The environment of HII galaxies

Abstract

Recent morphological studies (Telles \& Terlevich 1994) of HII galaxies, i.e. dwarf galaxies dominated by a very luminous starburst, have indicated that luminous HII galaxies tend to show distorted morphology suggestive of tidal interactions triggering the present starburst while low luminosity HII galaxies tend to be instead symmetric and regular. To check the tidal origin of the starburst in HII galaxies, we have searched for companions in the neighbourhood of a sample of 51 HII galaxies. We found that only 12 HII galaxies have a neighbour within a projected distance of 1 Mpc and 250 \kmsec~ in velocity difference, and of these 12, only 4 have a luminous (M$_B <$ --19) neighbour. Surprisingly, isolated HII galaxies tend to be of high luminosity and disturbed morphology while HII galaxies with neighbours tend to be low luminosity regular HII galaxies. Furthermore, the metal abundance and the equivalent width of the emission lines in HII galaxies do not depend on the presence of a companion. These results are opposed to simple expectations if interaction with a bright companion is the main mechanism triggering the starbursts. We have also found a loose group of HII galaxies with no luminous companion. For this, there is the additional difficulty of understanding how these starbursts are synchronized on time scales of less than $10^7$ yrs in systems separated by $\sim$ 1-2 Mpc.Comment: 7 pages, uuencoded, compressed, postscript file. (three figures included). Also available via anonymous ftp from ftp://cast0.ast.cam.ac.uk/pub/etelles/environ.ps.