Large-scale Features and Large-scale Features and Enigmas of the HI in the Enigmas of the HI in the

Magellanic Bridge.Magellanic Bridge.

Erik Muller (Arecibo Observatory)Erik Muller (Arecibo Observatory)AlsoAlso

Lister Stavley Smith (ATNF)Lister Stavley Smith (ATNF)

SynopsisSynopsisRecent observations of the HRecent observations of the HII Magellanic Bridge show Magellanic Bridge show

(Muller et al, 2003):(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 significant large scale structure, indications of the

occurrence of large-scale energy deposition occurrence of large-scale energy deposition events.events.

Obvious Large scale HObvious Large scale HII structures appear in the structures appear in the Bridge as:Bridge as:

1.1. A significant discontinuity appears in the velocity profile A significant discontinuity appears in the velocity profile of the Bridge, occurring at a declination of ~-73of the Bridge, occurring at a declination of ~-73oo

2.2. Generally complex HI profiles. A dominant bimodal Generally complex HI profiles. A dominant bimodal arrangement apparently originates in the SMC and arrangement apparently originates in the SMC and extends halfway along the SMC wing, into the Bridge.extends halfway along the SMC wing, into the Bridge.

3.3. A large and rim-brightened filament-loop appears off the A large and rim-brightened filament-loop appears off the NE edge of the SMC.NE edge of the SMC.

The HThe HII Dataset Dataset Mosaiced observations from ATCAMosaiced observations from ATCA Short spacings from ParkesShort spacings from Parkes Vel: 100-350 km/s (Helio)Vel: 100-350 km/s (Helio) dV ~1.63 km/sdV ~1.63 km/s Overall sens.: NOverall sens.: NHH ~1.7x10 ~1.7x101818 cm cm22

per channel.per channel. Resolution: (synth. beam) ~ 98”Resolution: (synth. beam) ~ 98”

SGP

Magellanic System in HI (Putman, 2000)

SMC

LMC

The Magellanic System. HThe Magellanic System. HII::

Peak intensity, [K] (Muller et al. 2003)

Velocity structureVelocity structure – Gaussian – Gaussian analysisanalysis

Previous analysis of HPrevious analysis of HI line profiles by McGee & I line profiles by McGee & Newton (1986):Newton (1986):

Measurements of the entire Magellanic Bridge Measurements of the entire Magellanic Bridge ( ~14( ~14oo, SMC to LMC) using Parkes 64m , SMC to LMC) using Parkes 64m (FWHP~15’) (FWHP~15’)

217 profiles, spaced with approximately 1217 profiles, spaced with approximately 1o o of of separation.separation.

Identification of two velocity components in the Identification of two velocity components in the BridgeBridge

Identification of a contiguous, three-component Identification of a contiguous, three-component arrangement of structure of the Harrangement of structure of the HII in Bridge as it in Bridge as it merges with the SMC.merges with the SMC.

Velocity structureVelocity structure – Gaussian – Gaussian analysisanalysis

Velocity structure - Velocity structure - Velocity Velocity CentriodsCentriods

Row 6

Row 5

Row 4

Row 3

Row 2

Row 1

Row 6

Row 5

Row 4

Row 3

Row 2

Row 1

Heliocentric Galactic

Velocity structure Velocity structure – Shift with – Shift with DeclinationDeclination

Peak intensity, Vel-Dec [K] (Muller et al. 2003)

Integrated intensity, Vel-Dec [K] (Muller et al. 2003)

Velocity structure Velocity structure – Shift with – Shift with DeclinationDeclination

Peak intensity, RA-Vel [K] (Bruns 2003)

LMC tip

SMC

SMC wing

Velocity structure Velocity structure – Numerical – Numerical simulations simulations (Gardiner, Sawa & Noguchi, 1994)(Gardiner, Sawa & Noguchi, 1994)

Formation of the Bridge began ~200Myr ago.

SMC Wing

LMC

SMC

High velocity spur

Velocity structure Velocity structure – Numerical – Numerical simulationssimulationsThe Hand-wavey bit.The Hand-wavey bit.

Velocity structure Velocity structure – Results of Spatial power spectrum analysis – Results of Spatial power spectrum analysis (SPS) (SPS) (Muller, et al. 2004)(Muller, et al. 2004)

ATCA+Parkes HI - Peak T

NE NW

SE SW

Velocity structure Velocity structure – Spatial power spectrum– Spatial power spectrum

SMC

Northern region: Deficient in large-scale velocity motions

Southern region: Similar to the SMC

(Muller et al. 2004)

Velocity structure Velocity structure – The SMC as an Armed Dwarf irregular?– The SMC as an Armed Dwarf irregular?

Distribution of HDistribution of HII is not inconsistent is not inconsistent with simulations. Need distance with simulations. Need distance information.information.

Spatial power spectrum also appear to Spatial power spectrum also appear to support the simulations. The two parts support the simulations. The two parts of the Bridge show extremely different of the Bridge show extremely different organisation of scale. organisation of scale.

The ‘Bridge’ is a transverse feature, The ‘Bridge’ is a transverse feature, where the turbulent component where the turbulent component dominates.dominates.

The radial arm is deficient in large-scale The radial arm is deficient in large-scale (i.e. fast) velocity component.(i.e. fast) velocity component.

Two ‘arms’ of the SMC superimpose on Two ‘arms’ of the SMC superimpose on the sky.the sky.

The radial arm: the ‘Bridge’, shows The radial arm: the ‘Bridge’, shows organisation of scale similar to the SMC.organisation of scale similar to the SMC.

Measured star positions

Wing BifurcationWing Bifurcation

~40 km/s

~25 km/s

~40 km/s

~35 km/s~55 km/s

Wing Bifurcation Wing Bifurcation – Candidate – Candidate ScenariosScenarios

Scenario I - Bifurcation forms AFTER tidal perturbationsScenario I - Bifurcation forms AFTER tidal perturbations Requires a series of velocity-correlated and roughly time-correlated (i.e. Requires a series of velocity-correlated and roughly time-correlated (i.e.

within 5-10 Myr or so) energy-injection events over ~4.7 kpcwithin 5-10 Myr or so) energy-injection events over ~4.7 kpc22.. The observed O type stellar population of the Bridge does not support this 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?as a stellar wind or SNe scenario very well. What else? HVCs? GRBs? Scenario II - Bifurcation forms BEFORE tidal perturbationsScenario II - Bifurcation forms BEFORE tidal perturbations

Forming void preferentially expands in the direction of the tidal Forming void preferentially expands in the direction of the tidal perturbation.perturbation.

Estimated age of responsible shell population will be difficult: Weavers Estimated age of responsible shell population will be difficult: Weavers assumptions are violated.assumptions are violated.

Some parameters:Some parameters: Expansion Velocity: 30-40 km/sExpansion Velocity: 30-40 km/s Length: ~1.5 – 2 kpcLength: ~1.5 – 2 kpc Swept-up mass: 8.1 x 10Swept-up mass: 8.1 x 1077 (roughly equal masses of ‘sheets’) (roughly equal masses of ‘sheets’) Approximate Kinetic Energy (1/2 MVApproximate Kinetic Energy (1/2 MV22): ):

9x109x105252 erg (2x10 erg (2x1052 52 erg/degerg/deg2 2 ~20 O-type stars) ~20 O-type stars)

Wing Bifurcation Wing Bifurcation – Known shell – Known shell candidatescandidates

ShellShell RARA

(J2000)(J2000)DecDec

(J2000)(J2000)Expansion Expansion VelocityVelocity

[km/s][km/s]

Heliocetric velocityHeliocetric velocity

[km/s][km/s]AgeAge

[Myr][Myr]RadiuRadiu

ss

[pc][pc]

SMC HSMC HII--495495

01:27:401:27:477

-73:02:16-73:02:16 17.617.6 158.0158.0 6.06.0 179179

SMC HSMC HII--498498

1:29:011:29:01 -73:15:01-73:15:01 8.08.0 273.1273.1 15.215.2 207207

SMC HSMC HII--499499

01:30:201:30:255

-73:27:44-73:27:44 14.214.2 160.7160.7 15.215.2 179179

SMC HSMC HII--498498

1:29:011:29:01 -73:15:01-73:15:01 8.08.0 273.1273.1 15.215.2 207207

(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 Ages of Bridge and shells are discrepant by an order of magnitude (at best)!(at best)!

The assumptions for shell expansion, outlined by Weaver (1977) are unlikely The assumptions for shell expansion, outlined by Weaver (1977) are unlikely to be true, the expected error due to is ~x2. to be true, the expected error due to is ~x2.

Presumably, the observed shell population is not responsible producing the Presumably, the observed shell population is not responsible producing the observed feature. 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)?

Loop filamentLoop filament

SGP

Magellanic System in HI (Putman, 2000)

SMC

LMC

Loop filamentLoop filament

Loop filamentLoop filament

Loop filamentLoop filament

Loop filament Loop filament – Candidate formation scenarios– 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 Stellar wind/SNe (Weaver, 1977, Chevalier, 1974), GRB (Wijers et al. 1998)(Wijers et al. 1998) A void generated by the action of stellar wind from large A void generated by the action of stellar wind from large

number of energetic O-type stars, or by the cumulative efforts number of energetic O-type stars, or by the cumulative efforts of many SNe.of many SNe.

Approximately 10% of Energy shed during a Neutron-Neutron Approximately 10% of Energy shed during a Neutron-Neutron Collision is expelled into the ISM as kinetic energy.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 An infalling HVC is capable of generating an approximately

spherical or cylindrical void in stratified ISM.spherical or cylindrical void in stratified ISM.

Loop filament Loop filament Some parameters:Some parameters:

Axis diameter: Axis diameter: Major ~1.60kpcMajor ~1.60kpcMinor ~1.02kpcMinor ~1.02kpc

Position angle: Position angle: 5050oo

Systemic velocity: Systemic velocity: 217.1 km/s 217.1 km/s Interior rms: Interior rms: 56 K.km/s56 K.km/s Swept-up mass: Swept-up mass: 4.5 x 104.5 x 106(1)6(1), 2.7x10, 2.7x107(2)7(2)

(1) Calculate from Area x ambient H(1) Calculate from Area x ambient HII surface density surface density(2) Calculate from total in loop and rim.(2) Calculate from total in loop and rim.

Loop filament Loop filament – L1 Legrange point– L1 Legrange point Use ‘typical’ mass ratio of LMC/SMC~ 6.6 - 10 (e.g. Gardiner, Use ‘typical’ mass ratio of LMC/SMC~ 6.6 - 10 (e.g. Gardiner,

Sawa & Noguchi, 1994). L1 occurs at 0.65 – 0.69 R Sawa & Noguchi, 1994). L1 occurs at 0.65 – 0.69 R (R=separation of LMC/SMC). (R=separation of LMC/SMC).

Observed position is ~ 0.2 – 0.25 R (HObserved position is ~ 0.2 – 0.25 R (HII data only) data only)

0.5

Loop filament Loop filament – L1 Legrange point– L1 Legrange point

Reasons that it is probably not Reasons that it is probably not (in order of decreasing plausibility):(in order of decreasing plausibility):

All reasonable estimates of the mass ratio of the LMC/SMC predict 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 that the L1 point is much closer to the LMC than is the observed hole.hole.

To date, no numerical simulations have shown such a structure. To date, no numerical simulations have shown such a structure. Therefore, it is not a gravitational feature.Therefore, it is not a gravitational feature.

If we adopt the results of the SPS, which suggest that the Northern 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 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.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 projected hole centre is not well aligned with the line joining the apparent HI centres of the SMC/LMCthe 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.

Loop filament Loop filament – An expanding H– An expanding HII hole?hole?

Expansion Velocity: 30-40 km/sExpansion Velocity: 30-40 km/s

Loop filament Loop filament – Stellar Wind, SNe, – Stellar Wind, SNe, GRB GRB

Energy and Age predictions by standard Kinetic, Stellar wind and SNe formalisms:

Energy AgeInternal kinetic 52.1 log ergs 62 MyrWeaver Method 53.5 log ergs 38 MyrChevalier Method 53.9 log ergs

Breakout at D=2kpcModified Weaver 71 MyrWoltjer 58 MyrDyson & Williams 51 Myr

TW=3/4(Rs/Vs) Tbo=3/b(Rbo*Mbo/VshMsh) Mbo=1.1x107Mo

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.

Loop filament Loop filament – Infalling HVCs– 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/Rc3Vc

2

Ekin=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)

Loop filament Loop filament – Infalling HVCs– Infalling HVCs

Clouds capable of creating hole in Magellanic Bridge. Does not include any losses.

Limit of Wakker Cloud

Half Hole size

Reasons that it is probably not Reasons that it is probably not (in order of decreasing plausibility):(in order of decreasing plausibility):

The mass, velocity and size and the impactor will The mass, velocity and size and the impactor will necessarily be very large, compared with the known HVC necessarily be very large, compared with the known HVC population and compared with the size of the hole itselfpopulation and compared with the size of the hole itself

The hole is quite neat: there does not appear to be any The hole is quite neat: there does not appear to be any debris which might be expected from an impact, nor are debris which might be expected from an impact, nor are there any visible morphological signatures of an impact.there any visible morphological signatures of an impact.

An HVC impact cannot be confidently ruled out with these An HVC impact cannot be confidently ruled out with these arguments.arguments.

Loop filament Loop filament – Infalling HVs– Infalling HVs

Large holes exist in other systems:Large holes exist in other systems: M101M101

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 mechanismStellar wind is invoked as the most likely evolution mechanism

NGC6822NGC6822 De Blok & Walter (2000) locate a 2.0x1.4kpc hole in the NGC6822 dwarf De Blok & Walter (2000) locate a 2.0x1.4kpc hole in the NGC6822 dwarf

GalaxyGalaxy Age is ~100 MyrAge is ~100 Myr Stellar wind following tidally-induced Starburst is considered to be a possible Stellar wind following tidally-induced Starburst is considered to be a possible

mechanismmechanism NGC1313NGC1313

Ryder et al (1995) locate a 1.6kpc shell in NCC1313Ryder et al (1995) locate a 1.6kpc shell in NCC1313 It has similar proportions to the Bridge hole, although it expands at twice the It has similar proportions to the Bridge hole, although it expands at twice the

rate of the Bridge hole.rate of the Bridge hole. SNes were considered for the expansion mechanism.SNes were considered for the expansion mechanism.

Shapleys constellation IIIShapleys constellation III Dopita, Mathewson & Ford locate a large shell in the LMC, at Constellation IIIDopita, 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 mechanismStellar wind is considered to be the expansion mechanism

Loop filament Loop filament – Other systems– Other systems

SummarySummaryThree large-scale structures have been identified in the high-resolution HI dataset of Muller et al, 2003.1. High velocity component2. Velocity birfurcation3. 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 ofExpanding 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 modelThe 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.

SummarySummary