A study of the HI expanding shell population of the Magellanic Bridge

Erik Muller: University of Wollongong

Australia Telescope National Facility

Supervisors: Bill Zealey (UOW)

Lister Staveley-Smith (ATNF)

A brief discussion of a high-resolution HI dataset OF The Magellanic Bridge, and of a survey for HI

expanding shells.

Peak Pixel map, Linear trans. func. Tmax=0.3 MJy/beam

The Magellanic System in HI:

To the Magellanic Stream

To the Leading Arm

LMC, R~50kpc

SMC, R~60kpc

The Magellanic Bridge l~14kpc

Parkes Multibeam data obtained by Putman, M.E.

HI Data collection & Reduction:• 144 mosaiced pointings with ATCA at ~16 minutes/pointing

• Scanning with Parkes multibeam (inner seven beams)

• ATCA Data reduced with MIRIAD

• Parkes data reduced on line with ‘LIVEDATA’

• ATCA + Parkes data merged post-convolution using IMMERGE (Stanimirovic, PhD, 1999)

• Resulting cube:– ~7ox7o region, Vel range~100-350 km/s (Heliocentric)

– RMS ~ 15.2 mJy/Beam (eq 1.7x1018 cm2 for each channel)

– 98” spatial resolution

– ~2x108 M (SMC ~4x108 M)

Velocity-DeclinationRight Ascension-Declination

Right Ascension-Velocity

Peak pixel maps of ATCA/Parkes HI datacube

RMS=15.2 mJy/beam (1.7x1018 atm cm-2)

Total observed HI Mass=200x106 M

8 km/s [VGSR]

38 km/s [VGSR]

?

Mass of centre region=72x106M

(2 x 4.7 x 5)kpc slab ρ=0.06 atm cm-3

(2 x 4.7)kpc cylinder ρ=0.2 atm cm-3

Shell selection Criteria• Adapted from Puche et. al. (1992)

i. A (rough) ring shape in all three projections (RA-Dec, RA-Vel, Dec-Vel), must be present across the velocity range occupied by the shell

ii. Shape is observable across at least three velocity channels (~5km/s)

iii. Ring Must be rim-brightened relative to ambient column density of channel

Criteria target rim brightened, expanding spherical structures (not cylindrical or blown out volumes)

To reduce subjectivity, criteria must be strictly satisfied!

•We assume a stellar wind model (Weaver, 1977)

HI Peak Pixel map. Size and location of 163 Magellanic Bridge HI expanding shells.

Crosses locate OB associations (Bica et al. 1995) Ret

HI, OB associations and Shells appear to correlate well.

Comparison with other systems

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

• SMC shell size distribution agrees with theoretical estimates for size distribution of shells formed through stellar winds (Oey & Clarke, 1997).

• Strict selection criteria manifests as a deficiency of large radii shells.

From Slope of log of population histogram: γx, αx = 1-γx

• Bridge shells, compared to the SMC population are (on average): – Marginally older, 60% smaller and expand 60% more slowly:

– Much less energetic:– Mean energy of Bridge: 48.1 (log erg)

– Mean energy for SMC: 51.8 (log erg)

Decreasing shell radius with increasing RA

Radius Discontinuity at MB/SMC transition (from strict critera)

MB and SMC HI shell populationDiscontinuity at MB/SMC transition (from strict critera)

Expansion Velocity

Decreasing expansion velocity with increasing RA

Dynamic age • Generally continuous age distribution

• Slight dominance by older shells at higher RA

HI around OB associations•HI ramps almost linearly to centre of OB positions

•Excess of HI <80pc of association centre (C/W Grondin & Demers, 1993.)

Diamonds: Mean variation of HI with distance from OB catalogued positions.Triangles: As for Diamonds, with positions of OB associations offset by 90pc (10 pixels) south

Error bars mark one standard error of the mean, vertical line marks resolution of Parkes observations by Matthewson, Cleary & Murray (1974)

Distribution of OB associations and HI shells

• Visually, Stars, HI and shells appear to correlate rather well.

• 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)

• Variation of the HI column density around OB associations shows an excess of HI within ~80pc of each association (!)

These call into question the commonly used wind/SNe modelDo alternative theories for shell-forming mechanisms help?

• Stellar wind and SNe driven shells (Weaver et al, 1977; Chevalier, 1974)

– Very poor correlation with Bridge Shells (see also Rhode et al. 1999)

– A local HI excess(!) appears co-incident with OB association positions.

• Gamma Ray Bursts (Efremov, Elmegreen & Hodge, 1998; Loeb & Perna, 1998)

– Relatively rare occurrence, unlikely to be applicable to all 163 shells in the Bridge.

– Expansion velocities are ~10-2 of predicted velocity.

Formation mechanisms of HI expanding Shells:

• HVC collisions (Tenorio-Tagle 1987, 1988; Ehlerova & Palous, 1996)

– Surface distribution of shells shows a tendency higher HI col. dens.

– HVC collisions in tenuous gas may create very asymmetrical and fragmented shells and would not be included in the survey!

– An further survey of incomplete shell-like features is necessary before much can be said about the effectiveness of this mechanism in the context of the Mag. Bridge.

• Ram pressure drag (Bureau et al, 2001)

– Hole produced this way would not form a complete spherical shell

– Shells would appear on ‘skin’ of HI mass, MB shells generally found to be deeply embedded.

Formation mechanisms of HI expanding Shells:

• Shell Kinematic age increases dramatically by ~40% at Right Ascension >2h 15m

•Shell expansion velocity steadily decreases with RA

•Shell radius drops by ~%40 at higher RA.

This represents an older, smaller, and more slowly expanding population of Shells RA >2h 15m

Why is there such an excess of old small, old shells at higher RA?

The Schizophrenic ISM of the Bridge?

The Schizophrenic ISM of the Bridge?

Mean Dispersion MeanColumn Density

RA-Vel: Peak pixel

Region of Older shell excess

This Survey excludes fragmented and incomplete shellsWhat does this excess of older shells at Higher RA indicate?:

-Are these regions relatively quiescent?-Are shells remaining intact within this region for a longer period?Need an additional survey of shell-like features (i.e. incomplete and fragmented shells)

SummaryShell census:

– Search and census of HI shell features reveal 163 expanding shells within a ~2x108 M of HI .

– Strict shell selection criteria must be satisfied• Survey has probably selected against large Radii shells.

– Bridge shells have kinematic ages similar to those found in the SMC, and logarithmic frequency distribution similar to other galaxies

– Distribution of shell centres and positions of OB associations is generally extremely poor.

• A HI excess appears locally around OB association positions (!)

– The effects of HVC impacts into the lower-density HI nearby to the Magellanic Bridge Requires more study.

Current theories of shell formation do not satisfactorily describe this Magellanic Bridge Expanding HI shell

population.