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 H I Magellanic Bridge show (Muller et al, 2003): Smooth connectivity of the SMC and LMC Smooth connectivity of the SMC and LMC significant large scale structure, indications of the occurrence of large-scale energy deposition events. significant large scale structure, indications of the occurrence of large-scale energy deposition events. Obvious Large scale H I structures appear in the Bridge as: 1. A significant discontinuity appears in the velocity profile of the Bridge, occurring at a declination of ~-73 o 2. Generally complex HI profiles. A dominant bimodal arrangement apparently originates in the SMC and extends halfway along the SMC wing, into the Bridge. 3. A large and rim-brightened filament-loop appears off the NE edge of the SMC.

3. The H I Dataset Mosaiced observations from ATCA Mosaiced observations from ATCA Short spacings from Parkes Short spacings from Parkes Vel: 100-350 km/s (Helio) Vel: 100-350 km/s (Helio) dV ~1.63 km/s dV ~1.63 km/s Overall sens.: N H ~1.7x10 18 cm 2 per channel. Overall sens.: N H ~1.7x10 18 cm 2 per channel. Resolution: (synth. beam) ~ 98” Resolution: (synth. beam) ~ 98”

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

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

6. Velocity structure – Gaussian analysis

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

8. Velocity structure – Shift with Declination Peak intensity, Vel-Dec [K] (Muller et al. 2003) Integrated 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. SMC Wing LMC SMC High velocity spur Velocity structure – Numerical simulations The Hand-wavey bit.

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

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

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

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

16. Wing Bifurcation – Candidate Scenarios Scenario I - Bifurcation forms AFTER tidal perturbations 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 kpc 2. Requires a series of velocity-correlated and roughly time-correlated (i.e. within 5-10 Myr or so) energy-injection events over ~4.7 kpc 2. 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? 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 Scenario II - Bifurcation forms BEFORE tidal perturbations Forming void preferentially expands in the direction of the tidal perturbation. Forming void preferentially expands in the direction of the tidal perturbation. Estimated age of responsible shell population will be difficult: Weavers assumptions are violated. Estimated age of responsible shell population will be difficult: Weavers assumptions are violated. Some parameters: Some parameters: Expansion Velocity: 30-40 km/s Expansion Velocity: 30-40 km/s Length: ~1.5 – 2 kpc Length: ~1.5 – 2 kpc Swept-up mass: 8.1 x 10 7 (roughly equal masses of ‘sheets’) Swept-up mass: 8.1 x 10 7 (roughly equal masses of ‘sheets’) Approximate Kinetic Energy (1/2 MV 2 ): Approximate Kinetic Energy (1/2 MV 2 ): 9x10 52 erg (2x10 52 erg/deg 2 ~20 O-type stars) 9x10 52 erg (2x10 52 erg/deg 2 ~20 O-type stars)

17. Wing Bifurcation – Known shell candidates ShellRA(J2000)Dec(J2000) Expansion Velocity [km/s] Heliocetric velocity [km/s]Age[Myr]Radius[pc] SMC H I -495 01:27:47-73:02:1617.6158.06.0179 SMC H I -498 1:29:01-73:15:018.0273.115.2207 SMC H I -499 01:30:25-73:27:4414.2160.715.2179 SMC H I -498 1:29:01-73:15:018.0273.115.2207 (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)! 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. 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. Presumably, the observed shell population is not responsible producing the observed feature. Secondary (or more?) shell formation?. Secondary (or more?) shell formation?. What and when was/were the original event(s)? What and when was/were the original event(s)?

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

19. Loop filament

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

23. Loop filament Some parameters: Some parameters: Axis diameter: Major ~1.60kpc Minor ~1.02kpc Axis diameter: Major ~1.60kpc Minor ~1.02kpc Position angle: 50 o Position angle: 50 o Systemic velocity: 217.1 km/s Systemic velocity: 217.1 km/s Interior rms: 56 K.km/s Interior rms: 56 K.km/s Swept-up mass: 4.5 x 10 6(1), 2.7x10 7(2) (1) Calculate from Area x ambient H I surface density (2) Calculate from total in loop and rim. Swept-up mass: 4.5 x 10 6(1), 2.7x10 7(2) (1) Calculate from Area x ambient H I surface density (2) Calculate from total in loop and rim.

24. 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). 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 (H I data only) Observed position is ~ 0.2 – 0.25 R (H I data only) 0.5

25. Loop filament – L1 Legrange point Reasons that it is probably not (in order of decreasing plausibility): 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. 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. 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. 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 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. It does not appear to have the expected shape of a L1 region.

26. Loop filament – An expanding H I hole? Expansion Velocity: 30-40 km/s 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: EnergyAge Internal kinetic52.1 log ergs62 Myr Weaver Method53.5 log ergs38 Myr Chevalier Method53.9 log ergs Breakout at D=2kpc Modified Weaver71 Myr Woltjer58 Myr Dyson & Williams51 Myr TW=3/4(Rs/Vs)Tbo=3/b(Rbo*Mbo/VshMsh)Mbo=1.1x10 7 Mo TWM=Rbo(1+(R/Rbo)1/4)Two=Tbo+(5Tbo/8)(3/5+[R/Rbo]1/4)TD&W=Tbo(R/4Vbo)([R/Rbo]1/4-1) 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 -43 E kin /R c 3 V c 2 E kin =3.1x10 53 erg (as a lower limit from Weaver, 1977). Some Constraints: “Typical” HVC mass is 10 5 and 10 6 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, V c ~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 Clouds capable of creating hole in Magellanic Bridge. Does not include any losses. Limit of Wakker Cloud Half Hole size

30. Reasons that it is probably not (in order of decreasing plausibility): 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 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. 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. Loop filament – Infalling HVs

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

32. Summary Three large-scale structures have been identified in the high-resolution HI dataset of Muller et al, 2003. 1.High velocity component 2.Velocity birfurcation 3.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