The Orbital Evolution of the Magellanic Clouds and the Implications for the Formation of the Stream

1
The Orbital Evolution of the Magellanic Clouds
and the Implications for the Formation of the
Stream
Gurtina Besla Harvard CfA

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

Putman et al (2003)
The Globular Cluster-Dwarf Galaxy Connection,
August 2007
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Outline

• Motivation New PM and halo models
• Results Clouds are on their first
• passage
• Implications for the Magellanic Stream

S
L
3
LMC Proper Motions
K1 measurement is accurate to 5
?N is not consistent with 0
New velocity 1.3 times higher
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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.
5
4-component MW model
Mvir 1012 M? Consistent with Klypin et al
(2002) Consistent with known obs. constraints
?Knapp et al 1985
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New MW model how do the old orbits change?
New MW model (static) new PM
Fiducial
378 km/s
450 km/s
T6-10 Gyr Apo400 kpc Only one previous passage
310 km/s
The LMC is on its FIRST passage about the MW
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Implications for the Formation of the Magellanic
Stream

• 1) Orbit traces the MS on the plane of the sky
  (uniquely requires LMCs ?N 0)

1) The LMCs past orbit does NOT trace the
current location of the MS on the plane of the sky
(?W, ?N) K1 vdM02
N
Magellanic Stream
LMC
GN96
W
E
Orbit deviates from the MS by 10º
SMC
This result is independent of the new HST
measurements AND the MW model.
Putman et al (2003)
S
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2) Vlsr orbit ? Vlsr MS
Putman et al (2003 P03)
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Implications for the Magellanic Stream (MS)
Issue the strength of the MW/L/SMC interaction
is severely limited
Tidal Stripping NOT VIABLE - No stars tidal
radius is too large along fiducial orbits. - Most
of the mass is lost at PERICENTER Ram Pressure
Stripping (?v2) LIKELY NOT VIABLE - Requires
high gas densities no Leading Arm Feature -
Instantaneous ram pressure is insufficient
without tides. ALTERNATIVES Stellar Feedback
Olano (2004), Nidever et al (2007), Lehner Howk
(2007) Intercloud Region Tidesram pressure
timescale is 300 Myr (formation of Magellanic
Bridge)
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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 (gt2 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.
  This result is independent of the halo model and
  the new velocities.
• All formation scenarios for the MS need to be
  re-evaluated in light of the new orbital history.