Flat Metallicity Profiles in Rotating Dwarf Galaxies

Abstract

dIrrs and flat, rotating dEs generally possess flat metallicity profiles while round dEs show strong metallicity gradients [8]. Unlike dEs, dIrrs also exhibit ongoing star formation (SF) [4], compatible with a continuous star formation history (SFH). We show results based on a large suite of Nbody-SPH simulations of isolated flat dwarf galaxies (DG), both rotating and non-rotating. These simulations show that using rotation to flatten a dwarf galaxy is particularly efficient in turning a so-called “breathing” SFH [15] into a more continuous SFH, and in producing flat metallicity profiles. Non-rotating dEs in a flattened dark-matter halo are not able to reproduce this. Thus it appears that angular momentum is a key factor in DG behaviour. Rotation causes a ‘centrifugal barrier’ which slows down the infall of gas, so the low-level SF (and feedback) is not centrally concentrated but occurs galaxy-wide, and prevents large-scale oscillations in the SFR. This mechanism of smearing out SF in time and space proves to be the principal reason for flat metallicity profiles, instead of the often referred to ‘fountain mechanism’ [1, 3, 5, 10]. We therefore propose a ‘centrifugal barrier mechanism’ which is able to explain the observations.