Molecular Clouds and Star Formation in the Magellanic Bridge and the SMC

Yasuo Fukui, Norikazu Mizuno( Nagoya University )

LMC

SMC

Magellanic Bridge

Putman et al. 1998

Magellanic BridgeMagellanic Bridge

HI connection between the Clouds (LMC-SMC) M(HI) Bridge ~ 5.5 x 107Msun (Putman et al. 1998)

Bridge was pulled from the SMC 0.2 Gyr ago via a tidal encounter between the two Clouds, (e.g., Murai & Fujimoto 1980, Gardiner, Sawa & Fujimoto 1994)

Yong Stars in the bridge (6-25 Myr) Blue stars and associations (e.g., Irwin 1990, Grondin 1992, Hambly et al. 1994)

Young star formation in the western region ( few Myr ) H filamentary shell (DEM171, Meaburn 1986)

Low Metallicity Z ~ 0.08 Zsun

Based on C, N, O, Mg, and Si (Rolleston et al. 1999) SMC metallicity Z ~ 0.25 Zsun (e.g., Russel and Dopita 1992)

Irwin et al. 1990

Meaburn 1986

DEM171, caused by a Wolf-Rayet star?

big bang

H・He

small galaxies collision & merging giant galaxies

Metallicity =>highMetallicity ： low

Nearby （ ~ 58 kpc ） and extremely low metallicity=> understanding evolution of galaxies star formation in the early universe and distant galaxies

Magellanic BridgeMagellanic Bridge

In this study, Star formation (molecular clouds) in the Bridge Future of the Bridge

Diameter: 4m

Beam size 2.6’ @115GHz ~ 40-50 pc@50-60kpc

Velo. Res. 0.1 km/s

Band Width 100 km/s

Tsys ~ 300 K (SSB)

Integration time

~ 20 hours/point

2002 Oct. -2003 July

Observation with NANTEN Observation with NANTEN

Las Campanas Observatory

Search for CO in the Magellanic Bridge

Toward a region of cold atomic hydrogen (Smoker et al. 2000) =>No CO and no stars

Kobulnicky & Dickey 1999

Search for CO in the Magellanic BridgeToward a HI peak with IRAS Source (Muller et al. 2003)

The first detection of CO from the Bridge !

280 290 300

Galactic longitude (degree)

-45

-40

-35

-30Gal

actic

latit

ude

(deg

ree)

N(HI) > 1021 cm-2

Putman et al. 1998

Bica & Schmitt 1995

Clusters and Associations

16 regions40 points16 regions40 points

IRAS100μmIRAS

100μm

Targets

HI and Dust peaks

HI(Muller er al.)

HI(Muller er al.)

Search for CO in the Magellanic Bridge with NANTEN

HIHI

IRAS100μmIRAS

100μm

ResultsResults

8 CO clouds1-7x103 Msun

8 CO clouds1-7x103 Msun

M(H2) ~2x104 Msun

M(HI) ~5x107 Msun

M(H2) ~2x104 Msun

M(HI) ~5x107 Msun

TCO

~ 10-60 mK

dV ~ 1- 4 km/s

TCO

~ 10-60 mK

dV ~ 1- 4 km/s

SMC SMC

V（ km/s ）

0 1 2 3 4 5 6

5

10

15

20

25

30

35

Log(Mco)

5

10

15

20

25

30

2.5 3.5 4.53 4 5

Molecular Clouds in the Bridge

V ～ 1.5km/s, 　Mcloud ～ 5000 Msun

(Nakagawa et al. 2005)

Properties of Molecular clouds in the Bridge are similar to the clouds in the far outer Galaxy (Galactic Warp region)

with IRAS sources

Galactic Warp

R(kpc)

10 14 16 18 20

z(pc

)

500

0

-500

-1500

-1000Color:HIRed cross:CO

Nakagawa et al. （ 2005）

HI: Muller et al. 2003

Star Formation in the BridgeStar Formation in the Bridge(Bica & Schmitt 1995)

(Meaburn 1986)

Galaxy collision and formation of Tidal Dwarf Galaxy

Braine et al. 2001

TDG: far （ 20-135 Mpc ） , collision of disk galaxies, high metallicity

Bridge: near （ ~ 60 kpc ） , collision of small irregulars, low metallicity

Magellanic Bridge evolve into a new dwarf galaxy?（ Wiklind 2002 Nature ）

- Bica & Schmitt (1995) identified 3 clumps D1, D2, D3.- similar to dwarf spheroidals (e.g, Fornax, Draco, Ursa Minor)- D1 has two populous clusters (NGC796, L114)

D1 may evolve into a dwarf spheroidal (Bica & Schmitt 1995)

Star formation rate in the Magellanic Bridge

Mass of CO clouds in the Bridge 　 Mco ~ 1-7×103 Msun similar to Taurus molecular cloud

SFR in Taurus molecular cloud N*~ 100, <m*> ~ 0.7Msun, τ ~106 yr, M~ 7 ×103 Msun

=>1 ×10-8 Msun,star/Msun,cloud /yr

SFR estimate from molecular cloud mass SFR ~ 2 ×10-4 Msun/yr

SFR estimate from OB association in the Bridge ~ 100 OB association, M* ~ 100 Msun, τ~ 10-25Myr SFR ~ 6 ×10-4 Msun/yr

Very Small Magellanic dwarf galaxy?Very Small Magellanic dwarf galaxy?

Star formation is on-going and widespread 　 SFR ~ 2-6×10-4 Msun/yr, a few Gyr ~106 Msun

Gravitationally stable system M(HI)~ 5×107Msun => dV ~ 30 km/s dVCO, HI ~ 20-30 km/s

Relation with LMC and SMC past collision interval 0.2-1.5 Gyr 　 (e.g., Gardiner et al. 1994, Yoshizawa & Noguchi 2003)

Mass ratio　 Mstar/M(HI) ~ 0.02 ~ low-end for dwarf galaxies (Mateo 1998)

SummarySummary

• 8 CO clouds detected in the Magellanic Bridge - site of star formation - dust emission trace CO than HI - close to young associations - dV~ 2km/s, Mcloud ~ 1-7 × 103 Msun

To understand star formation process in the early universe ( low metallicity, HI rich, interacting system, dwarf galaxy )• New small dwarf galaxy will be formed in the Bridge

- collision of small galaxies also important in forming new galaxy - SFR~ 2-6×10-4 Msun/yr, gravitationally boundUnique close-by laboratory to study galaxy formation with high resolution and high sensitivity

Small Magellanic Cloud (D ~ 60 kpc)Small Magellanic Cloud (D ~ 60 kpc)

・ Lower metallicity (Dufour 1984) 　 SMC ~ 1/10 Zsun・ Higher gas to dust ratio (Koornneef 1984)

17 times higher than in our galaxy

Taylor et al. (1998)

No CO detections

Threshold for CO detection in galaxies

Observation with NANTEN Observation with NANTEN

•Diameter: 4m

•Beam size 2.6 arcmin (@115GHz) ~ 50 pc@60kpc•Velo. Res. 0.1 km/s

•Band Width 100 km/s

•Tsys ~ 300 K (SSB)

•Total integration time ~ 1 hours

1998 Oct. -2003 July

Las Campanas Observatory

Mizuno et al. 2001

27 CO clouds identified (10 clouds detected at one observed point. )

Mass ： 6.0×104 ～ 9.3×105 Msun

Radius ： 35~80 pc

Total molecular mass ~4.5×106 Msun

~ 0.7 % of HI mass 6.5 ×108 Msun (Hindman 1967)

Total molecular mass5 ×106 Msun

< 1% of the total atomic mass

NE region

N(HI) ~ ６ ×1021(cm-2) N(HI) ~ 9×1021(cm-2)

14/20 show a single peak in HI

SW region

All observed points (51) show multiple peaks in HI

Image: Spitzer 3.6(blue), 4.5(green), and 8.0(red) um

NANTEN &

Spitzer

NANTEN &

Spitzer

Contour: NANTEN

Spitzer Imaging Survey of the SMC(S3MC) by Bolatto et al.

Properties of molecular clouds in the SMCProperties of molecular clouds in the SMC

Weakness of CO Less C and O abundances => slower CO formation Less dust => faster CO photodissociation

No diffuse CO in the inter-cloud medium CO molecules can only survive in the dense region

Weakness of CO Less C and O abundances => slower CO formation Less dust => faster CO photodissociation

No diffuse CO in the inter-cloud medium CO molecules can only survive in the dense region

Star formation in the SMCStar formation in the SMC

Comparisons of the CO clouds with clusters, associations, and emission nebulae.

SW region --- small, but bright HII regions (e.g., N12, N27) on-going formation of massive stars in large CO clouds NE region --- large evolved HII region (e.g., N66) smaller CO clouds Wing region --- small bright HII regions are associated CO clouds

Comparisons of the CO clouds with clusters, associations, and emission nebulae.

SW region --- small, but bright HII regions (e.g., N12, N27) on-going formation of massive stars in large CO clouds NE region --- large evolved HII region (e.g., N66) smaller CO clouds Wing region --- small bright HII regions are associated CO clouds

HH

Rubio et al. 2004

SIMBA 1.2mm CO(2-1)

CO emission arise from high column density clumps ?

(Lequeux et al 1994)

In the Galaxy, CO appears at lower distance from the surface of the cloud

Putman et al. 1998

LMC

SMC

Magellanic Bridge

orange： IRAS 100μm

Star formationStar formation

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ǙDZÇÃÉsÉNÉ`ÉÉÇ å©ÇÈÇΩÇflÇ…ÇÕïKóvÇ≈Ç∑ÅBYoshizawa & Noguchi 2003

QuickTime˛ Ç∆TIFFÅià≥èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ

ǙDZÇÃÉsÉNÉ`ÉÉÇ å©ÇÈÇΩÇflÇ…ÇÕïKóvÇ≈Ç∑ÅB

Yoshizawa & Noguchi 2003

Right Ascension (J2000)

Dec

linat

ion

(J20

00)

2h20m 1h40m

-74°

-75°

Contour:IRAS100μm

2h0m

Contour:IRAS100μm

Color :HI (Muller )２ １43

Spectra of the HI and CO in the Bridge

1

3 4

- HI -CO

V ： 1.50km/s

V：0.77km/s

線幅：0.91km/s

V ： 2.08km/s

2

43

1

検出スペクトル

1 2

3 4

- HI -CO

- HI -CO

- HI -CO

- HI -CO

線幅： 1.50km/s

線幅： 0.77km/s

線幅： 0.91km/s

線幅： 2.08km/s

分子雲の広がり　～ 5 点十字観測～

2’ （ 35pc ）

M31 M33LMC SMC ブリッジ

NGC6822

銀河系 M32 Fornax UrsaMinor

Draco SculptorCarinaNGC 147

NGC 185

Galaxy name

太陽質量1012

1011

1010

109

108

107

（ Mateo 1998 ）

Studies of the SMCStudies of the SMC

understanding evolution of galaxies star formation in the early universe and

distant galaxies

understanding evolution of galaxies star formation in the early universe and

distant galaxies

HI Stanimirovic et al. 1998

Bica et al. 1995