Testing ΛCDM with Dwarf Galaxy Morphology

Abstract

The leading tensions to the collisionless cold dark matter (CDM) paradigm are the "small-scale controversies," discrepancies between observations at the dwarf-galactic scale, and their simulational counterparts. In this work we consider methods to infer 3D morphological information on Local Group dwarf spheroidals and test the fitness of CDM+hydrodynamics simulations to the observed galaxy shapes. We find that the subpopulation of dwarf galaxies with mass-to-light ratio $\gtrsim 100{M}_{\odot }/{L}_{\odot }$ reflects an oblate morphology. This is discrepant with the dwarf galaxies with mass-to-light ratios $\lesssim 100{M}_{\odot }/{L}_{\odot }$ , which reflect prolate morphologies, as well as simulations of CDM-sourced bright isolated galaxies that are explicitly prolate. Although more simulations and data are called for if evidence of oblate pressure-supported stellar distributions persists in observed galaxies while being absent from simulations, we argue that an underlying oblate non-CDM dark matter halo may be required and present this as motivation for future studies.