THE ENVIRONMENT OF H-II GALAXIES

Recent morphological studies by Telles & Terlevich of H II galaxies, i .e. dwarf galaxies dominated by a very luminous starburst, have indica ted that luminous H II galaxies tend to show disturbed morphology sugg estive of tidal interactions triggering the present starburst, while l ow-luminosity H II galaxies tend, instead, to be symmetric and regular . To check the tidal origin of the starburst in H II galaxies, we have searched for companions in the neighbourhood of a sample of 51 H II g alaxies. We found that only 12 H II galaxies have a neighbour within a projected distance of 1 Mpc and 250 km s(-1) in velocity difference a nd, of these 12, only four have a luminous (M(B) < - 19) neighbour. Su rprisingly, isolated H II galaxies tend to be of high luminosity and d isturbed morphology, while H II galaxies with neighbours tend to be lo w-luminosity regular H II galaxies. Furthermore, the metal abundance a nd the equivalent width of the emission lines in H II galaxies do not depend on the presence of a companion. These results are opposed to wh at is expected if interaction with a bright companion is the main mech anism triggering the starbursts. We have also found a loose group of H II galaxies with no luminous companion. For this, there is the additi onal difficulty of understanding how these starbursts are synchronized on time-scales of less than 10(7) yr in systems separated by similar to 1-2 Mpc.