The geometry and origin of ultra-diffuse ghost galaxies

TL;DR

This study reveals that ultra-diffuse galaxies in the Coma cluster are predominantly prolate, elongated structures, challenging the idea they are thickened disks, and provides constraints for their formation models.

Contribution

It determines the intrinsic shape distribution of UDGs, showing they are prolate rather than oblate, which informs theories of their origin.

Findings

UDGs are mainly prolate, elongated structures.

Oblate, disk-like shapes are statistically ruled out.

The axial ratio distribution is flat between 0.4 and 0.9.

Abstract

The geometry and intrinsic ellipticity distribution of ultra diffuse galaxies (UDGs) is determined from the line-of-sight distribution of axial ratios q of a large sample of UDGs, detected by Koda et al. (2015) in the Coma cluster. With high significance the data rules out an oblate, disk-like geometry, characterised by major axi a=b>c. The data is however in good agreement with prolate shapes, corresponding to a=b<c. This indicates that UDGs are not thickened, rotating, axisymmetric disks, puffed up by violent processes. Instead they are anisotropic elongated cigar- or bar-like structures, similar to the prolate dwarf spheroidal galaxy population of the Local Group. The intrinsic distribution of axial ratios of the Coma UDGs is flat in the range of 0.4 <= a/c <= 0.9 which will provide important constraints for theoretical models of their origin. Formation scenarios that could explain the extended prolate nature of UDGs are discussed.