In recent years , many studies have reported substantial populations of large galaxies with low surface brightness 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 distinguish between models is whether UDGs have counterparts in host haloes with lower masses , and if so , what their abundance as a function of halo mass is .
We here 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 \sim 25.5 mag arcsec ^ { -2 } in the r band .
We are able to measure a significant overdensity of UDGs ( with sizes r _ { \mathrm { eff } } \geq 1.5 kpc ) in galaxy groups down to M _ { 200 } = 10 ^ { 12 } \mathrm { M _ { \odot } } , a regime where approximately only one in ten 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 : \mathrm { N _ { UDG } } ( \mathrm { R } < R _ { 200 } ) \propto M _ { 200 } ^ { 1.11 \pm 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 \mathrm { N _ { Bright } } ( \mathrm { R } < R _ { 200 } ) \propto M _ { 200 } ^ { 0.78 \pm 0.05 } .
This shows that compared to bright galaxies , UDGs are relatively more abundant in massive clusters than in groups .
We discuss the implications , but it is still unclear whether this difference is related to a higher destruction rate of UDGs in groups or if massive haloes have a positive effect on UDG formation .