Jiaxuan Li, Jenny E. Greene, Shany Danieli, Scott G. Carlsten, Marla Geha
Abstract The abundance of satellite galaxies provides a direct test of ΛCDM and galaxy formation physics on small scales. While satellites of Milky Way-mass galaxies are well studied, those of dwarf galaxies remain largely unexplored. We present a systematic search for satellites around the isolated dwarf galaxy DDO 161 ( M ⋆ ≈ 10 8.4 M ⊙ ) at a distance of 6 Mpc. We identify eight satellite candidates within the projected virial radius and confirm three new satellites through surface brightness fluctuation distance measurements from deep Magellan imaging data. Together with its confirmed satellite UGCA 319, DDO 161 has four confirmed satellites above M ⋆ sat > 1 0 5.4 M ⊙ , making it the most satellite-rich dwarf galaxy known to date. We compare this system with predictions from the TNG50 cosmological simulation, combined with currently established galaxy–halo connection models calibrated on Milky Way satellites, and find that DDO 161 has a satellite abundance far exceeding all current expectations. The rich satellite system of DDO 161 offers new insight into how low-mass galaxies occupy dark matter halos in low-density environments and may provide new constraints on the nature of dark matter.