Lopsidedness in dwarf irregular galaxies

We quantify the amplitude of the lopsidedness, the azimuthal angular asymme try index and the concentration of star-forming regions, as represented by the distribution of the H alpha emission, in a sample of 78 late-type irreg ular galaxies. We bin the observed galaxies into two groups representing bl ue compact galaxies (BCDs) and low-surface-brightness dwarf galaxies (LSBs) . The light distribution is analysed with a novel algorithm, which allows d etection of details in the light distribution pattern. We find that while t he asymmetry of the underlying continuum light, representing the older stel lar generations, is relatively small, the H alpha emission is very asymmetr ic and is correlated in position angle with the continuum light. We show th at the concentration of continuum light is correlated with the H alpha conc entration; this implies that the young star formation has the same spatial properties as the older stellar populations, but that these properties are more strongly expressed by the young stars. We test a model of random star formation over the extent of a galaxy by simulating H II regions in artific ial dwarf galaxies. A galaxy is traced by assuming red star clusters distri buted on an underlying exponential disc of radius twice the scalelength. Th e disc is allowed to change in apparent magnitude, scaleradius, position an gle and ellipticity. We compare the asymmetry- concentration distribution p redicted by the simulations with the real observed distribution; we find th at only LSBs match the distribution predicted by the model. The reason is t hat, independently of the number of H II regions, LSBs show no particular l ocation of H II regions, whereas BCDs show current star formation activity restricted very much to the central parts of the galaxies. A consideration of the properties of the continuum light leads to the conclusion that most of LSBs can be approximated by exponential discs of radius twice their scal elength; BCDs call, however, for much more concentrated underlying systems, with smaller scalelengths than assumed in the simulations. The implication is that random star formation over the full extent of a galaxy may be gene rated in LSB dwarf irregular galaxies but not in BCD galaxies.