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Register a new userConnecting the Extremes: A Story of Supermassive Black Holes and Ultralight Dark Matter
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The formation of ultra rare supermassive black holes (SMBHs), with masses of $mathcal O(10^9,M_odot)$, in the first billion years of the Universe remains an open question in astrophysics. At the same time, ultralight dark matter (DM) with mass in the vicinity of $mathcal O(10^{-20}~text{eV})$ has been motivated by small scale DM distributions. Though this type of DM is constrained by various astrophysical considerations, certain observations could be pointing to modest evidence for it. We present a model with a confining first order phase transition at $sim 10$ keV temperatures, facilitating production of $mathcal O(10^9,M_odot)$ primordial SMBHs. Such a phase transition can also naturally lead to the implied mass for a motivated ultralight axion DM candidate, suggesting that SMBHs and ultralight DM may be two sides of the same cosmic coin. We consider constraints and avenues to discovery from superradiance and a modification to $N_{rm eff}$. On general grounds, we also expect primordial gravitational waves -- from the assumed first order phase transition -- characterized by frequencies of $mathcal O(10^{-12}-10^{-9}~text{Hz})$. This frequency regime is largely uncharted, but could be accessible to pulsar timing arrays if the primordial gravitational waves are at the higher end of this frequency range, as could be the case in our assumed confining phase transition.
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Measurements of the dynamical environment of supermassive black holes (SMBHs) are becoming abundant and precise. We use such measurements to look for ultralight dark matter (ULDM), which is predicted to form dense cores (solitons) in the centre of galactic halos. We search for the gravitational imprint of an ULDM soliton on stellar orbits near Sgr A* and by combining stellar velocity measurements with Event Horizon Telescope imaging of M87*. Finding no positive evidence, we set limits on the soliton mass for different values of the ULDM particle mass $m$. The constraints we derive exclude the solitons predicted by a naive extrapolation of the soliton-halo relation, found in DM-only numerical simulations, for $2times10^{-20}~{rm eV}lesssim mlesssim8times10^{-19}~{rm eV}$ (from Sgr A*) and $mlesssim4times10^{-22}~{rm eV}$ (from M87*). However, we present theoretical arguments suggesting that an extrapolation of the soliton-halo relation may not be adequate: in some regions of the parameter space, the dynamical effect of the SMBH could cause this extrapolation to over-predict the soliton mass by orders of magnitude.
The energy and momentum deposited by the radiation from accretion onto the supermassive black holes (BHs) that reside at the centres of virtually all galaxies can halt or even reverse gas inflow, providing a natural mechanism for supermassive BHs to regulate their growth and to couple their properties to those of their host galaxies. However, it remains unclear whether this self-regulation occurs on the scale at which the BH is gravitationally dominant, on that of the stellar bulge, the galaxy, or that of the entire dark matter halo. To answer this question, we use self-consistent simulations of the co-evolution of the BH and galaxy populations that reproduce the observed correlations between the masses of the BHs and the properties of their host galaxies. We first confirm unambiguously that the BHs regulate their growth: the amount of energy that the BHs inject into their surroundings remains unchanged when the fraction of the accreted rest mass energy that is injected, is varied by four orders of magnitude. The BHs simply adjust their masses so as to inject the same amount of energy. We then use simulations with artificially reduced star formation rates to demonstrate explicitly that BH mass is not set by the stellar mass. Instead, we find that it is determined by the mass of the dark matter halo with a secondary dependence on the halo concentration, of the form that would be expected if the halo binding energy were the fundamental property that controls the mass of the BH. We predict that the logarithmic slope of the relation between dark matter halo mass and black hole mass is 1.55+/-0.05 and that the scatter around the mean relation in part reflects the scatter in the halo concentration-mass relation.
A perfect irrotational fluid with the equation of state of dust, Irrotational Dark Matter (IDM), is incapable of virializing and instead forms a cosmoskeleton of filaments with supermassive black holes at the joints. This stark difference from the standard cold dark matter (CDM) scenario arises because IDM must exhibit potential flow at all times, preventing shell-crossing from occurring. This scenario is applicable to general non-oscillating scalar-field theories with a small sound speed. Our model of combined IDM and CDM components thereby provides a solution to the problem of forming the observed billion-solar-mass black holes at redshifts of six and higher. In particular, as a result of the reduced vortical flow, the growth of the black holes is expected to be more rapid at later times as compared to the standard scenario.
Warm dark matter has recently become increasingly constrained by observational inferences about the low-mass end of the subhalo mass function, which would be suppressed by dark matter free streaming in the early Universe. In this work, we point out that a constraint can be placed on ultralight bosonic dark matter (often referred to as fuzzy dark matter) based on similar considerations. Recent limits on warm dark matter from strong gravitational lensing of quasars and from fluctuations in stellar streams separately translate to a lower limit of $sim 2.1 times 10^{-21}$ eV on the mass of an ultralight boson comprising all dark matter. These limits are complementary to constraints on ultralight dark matter from the Lyman-$alpha$ forest and are subject to a completely different set of assumptions and systematic uncertainties. Taken together, these probes strongly suggest that dark matter with a mass $sim 10^{-22}$ eV is not a viable way to reconcile differences between cold dark matter simulations and observations of structure on small scales.
We analyze the intriguing possibility to explain both dark mass components in a galaxy: the dark matter (DM) halo and the supermassive dark compact object lying at the center, by a unified approach in terms of a quasi-relaxed system of massive, neutral fermions in general relativity. The solutions to the mass distribution of such a model that fulfill realistic halo boundary conditions inferred from observations, develop a highly-density core supported by the fermion degeneracy pressure able to mimic massive black holes at the center of galaxies. Remarkably, these dense core-diluted halo configurations can explain the dynamics of the closest stars around Milky Ways center (SgrA*) all the way to the halo rotation curve, without spoiling the baryonic bulge-disk components, for a narrow particle mass range $mc^2 sim 10$-$10^2$~keV.
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