HI observations of the Magellanic Bridge

Erik Muller UOW/ATNF

Supervisors: Bill Zealey (UOW)Lister Staveley-Smith (ATNF)

Overview: HI observations of the Magellanic Bridge

• HI Data collection and reduction• Magellanic Bridge HI expanding Shell census• Evidence for star-formation within the

Magellanic Bridge: results of a search for CO emission in the Magellanic Bridge

• A quick look at H (maybe..) • Statistical tools applied to Bridge HI dataset

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 150-

200 Myr old - eg. Gardiner & Noguchi 1996)– Populated by young O-B (>7 Myr), as well as older, stars. (eg. Irwin et al, 1995)

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

The Magellanic System in HI:Multibeam, Parkes.

To the Magellanic Stream

To the Leading Arm

LMC

SMCThe Magellanic Bridge

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~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

Ret.

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

Bridge HI Shell census (and some stars):• Shell formation mechanisms • Shell census:

– Search and census of HI shell features reveal 163 expanding shells within a ~1.9x108 M of HI .

– Strict shell selection criteria must be satisfied – Bridge shells have kinematic ages similar to those found in

the SMC, and parameters similar to other galaxies– Distribution of shell centres and positions of OB associations

is generally very poor.– High HI column density and spatial distribution of OB

associations correlate well– HI and spatial distribution of Shell centres correlate well, no

indication of local HI deficiency around OB associations.

• 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.

Formation mechanisms of HI expanding Shells:

• 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), This must be considerably less in the case of the Magellanic Bridge where the stellar population is much smaller. Given the age of the Bridge (~200Myr), we therefore do not expect to observe a large population of shells formed through GRB.

Formation mechanisms of HI expanding Shells (cont):

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

i. 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

ii. Expansion must be present across at least three velocity channels (~5km/s)

iii. 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

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

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

Comparison of Magellanic Bridge shells to SMC population:

Magellanic Bridge SMCMean Stdev Mean Stdev

Kinematic Age (Myr) 6.2 3.4 5.7 2.8Shell Radius (pc) 58.6 33.2 91.9 65.5Expansion Velocity (km/s) 6.5 3.8 10.3 6.3Energy (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.

Ls=1.5x105

Rs

100pc

5

Ts

106yr

-3

no

cm-3 L Ts=52

RsVexp

(Weaver 1977)

•Discontinuity at MB/SMC transition (effect of selection critera)

•Decreasing shell radius with increasing RA

Radius •Discontinuity at MB/SMC transition (effect of selection critera)

•Decreasing expansion velocity with increasing RA

Expansion Velocity

•Generally continuous age distribution

•Slight excess at Higher RA

Dynamic age

MB and SMC HI shell population

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 163Expansion 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.

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)• 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).

•OB positions generally correspond regions of higher HI

•50% of OB associations correlate with a Mean HI column density>1.2x1021 atm cm-2

•Be wary of selection effects

Black: Left and mean integrated HI in 1.5’x1.5’ region, centred on 198 OB positions.White: Right and entire integrated intensity map, binned into 1.5’2 areas.

Distribution of OB, shells and HI

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, 1993. (No discernable a hole in the local HI)

•Spherical geometry implies ??M is contained within 80pc).

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

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.

Formation mechanisms of Bridge Shells • HVC impacts

– The distribution of holes shows preference for high HI column density (not withstanding selection effects)

– 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.

So which is it?

Star-formation sites in the Magellanic Bridge

• The Bridge has a stellar population (Tidal remnant)• SMC has v. low Metallicity (Rubio et al 1993)

– Bridge has Metallicity even lower than SMC (Lehner et al 2001, Smoker et al 2000)

• O-B star ages range >7 Myr• Bridge formation began ~150-200 Myr ago• Stars are forming in the Bridge

– Where are they forming? - Any molecular clouds?

Candidate 12CO(1-0) emission sites

60m

•Local 60m Maximum = dust = surface chemistry.

HI Integrated intensity

•Local HI Maximum = absorbing layer and raw material

HI Int. Int., 60m contours: (0.4-3.4 +1.9MJy/str)

Pointing Offsets by 1 BW (~45”)

• 12CO(1-0) detected in emission in the Magellanic Bridge– Star-formation through molecular cloud collapse– Star-formation is a current, active process in the Bridge– Stellar population develop in situ, and are not transported from remote

location.

• Evidence of Star-formation in a low-metallicity, tidal structure

HI and CO spectra suggest the CO emission region is imbedded within an HI cloud:Right: CO Spectra at pointing 1 (solid line and Left axis) overlaid on HI spectra (dotted line and right axis).

Hα structures in the Magellanic Bridge.– One shell has been previously detected in Hα emission that

corresponds to an HI shell (Meaburn 1985)• Age calculations from spectroscopic studies disagree with HI shell

age estimates by ~10%. (Graham, Meaburn & Bryce (2001)• Contains a UV source ( FAUST 392)

– Other Hα + HI shells and Hα filaments have since been found

• All existing Hα stuctures in the Bridge are nearby to a FAUST object. (within 2-3”)

• Pending further investigations

– HI + Hα shells are not significantly different from mean Magellanic Bridge HI shell in terms of radii, expansion velocity, energy etc.

Hα structures

Statistical tools:• Statistical tools provide a means to

– compare populations of similar objects between different systems

– Understand and model general trends and behaviours.– Distinguish between sub-populations

• Spectral correlation function (SCF): Measures spectral similarity as a function of radial separation

• Power spectrum analysis (PS): Measures power as a function of scale, and as a function of velocity range.

• Both SCF and PS have been used to infer information about the third spatial dimension.

Spectral Tools 1:• Specral Correlation function:

– Compares two spectra separated by Δr, and makes an estimate of their ‘similarity’

– A 2D map of mean SCF shows rate of change (or degree of corrleation) of SCF with Δr and θ

– Has been used to confirm a characteristic length for the scale height of the LMC, by measuring the radius of decorrelation (Padoan et al. 2001)

– SCF shows that MB spectra has a longer decorrelation length in the east-west direction. (Tidal stretching)

• Spatial power spectrum– Used to show the range of spatial scales present in

source– Highlights any process favouring a particular scale.

(Eg. Elmegreen, Kim, Staveley-Smith, 2001)– Using velocity averaging, is can be used to show the

relative contributions of density and velocity dominated fluctuations. (Lazarian & Pogosyan, 2001)

Spectral Tools 2:

Spectral Correlation functionHow it works:

r

oo S

rSS

,()( r

rr r

ro vrTvT

vrTvTrS

22

2

),(),(),(),(

1),(rr

rrr

)(11)(,0 rQ

S N r

WdvvT

NQ r

2),(1)(r

r

Δr Δr Δr

(A spatial Structure function)

SCF output maps:

T maps

SCF maps

55 pixels

37 pixels

•Dotted vertical lines show radius of decorrelation•+ve and –ve fit departures

•+ve departures at ~250-380pc (14’-22’ at 60kpc)•-ve departures for sub images where signal is lower and less well distributed throughout.

Fits in E-W and N-S directions (central 5 rows/columns)

ΣT=7.5x105 K.km/s ΣT=8.4x105 K.km/s ΣT=9.4x105 K.km/s

ΣT=1.0x106 K.km/s

ΣT=1.0x106 K.km/s ΣT=1.1x106 K.km/s ΣT=1.1x106 K.km/s ΣT=1.1x106 K.km/s

Spatial Power spectrum• Measures the rate of change of power with spatial scale • Works on Fourier inverted image data (edges are rounded by

convol with a gaussian)• Channels with significant signal selected (60 channels)• Filtered to reduce leakage from low spatial frequencies (image

convolved with 3x3 unsharp mask, then divided back into FFT data)

• Un-observed UV data is masked out.• Power-law fit to dataset (γ) (IDL poly_fit). • A range of velocity increments are examined to determine the

relative contributions of density (thin regime) and velocity (thick regime) fluctuations.

Spatial Power spectrum cont.

ATCA + Parkes data(+Gaussian rounding)

FFT (im2+r2)

Power law fit for

Brig

htne

ss2 [

K2 ]

Spatial Power spectrum cont.

γ – velocity binsize

Transition from thin to thick regime(velocity to density dominated regime)

General result:• All Power spectra, for all velocity bins are featureless and well

fit with by a single power law:• No processes present that lead to a dominant scale (c/w LMC)• More ‘3 dimensional’ than the LMC (Similar to SMC). i.e. no

characteristic thickness.• Power spectra steepen for increasing velocity bin size

(ΔV~<20km/s)• Transition from ‘thin’ velocity dominated (spectral ΔV ~< integrated

ΔV thickness) to thick, density dominated regime.• γ changes from ~-2.90 - ~-3.35, consistent with Kolmogorov

Turbulence. (Lazarian & Pogosyan, 2000)• Source of turbulence?

– Processes that do not show a scale preference:• Stirring & instabilites from tidal force of LMC and SMC?• Energy deposition into ISM from stellar population?

PS from other systems:• LMC (Elmegreen, Kim & Staveley-Smith, 2001)

• much steeper; γ ~<2.7 (Entire velocity range, two linear fits)• LMC spectra turns over at r~100pc

– attributed to line-of-sight thickness of LMC.

• SMC (Stanimirovic, Lazarian, 2001)• SMC and MB cover same range of γ:

– γSMC~ 3.4 at ΔV ~100km/s– γMB~ 3.3 at ΔV ~100km/s

• linear (featureless) over entire range of Δv• does not appear to approach a characteristic Δv

• Galaxy (Dickey et al. 2001)• Analysed for smaller range of Δv (0-20 km/s)• Inner Galaxy γ ~ -2.5 - -4, consistent with Kolmogorov turbulence.

• All systems show steepening of γ with ΔV.

SMC and Galaxy γ with ΔV

SMC γ with ΔV. (Stanimirovic & Lazarian, 2001)

Galaxy γ with ΔV. (Dickey et al 2001) (N.B. Inverted γ scale, linear ΔV scale)

Overall Summary:• General appearance:

– 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

• Shells, stars and HI :– 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 – 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 deeply embedded throughout the HI Bridge.

– Alternatives ??

Overall Summary:

• 12CO(1-0) detected in emission for the first time in the Magellanic Bridge– Indication of star formation from molecular cloud collapse

(c/w shock triggered star formation)– Star formation within tidal structure and a very low

metallicity environment• This presents a unique and opportunity to study star-formation under

these conditions at high spatial resolution

• Ha filaments throughout the Bridge:– A few are shown to be associated with an expanding HI

structure.– All have FAUST objects nearby, the likely source of

ionisation.

Overall Summary:

• SCF– In general, decorrelation of spectra separated by Δr occurs at ~200-400pc

• Estimated thickness of MB is ~5kpc, based on distance measurements for two OB associations separated by ~7’ (Demers & Battinelli, 1998)

• Results of SCF are difficult to interpret in the same way for LMC, PS analysis may help.

– SCF behaves strangely for datacubes containing low S/N– The line of minimum rate of change of SCF is points almost, but not

quite, E-W, towards the SMC and LMC.

• Power Spectra– There is no suggestion of a departure from a power law fit to MB spatial

power spectra, despite a decorrelation at ~200-400pc found using SCF. (c/w Padoan et al, 2001)

– PS shows transition from γ =~-2.9 to γ =-3.35, through thin to thick regime, consistent with Kolmogorov turbulence.

Overall Summary: