A Lonely Giant: The Sparse Satellite Population of M94 Challenges Galaxy Formation
Abstract
The dwarf satellites of Milky Way (MW)-mass galaxies are important probes of galaxy formation. Satellites are predicted to live in the lowest-mass dark matter halos, which simulations predict are found in abundance around MW-mass galaxies. Recent observational advances have begun to allow us to place our own MW's satellite population in context with other galaxies, and compare to these model predictions. We have conducted a deep Subaru Hyper Suprime Cam (HSC) survey satellite population of the MW-mass galaxy M94. Our survey extends to an effective radius of 150 kpc in g-band. Surprisingly, we find that M94 hosts a satellite population unlike any other known galaxy: it possesses only two low-mass satellites, both <10^6 M_sun, compared to ~10 around the MW. This is quite striking, as the current highest-resolution, limited-run hydrodynamical simulations such as FIRE fail to predict such broad scatter in the satellite populations of MW analogs. Using 'standard' halo occupation, we find that such a sparse satellite population occurs in <0.1% of MW-mass systems in the cosmological EAGLE simulation, which hosts thousands of MW-mass halos. In order to produce an M94-like system more frequently we find that satellite galaxy formation must be much more stochastic than is currently predicted, requiring a dramatic increase to the slope and scatter of the SMHM relation. Surprisingly, the SMHM relation must even be altered above the `too big to fail' mass. The sparse satellite population of M94 thus advocates for a major modification to ideas of how the satellites around MW-mass galaxies form.
- Publication:
-
American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23333903S