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FORMATION OF MASSIVE CLOUDS AND DWARF GALAXIES DURING TIDAL 

ENCOUNTERS 


Michele Kaufman (Ohio St. U.), Bruce G. Elmegreen (IBM), Magnus Thomasson (Nordita), 
and Debra M. Elmegreen (Vassar) 

Gerola et al. (1983) propose that isolated dwarf galaxies can form during galaxy interactions. 
As evidence of this process, Mirabel et al. (1991) find 10 9 M© clouds and star formation 
complexes at the outer ends of the tidal arms in the Antennae and Superantennae galaxies. 
We describe observations of HI clouds with mass > 10 8 M© in the interacting galaxy pair 
IC 2163/NGC 2207. This pair is important because we believe it represents an early stage in 
the formation of giant clouds during an encounter. We use a gravitational instability model 
to explain why the observed clouds are so massive and discuss a two-dimensional N-body 
simulation of an encounter that produces giant clouds. For more details see Elmegreen et al. 
(1992a). 

VLA HI observations (Elmegreen et al. 1992b) reveal 10 giant clouds with HI mass in 
the range 1-5 x 10 8 M© in the outer parts and in the main disks of the galaxy pair 
IC 2163/NGC 2207 (see Fig. 1). One of the giant clouds in NGC 2207 lies between the 
optical spiral arms and shows no evidence of present star formation; this suggests it may 
be young. The measured velocity dispersion in each of these 10 clouds is typically about 
40 km/s, the same as in much of the main disk of NGC 2207, but a factor of 4 times higher 
than for clouds in normal disk galaxies. The high velocity dispersion, which results from 
the agitation of the ISM by the galaxy interactions, appears to be the key to understanding 
why these giant clouds are at least 10 times more massive than the largest clouds in normal 
galaxies. In gravitational instability, the Jeans mass scales as the fourth power of the effec- 
tive velocity dispersion. Giant clouds can form where the value of the Jeans mass is suitable 
and where the local value of the instability parameter Q for the gas is below threshold, e.g., 
in the outer regions and on the tidal arms. The time scale for these giant clouds to form 
by gravitational instability is 50-100 million years, and significant star formation should 
commence in about twice that time. This time scale is consistent with other evidence that 
IC 2163/NGC 2207 is a recent encounter. 

We present results of an N-body gas -f- star simulation of an encounter between galaxies with 
extended gas disks and nearly equal mass. Giant complexes with a mass of about 10 8 M© 
and an internal velocity dispersion of about 25 km/s form in the tidal tail. The largest 
clumps in the simulations have a mass consistent with the Jeans mass and with the values 
observed in IC 2163/NGC 2207. In a simulation with a companion mass equal to 1.4 times 
the mass of the galaxy, giant complexes in the tidal tail escape to form independent dwarf 
galaxies. 

This work was supported in part by NSF Grant AST-8914969 to M.K. 

Elmegreen, B.G., Kaufman, M, Thomasson, M. 1992a, submitted to Ap. J. 

Elmegreen, D.M., Kaufman, M., Brinks, E., Elmegreen, B.G. 1992b, in prep. 

Gerola, H., Carnevali, P., Salpeter, E.E. 1983, Ap. J. 268, L75. 

Mirabel, I.F., Dottori, H., Lutz, D. 1991, Astr. Astrophys. 243, 367. 



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