First Star-Forming Structures in Fuzzy Cosmic Filaments

In hierarchical models of structure formation, the first galaxies form in low-mass dark matter potential wells, probing the behavior of dark matter on kiloparsec scales. Even though these objects are below the detection threshold of current telescopes, future missions will open an observational window into this emergent world. In this Letter, we investigate how the first galaxies are assembled in a “fuzzy” dark matter (FDM) cosmology where dark matter is an ultralight $\sim {10}^{-22}\mathrm{eV}$ boson and the primordial stars are expected to form along dense dark matter filaments. Using a first-of-its-kind cosmological hydrodynamical simulation, we explore the interplay between baryonic physics and unique wavelike features inherent to FDM. In our simulation, the dark matter filaments show coherent interference patterns on the boson de Broglie scale and develop cylindrical solitonlike cores, which are unstable under gravity and collapse into kiloparsec-scale spherical solitons. Features of the dark matter distribution are largely unaffected by the baryonic feedback. On the contrary, the distributions of gas and stars, which do form along the entire filament, exhibit central cores imprinted by dark matter—a smoking gun signature of FDM.

Figure 1

Anatomy of a cosmic filament. We show, for CDM, WDM, and FDM cosmologies, (a) the projected dark matter distribution in the simulation domain at redshift $z=5.5$, (b) projections of dark matter, gas, and stars in a filament, and (c) slices of the dark matter through a filament. In CDM the dark matter fragments into subhalos on all scales. WDM exhibits rich caustic structures. FDM has interference patterns at the scales of the de Broglie wavelength, which regularize caustic singularities. These differences in small-scale structure will help constrain the elusive nature of dark matter.

Figure 2

The structure of FDM filaments: collapse of a cylindrical filament to a spherical soliton. FDM (blue) radial profiles [dark matter (dash); gas (double dash); stars (dash star)] are shown through a cross section of a filament at $z=5.5$ [shown in inset (a)]. (b) The dark matter filament has previously ($z=7$) gone unstable from a cylindrical solution and formed a soliton core [the yellow (red) lines are cylindrical (spherical) profiles of Eq. (3)]. Gas traces the dark matter on all scales, while stars form steeper profiles in the filament “spine,” but are still cored in the center. In contrast, WDM (green) exhibits cuspy profiles. In CDM (not shown), the filament fragments into multiple subhalos, so the cross section profile is ill defined. Characteristic power law dependencies are shown with gray lines.