1. HI observations of the Magellanic Bridge
Erik Muller UOW/ATNF
Supervisors: Bill Zealey (UOW)
Lister Staveley-Smith (ATNF)

2. Overview: HI observations of the Magellanic Bridge
The Magellanic system
HI Data collection and reduction
Shell formation mechanisms
Magellanic Bridge HI expanding Shell census
Selection criteria
Statistical results
OB Stellar associations, HI shells and HI gas
Shell formation mechanisms applied to the Bridge shell population.

3. The Magellanic System:
Detected in HI (spin-flip transition of Neutral Hydrogen) by Kerr, Hindman & Robinson, (1954), Parkes Telescope (ATNF).
Magellanic clouds are ~60kpc (SMC) to ~50kpc (LMC)
Their nearness makes them an excellent laboratory in which to observe physical processes with high spatial resolution
Magellanic system comprises five elements:
Large Magellanic Cloud (LMC) (Kim 1998)
Small Magellanic Cloud (SMC) (Staveley-Smith 1998, Stanimirovic 1999)
The Magellanic Stream (Putman, Gibson, Stanimirovic etc. 1998)
The Leading Arm (Putman 2000)
The Magellanic Bridge (Mathewson & Cleary 1984)
Bridge spans the ~14kpc from western edge of LMC to eastern edge of SMC
Formed through tidal interaction of SMC with LMC (Simulations predict Myr old - eg. Gardiner & Noguchi 1996)
Populated by young O-B (>7 Myr), as well as older, stars. (eg. Irwin et al, 1995)

4. The Magellanic System in HI: Multibeam, Parkes.
To the Magellanic Stream
To the Leading Arm
LMC
SMC
The Magellanic Bridge
Peak Pixel map, Linear trans. func. Tmax=0.3 MJy/beam

5. HI Data collection & Reduction:
144 pointings with ATCA (375m configuration)
~16 minutes/pointing
Scanning with Parkes multibeam (inner seven beams)
Scanning rate: 1o/min
ATCA Data reduced with MIRIAD
conventional procedures for data flagging and calibration
Parkes and ATCA data merged post-convolution using IMMERGE (Stanimirovic, PhD, 1999)
Parkes data reduced on line with ‘LIVEDATA’
Bandpass calibrations, velocity corrections
Resulting cube:
~7ox7o region, Vel range~ km/s (Heliocentric)
RMS ~ 15.2 mJy/Beam (eq 1.7x1018 cm2 for each channel)
98” spatial resolution
~2x108 M (SMC ~4x108 M)

6. Right Ascension-Declination
RMS=15.2 mJy/beam
(1.7x1018 atm cm-2)
Velocity-Declination ?
Mass of centre region=72x106M
(2 x 4.7)kpc cylinder ρ=0.2 atm cm-3 (2 x 4.7 x 5)kpc slab ρ=0.06 atm cm-3
Right Ascension-Velocity
Peak pixel maps of ATCA/Parkes HI datacube Total observed HI Mass=200x106 M
8 km/s [VGSR]
38 km/s [VGSR]

7. Formation mechanisms of HI expanding Shells:
Stellar wind and SNe driven shells (Weaver et al, 1977):
Hot, energetic stars ionise local gas, and blow open an expanding sphere of hot gas.
Detailed study by Rhode et al (1999) using HI data of Holmburg II galaxy find that the distribution and brightness of HOII clusters do not support the idea of expansion from SNe.
HVC collisions (Tenorio-Tagle 1987, 1988, Ehlerova & Palous, 1996)
Capable of producing low energy, spherical expanding structures for impacts by low Ek clouds. Rc ~10pc
Difficult/impossible to differentiate from stellar wind formation mechanism.
Gamma Ray Bursts (Efremov, Elmegreen & Hodge, 1998, Loeb & Perna, 1998)
Release relatively large amounts of energy (10% of progenitor mass) ~1053 erg
Shells formed from GRB are more energetic for lower radii and more quickly expanding shells.
GBR frequency in a our galaxy ~0.1 Myr –1 (Portegies Zwart, & Spreeuw, 1996),

8. Shell selection Criteria
Adapted from Puche et. al. (1992)
A (rough) ring shape must be observed in all three projections (RA-Dec, RA-Vel, Dec-Vel), and must be present across the velocity range occupied by the shell
Expansion must be present across at least three velocity channels (~5km/s)
The rim of the ring has good contrast with ambient column density of channel (i.e. the shell is rim brightened).
Criteria target rim brightened, expanding spherical structures (not cylindrical or blown out volumes)
To reduce subjectivity, criteria must be strictly satisfied

9. HI Peak Pixel map. Size and location of 163 Magellanic Bridge HI expanding shells. Crosses locate OB associations (Bica et al. 1995) Ret

10. Comparison of Magellanic Bridge shells to SMC population:
Dynamic Age
Luminosity:
(Weaver 1977)
Magellanic Bridge
SMC
Mean
Stdev
Kinematic Age (Myr)
6.2
3.4
5.7
2.8
Shell Radius (pc)
58.6
33.2
91.9
65.5
Expansion Velocity (km/s)
6.5
3.8
10.3
6.3
Energy (log [ergs])
48.1
51.8 (n=1 cm3)
Bridge shells, compared to the SMC population are (on average):
Marginally older (!)
60% smaller + expand 60% more slowly = Much less energetic.

11. MB and SMC HI shell population
Discontinuity at MB/SMC transition (effect of selection criteria)
Decreasing shell radius with increasing RA
Radius
Discontinuity at MB/SMC transition (effect of selection criteria)
Decreasing expansion velocity with increasing RA
Expansion Velocity
Generally continuous age distribution
Slight excess at Higher RA
Dynamic age

12. Comparison of power law parameters of expanding HI structures from other surveys
αx = 1-γx
Holmberg II
(Puche et al. 1992)
SMC
(Staveley-Smith et al. 1997)
Magellanic Bridge
Number of Shells 51
509
163
Expansion Velocity αv
2.9±0.6
2.8±0.4
2.6±0.6
Shell Radius αr
2.0±0.2
2.2±0.3
3.6±0.4
αv is in agreement with other systems
αr is much steeper for the Magellanic Bridge population
Due to a strict selection criteria that manifests as an overall deficiency of small radii shells, and ultimately as an older shell population.

13. Distribution of OB associations and HI shells
Visually, OB associations, HI and shell centres appear to correlate reasonably well. Map
A more quantitative study shows that:
~50% of shells have one or more OB association within 8’ (140pc)
~18% of shells have one or more OB associations within 3.5’ (60pc) (mean shell radius)
~40% of shells have at least one or more associations within one radius
Poor spatial correlation statistic – Are these associations really responsible for HI shell expansion?
Alternatives include Gamma ray bursts (Efremov, Elmegreen and Hodge, 1998), HVC collisions (Tenorio-Tagle 1981, 1987), ram pressure drag (Bureau et al, 2001).

14. HI around OB associations
HI ramps almost linearly to centre of OB positions
Excess of HI <80pc of association centre, in disagreement with Grondin & Demers, (No discernable depletion of the local HI)
Diamonds: Mean HI averaged in concentric annuli around OB catalogued positions.
Triangles: Mean HI averaged in concentric annuli, offset 90pc (10 pixels) south of OB centres
Error bars mark one standard error of the mean, vertical line marks resolution of Parkes observations by Matthewson, Cleary & Murray (1974)

15. Formation mechanisms of Bridge Shells
Stellar Wind and SNe
The most recent burst of starformation 10-25Myr ago (Demers & Battinelli 1998) , C/W mean shell kinematic age ~6Myr
‘Constant energy input rate is generally invalid’ (Shull, & Saken 1995)
Input from WR and stellar wind at 3~10Myr for coeval and non-coeval associations, increased expansion velocity, and mis-estimation of ‘true’ age by up to 40% - lower limit of starburst date by Demers & Battinelli
Bridge Associations & Clusters are very poorly populated, typically N ~ 8 (N increases towards SMC)
Some Assocations & Clusters ‘may be of type later than O-B’ (priv comm. Bica 2002)
Poor spatial correlation of OB associations, clusters and expanding shells in the Magellanic Bridge
Given a ‘normal’ IMF, we would expect a significant energy input from SNe after ~5Myr. Shells of this age not found around, or even near, most observed Magellanic Bridge OB associations – why not?.

16. Formation mechanisms of Bridge Shells
HVC impacts
The distribution of holes shows preference for high HI column density (not withstanding selection effects)
There is no reason for HVCs to preferentially impact in a specific region.
Many shells are deeply embedded in the HI, rather than being found near the surface.
GRB
Under this model, mean shell energy is ~1.3x1051erg, Mean Kinematic age is 1.2x105 yr (c/w 1.3x1048 erg and 6.1Myr for stellar wind model), expansion velocities are ~10-2 of predicted velocity.

17. Summary: General appearance: Shell survey:
ATCA and Parkes have uncovered chaotic and intricate structure of HI comprising the Magellanic Bridge.
Loops, filaments and clumps observable to smallest scales of 98” (~29pc)
Much of the Bridge is bifurcated into two velocity sheets, converging at ~2hr 30min
Large loop R~1kpc off the northeastern edge of SMC.
Shell survey:
163 shells found within the Magellanic Bridge
Kinematic age is consistent with that of , shells of the SMC although Magellanic Bridge shells are considerably smaller and less energetic.
Power law distribution of expansion velocity is consistent with HoII and SMC.
Strict selection criteria is insensitive to incomplete and fragmented shells

18. Summary: Shells, stars and HI : Shell formation:
Good correlation of HI with OB assocations and Clusters, and also with HI shells (NB. Selection criteria), Poor correlation of OB associations and clusters with expanding shell centres
HI distribution about OB associations and Clusters shows a mean excess at short radii (<80pc), and a decreasing slope with increasing radii
Shell formation:
Shell Energies and spatial distribution do not agree with theories of formation by stellar wind by OB associations and Clusters or by SNe
Theoretical frequency of GBRs is too low to be generally applied to Magellanic Bridge shells.
HVCs are capable of producing the observed structures, however, the surface distribution shows preferential distribution (selection effects!), and many shells are found too deeply embedded throughout the HI Bridge.
Alternatives ??