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Astron. Astrophys. 361, 500-506 (2000)

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5. Origin and future of the molecular complex

Evidently, the galaxies NGC 3077, M 81, and M 82 have undergone some violent interaction in the past. Visible signs of this interaction are the long tidal arms seen in the 21 cm line of atomic hydrogen (van der Hulst 1979; Yun et al. 1994) connecting the galaxies. According to numerical simulations the closest encounter between NGC 3077 and M 81, which presumably redistributed much of the interstellar gas in the outskirts of NGC 3077, happened some [FORMULA] years ago (Yun et al. 1993).

Now, as already discussed in Walter & Heithausen (1999) it should be stressed that it is very surprising that a huge amount of molecular gas is present at all in the tidal arm. Not only are molecular gas and hence metals present far off a galaxy but the total mass of the molecular gas in the tidal arm ([FORMULA] [FORMULA]) is possibly higher than the entire molecular mass within NGC 3077 itself ([FORMULA] [FORMULA], Becker et al. 1989). It is not clear if the same ratio also holds for the total metal content in the tidal arm and in NGC 3077. In any case our finding implies that huge amounts of metal enriched material can be removed from a galaxy due to tidal interactions. This has not only important consequences for the chemical history of a single galaxy which might have undergone some interaction - interactions also seem to be able to enrich the intergalactic medium.

Regarding the molecular complex, an important question is which process condenses the molecular gas. Are the complexes pressure confined or gravitationally bound? While it is hard to imagine that a structure of kpc size is pressure confined, the intergalactic pressure is most likely too low. If we use the results from our excitation study (Sect. 4.1) the internal pressure in the molecular gas is [FORMULA] cm-3 K. From X-ray observations with ROSAT Bi et al. (1994) found an intergalactic volume density of less than [FORMULA] cm-3 in the region of the tidal arm around NGC 3077. The intergalactic gas thus must have a temperature of more than [FORMULA] K to confine the molecular gas, which is unlikely. With an adopted temperature of [FORMULA] K, similar to that of the galactic corona (Wolfire et al. 1995), the intergalactic pressure is less than [FORMULA] K cm-3, too low to confine the complexes.

We thus conclude that the molecular clouds are indeed gravitationally stable objects. The low observed main beam brightness temperature for all clouds indicates a low beam filling. If complex #1 is as cold as derived from our excitation study (Sect. 4.1) the observed mainbeam brightness temperature of only about 70 mK translates into a beam filling of [FORMULA]. This implies that the large complex will probably break up into several smaller clouds when observed at higher angular resolution. Because the complex is extended the arguments hold for all of the observed positions. This implies that the single clouds are spread over the area with an equivalent radius of 700 pc, and are not confined to one single massive cloud. High angular resolution interferometric CO studies will allow to investigate this situation further.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000
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