMC may not be bound to each other (highly eccentric)
+

Perturbations from SMC are negligible: MSMC/MLMC ~ 1/10


Implications for the Magellanic Stream (MS)
Issue: the strength of the MW/L/SMC interaction is severely limited
Tidal Stripping: NOT VIABLE - No stars associated with the MS - Tidal radius is too large along fiducial orbits. - Most of the mass is lost at PERICENTER Ram Pressure Stripping (v2): NOT VIABLE - Requires high gas densities & no Leading Arm Feature - Instantaneous ram pressure is insufficient. SMC bound: Stellar Feedback ? - see Olano (2004) & Nidever et al (2007) SMC unbound: Stream originates from SMC ?


Conclusions
The new PM measurements by Kallivayalil et al (2006) strongly suggest that the Clouds are on their first passage about the MW. OR The MW is substantially more massive than previously believed (>2 x 1012 M ) and the proper motions are discrepant by 4. The past orbits of the Clouds DO NOT trace the line of sight velocities or location of the MS unless the SMC is not bound to the LMC. All formation scenarios for the MS need to be reevaluated in light of the new orbital history.