The abundance of ultra-diffuse galaxies from groups to clusters: UDGs
  are relatively more common in more massive haloes
  In recent years, multiple studies have reported substantial populations of
large, low-surface-brightness galaxies in local galaxy clusters. Various
theories that aim to explain the presence of such ultra-diffuse galaxies (UDGs)
have since been proposed. A key question that will help to differentiate
between models is whether UDGs have counterparts in lower-mass host haloes, and
what their abundance as a function of halo mass is. In this study we extend our
previous study of UDGs in galaxy clusters to galaxy groups. We measure the
abundance of UDGs in 325 spectroscopically-selected groups from the Galaxy And
Mass Assembly (GAMA) survey. We make use of the overlapping imaging from the
ESO Kilo-Degree Survey (KiDS), from which we can identify galaxies with mean
surface brightnesses within their effective radii down to ~25.5 mag
arcsec$^{-2}$ in the r-band. We are able to measure a significant overdensity
of UDGs (with sizes r_eff > 1.5 kpc) in galaxy groups down to M200=10^12 Msun,
a regime where approximately only 1 in 10 groups contains a UDG that we can
detect. We combine measurements of the abundance of UDGs in haloes that cover
three orders of magnitude in halo mass, finding that their numbers scale quite
steeply with halo mass; N_UDG (R<R200) $\propto$ M200^(1.11+/-0.07). To better
interpret this, we also measure the mass-richness relation for brighter
galaxies down to $M^*_r$+2.5 in the same GAMA groups, and find a much shallower
relation of N_Bright (R<R200) $\propto$ M200^(0.78+/-0.05). This shows that
UDGs are relatively more abundant, compared to bright galaxies, in massive
clusters than in groups. We discuss implications, but whether this difference
is related to a higher destruction rate of UDGs in groups, or whether massive
haloes have a positive effect on their formation, is not yet clear.
