Vistas t , t Astrono,ny. Vol. 30, pp. 243.-251, 19g7 (K)bl3 6~5~'$7 $11 Ill) + .50 Printed in Great Britain. All rights reserved, Copyright © 19g~ Perg,lmon Press pie.

L A R G E - S C A L E FILAMENTS OF STAR F O R M A T I O N IN THE L A R G E M A G E L L A N I C

C L O U D

J. V, Feitzinger,* R. F. Haynes,** U. Klein,* .... R. Wielebinski. and M. Perschke*

*Aslronomischcs Institute, Ruhr-Universitfil Bochum, Poslfach 102148. D~.I.630 Bochum, F .R.G.

**Division of Radiophysics, CSIRO, P.O. Box 76, Epping, NSW 2121, Australia ***Radioaslronornisches lnstitut der Universit~il Bonn. Auf dem Hiigel 71,

D-53111) Bonn 1. F .R.G. +Max-Planck-lnstitut flit Radioastronomic, Auf dem Hiigel 69,

D-53110 Bonn 1, F .R.G.

SUMMARY A new high sensitivity radio survey of the Large Magellanic Cloud (I,MC) has been undertaken at

1.4 GHz. Evidence for large-scale star-forming filaments has come from this new data. The radio results are complemented by an IRAS image of the LMC at 100 ibm which shows the same large-scale filaments. Additional calculations in the frame of stochastic selfpropagating star formation models are presented. The implications for the morphology of the LMC are discussed.

I. The immediate past history of star formation and the distribution of dust in the LMC show coherent regular strings. As many as six or seven very extensive star-forming features cart be recognized.

2. The optical bar of the I,MC is not directly related to the star-formlng pattern; it appears to reflect an earlier period in the evolution of the galaxy.

3. The stochastic selfpropagating star formation models simulate very realistically the observed distribution of young stars and molecular cloud or dust features but they do not explain the function of the 30 Doradus nebula which is also a site of a burst of s tar formation.

I. INTRODOCTION

In this paper we investigate the morphology of the LMC using in particular the recent radio

continuum data of Haynes et al. (19861 and the IRAS images from the a l l -sky survey (Beichman et

at., 19841. The 1.4 GHz radio continuum map is the first high-sensit ivity, high dynamic range

il 'vestlgation of the LMC since the pioneering observations of Mathewson and Healey (19641. In the

last 20 years both receiver sensitivity has been improved and new data processing techniques

developed giving us a new map with superior information about the distribution of the radio

corttinuum. The first far infrared (FIR) images of the I,MC became available from the HCONI IRAS

al l -sky survey and are directly comparable with the radio continuum data. Previous studies of the

LMC in radio continuum had limited sensitivity (Mathewson and Healey, 1964) or covered only small

regions within the galaxy (Mills and Turtle, 19841. Studies of the distribution o," the neutral

hydrogen (McGee and Milton, 1966; Rohlfs et al., 1984) covered the inner parts of the LM('; the ',30

Doradus region and the prominent bar.

Using optical data Schmidt-Kaler (1977) found filaments and suggested that the LMC has a spiral

structure. Feitzinger 11980) reviewed Magellanic~type galaxies and suggested that the spiral

filaments are sites of recent star formation and that they are not connected with density waves. The

new radio continuum and FIR data show again the existence of these filaments. In view of recently

discovered correlations between FIR flux and radio continuum (both being due to star formation

processes) we reinvestigate the filamentary structures in the LMC. We suggest that these filaments

are a result of stochastic s tar formation.

243

244 J . V . I :c i tzmgcr et al.

2. MORPHOLOGY OF THE LMC AT 1.4 GHz

in F i g u r e l a we d i s p l a y t h e d i s t r i b u t i o n o f t h e r a d i o c o n t i n u u m e m i s s i o n o f t h e LMC a t 1.4 GHz.

T h i s m a p is i d e n t i c a l to t h a t p r e s e n t e d b y H a y n e s e t a l . ( 1 9 8 6 ) , a n d s h o w n h e r e a s a g r a y - s c a l e

r a d i o b r i g h t n e s s i m a g e . T h e a n g u l a r r e s o l u t i o n is 15 ' a t h a l f p o w e r ( 2 4 0 pc a t a n a s s u m e d d i s t a n c e

o f t h e LMC o f 55 k p c ) . T h e m a p is a p p r o x i m a t e l y 7 t i m e s m o r e s e n s i t i v e t h a n t h e p r e v i o u s 1.4 GIIz

s u r v e y o f M a t h e w s o n a n d H e a l e y ( 1 9 6 4 ) . T h i s i m p r o v e d s e n s i t i v i t y a l l o w s t h e s t u d y o f t h e r a d i o

m o r p h o l o g y m o r e t h o r o u g h l y t h a n h a s b e e n p o s s i b l e b e f o r e . T h e p i c t u r e s h o w s a n e x t e n d e d ' b a s e

d i s k ' , m e a s u r i n g 7-.4 (7.1 kpc ) in t h e n o r t h - s o u t h a n d 5".9 (5 .7 kpc ) in t h e e a s t - w e s [ d i r e c t i o n s ,

t h u s b e i n g s o m e w h a t e l o n g a t e d n o r t h - s o u t h . S u p e r i m p o s e d o n t o t h i s o v e r a l l e n v e l o p e o f ( p r e s u m a b l y

n o n t h e r m a l ) e m i s s i o n we f i n d a v a s t n u m b e r o f s o u r c e s a n d s o u r c e c o m p l e x e s m a n y o f w h i c h a p p e a r

to fo rm c h a i n s e x t e n d i n g o v e r s e v e r a l kpc . T h e m o s t p r o m i n e n t r e g i o n is 3 0 Dor w h i c h a p a r t f r om

b e i n g t h e m o s t i n t e n s e e m i s s i o n r e g i o n In t h e LMC a p p e a r s to be t h e o r i g i n o f a n u m b e r o f

c o n s p i c u o u s r a d i o c o n t i n u u m f e a t u r e s . T h e s e f e a t u r e s a r e h i g h l i g h t e d in F i g u r e 2 a s r i d g e l i n e s

s u p e r i m p o s e d o n t o a c o n t o u r p l o t o f t h e LMC r a d i o e m i s s i o n . As wi l l be d i s c u s s e d b e l o w t h e s e

f e a t u r e s c o i n c i d e p r e c i s e l y w i t h t h e s t a r - f o r m i n g f i l a m e n t s w h i c h h a v e b e e n f r e q u e n t l y d i s c u s s e d

u s i n g c o n s t i t u e n t s w h i c h d e l i n e a t e t h e r e c e n t s t a r f o r m a t i o n h i s t o r y o f t h e LMC. F o u r o f t h e s e

f e a t u r e s a r e e a s y to i d e n t i f y ( t h o s e l a b e l l e d B, C, D, a n d E in F i g u r e 2) . T h e r a d i o c o n t i n u u m

m o r p h o l o g y o f t h e LMC t h u s c l o s e l y r e s e m b l e s t h e a s y m m e t r i c p a t t e r n o f t h e e x t r e m e p o p u l a t i o n l

c o n s t i t u e n t s . T h e t y p i c a l l e n g t h o f t h e w e l l - d e f i n e d g l o b a l s t r u c t u r e s is 2 - 3 kpc . T h e s e s t r i n g s c a n

be a s s o c i a t e d w i t h t h e s u p e r g i a n t HI s h e l l s ( M e a b u r n , 1981) .

It is i m p o r t a n t to n o t e ( F i g u r e l a ) t h a t in t h e e a s t e r n p a r t o f t h e g a l a x y t h e b r i g h t n e s s

e x h i b i t s a r a t h e r s h a r p g r a d i e n t - - q u i c k l y d r o p p i n g f rom i t s m a x i m u m a n d s h o w i n g no v i s i b l e

f e a t u r e s m w h i l e t o w a r d s t h e w e s t e r n s i d e i t f a d e s a w a y v e r y s l o w l y w i t h a l l t h e p r o m i n e n t f e a t u r e s

r u n n i n g o u t t o w a r d s t h e s o u t h e r n , w e s t e r n a n d n o r t h e r n d i r e c t i o n s . T h e a p p e a r a n c e o f t h e r a d i o

m o r p h o l o g y is r e m i n i s c e n t o f t h e a s y m m e t r i c s p i r a l g a l a x y NGC 2 2 7 6 ( C o n d o n , 1983 ) .

F i g u r e l a . R a d i o b r i g h t n e s s i m a g e o f t h e LMC a t 1.4 GHz F i g u r e l b . A 100 ]~m IRAS s k y f l u x p l a t e (H CON 3) c e n t r e d on t h e LMC,

Largc-Scalc Filaments of Star Format ion in the LMC 245

3. A COMPARISON WITH THE I00 /~m IRAS DATA

Given a correlation between the distribution of dust and regions of star formation it is

important to compare the radio image of Figure la with the recently released results of the IrAS

survey. In Figure Ib we reproduce the 100 ~m sky flux plate from the HCONI IRAS survey (Beichman

et al., 1984) corresponding to the area surveyed by us at 1.4 GHz. This I00 ~um IRAS image was

produced by transforming the coordinate system of the IRAS PL I04 H3 image to ct-5 coordinates

using the Astronomical Image Processing System of Bonn Unlversity. The map was then smoothed to a

15' Gausslan beam to match the resolution of the 1.4 GHz map. Both maps have then been re-

tabulated to the same size and sampling interval to be comparable.

C o m p a r i n g t h e f i g u r e s we s e e t h a t t h e cool 100 ~m e m i s s i o n from d i s t r i b u t e d d u s t in t h e LMC is

f ound to be c l o s e l y c o r r e l a t e d w i t h t h e r i d g e s of m a x i m u m 1.4 GHz r a d i o c o n t i n u u m e m i s s i o n . T h e

it)() ~m m a p s show f ine s t r u c t u r e w i t h i n t h e d u s t d i s t r i b u t i o n wh i l e a l so p r o v i d i n g d e f i n i t i v e s u p p o r t

fo r t h e e x i s t e n c e of l o o s e l y c o n n e c t e d s t a r - f o r m i n g s i t e s w i th a c o h e r e n c e s c a l e of 2 - 4 kpc

( F e i t z i n g e r and B r a u n s f u r t h , 1984). T h e p l a c e s of s t a r f o r m a t i o n r e p r e s e n t i n g t h e i m m e d i a t e p a s t

h i s t o r y (z 108 y e a r s ) , t h e r a d i o c o n t i n u u m e m i s s i o n a n d t h e d i s t r i b u t i o n of d u s t in t h e LMC fol low

t h e s a m e r i d g e l ines . Our f e a t u r e s A, B, D, E, F ( F i g u r e 2) c a n be e a s i l y l o c a t e d on t h e IRAS 100 JJm

m a p s . T h e i n d i v i d u a l b lobs a n d c h a i n s of d u s t m a r k e d by t h e FIR e m i s s i o n h a v e a s c a l e l en i : th of

300 pc to 1500 pc. T h i s m a t c h e s e x a c t l y to a l e n g t h - t o - w i d t h r a t i o of [,/W = 3.5 a s d e d u c e d by t h e

E l m e g r e e n s (1983) for a s a m p l e of S b - S c d g a l a x i e s . Such a r a t i o c h a r a c t e r i z e s a s u p e r c l o u d s t r u c t u r e

in t h e i n t e r s t e l l a r m e d i u m . 30 D o r a d u s a p p e a r s to d o m i n a t e t h e s t r u c t u r e d e f i n e d by t h e 100 #m

e m i s s i o n , and t h e o p t i c a l b a r of t h e LMC ( s e e F i g u r e lb ) does no t s t a n d o u t a s a ma jo r f e a t u r e a t

1 O0 p.m.

4. STAR-FORMING FILAMENTS

In F i g u r e 2 we show a s k e t c h of t h e r i d g e l i n e s b a s e d on t h e r ad io a n d i n f r a r e d d a t a . At l e a s t

s ix , and p o s s i b l y s e v e n , r i d g e s a r e v i s i b l e . All t h e s e r i d g e s s e e m to o r i g i n a t e in t h e 30 D o r a d u s

n e b u l a . T h e r i d g e s d e n o t e d by A a nd B in F i g u r e 2 c o m m e n c e a t a p o s i t i o n a n g l e of 140" an d 175"

r e s p e c t i v e l y . T h e r i d g e d e n o t e d by C a t PA -- 200" is more o b v i o u s a t some d i s t a n c e a w a y f rom t h e

"lO D o r a d u s complex . T h e r i d g e D wh ic h s t a r t s a t PA = 230" p a s s e s t h r o u g h t h e ' b a r ' of t h e LMC an d

j o i n s m a n y c o m p l e x e s . T h e two f e a t u r e s d e n o t e d by E1 a n d E2 a r e v e r y c o n s p i c u o u s , p a r t i c u l a r l y in

r a d i o c o n t i n u u m . T h e b i f u r c a t i o n o c c u r s a t a b o u t a so --- 05h07 m, 6so = - 6 8 " 3 5 ' w i t h t h e f e a t u r e E2

t r a c i n g a n u m b e r of c o m p l e x e s . Such b i f u r c a t i o n s a r e q u i t e common a m o n g t h e o p t i c a l s t a r - f o r m i n g

p a t t e r n s in g a l a x i e s (Ml01 a nd NGC 6946 b e i n g p r o m i n e n t e x a m p l e s ) a n d a r e a c o n s e q u e n c e of

s t o c h a s t i c s t a r f o r m a t i o n . T h e one s e e n h e r e in t h e LMC w a r r a n t s f u r t h e r s t u d y .

The star-forming pattern shown in Figure 2 has been compared with the distribution of ionized

hydrogen revealed by H(~ measurements (Davies et al., 1976), and with the distribution of neutrM

hydrogen (McGee and Milton, 1964; Rohlfs et al., 1984). The distribution of Htx aligns well with the

star-forming structure proposed; even the bifurcation of arm E can be recognized in the Hc~

distribution. The young stellar populations (Isserstedt, 1984; Isserstedt and Kohl, 1984) are not as

uniformly distributed. There are obviously greater strips of filaments of star-forming activity loosely

connected. Extremely young objects, for example the Wolf-Rayet stars with ages of some 10' years

follow also these distributions. The asymmetry and clumpiness on the one hand and the coherence on

tile other hand is revealed if we sample all objects with ages less than 2 10 a years and outline also

the major regions of the UV 1200 A - 2100 A emission (Martin et al., 1976). The UV brightness dis-

tribution follows clearly the FIR and radio continuum emission and also the spatial distribut, ion of

the early type stars and lilt regions. The pictures of the Very-Wide-Field Camera at 1930 A of the

LMC in the course of the Spacelab l mission (Pierre et al., 1986) reveal the same global structure.

The UV couterparts and extensions of the B, C features can well be located as an arc-shaped [iV

intensity ridge in the south of the I,MC (compare Figure l of Pierre et at., 1986). The new results on

the LMC spiral structure based on ultraviolet images (Smith et al., 1987) fully confirm our spiral

246 J . V . F e i t z i n g e r et el .

LMC 1400 MHz I _ _ _ t - . . . . . . '. . . . . . . . . ~_

I . . . . . . . . . . . . . . . . : < : i . . . . <- : , . ' , < . . ~ - . . . . : . : . - , . .............. - . , . . , : , _ ~ ' - . , ~ ~ . . . ~ , . . ~ . . . . . . . !

6 3 0 , - . -~ , , ~ . . , , . • , - c '. : ' f ~ 4 " ~ , - - - , ,- ~: .' . . . - ' / " :.. t

l ' ~ . ~ . . . . . . . . . j_ -. ° ,t : ~ ; - ~ : = ~ , . , . . . . . . . . . . . . . ~ . ~ '.-~ ...... / , , : ~ . . , ~.:_,.~?~ :~< > ~ . ~ _ , - - ..p~ / > , . , . . : , ~ ~ . ~ : ~ _ j

'" J - V " ~ ~ , , " 'J~-%_ ~ : , ~ . ~ : : ' ~ , = ~ - ~ : ~ . ~ . _C~ ' I , ~ - ~ - , ~-~, ~ /

~ - : , ' . - . . . . . . L. ' J ' ~ < " : = ~ < - < ~ ' ~ • ~ " ~ . . . . . . " " t : ~ i - - ~ : : : £ . - ~ ' - . ) , : ~ , - - " - - - ~ " : > , ' . - , \ ~ - w V j . ~ ~ ,"

I- ,<. . . . . I . . . . . . . . . . . . . . i . . . . -

. . . : . - v - " . . , . ' . ~ . . . . ~ ' . . - - ~ > ~ . - - ,

...... i . . . . . . . . ' - ........ - . . . . o .... • - " ": - ..' \ ' i '. "" " - ~ ; . . ' \ f '.. / ' " ' " ' i

. ' , ~ - . ~ : - ~ ' . ~ : . ' ~ ' - - ~ ~ " T ~ - : - . ) T J - - T ' ~ V : ~ - ~ , ---~ -4.... . . : / - ~ " 1 " <>-, .............. ~":: , : - , ~ ~ R ~ ~ ~ - ~ ' z ....... ~ :~ .

-67 ' , . . ~ ..-:.'. 7,. ' . . . . , .~ , ,---- ,~ , ' • I . ........... <- <---- -,:~>.~. ,;-. ~ - ] ~ , ,_~., ~ , ~ , . b O ' ' ' ~ - ~ " ~ ~ " ~ ~ . ' / ~ . . : ; : ¥ ~ L , , ~ - " • , "

. . . . . . . .

. . . , . • . . ~ ~ ; : . . ~ ~ ." , ' : < , ~ : , , . , ~ . - : , . , ~ • ~ . . .

. . o l . . . . . . . . . • . ~ . . ~ ~ ' ~ : - ~ . ~ , , I -b ') ! . ~ " . < f ' ~ ! . / - "L:~-- -,<;:=" ~ ' f , .

- ' ]0 ' . ° • " ~:: ' j , ~ i ~ i ~ - . i > ~ ; " I':'~ ' ' -~" " : " ; : ; ~ ' ~ \ . . . . . . -C'-%' • ' "" 1 , , , , . ~ C ~ - - - , . ~ , , r ~ . ~ . . : . ~. ; ,

' r " ~ ~ ~ ~ ' - - / , C ~ S " ' ~ - ; . .. I . . . . ~ - ~ ~ . . . : . ! / y_. : . ~ . i : ~ . . , . " . - . . , . " , ~ < : ; ~ ,

-71° ": " " ~ ~ . . . . . . . . . ~-~:r ~ ~ , .. .. ..;,.-'.--:.. : - : . . . . . - .:--.. " . . . . . . . r ~ . : : . ~ : ~, .' , ~ - : . ~ J ~ < : " . . . . . . ~

I . . . . . . ~ ~ . ' < " ~ . t . n ~ . ~ ~ . . . I • . ~,~ ' ~ i ~ . - : . . - , - . r , i , " - ~- < \ ~ ' - - . . .

~ o , . . L.-:-:~' t \ ' . :'- - ~ t - , - - = ~ b . . . . ~ . . . . . . . ~"<--~--.:... ~ ",Y,'J~", - ~ . (< : . . . ~ . ;. .. . .. .- ! : " ) / <: . j ~ . ~ - - ~ ~,~.~ ~ . : . ~ . - . ,

• .~ . .~ . . ~ , ~ . .. ~.. ~ < , . - - . . . . . . . . . ~ -, . ,,: ~ . ~ # ~ ' ' - k - / '<f :- ,:

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36 m

. , , \ 5 . . . . . . % . - - 7 - ~ - ~ " . " . . . . . . . . . . . . . . . . ) • . . / ' ,

. . . . . . . , . ~., % ~ , , ' . , , / - - .. • . , . . . .

" ~ ' ~ - ' - " ; . . -. , " : ' . . . . . : - - I . " . . . . . . . . . . . ~ " .-, •

21, m '12 m 06bOO m 48" 36 m 2 t2 1 2 " 0 5 h 0 0 m t,8 m 36 m 2 t ? 12 ~ 0L)h00 m ~8 m

F i g u r e 2. S t a r f o r m i n g f i l a m e n t s a s d e l i n e a t e d b y t h e r i d g e s o f r a d i o c o n t i n u u m , s k e t c h e d o n t o a c o n t o u r m a p o f t h e I,MC a t 1 .4 GHz. T h e r a d i o c o n t o u r s a r e : 0 ( d a s h e d ) , O . lO , 0 . 2 5 . . . . . l .O , 1.2 . . . . . 2 . 0 , 2 . 5 . . . . . 4 . 0 , 4 . 5 . . . . . iO, 12, . . . , 2 0 , 22 . . . . . 3 0 , 3 2 J y p e r b e a m a r e a . T h e " h a l f - p o w e r " b e a m w i d t h ( 1 5 ' a r c ) i s i n d i c a t e d b y t h e h a t c h e d c i r c l e in t h e l o w e r l e f t o f t h e m a p . C o o r d i n a t e s a r e r i g h t a s -

c e n s i o n a n d d e c l i n a t i o n ( e p o c h 1 9 5 0 . 0 ) .

p a t t e r n a n d a l s o t h e s t o c h a s t i c m o d e l . T h e a p p e a r a n c e o f s u c h a p a t t e r n o f s t a r - f o r m i n g f i l a m e n t s

( s p i r a l a r m f i l a m e n t s ) i s t y p i c a l f o r l a t e t y p e g a l a x i e s ( F e i t z i n g e r , 1980 ; H u n t e r , 1982 ; G a l l a g h e r a n d

H u n t e r , 1 9 8 4 ) .

N e u t r a l h y d r o g e n f e a t u r e s c o i n c i d e p a r t l y w i t h f e a t u r e s A, B, C, D a n d E. T h e i n t e n s i t y o f t h e

n e u t r a l h y d r o g e n l i n e ( M c G e e a n d M i l t o n , 1 9 6 6 ) a t t h e m e a n r a d i a l v e l o c t t y o f 2 4 3 k m s - ' i s c l o s e l y

r e t a t e d t o t h e D a n d p a r t i c u l a r l y t o t h e A a n d B f e a t u r e s o f F i g u r e 2. S i m i l a r l y , t h e 3 0 0 k m s - * g a s

r e l a t e s w e l l to t h e F, E l a n d E2 f e a t u r e s a s w e l l a s t o t h e o u t e r p a r t o f D. T h e i n t e r m e d i a t e

v e l o c i t y h y d r o g e n g a s a t 2 7 3 km s - ' i s r e l a t e d t o t h e o u t e r r e g i o n s o f l) a n d B, w h i l e n e a r e r t o 3 0

D o r a d u s t h e s a m e v e l o c i t y g a s a p p e a r s to be a s s o c i a t e d w i t h f e a t u r e s F a n d G w i t h i n a b o u t 1 .5 k p c

o f 3 0 D o r a d u s ,

T h e c o r r e l a t i o n o f t h e FIR a n d r a d i o c o n t i n u u m f e a t u r e s w i t h t h e HI c o l u m n d e n s i t y a n d t h e

d i f f e r e n t v e l o c i t y r e g i m e s i s n o t v e r y t i g h t in a s p a t i a l s e n s e ( F e i t z i n g e r , 1 9 8 7 ) . I n s t e a d t h e m a x i m a

c a n a l w a y s be f o u n d s o m e w h a t s h i f t e d . T h e s t a r - f o r m i n g s i t e s a r e l o c a t e d o n t h e e d g e s o f t h e g i a n t

HI c l u m p s .

Large-ScaLe Filaments of Star Format ion in the LMC 247

In a r e c e n t p a p e r by Wood e t al . (1985) i t h a s b e e n e s t a b l i s h e d t h a t two d i s t i n c t s t a r

p o p u l a t i o n s e x i s t in an e a s t e r l y p a r t of t h e ba r . T h e m a j o r i t y of t h e s t a r s a r e of ag e * 10 ~ y e a r s

b u t a s m a l l e r p o p u l a t i o n of more m a s s i v e s t a r s of ag e - 6 x l0 T y e a r s a l s o e x i s t s . F e a t u r e El

t r a v e r s e s s e c t i o n s of t h e r e g i o n s t u d i e d by Wood e t a l . , an d s i n c e o u r 1.4 GHz s u r v y is e x p e c t e d to

d e t e c t e m i s s i o n f rom r e g i o n s in w h i c h y o u n g s t a r s a r e b e i n g c r e a t e d i t is n o t s u r p r i s i n g t h a t t h e two

d i s t i n c t p o p u l a t i o n s of s t a r s h a v e b e e n f o u n d in t h i s r eg io n , a r e g i o n w h e r e f e a t u r e E a l s o l ies

c o i n c i d e n t w i th t h e o p t i c a l ba r . T h e b a r r e g i o n m a y s e r v e to e n h a n c e s t a r f o r m a t i o n b u t i t is n o t

t h e h o s t for t h e bu lk of new s t a r s b e i n g fo rmed in t h e LMC.

5. STOCHASTIC STAR FORMATION MODELS

It is e v i d e n t f rom t h e d i s t r i b u t i o n of t h e c e p h e i d s an d s u p e r g i a n t s , d i v i d e d in to d e f i n e d a g e

g r o u p s ( I s s e r s t e d t , 19~4; I s s e r s t e d t a n d Koh| , 1984) t h a t s t a r f o r m a t i o n in t h e I,MC o c c u r e d

s t o c h a s t i c a l l y in v a r y i n g r e g i o n s , w i th a t e n d e n c y of c o n c e n t r a t i o n s i n to i s o l a t e d s p a c e - t i m e ce l l s .

E v o l u t i o n a r y e f f e c t s a r e a l so o b s e r v e d . A f ine e x a m p l e is t h e S h a p l e y III c o n s t e l l a t i o n , w h e r e s t a r

f o r m a t i o n a p p e a r e d to s t a r t a p p r o x i m a t e l y 17 Myr ago n e a r t h e c e n t r e an d h a s p r o p a g a t e d o u t w a r d s

as a s t o c h a s t i c a nd s e l f - l i m i t i n g f eed b a c k p r o c e s s (Do p i t a e t a l . , 1985; Dopl ta , 1985). T h e

m o r p h o l o g i c a l a p p e a r a n c e of t h i s r e g i o n in al l a v a i l a b l e w a v e l e n g t h r e g i m e s s u p p o r t s u c h a p i c t u r e .

T h e s t o c h a s t i c s e l f p r o p a g a t i n g s t a r f o r m a t i o n t h e o r y is a b l e to s i m u l a t e s u c h s t r u c t u r e s . T h e

m o d e l s of F e i t z i n g e r e t al . (1981; s e e a l so S e l d e n a n d Ge ro l a , 1982) g a v e p a t t e r n s of too g r e a t a

t ime r e g u l a r i t y a nd s t e a d i n e s s in t h e c a s e of t h e LMC. T h e mod i f i ed m o d e l s ( F e i t z i n g e r a n d P e r s c h k e ,

in p r e p a r a t i o n ) w i t h s t o c h a s t i c m o l e c u l a r c l o u d f o r m a t i o n , t h r e e g a s p h a s e s a n d d e t a i l e d g a s

d i s t r i b u t i o n s in n e i g h b o u r i n g ce l l s a n d p e r p e n d i c u l a r to t h e d i sk a r o u n d s t a r - f o r m i n g s i t e s , s i m u l a t e

m u c h b e t t e r t h e p e r c o l a t i n g s t a r f o r m a t i o n in t h e LMC. A f l a t r o t a t i o n c u r v e w i t h Vma x "~- 60 km s - '

is u sed . T h e cel l s i z e is 200 pc. T h e t i m e s c a l e s r e g u l a t i n g t h e s t a r f o r m a t i o n p r o c e s s e s a r e t h e

c oo l i ng t ime s c a l e s of t h e g a s p h a s e s a nd t h e f o r m a t i o n t i m e s c a l e s of t h e m o l e c u l a r c l o u d s .

A s y m m e t r i c a l l y l o c a t e d f i l a m e n t s of l oos e ly c o n n e c t e d s t a r - f o r m i n g r e g i o n s a r e p r o d u c e d ( F i g u r e 3).

T h e d i s t r i b u t i o n of t h e y o u n g s t r u c t u r e s c h a n g e s d r a s t i c a l l y on t ime s c a l e s of 10 a y e a r s . Such a

F i g u r e 3. S t o c h a s t i c s t a r f o r m a t i o n model for M a g e l l a n i c t y p e g a l a x i e s . T h e a r b i t r a r i l y s e l e c t e d t ime s t e p s s h o w n a r e 105, 106, 123, 124, 132, 364 IO T y e a r s (one t ime s t e p is IO T y e a r s ) . 10 t i m e s t e p s a r e p l o t t e d in e v e r y f r a m e . T h e disk d i a m e t e r is }0 kpc.

248 J . V . Fc i tz inger et al.

b e h a v i o u r is t y p i c a l f o r l a t e t y p e i r r e g u l a r s t e l l a r s y s t e m s a n d b e c o m e s e x t r e m e l y p r o n o u n c e d in

d w a r f g a l a x i e s ( H u n t e r a n d G a l l a g h e r , 1985b ) . T h e c a l c u l a t i o n s s h o w v e r y r a p i d v a r i a t i o n s in t h e

d i s t r i b u t i o n o f t h e y o u n g s t a r s , i .e . in t h e m o r p h o l o g i c a l a p p e a r a n c e o f t h e s y s t e m . P a t t e r n c h a n g e s

a s t h e r e s u l t o f j u m p i n g s t a r f o r m a t i o n a r e c o m p a t i b l e w i t h t h e f i n d i n g o f I s s e r s t e d t ( 1 9 8 4 ) on t h e

m i g r a t i o n o f t h e s t a r - f o r m i n g s i t e s a n d i n v a l i d a t e t h e a r g u m e n t s a g a i n s t t h e s t o c h a s t i c i t y a n d

c o n f i r m s t h e s t o c h a s t i c a ~ s a t z a s u s a b l e f o r t h e 1,MC a n d l a t e t y p e g a l a x i e s .

T h e d u s t a n d m o l e c u l a r c l o u d d i s t r i b u t i o n ( F i g u r e 4) is s t r o n g l y c o r r e l a t e d w i t h t h e s t a r - f o r m i n g

s i t e s . T h e m o l e c u l a r g a s is wel l d e l i n e a t e d in f i l a m e n t s i n t i m a t e l y n e i g h b o u r i n g (] ce l l s i z e ) o r

i n t e r m i n g l e d w i t h t h e s t a r - f o r m i n g s i t e s . I f we c o m p a r e t h e s e d i s t r i b u t i o n s w i t h t h e FIR e m i s s i o n ,

e s p e c i a l l y w i t h t h e d u s t c o l u m n d e n s i t y m a p s p r o d u c e d by I s r a e l a n d S c h w e r i n g ( 1 9 8 6 ) , s i m i l a r

p a t t e r n s , s c a l e s a n d c o h e r e n c e o f i n d i v i d u a l f e a t u r e s c a n be r e c o g n i z e d .

T = 1 0 5

+ . . - ? - ,++ ,~ : :~" - ,

~.~, : , . , ~

., . I,,~ ,1. ' . :+ ¢++ : - . . ; . . .

'~:>,L.-.~,,.' :"'~.. ~ ' : , ":~.2~:":-*+~

Fli~ure 4. T h e d i s t r i b u t i o n o f m o l e c u l a r g a s ( d u s t ) .

T h e e n e r g e t i c s o f t h e s t o c h a s t i c p e r c o l a t i n g s t a r f o r m a t i o n p r o c e s s e s c o m e s f rom t h e d i f f e r e n t i a l

r o t a t i o n a n d t h e e n e r g y i n p u t o f s u p e r n o v a , WR, O a n d B s t a r s . T h i s e n e r g y is r e s p o n s i b l e f o r t h e

d r i v i n g a n d s t r u c t u r i n g m e c h a n i s m s o f t h e m a n i f o l d o f g a s p h a s e s a n d m o r p h o l o g i c a l f o r m s . T h e

a v e r a g e r a t e o f w i n d a n d r a d i a t i v e e n e r g y i n p u t to t h e i n t e r s t e l l a r m e d i u m in t h e LMC is Eeu t = 2

l04a e r g / s / k p c 2 (1 .2 lO 39 e r g / s e c / k p c 2 f o r t h e s o l a r n e i g h b o u r h o o d ) . T h i s g i v e s a r a t i o E to t to u n i t

h y d r o g e n m a s s , n o r m a l i z e d to t h e s o l a r n e i g h b o u r h o o d , o f 1 ( T a r r a b , 1983 ) . In t h e f r a m e of t h e

s e l f r e g u l a t i n g s t o c h a s t i c s t a r f o r m a t i o n p r o c e s s e s t .his h i g h e n e r g y i n p u t is t h e r e a s o n fo r t h e

o n g o i n g l a r g e s t a r - f o r m i n g a c t i v i t y o f t h e LMC. T h e s t a r - f o r m i n g p r o c e s s in i n d i v i d u a l c e l l s , i .e.

m o r p h o l o g y , e x c i t a t i o n , d u s t , s t e l l a r c o n t e n t g i v e t h e k e y i n f o r m a t i o n on t h e s t a r - f o r m i n g s c e n a r i o .

In s p i t e o f t h e a p p a r e n t c h a o s , s t a r - f o r m i n g r e g i o n s a r e d i s t r i b u t e d o v e r t h e d i s k o f t h e m o d e l

o f t h e LMC w i t h d e f i n i t e c l u m p i n g s c a l e s . S t a r f o r m a t i o n in t h e mode l m i g r a t e s in a s y s t e m a t i c

m a n n e r .

Large-Sca le F i laments of Star F o r m a t i o n in the I M C 249

6. IMPORTANCE OF 3 0 DORADUS TO THE LMC

O n e m a i n c h a r a c t e r i s t i c o f t h e s t a r - f o r m i n g s i t e s in t h e LMC is t h e i r a s y m m e t r i c l o c a t i o n w i t h

r e s p e c t to t h e s y m m e t r y c e n t r e s o f t h i s s t e l l a r s y s t e m , i .e . t h e b a r o r t h e b a r a x i s , o r t h e r o t a t i o n

c e n t r e . T h e e x p e r i e n c e c o m i n g f rom o b s e r v a t i o n s o f t h e d i s t r i b u t i o n o f s t a r - f o r m i n g s i t e s in

M a g e l l a n i c - t y p e g a l a x i e s s h o w s t h a t g e n e r a l l y a s y m m e t r i c d i s t r i b u t i o n s a r e f o u n d . Good e x a m p l e s a r e

NGC 2 5 3 7 , 4 0 2 7 , 7 1 0 7 , 7 1 6 2 A . In l a t e - t y p e g a l a x i e s on t h e b o r d e r l i n e to d w a r f s y s t e m s ( w i t h

e x t r e m e l y low r o t a t i o n v e l o c i t i e s ) i t is t y p i c a l t h a t t h e s t a r - f o r m i n g f i l a m e n t s a r e i r r e g u l a r l y

d i s t r i b u t e d . A r e p r e s e n t a t i v e c a s e is DDO 155 (de V a u c o u l e u r s a n d Moss , 1983) . T h e s t a r - f o r m i n g

a c t i v i t y is c o n c e n t r a t e d on o n e s i d e o f t h e s y s t e m . F o r l a t e - t y p e g a l a x i e s t h e m o d e l c a l c u l a t i o n in

t h e f r a m e o f t h e s t o c h a s t i c s e l f p r o p a g a t i n g s t a r f o r m a t i o n a l w a y s p r o d u c e s a s y m m e t r i c a l l y l o c a t e d

f i l a m e n t s . T h e t r a n s i t i o n to d w a r f s y s t e m s is s t e a d y a n d is a s i z e e f f e c t ( n u m b e r o f s t a r - f o r m i n g

c e l l s ) a s f a r a s p e r c o l a t i n g p h e n o m e n a a r e c o n c e r n e d ( S e l d e n e t a l . , 1980 ) . Sma l l s y s t e m s ( d i a m e t e r

l e s s t h a n 3 - 4 kpc ) p r e f e r a b u r s t i n g m o d e o f s t a r f o r m a t i o n w i t h i s o l a t e d i s l a n d s o f s t a r - f o r m i n g

a c t i v i t y .

T h e n a t u r e o f 3 0 D o r a d u s a n d i t s r e l a t i o n s h i p to t h e o v e r a l l s t r u c t u r e o f t h e LMC is s t i l l

c o n t r o v e r s i a l (Mil ls e t a l . , 1978) , t h o u g h m o s t a u t h o r s a g r e e t h a t i t is a c e n t r e o f p r o d i g i o u s s t a r

f o r m a t i o n a c t i v i t y . T h e mode l c a l c u l a t i o n s p r e s e n t e d a b o v e p r o d u c e a l s o k n o t s o f s h o r t s t a r f o r m a t i o n

b u r s t s . S u c h b u r s t s a r e n o t f r e q u e n t , b u t t h e y a r e a l s o n o t t h e g r e a t e x c e p t i o n . T h e y s e e m to

d o m i n a t e t h e p a t t e r n o c c a s i o n a l l y f o r 10 ~ y e a r s . T h e r e is no g e n e t i c r e l a t i o n o f t h e s e b u r s t i n g s p a c e

c e l l s to t h e g l o b a l d i s t r i b u t i o n o f t h e s t a r - f o r m i n g s i t e s . S u c h s t a r f o r m a t i o n e v e n t s a r e a l o c a l

p r o c e s s a n d s m a l l s y s t e m s c a n a l s o m a k e g r e a t a c t i v i t y ( H u n t e r a n d G a l l a g h e r , 1 9 8 5 a , b ) . T h e l o c a l

s t a r - f o r m i n g a c t i v i t y , a s in t h e c a s e o f 30 D o r a d u s , c a n p r o d u c e l a r g e a m o u n t s o f h o t g a s l e a d i n g to

t h e i m p r e s s i o n o f a h o t g a s e o u s h a l o a r o u n d t h e LMC ( F e i t z i n g e r a n d S c h m i d t - l < a l e r , 1982 ) . In a n y

c a s e . t h e v e l o c i t y f i e l d o f t h e g a s in t h e i m m e d i a t e n e i g h b o u r h o u d of s u c h b u r s t s wi l l be i n f l u e n c e d

by t h e h u g e a m o u n t o f e n e r g y d e p o s i t e d a t s u c h s i t e s ( F e i t z l n g e r e t a l . , 1984) .

T h i s c o r r o b o r a t e s t h e r e s u l t o f E l m e g r e e n a n d E l m e g r e e n ( 1 9 7 0 ) t h a t in a p p r o x i m a t e l y h a l f o f a l l

b a r r e d M a g e l l a n i c g a l a x i e s t h e l a r g e s t HII r e g i o n s a r e l o c a t e d n e a r t h e e n d s o f t h e i r b a r s . T h e

e n h a n c e d s t a r f o r m a t i o n r a t e m a y h a v e b e e n t r i g g e r e d b e s i d e s t h e s t o c h a s t i c p r o c e s s e s b y t h e

l a r g e - s c a l e c o m p r e s s i o n o f g a s m o v i n g a r o u n d or s t r e a m i n g o u t o f t h e b a r ( F e l t z i n g e r , 1980 ) . T h i s is

a l s o in l i n e w i t h t h e f a c t t h a t c o m m o n p h y s i c a l p r o c e s s e s i n t r i n s i c to g i a n t s t a r - f o r m i n g g a s c l o u d s

d o m i n a t e o v e r t h e i n f l u e n c e s d u e to g a l a c t i c e n v i r o n m e n t s in c o n t r o l l i n g t h e p r o d u c t i o n a n d

e v o l u t i o n o f s u p e r g i a n t HIt r e g i o n s ( H u n t e r a n d G a l l a g h e r , 1985b ) .

T h e p o l a r i z a t i o n m e a s u r e m e n t s o f S c h m l d t ( 1 9 7 0 , 1976) a n d M a t h e w s o n a n d F o r d ( 1 9 7 0 ) i n d i c a t e

t h a t t h e magn ,~ t i c f i e l d d i r e c t i o n in t h e LMC is a s y m m e t r i c a n d i t s m a i n o r i e n t a t i o n r u n s t o w a r d s t h e

Smal l M a g e l l a n i c C l o u d . T h e h i g h e s t f i e ld s t r e n g t h is l o c a t e d in t i le 3 0 Dor r e g i o n . No s y m m e t r y w i t h

r e s p e c t to t h e b a r o r t h e r o t a t i o n c e n t r e c a n be d e d u c e d . T h e i n t e r a c t i o n b e t w e e n t h e g l o b a l

m a g n e t i c f i e l d s t r u c t u r e s a n d t h e p r o p a g a t i n g s t a r f o r m a t i o n p r o c e s s e s w e r e r e c e n t l y d e m o n s t r a t e d

( C h i a n g a n d E I m e g r e e n , 1982; F e i t z i n g e r , 1 9 8 7 ) T h e a n i s o t r o p i c m a g n e t i c f i e l d p r e s s u r e f a v o u r s a

m a s s m o t i o n e f t h e g a s e o u s c o m p o n e n t a l o n g t h e f i e l d l i n e s . T h i s a n i s o t r o p y r e i n f o r c e s t h e d i r e c t i o n

o f t h e p r o p a g a t i n g s t a r f o r m a t i o n ~ T h e r e f o r e we h a v e g o o d r e a s o n s to a c c e p t t h a t t h e a s y m m e t r i c

l o c a t i o n o f t h e s t a r - f o r m i n g f i l a m e n t s is a l s o c o n n e c t e d w i t h t h e a s y m m e t r i c m a g n e t i c f i e l d s t r u c t u r e

o f t h e I,MC. T h e a r g u m e n t s g i v e n f a v o u r t h e s t o c h a s t i c i n t e r p r e t a t i o n o f t h e a s y m m e t r i c d i s t r i b u t i o n

o f t h e s t a r - f o r m i n ~ s i t e s ( f o r a p a r t i c u l a r e p o c h ) .

To introduce a more traditional interpretation in the sense of a causal connection to the

asymmetry we give the following arguments. The atlas of peculiar galaxies (Arp, 1966; Arp and

Madorc. 1985) conlains a small class of galaxies with spiral structure compressed to one side and

enhanced alon~ the other side and the outer perimeter. The nuclei always exhibit an appreciable

offset, with respect to the overall structure (Madore, 1986). Examples in Arp's atlas (1966) are No. 5,

250 J . V . Fcilzinger et al.

6, 119. In the case of the LMC the a s sumpt ion is not too h y p o t h e t i c a l t h a t the ram p r e s s u r e be tween

the g a l a c t i c halo and the LMC d i s t o r t s the gas d i s t r i b u t i o n .

There is a conspicuous extension of the 1.4 GHz radio emission from the SMC toward the LMC

(Haynes et al., 1985), which would favour the view that tidal interaction is causing the asymmetry.

We note that the neutral hydrogen emissivity is also greatest in the 30 Doradus region and that the

distribution of hydrogen is asymmetric with respect to this intense region (Rohlfs et al., 1984). We

also note that the part of the LMC with the highest radio brightness is toward the concave side of

the ridge lines of star formation shown in Figure 2, and that both the enhancement of radio

emissivity and the warping of the disk which may be present there could be caused by ram pressure

as the I,MC moves through the galactic halo in this direction (Feitzinger et al., 1977; Lin and

Lynden-Be]l, 1982). A strong warp on the eastern edge as proposed by Feitzinger (1980, see his

Figure 17) could then account for the apparent absence of prominent star-forming sites in that

direction and the great velocity anomaly in HI. Alternatively, the warp could result from interaction

between the Small Magellanic Cloud and the LMC (Fujimoto and Mural, 1984). This interpretation is

not in conflict with the arguments given further above. A Iopslded gas distribution will bear lopsided

star-forming filaments.

ACKNOWLEDGEMENTS

The authors thank Drs. Rob Roger, Mike Kesteven, Jim Roberts, Ros Haynes and particularly Dick

McGee for helpful discussions. U.K. and R.W. are indebted to the staff of the CSIRO Division of

Radiophysics at Epping and Parkes for their great support and hospitality.

REFERENCES

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