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Register a new userSeeding supermassive black holes with a non-vortical dark-matter subcomponent
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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.
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Observations show that supermassive black holes (SMBHs) with a mass of $sim10^9 M_odot$ exist when the Universe is just $6%$ of its current age. We propose a scenario where a self-interacting dark matter halo experiences gravothermal instability and its central region collapses into a seed black hole. The presence of baryons in protogalaxies could significantly accelerate the gravothermal evolution of the halo and shorten collapse timescales. The central halo could dissipate its angular momentum remnant via viscosity induced by the self-interactions. The host halo must be on high tails of density fluctuations, implying that high-$z$ SMBHs are expected to be rare in this scenario. We further derive conditions for triggering general relativistic instability of the collapsed region. Our results indicate that self-interacting dark matter can provide a unified explanation for diverse dark matter distributions in galaxies today and the origin of SMBHs at redshifts $zsim6-7$.
Scalar-tensor theories of gravity generally violate the strong equivalence principle, namely compact objects have a suppressed coupling to the scalar force, causing them to fall slower. A black hole is the extreme example where such a coupling vanishes, i.e. black hole has no scalar hair. Following earlier work, we explore observational scenarios for detecting strong equivalence principle violation, focusing on galileon gravity as an example. For galaxies in-falling towards galaxy clusters, the supermassive black hole can be offset from the galaxy center away from the direction of the cluster. Hence, well resolved images of galaxies around nearby clusters can be used to identify the displaced black hole via the star cluster bound to it. We show that this signal is accessible with imaging surveys, both ongoing ones such as the Dark Energy Survey, and future ground and space based surveys. Already, the observation of the central black hole in M~87 places new constraints on the galileon parameters, which we present here. $mathcal{O}(1)$ matter couplings are disfavored for a large region of the parameter space. We also find a novel phenomenon whereby the black hole can escape the galaxy completely in less than one billion years.
By examining the locations of central black holes in two elliptical galaxies, M,32 and M,87, we derive constraints on the violation of the strong equivalence principle for purely gravitational objects, i.e. black holes, of less than about two-thirds, $eta_N<0.68$ from the gravitational interaction of M,87 with its neighbours in the Virgo cluster. Although M,32 appears to be a good candidate for this technique, the high concentration of stars near its centre substantially weakens the constraints. On the other hand, if a central black hole is found in NGC 205 or one of the other satellite ellipticals of M,31, substantially better constraints could be obtained. In all cases the constraints could improve dramatically with better astrometry.
We discuss formation of dark matter (DM) mini-halos around primordial black holes (PBHs) and its implication on DM direct detection experiments, including axion searches. Motivated by LIGO observations, we consider $f_{textrm{DM}} simeq 0.01$ as the fraction of DM in PBHs with masses $10 M_{odot} - 70 M_{odot}$. In this case, we expect the presence of dressed PBHs after Milky Way halo formation with mini-halo masses peaked around $M_{textrm{halo}} sim (50-55) M_{textrm{PBH}}$. We analyze the effect of tidal forces acting on dressed PBHs within the Milky Way galaxy. In the solar neighborhood, the mini-halos are resistant against tidal disruption from the mean-field potential of the galaxy and encounters with stars, but they undergo a small level of disruption caused by disk shocking. The presence of mini-halos around LIGO-motivated PBHs today could reduce by half the local dark matter background. High-resolution simulations are encouraged. If the proposed scenario is realized, chances of direct detection of DM would decrease.
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has yielded unprecedented measurements of the soft gamma-ray spectrum of our Galaxy. Here we use those measurements to set constraints on dark matter (DM) that decays or annihilates into photons with energies $Eapprox 0.02-2$ MeV. First, we revisit the constraints on particle DM that decays or annihilates to photon pairs. In particular, for decaying DM, we find that previous limits were overstated by roughly an order of magnitude. Our new, conservative analysis finds that the DM lifetime must satisfy $taugtrsim 5times 10^{26},{rm s}times (m_{chi}/rm MeV)^{-1}$ for DM masses $m_{chi}=0.054-3.6$ MeV. For MeV-scale DM that annihilates into photons INTEGRAL sets the strongest constraints to date. Second, we target ultralight primordial black holes (PBHs) through their Hawking radiation. This makes them appear as decaying DM with a photon spectrum peaking at $Eapprox 5.77/(8pi G M_{rm PBH})$, for a PBH of mass $M_{rm PBH}$. We use the INTEGRAL data to demonstrate that, at 95% C.L., PBHs with masses less than $1.2times 10^{17}$ g cannot comprise all of the DM, setting the tightest bound to date on ultralight PBHs.
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