First Star-Forming Structures in Fuzzy Cosmic Filaments

Abstract

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 (kpc) 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 ∼ 10 −22 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 kpc-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.Introduction. The nearly century-old dark matter problem is one of the most intriguing mysteries in modern physics. We do not know the nature of 84 percent of matter in the Universe, yet it is thought to govern cosmic structure and hold galaxies and clusters together [1]. Observations show that on scales larger than a few megaparsecs (Mpc), the behavior of dark matter is consistent with it being collisionless [2,3]. However, on scales at and below the size of dwarf galaxies (few kpc) dark matter is not well constrained [4], allowing for many plausible theories with exotic small-scale physics and particle masses spanning over 30 orders of magnitude [5][6][7][8][9][10]. The first star-forming regions in the Universe -more susceptible to dark matter's small-scale behavior than much heavier present-day galaxies -will be revealed by next generation space telescopes and offer a unique probe of the nature of this elusive component.A leading hypothesis for the dark matter 'back-bone' of the Universe is cold dark matter (CDM), such as a thermally-produced weakly interacting massive particle (WIMP) of mass eV. CDM is collisionless and Jeans unstable to forming structure on all astrophysical scales down to a particle physics model-dependent small-scale

Galaxy formation with BECDM – II. Cosmic filaments and first galaxies

“…Thus, the box size for a cosmological simulation is limited to a few comoving Mpc for a resolution of 1024 3 . This limits applications of spectral methods to systems with low values of maximal velocities, such as low-mass first galaxies (in haloes of mass < 10 11 M ), such as the work presented by Mocz et al (2019) and the one here. Due to the resolution requirements we stop our simulations at z = 5.5.…”

“…As a result, such dark matter is expected to form solitonic cores (rather than cusps) in galaxy centers and render fewer dwarf galaxies than what is predicted for ΛCDM (Hu, Barkana & Gruzinov 2000;Goodman 2000;Guzmán & Matos 2000;Matos & Arturo Ureña-López 2001;Hui et al 2017). Furthermore, filaments, and not necessarily haloes as in ΛCDM, are the sites of primordial star formation in BECDM (Hirano, Sullivan & Bromm 2017;Mocz et al 2019), in a similar way to how the same process occurs in WDM models (Yoshida et al 2003b;Gao & Theuns 2007). Finally, due to the quantum nature of BECDM, the cosmic web exhibits interference patterns Mocz et al 2019).…”

“…This is due to the high particle resolution used (the cores formed in "WDM" are physical and not due to numerical effects because the same cores form in BECDM, which is unaffected by discreteness noise). Mocz et al (2019) showed that unstable cylindrical soliton-like core can be found in the center of dark matter filaments. This cylindrical structure is unstable and can fragment and form spherical solitons.…”

“…Furthermore, filaments, and not necessarily haloes as in ΛCDM, are the sites of primordial star formation in BECDM (Hirano, Sullivan & Bromm 2017;Mocz et al 2019), in a similar way to how the same process occurs in WDM models (Yoshida et al 2003b;Gao & Theuns 2007). Finally, due to the quantum nature of BECDM, the cosmic web exhibits interference patterns Mocz et al 2019). The bosons may also experience self-interactions, which may further affect cosmic structure formation (Chavanis 2018;Desjacques, Kehagias & Riotto 2018, not considered in the current work).…”

“…For instance, the fact that first star forming objects in WDM are filamentary (Yoshida et al 2003b;Gao & Theuns 2007) suggests that also in BECDM first stars will form in dense filaments rather than spherical haloes (Hirano, Sullivan & Bromm 2017). In this paper we explore structure and galaxy formation in BECDM using the first fully self-consistent simulations of BECDM coupled to baryonic physics, which were recently presented by Mocz et al (2019). Such simulations are indispensable for future validation or rejection of BECDM.…”