Black hole density
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Black Hole Density: Insights from Recent Research
Local Black Hole Mass Density
Recent studies have significantly revised our understanding of the local mass density in black holes. New scaling relations suggest that the local black hole mass density is five times higher than previously estimated. This increase has profound implications for the properties and demographics of Active Galactic Nuclei (AGN) accretion. One hypothesis to explain this higher density is that black hole growth occurs predominantly through radiatively inefficient channels, such as super-Eddington accretion. Additionally, a larger fraction of heavily obscured, Compton-thick AGNs could also account for the increased density without violating the spectral energy density limits of hard X-ray and mid-infrared backgrounds1.
Primordial Black Holes and Dark Matter Density Spikes
Primordial black holes (PBHs), formed in the early universe, have been a subject of intense study due to their potential role in dark matter. Dark matter particles can accrete around these black holes, forming density spikes with densities much higher than the cosmological average. These spikes are formed immediately after the black holes' creation during the radiation-dominated stage of the universe. The density spikes around PBHs are expected to be bright gamma-ray sources, and their signals can be compared with Fermi-LAT data to constrain the present-day cosmological density parameter for PBHs2 7.
Constraints on Primordial Black Hole Density
X-ray data has been used to set new upper limits on the abundance of PBHs. Interactions between PBHs and the interstellar medium should produce significant X-ray fluxes, contributing to the observed number density of compact X-ray objects in galaxies. These data constrain the PBH number density in the mass range from a few solar masses to 2×10^7 solar masses. The density required to account for black holes detected by LIGO is marginally allowed by these constraints3.
Effects of Supermassive Black Holes on Dark Matter Density
Supermassive black holes (SMBHs) at the centers of galaxies can significantly influence the density profiles of surrounding dark matter. The gravitational pull of an SMBH can "squeeze" the density profile of a fuzzy dark matter (FDM) soliton core, decreasing its core radius and increasing its central density. This effect is more pronounced with larger black hole masses, and the resulting density profile can be likened to that of a hydrogen atom, with the radius inversely proportional to the black hole mass5.
Indirect Evidence for Dark Matter Density Spikes
There is indirect evidence suggesting the presence of dark matter density spikes around stellar-mass black holes. Observations of abnormally fast orbital decays in binary systems, such as A0620-00 and XTE J1118+480, can be explained by the dynamical friction between dark matter and companion stars. The calculated spike indices for these systems align with predictions from the stellar heating model, providing possible indirect evidence for dark matter density spikes around stellar-mass black holes6.
Conclusion
The study of black hole density, both local and primordial, reveals complex interactions between black holes and their surrounding environments. The revised local black hole mass density, the formation of dark matter density spikes around primordial and stellar-mass black holes, and the constraints on PBH density from X-ray data all contribute to a deeper understanding of black hole growth and dark matter distribution in the universe. These findings underscore the importance of multi-wavelength observations and theoretical models in unraveling the mysteries of black hole density and its implications for cosmology.
Sources and full results
Most relevant research papers on this topic
Mass without radiation: Heavily obscured AGNs, the X-ray background, and the black hole mass density
The local mass density in black holes can be significantly higher than previously determined, allowing for more heavily buried, Compton-thick AGNs without violating the limit imposed by the hard X-ray and mid-infrared backgrounds.
In-depth analysis of the clustering of dark matter particles around primordial black holes. Part I. Density profiles
Primordial black holes may have accreted non-relativistic dark matter around them, leading to a dark matter mini-spike with a density orders of magnitude larger than the cosmological one.
New X-ray bound on density of primordial black holes
Primordial black hole density is limited to a few M to 2 107 M, with a marginal allowance for the origin of black holes detected by LIGO.
Highly CitedTHE OBSCURED GROWTH OF MASSIVE BLACK HOLES
Massive black holes form galaxy bulges and central black holes through the emission of a slow wind and a quasar-like spectrum, with the wind exerting the most force on the gas, terminating growth both black hole and galaxy when the Thomson depth drops to about unity.
Highly CitedFuzzy dark matter soliton cores around supermassive black holes
Supermassive black holes'squeeze' fuzzy dark matter soliton cores, decreasing core radius and increasing central density, constraining their particle mass range.
Indirect Evidence for Dark Matter Density Spikes around Stellar-mass Black Holes
Dark matter density spikes may exist around nearby stellar-mass black holes, potentially explaining the abnormal orbital decays in nearby black hole X-ray binaries.
Dark matter density spikes around primordial black holes
Density spikes form from dark matter particles around primordial black holes, constraining the present-day cosmological density parameter for black holes with masses MBH 108M by up to BH 108, depending on MBH.
How dense is a black hole
Supermassive black holes can have a very low density, challenging the assumption that black holes must be extremely dense to form.
Density matrix of radiation of a black hole with a fluctuating horizon
Quantum fluctuations in black hole horizons modify the density matrix of radiation, potentially revealing correlations between radiation and the black hole.
Primordial Black Holes as a Dark Matter Candidate Are Severely Constrained by the Galactic Center 511 keV γ-Ray Line.
Primordial black holes contribute to less than 1% of the dark matter density, as strongly constraint by the Galactic Center 511 keV -ray line.
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