Multifield ultralight dark matter

Ultralight dark matter (ULDM) is usually taken to be a single scalar field. Here we explore the possibility that ULDM consists of N light scalar fields with only gravitational interactions. This configuration is more consistent with the underlying particle physics motivations for these scenarios than a single ultralight field. ULDM halos have a characteristic granular structure that increases stellar velocity dispersion and can be used as observational constraints on ULDM models. In multifield simulations, we find that inside a halo the amplitude of the total density fluctuations decreases as 1= p and that the fields do not become ffiffiffiN significantly correlated over cosmological timescales. Smoother halos heat stellar orbits less efficiently, reducing the velocity dispersion relative to the single field case and thus weakening the observational constraints on the field mass. Analytically, we show that for N equal-mass fields with mass m the ULDM contribution to the stellar velocity dispersion scales as 1=(Nm3). Lighter fields heat the most efficiently and if the smallest mass mL is significantly below the other field masses the dispersion scales as 1=(N2m3L).

Automated summary

Ultralight dark matter (ULDM) is usually assumed to be a single scalar field, but we explore the possibility that it consists of N light scalar fields with only gravitational interactions, which is more consistent with the underlying particle physics motivations for these scenarios. In multifield simulations, we find that the amplitude of the total density fluctuations inside a ULDM halo decreases as 1/sqrt(N) and the fields do not significantly correlate over cosmological timescales. Smoother halos heat stellar orbits less efficiently, weakening the observational constraints on the field mass.