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Register a new userGhostly Galaxies as Solitons of Bose-Einstein Dark Matter
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The large dark cores of common dwarf galaxies are unexplained by the standard heavy particle interpretation of dark matter. This puzzle is exacerbated by the discovery of a very large but barely visible, dark matter dominated galaxy Antlia II orbiting the Milky Way, uncovered by tracking star motions with the {t Gaia} satellite. Although Antlia II has a low mass, its visible radius is more than double any known dwarf galaxy, with an unprecedentedly low density core. We show that Antlia II favors dark matter as a Bose-Einstein condensate, for which the ground state is a stable soliton with a core radius given by the de Broglie wavelength. The lower the galaxy mass, the larger the de Broglie wavelength, so the least massive galaxies should have the widest soliton cores of lowest density. An ultra-light boson of $m_psi sim 1.1 times10^{-22}$ eV, accounts well for the large size and slowly moving stars within Antlia II, and agrees with boson mass estimates derived from the denser cores of more massive dwarf galaxies. For this very light boson, Antlia II is close to the lower limiting Jeans scale for galaxy formation permitted by the Uncertainty Principle, so other examples are expected but none significantly larger in size. This simple explanation for the puzzling dark cores of dwarf galaxies implies dark matter as an ultra-light boson, such as an axion generic in String Theory.
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Galactic dark matter is modelled by a scalar field in order to effectively modify Keplers law without changing standard Newtonian gravity. In particular, a solvable toy model with a self-interaction U(Phi) borrowed from non-topological solitons produces already qualitatively correct rotation curves and scaling relations. Although relativistic effects in the halo are very small, we indicate corrections arising from the general relativistic formulation. Thereby, we can also probe the weak gravitational lensing of our soliton type halo. For cold scalar fields, it corresponds to a gravitationally confined Boson-Einstein condensate, but of galactic dimensions.
The presence of large dark matter cores in dwarf galaxies has long been puzzling and many are now known to be surrounded by an extensive halo of stars. Distinctive core-halo structure is characteristic of dark matter as a Bose Einstein condensate, $psi$DM, with a dense, soliton core predicted in every galaxy, representing the ground state, surrounded by a large, tenuous halo of excited density waves. A marked density transition is predicted between the core and the halo set by the de Broglie wavelength, as the soliton core is a prominent standing wave that is denser by over an order of magnitude than the surrounding halo. Here we identify this predicted behavior in the stellar profiles of the well known isolated dwarfs that lie outside the Milky Way, each with a clear density transition at $simeq 1.0~{rm kpc}$, implying a very light boson, $m_{psi} simeq 10^{-22}$eV. The classical dwarf galaxies orbiting within the Milky Way also show this predicted core-halo structure but with larger density transitions of over two orders of magnitude, that we show implies tidal stripping of dwarf galaxies by the Milky way, as the tenuous halo is more easily stripped than the stable soliton core. We conclude that dark matter as a light boson explains the observed family of classical dwarf profiles with tidal stripping included, in contrast to the standard heavy particle interpretation where low mass galaxies should be concentrated and core-less, quite unlike the core-halo structure observed.
We study the properties of Bose-Einstein Condensate (BEC) systems consisting of two scalars, focusing on both the case where the BEC is stellar scale as well as the case when it is galactic scale. After studying the stability of such systems and making contact with existing single scalar limits, we undertake a numerical study of the two interacting scalars using Einstein-Klein-Gordon (EKG) equations, including both non-gravitational self-interactions and interactions between the species. We show that the presence of extra scalars and possible interactions between them can leave unique imprints on the BEC system mass profile, especially when the system transitions from being dominated by one scalar to being dominated by the other. At stellar scales (nonlinear regime,) we observe that a repulsive interaction between the two scalars of the type $+phi_1^2 phi_2^2$ can stabilize the BEC system and support it up to high compactness, a phenomenon only known to exist in the $+phi^4$ system. We provide simple analytic understanding of this behavior and point out that it can lead to interesting gravitational wave signals at LIGO-Virgo. At galactic scales, on the other hand, we show that two-scalar BECs can address the scaling problem that arises when one uses ultralight dark matter mass profiles to fit observed galactic core mass profiles. In the end, we construct a particle model of two ultralight scalars with the repulsive $phi_1^2 phi_2^2$ interaction using collective symmetry breaking. We develop a fast numerical code that utilizes the relaxation method to solve the EKG system, which can be easily generalized to multiple scalars.
An intriguing alternative to cold dark matter (CDM) is that the dark matter is a light ( $m sim 10^{-22}$ eV) boson having a de Broglie wavelength $lambda sim 1$ kpc, often called fuzzy dark matter (FDM). We describe the arguments from particle physics that motivate FDM, review previous work on its astrophysical signatures, and analyze several unexplored aspects of its behavior. In particular, (i) FDM halos smaller than about $10^7 (m/10^{-22} {rm eV})^{-3/2} M_odot$ do not form. (ii) FDM halos are comprised of a core that is a stationary, minimum-energy configuration called a soliton, surrounded by an envelope that resembles a CDM halo. (iii) The transition between soliton and envelope is determined by a relaxation process analogous to two-body relaxation in gravitating systems, which proceeds as if the halo were composed of particles with mass $sim rholambda^3$ where $rho$ is the halo density. (iv) Relaxation may have substantial effects on the stellar disk and bulge in the inner parts of disk galaxies. (v) Relaxation can produce FDM disks but an FDM disk in the solar neighborhood must have a half-thickness of at least $300 (m/10^{-22} {rm eV})^{-2/3}$ pc. (vi) Solitonic FDM sub-halos evaporate by tunneling through the tidal radius and this limits the minimum sub-halo mass inside 30 kpc of the Milky Way to roughly $10^8 (m/10^{-22} {rm eV})^{-3/2} M_odot$. (vii) If the dark matter in the Fornax dwarf galaxy is composed of CDM, most of the globular clusters observed in that galaxy should have long ago spiraled to its center, and this problem is resolved if the dark matter is FDM.
The dramatic size evolution of early-type galaxies from z ~ 2 to 0 poses a new challenge in the theory of galaxy formation, which may not be explained by the standard picture. It is shown here that the size evolution can be explained if the non-baryonic cold dark matter is composed of compact objects having a mass scale of ~10^5 M_sun. This form of dark matter is consistent with or only weakly constrained by the currently available observations. The kinetic energy of the dark compact objects is transferred to stars by dynamical friction, and stars around the effective radius are pushed out to larger radii, resulting in a pure size evolution. This scenario has several good properties to explain the observations, including the ubiquitous nature of size evolution and faster disappearance of higher density galaxies.
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