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Large-scale Features and Enigmas of the HI in the Magellanic Bridge.

Large-scale Features and Enigmas of the HI in the Magellanic Bridge. Erik Muller (Arecibo Observatory) Also Lister Stavley Smith (ATNF). Synopsis. Recent observations of the H I Magellanic Bridge show (Muller et al, 2003): Smooth connectivity of the SMC and LMC

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Large-scale Features and Enigmas of the HI in the Magellanic Bridge.

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  1. Large-scale Features and Enigmas of the HI in the Magellanic Bridge. Erik Muller (Arecibo Observatory) Also Lister Stavley Smith (ATNF)

  2. Synopsis Recent observations of the HI Magellanic Bridge show (Muller et al, 2003): • Smooth connectivity of the SMC and LMC • significant large scale structure, indications of the occurrence of large-scale energy deposition events. Obvious Large scale HI structures appear in the Bridge as: • A significant discontinuity appears in the velocity profile of the Bridge, occurring at a declination of ~-73o • Generally complex HI profiles. A dominant bimodal arrangement apparently originates in the SMC and extends halfway along the SMC wing, into the Bridge. • A large and rim-brightened filament-loop appears off the NE edge of the SMC.

  3. The HI Dataset • Mosaiced observations from ATCA • Short spacings from Parkes • Vel: 100-350 km/s (Helio) • dV ~1.63 km/s • Overall sens.: NH ~1.7x1018 cm2 per channel. • Resolution: (synth. beam) ~ 98”

  4. Peak intensity, [K] (Muller et al. 2003) SGP LMC SMC Magellanic System in HI (Putman, 2000) The Magellanic System. HI:

  5. Velocity structure– Gaussian analysis Previous analysis of HI line profiles by McGee & Newton (1986): • Measurements of the entire Magellanic Bridge ( ~14o, SMC to LMC) using Parkes 64m (FWHP~15’) • 217 profiles, spaced with approximately 1o of separation. • Identification of two velocity components in the Bridge • Identification of a contiguous, three-component arrangement of structure of the HI in Bridge as it merges with the SMC.

  6. Velocity structure– Gaussian analysis

  7. Velocity structure - Velocity Centriods Heliocentric Galactic Row 6 Row 6 Row 5 Row 5 Row 4 Row 4 Row 3 Row 3 Row 2 Row 2 Row 1 Row 1

  8. Velocity structure – Shift with Declination Integrated intensity, Vel-Dec [K] (Muller et al. 2003) Peak intensity, Vel-Dec [K] (Muller et al. 2003)

  9. Velocity structure – Shift with Declination Peak intensity, RA-Vel [K] (Bruns 2003) LMC tip SMC SMC wing

  10. Velocity structure – Numerical simulations (Gardiner, Sawa & Noguchi, 1994) Formation of the Bridge began ~200Myr ago.

  11. Velocity structure – Numerical simulationsThe Hand-wavey bit. High velocity spur SMC LMC SMC Wing

  12. ATCA+Parkes HI - Peak T NE NW SE SW Velocity structure – Results of Spatial power spectrum analysis (SPS) (Muller, et al. 2004)

  13. Northern region: Deficient in large-scale velocity motions SMC Southern region: Similar to the SMC Velocity structure – Spatial power spectrum (Muller et al. 2004)

  14. Measured star positions Velocity structure – The SMC as an Armed Dwarf irregular? • Distribution of HI is not inconsistent with simulations. Need distance information. • Spatial power spectrum also appear to support the simulations. The two parts of the Bridge show extremely different organisation of scale. • The ‘Bridge’ is a transverse feature, where the turbulent component dominates. • The radial arm is deficient in large-scale (i.e. fast) velocity component. • Two ‘arms’ of the SMC superimpose on the sky. • The radial arm: the ‘Bridge’, shows organisation of scale similar to the SMC.

  15. ~40 km/s ~55 km/s ~35 km/s ~40 km/s ~25 km/s Wing Bifurcation

  16. Wing Bifurcation – Candidate Scenarios • Some parameters: • Expansion Velocity: 30-40 km/s • Length: ~1.5 – 2 kpc • Swept-up mass: 8.1 x 107 (roughly equal masses of ‘sheets’) • Approximate Kinetic Energy (1/2 MV2): • 9x1052 erg (2x1052 erg/deg2 ~20 O-type stars) • Scenario I - Bifurcation forms AFTER tidal perturbations • Requires a series of velocity-correlated and roughly time-correlated (i.e. within 5-10 Myr or so) energy-injection events over ~4.7 kpc2. • The observed O type stellar population of the Bridge does not support this as a stellar wind or SNe scenario very well. What else? HVCs? GRBs? • Scenario II - Bifurcation forms BEFORE tidal perturbations • Forming void preferentially expands in the direction of the tidal perturbation. • Estimated age of responsible shell population will be difficult: Weavers assumptions are violated.

  17. Wing Bifurcation – Known shell candidates (From Staveley-Smith et al, 1997 – using the shell model formalism by Weaver, 1977) • Ages of Bridge and shells are discrepant by an order of magnitude (at best)! • The assumptions for shell expansion, outlined by Weaver (1977) are unlikely to be true, the expected error due to is ~x2. • Presumably, the observed shell population is not responsible producing the observed feature. • Secondary (or more?) shell formation?. • What and when was/were the original event(s)?

  18. SGP LMC SMC Magellanic System in HI (Putman, 2000) Loop filament

  19. Loop filament

  20. Loop filament

  21. Loop filament

  22. Loop filament – Candidate formation scenarios • Legrange point (Wayte, 1994) • The loop is centred on the unstable LMC-SMC L1 point. • Stellar wind/SNe (Weaver, 1977, Chevalier, 1974), GRB (Wijers et al. 1998) • A void generated by the action of stellar wind from large number of energetic O-type stars, or by the cumulative efforts of many SNe. • Approximately 10% of Energy shed during a Neutron-Neutron Collision is expelled into the ISM as kinetic energy. • Infalling HVC (Tenorio-Tagle, 1986) • An infalling HVC is capable of generating an approximately spherical or cylindrical void in stratified ISM.

  23. Loop filament • Some parameters: • Axis diameter: Major ~1.60kpc Minor ~1.02kpc • Position angle: 50o • Systemic velocity: 217.1 km/s • Interior rms: 56 K.km/s • Swept-up mass: 4.5 x 106(1), 2.7x107(2)(1) Calculate from Area x ambient HI surface density(2) Calculate from total in loop and rim.

  24. 0.5 Loop filament – L1 Legrange point • Use ‘typical’ mass ratio of LMC/SMC~ 6.6 - 10 (e.g. Gardiner, Sawa & Noguchi, 1994). L1 occurs at 0.65 – 0.69 R (R=separation of LMC/SMC). • Observed position is ~ 0.2 – 0.25 R (HI data only)

  25. Loop filament – L1 Legrange point • Reasons that it is probably not (in order of decreasing plausibility): • All reasonable estimates of the mass ratio of the LMC/SMC predict that the L1 point is much closer to the LMC than is the observed hole. • To date, no numerical simulations have shown such a structure. Therefore, it is not a gravitational feature. • If we adopt the results of the SPS, which suggest that the Northern part of the Bridge is a radially extending arm, then the hole is very far from the line joining the mass centres of the SMC/LMC. • The projected hole centre is not well aligned with the line joining the apparent HI centres of the SMC/LMC • It does not appear to have the expected shape of a L1 region.

  26. Loop filament – An expanding HI hole? • Expansion Velocity: 30-40 km/s

  27. Loop filament – Stellar Wind, SNe, GRB Energy and Age predictions by standard Kinetic, Stellar wind and SNe formalisms: Energy Age Internal kinetic 52.1 log ergs 62 Myr Weaver Method 53.5 log ergs 38 Myr Chevalier Method 53.9 log ergs Breakout at D=2kpc Modified Weaver 71 Myr Woltjer 58 Myr Dyson & Williams 51 Myr • Weaver and Chevalier both require the equivalent of ~100-1000 O-Type stars in this part of the Bridge. • The mean population of a Bridge OB association is ~8 (Bica, Priv, comm. 2003). It • Perhaps a large renegade association from the SMC? • The GRB scenario fail for the same reason.

  28. Loop filament – Infalling HVCs • An small-mass object impinging on a larger, stratified gas layer will loose its KE to the medium. • Depending on the density of the impactor and the velocity, the formed feature will be roughly elliptical or cylindrical (blowout) • Following Tenorio-Tagle (1986), we use n[cm-3]=9.78x10-43Ekin/Rc3Vc2Ekin=3.1x1053 erg (as a lower limit from Weaver, 1977). • Some Constraints: • “Typical” HVC mass is 105 and 106 Mo (e.g. Van Woerden, 1999) • Very small and dense HVC: Radius~7.5pc, ρ~20 cm-3 Wakker, Oosterloo & Putman, 2002) N.B. For this particular cloud to create the observed hole, Vc~2300+/- 100 km/s [Helio] • For HVCs around the Magellanic System ρ~5 cm-3 (cold) ρ~5 cm-3 (hot) (Bruns, Priv Comm, 2003)

  29. Loop filament – Infalling HVCs Limit of Wakker Cloud Half Hole size Clouds capable of creating hole in Magellanic Bridge. Does not include any losses.

  30. Loop filament – Infalling HVs • Reasons that it is probably not (in order of decreasing plausibility): • The mass, velocity and size and the impactor will necessarily be very large, compared with the known HVC population and compared with the size of the hole itself • The hole is quite neat: there does not appear to be any debris which might be expected from an impact, nor are there any visible morphological signatures of an impact. An HVC impact cannot be confidently ruled out with these arguments.

  31. Loop filament – Other systems • Large holes exist in other systems: • M101 • Kamphuis, Sancisi & Van der Hulst (1991) locate a 1.5 kpc hole in M101. • Estimated age is ~150Myrs (comparable to the age of the Magellanic Bridge). • Stellar wind is invoked as the most likely evolution mechanism • NGC6822 • De Blok & Walter (2000) locate a 2.0x1.4kpc hole in the NGC6822 dwarf Galaxy • Age is ~100 Myr • Stellar wind following tidally-induced Starburst is considered to be a possible mechanism • NGC1313 • Ryder et al (1995) locate a 1.6kpc shell in NCC1313 • It has similar proportions to the Bridge hole, although it expands at twice the rate of the Bridge hole. • SNes were considered for the expansion mechanism. • Shapleys constellation III • Dopita, Mathewson & Ford locate a large shell in the LMC, at Constellation III • Well formed, similar proportions to the Bridge hole. • Stellar wind is considered to be the expansion mechanism

  32. Summary • Three large-scale structures have been identified in the high-resolution HI dataset of Muller et al, 2003. • High velocity component • Velocity birfurcation • Loop • The high velocity component appears to be somewhat compliant with simulations, which predict that • the SMC is ‘armed’. The second ‘Arm’ extends roughly radially, and is projected on top of the • Transverse arm. Later work by Muller et al. (2004) confirm that these components have more dissimilar • organisation of structure than should be expected from adjacent regions. Probably this is not too enigmatic • The bifircation of the Lower veocity part appears to originate in the SMC, probably a conglomeration of • Expanding stellar-wind shells. The formation of the Bridge is older than that of the calculated by the • Weaver model of stellar-wind-driven expansion, although the key assumptions are sure to be violated. • It is probably not gravitational, so what initially created this feature? When did it occur? • The large hole near the SMC is too large to be explained using the oft-invoked stellar wind or SNe model • The size, and/or density of the HVC necessary to generate the observed feature are on the high side of • Plausability, although such a thing is not impossible. This feature is not clearly reproduced in Numerical • Simulations.

  33. Summary

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