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

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Extensive molecular gas in the tidal arms near NGC 3077 - birth of a dwarf galaxy?

Andreas Heithausen 1 and Fabian Walter 2

1 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany (heith@astro.uni-bonn.de)
2 California Institute of Technology, Astronomy Department 105-24, Pasadena, CA 91125, USA (fw@astro.caltech.edu)

Received 28 April 2000 / Accepted 21 June 2000


Using the IRAM 30 m radio telescope we have mapped the tidal arm feature south-east of NGC 3077 where we recently detected molecular gas in the CO (J=1[FORMULA]0) and (2[FORMULA]1) transitions. We find that the molecular gas is much more extended than previously thought (several kpc). The CO emission can be separated into at least 3 distinct complexes with equivalent radii between 250 pc and 700 pc and all well confined over a narrow range in velocity - the newly detected complexes therefore range among the largest molecular complexes in the local universe. For one complex we have also obtained a CO (3[FORMULA]2) spectrum using the KOSMA 3 m radio telescope; utilizing an LVG model we find that the kinetic temperature for this complex must be about 10 K, and the H2 volume density between 600 and 10000 cm-3. Mass estimates based on virialization yield a total mass for the complexes of order [FORMULA] [FORMULA], i.e. more than the estimated molecular mass within NGC 3077 itself. This implies that interactions between galaxies can efficiently remove heavy elements and molecules from a galaxy and enrich the intergalactic medium. A comparison of the distribution of HI and CO shows no clear correlation. However, CO is only found in regions where the HI column density exceeds [FORMULA] cm-2. HI masses for the molecular complexes mapped are of the same order as the corresponding molecular masses. Because the intergalactic pressure is most likely too low to confine the complexes we conclude that they are gravitationally bound. Since the tidal arm with its molecular complexes has all the ingredients to form stars in the future, we are thus presumably witnessing the birth of a dwarf galaxy. This process could be important for the formation of dwarf galaxies especially at larger look-back times in the universe where galaxy interactions may have been more frequent.

Key words: ISM: molecules – galaxies: dwarf – galaxies: formation – galaxies: individual: NGC 3077 – galaxies: ISM – radio lines: ISM

Send offprint requests to: A. Heithausen

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000