A Possible "Too-Many-Satellites" Problem in the Isolated Dwarf Galaxy DDO 161

TL;DR

This study discovers an unusually high number of satellite galaxies around the isolated dwarf galaxy DDO 161, challenging existing galaxy formation models and providing new insights into dark matter and low-mass galaxy behavior.

Contribution

It presents the first detailed satellite census of an isolated dwarf galaxy and compares it with cosmological simulations, revealing a significant discrepancy with theoretical expectations.

Findings

DDO 161 has four confirmed satellites, the most for a dwarf galaxy.

The satellite count exceeds predictions from current cosmological models.

This system challenges existing theories of galaxy formation and dark matter.

Abstract

The abundance of satellite galaxies provides a direct test of $\Lambda$CDM 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 ($M_\star \approx 10^{8.4}\, M_\odot$) 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 $M_{\star}^{\rm sat} > 10^{5.4}\, M_\odot$, 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.