The Orbital Evolution of the Magellanic Clouds and
The Orbital Evolution of the Magellanic Clouds and the Implications for the Formation of the Stream Gurtina Besla Harvard Cf. A Putman et al (2003) Collaborators: Nitya Kallivayalil Lars Hernquist Brant Robertson T. J. Cox Roeland P. van der Marel Charles Alcock The Globular Cluster-Dwarf Galaxy Connection, August 2007
Outline § Motivation: New PM and halo models § Results: Clouds are on their first passage § Implications for the Magellanic Stream S L
LMC Proper Motions: K 1 measurement is accurate to 5% W (mas/yr) N (mas/yr) Van der Gardiner & Kallivayalil Marel et al Noguchi et al 2006 a 2002 1996 (K 1) (vd. M 02) N is not consistent with 0(GN 96) -2. 03 (± 0. 08) -1. 68 (± 0. 16) -1. 72 0. 44 (± 0. 05) 0. 34 (± 0. 16) 0. 12 Total Vel. (km/s) 378 (± 18) 293 (± 39) 297 Radial Vel. (km/s) 89 (± 4) 84 (± 7) 82 Tangential Vel. (km/s) 367 (± 18) 281 (± 41) 287 New velocity ~1. 3 times higher
Implications of the K 1 results for the Classical Picture Isothermal sphere model GN 96, vd. M 02 : Apo = 110 -120 kpc T = 1. 5 Gyr K 1 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: model (static) how do + orbits new PM: the old 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
Implications for the Formation of the Magellanic Stream 1) The LMC’s past orbit NOTplane trace the current 1) Orbit traces the MSdoes on the of the sky location of the MS on the. LMC’s plane of (uniquely requires Nthe = sky 0) ( W, N) K 1; vd. M 02 LMC N Magellanic Stream GN 96 W E Orbit deviates from the MS by ~ 10º SMC This result is independent of the Putman new et. HST al (2003) measurements AND the MW model. S
2) Vlsr orbit ≠ Vlsr MS Putman et al (2003; P 03)
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 ( v 2): 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: Tides+ram pressure; timescale is ~300 Myr (formation of Magellanic
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. This result is independent of the halo model and the new velocities. All formation scenarios for the MS need to be reevaluated in light of the new orbital history.
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