The intrinsic shapes of dwarf irregular galaxies

TL;DR

This study investigates the intrinsic three-dimensional shapes of dwarf irregular galaxies using axial ratio data, revealing a systematic change from thin discs in brighter dwarfs to spheroids in the faintest ones, linked to their kinematic properties.

Contribution

It provides the first detailed analysis of how dwarf irregular galaxy shapes vary systematically with luminosity, highlighting the transition from disc-like to spheroidal structures.

Findings

Brighter dwarfs are thin elliptical discs with axial ratio ~0.8.

Intermediate luminosity dwarfs have thicker elliptical discs with axial ratio ~0.7.

Faintest dwarfs are oblate spheroids with axial ratio ~0.5.

Abstract

We use the measured B band axial ratios of galaxies from an updated catalog of Local Volume galaxies to determine the intrinsic shape of dwarf irregular galaxies (de Vacouleurs' morphological types 8, 9 and 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 ~ 0.2), with the disc being slightly elliptical (axial ratio ~ 0.8). At intermediate luminosity, viz. -14.8 < M_B < -12.6, the galaxies are still characterized by elliptical discs (axial ratio ~ 0.7), but the discs are somewhat thicker (thickness ~ 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 ~ 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.