The intrinsic shapes of dwarf irregular galaxies

We use the measured B-band axial ratios of galaxies from an updated catalogue of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vaucouleurs' morphological types 8-10). We find that the shapes change systematically with luminosity, with fainter galaxies being thicker. In particular, we divide our sample into sub-samples and find that the most luminous dwarfs (-19.6 < M-B < -14.8) have thin discs (thickness similar to 0.2), with the disc being slightly elliptical (axial ratio similar to 0.8). At intermediate luminosity, viz. -14.8 < M-B < -12.6, the galaxies are still characterized by elliptical discs (axial ratio similar to 0.7), but the discs are somewhat thicker (thickness similar to 0.4). The faintest dwarfs, viz. those with -12.6 < M-B < -6.7 are well described as being oblate spheroids with an axial ratio similar to 0.5. The increasing thickness of the stellar discs of dwarf irregulars with decreasing luminosity is compatible with the increasing ratio of velocity dispersion to rotational velocity with decreasing galaxy size.