Thin discs, thick dwarfs and the effects of stellar feedback

TL;DR

This paper explores how stellar feedback and mass influence the thickness of galaxy discs, revealing a mass threshold below which galaxies become systematically thicker, supported by observations and hydrodynamical simulations.

Contribution

It identifies a critical stellar mass where galaxy shapes transition to thicker forms, linking feedback effects to galaxy morphology in low-mass systems.

Findings

Low-mass galaxies (below 2x10^9 M_sun) are systematically thicker.

The b/a distribution shows a characteristic 'U-shape' indicating shape diversity.

Hydrodynamical simulations support the role of turbulence and feedback in shaping galaxy thickness.

Abstract

We investigate the role of stellar mass in shaping the intrinsic thickness of galaxy discs by determining the probability distribution of apparent axis ratios (b/a) for two different samples that probe the faint end of the galaxy luminosity function. We find that the b/a distribution has a characteristic 'U-shape' and identify a limiting mass M_* ~ 2x10^9 M_sun below which low-mass galaxies start to be systematically thicker. This tendency holds for very faint (M_B ~ -8) dwarfs in the Local Volume, which are essentially spheroidal systems. We argue that galaxy shape is the result of the complex interplay between mass, specific angular momentum and stellar feedback effects. Thus, the increasing importance of turbulent motions in lower mass galaxies leads to the formation of thicker systems, a result supported by the latest hydrodynamical simulations of dwarf galaxy formation and other theoretical expectations. We discuss several implications of this finding, including the formation of bars in faint galaxies, the deprojection of HI line profiles and simulations of environmental effects on the dwarf galaxy population.