Observational Properties of Field UDGs: Colours and Number Densities

TL;DR

This study investigates the properties and abundance of ultra-diffuse galaxies in the field, revealing they are mostly bluer and possibly form through secular processes, with an estimated upper-limit on their density.

Contribution

It provides the first statistical analysis of field UDGs using deep optical surveys, challenging the idea that they are predominantly quiescent and cluster-like.

Findings

Most field UDGs are bluer than cluster UDGs.

Estimated upper-limit density of field UDGs is 8±3×10^{-3} cMpc^{-3}.

Secular formation channels may significantly contribute to UDG populations.

Abstract

While much of the focus around Ultra-Diffuse Galaxies (UDGs) has been given to those in galaxy groups and clusters, relatively little is known about them in less-dense environments. These isolated UDGs provide fundamental insights into UDG formation because environmentally driven evolution and survivability play less of a role in determining their physical and observable properties. We have recently conducted a statistical analysis of UDGs in the field using a new catalogue of sources detected in the deep Kilo-Degree Survey (KiDS) and Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) optical imaging surveys. Using an empirical model to assess our contamination from interloping sources, we show that a scenario in which cluster-like quiescent UDGs occupy a large fraction of the field UDG population is unlikely, with most being significantly bluer and some showing signs of localised star formation. We estimate an upper-limit on the total field abundance of UDGs of 8$\pm$3$\times10^{-3}$cMpc$^{-3}$ within our selection range. The mass formation efficiency of UDGs implied by this upper-limit is similar to what is measured in groups and clusters, meaning that secular formation channels may significantly contribute to the overall UDG population.