Systematically Measuring Ultradiffuse Galaxies (SMUDGes). IV. Ultradiffuse Satellites of Milky Way Analogs

To better understand the formation of large, low-surface-brightness galaxies, we measure the correlation function between ultradiffuse galaxy (UDG) candidates and Milky Way analogs (MWAs). We find that: (1) the projected radial distribution of UDG satellites (projected surface density ∝r ^{−0.84±0.06}) is consistent with that of normal satellite galaxies; (2) the number of UDG satellites per MWA (S _UDG) is ∼0.5 ± 0.1 over projected radii from 20 to 250 kpc and −17 < M _r < −13.5; (3) S _UDG is consistent with a linear extrapolation of the relationship between the number of UDGs per halo versus halo mass obtained over galaxy group and cluster scales; (4) red UDG satellites dominate the population of UDG satellites (∼80%); (5) over the range of satellite magnitudes studied, UDG satellites comprise ∼10% of the satellite galaxy population of MWAs; and (6) a significant fraction of these (∼13%) have estimated total masses >10^10.9 M _⊙ or, equivalently, at least half the halo mass of the LMC, and populate a large fraction (∼18%) of the expected subhalos down to these masses. All of these results suggest a close association between the overall low-mass galaxy population and UDGs, which we interpret as favoring models where UDG formation principally occurs within the general context of low-mass galaxy formation over models invoking more exotic physical processes specifically invoked to form UDGs.