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Anton N. Baushev

Publications and source records attributed to Anton N. Baushev.

16 recordsLinked to original sources
arXiv2024

How empty are the voids?

We find an analytical solution for the minimal matter density of a void, its central density. It turns out that the voids are not so empty: most of the voids have the central underdensity $Δ_c \sim -50\%$ (which means that the matter density in their centers is only two times lower than in the Universe on average). For small voids (of radius $R_0\simeq 5-10$~{Mpc}), the underdensity can be significantly greater, but the number of voids decreases rapidly with increasing of $|Δ_c|$ over $50\%$, and voids with $Δ_c < -80\%$ are practically absent. The large voids ($R_0\ge 40$~{Mpc}) always have $|Δ_c| < 50\%$.

astro-ph.CO↗
arXiv2021

The central region of a void: an analytical solution

We offer an exact analytical equation for the void central region. We show that the central density is solely determined by the amplitude of the initial perturbation. Our results suggest that N-body simulations somewhat overestimate the emptiness of voids: the majority of them should have the central underdensity $δ_c > -73\%$, and there should be almost no voids with $δ_c < -88\%$. The central region of a void is a part of an open Friedmann's 'universe', and its evolution differs drastically from the Universe evolution: there is a long stage when the curvature term dominates, which prevents the formation of galaxy clusters and massive galaxies inside voids. The density profile in the void center should be very flat. We discuss some void models obtained by N-body simulations and offer some ways to improve them. We also show that the dark energy makes the voids less underdense.

astro-ph.CO↗
arXiv2021

The relic neutrino composition as seen from Earth

Being generated, the relic neutrino background contained equal fractions of electron $ν_e$, muon $ν_μ$, and taon $ν_τ$ neutrinos. We show that the gravitational field of our Galaxy and other nearby cosmic objects changes this composition near the Solar System, enriching it with the heaviest neutrino $nu_3$. This mass state is almost free of the electron component (only $\sim 2\%$ of $ν_e$) and contains more muon component than the tau one. As a result, the relic background becomes enriched with taon and particularly muon neutrinos. The electron relic neutrinos are the rarest for a terrestrial observer: instead of $1/3$, the relic background may contain only $\gtrsim 20\%$ of them.

hep-ph↗
arXiv2020

The Hubble stream near a massive object: the exact analytical solution for the spherically-symmetric case

The gravitational field of a massive object (for instance, of a galaxy group or cluster) disturbs the Hubble stream, decreasing its speed. Dependence $v(r_0)$ of the radial velocity of the stream from the present-day radius $r_0$ can be directly observed and may provide valuable information about the cluster properties. We offer an exact analytical relationship $v(r_0)$ for a spherically-symmetric system.

astro-ph.CO↗
arXiv2018

Why does Einasto profile index $n\sim 6$ occur so frequently?

We consider the behavior of spherically symmetric Einasto halos composed of gravitating particles in the Fokker-Planck approximation. This approach allows us to consider the undesirable influence of close encounters in the N-body simulations more adequately than the generally accepted criteria. The Einasto profile with index $n \approx 6$ is a stationary solution of the Fokker-Planck equation in the halo center. There are some reasons to believe that the solution is an attractor. Then the Fokker-Planck diffusion tends to transform a density profile to the equilibrium one with the Einasto index $n \approx 6$. We suggest this effect as a possible reason why the Einasto index $n \approx 6$ occurs so frequently in the interpretation of N-body simulation results. The results obtained cast doubt on generally accepted criteria of N-body simulation convergence.

astro-ph.GA↗
arXiv2017

Galaxy collisions as a mechanism of ultra diffuse galaxy (UDG) formation

We suggest a possible mechanism of ultra diffuse galaxy formation: the UDGs may occur as a result of a central collision of galaxies. If the galaxies are young and contain a lot of gas, the collision may kick all the gas off the systems and thus strongly suppress any further star formation. As a result, the galaxies now have a very low surface brightness and other properties typical of the ultra diffuse galaxies. We use the Coma cluster (where numerous UDGs were recently discovered) to test the efficiency of the process. The mechanism works very well and can transform a significant fraction of the cluster population into ultra diffuse galaxies. The UDGs formed by the process concentrate towards the center of the cluster, and their globular cluster systems remain undamaged, in accordance with observational results. The projected surface density of UDGs in the cluster may help us to recognize the mechanism of UDG formation, or clarify relative contributions of several possible competitive mechanisms at work.

astro-ph.GA↗
arXiv2015

Can the dark matter annihilation signal be significantly boosted by substructures?

A very general cosmological consideration suggests that, along with galactic dark matter halos, much smaller dark matter structures may exist. These structures are usually called 'clumps', and their mass extends to $10^{-6} M_\odot$ or even lower. The clumps should give the main contribution into the signal of dark matter annihilation, provided that they have survived until the present time. Recent observations favor a cored profile for low-mass astrophysical halos. We consider cored clumps and show that they are significantly less firm than the standard NFW ones. In contrast to the standard scenario, the cored clumps should have been completely destroyed inside $\sim 20$~{kpc} from the Milky Way center. The dwarf spheroidals should not contain any dark matter clumps. On the other hand, even under the most pessimistic assumption about the clump structure, the clumps should have survived in the Milky Way at a distance exceeding $50$~{kpc} from the center, as well as in low-density cosmic structures. There they significantly boost the dark matter annihilation. We show that at least $~70\%$ of the clumps endured the primordial structure formation should still exist untouched in the present-day Universe.

astro-ph.CO↗
arXiv2014

The real and apparent convergence of N-body simulations of the dark matter structures: is the Navarro-Frenk-White profile real?

We consider the reasons why a cuspy NFW-like profile persistently occurs in N-body simulations, in contradiction to some astronomical observations. The routine method of testing the convergence of N-body simulations (in particular, the negligibility of two-body scattering effect) is to find the conditions under which the shape of the formed structures is insensitive to numerical parameters. The results obtained with this approach suggest a surprisingly minor role of the particle collisions: the central density profile remains untouched and close to NFW, even if the simulation time significantly exceeds the collisional relaxation time $τ_r$. We analyze the test body distribution in the halo center with help of the Fokker-Planck equation. It turns out that the Fokker-Planck diffusion transforms any reasonable initial distribution into NFW-like profile $ρ\propto r^{-1}$ in a time shorter than $τ_r$. On the contrary, profile $ρ\propto r^{-1}$ should survive much longer, being a sort of attractor: the Fokker-Planck diffusion is self-compensated in this case. Thus the test body scattering may create a stable NFW-like pseudosolution that can be mixed up with the real convergence. This fact might help to eliminate the well-known 'cusp vs. core' problem.

astro-ph.CO↗
arXiv2014

Relaxation of dark matter halos: how to match observational data?

We show that moderate energy relaxation in the formation of dark matter halos invariably leads to profiles that match those observed in the central regions of galaxies. The density profile of the central region is universal and insensitive to either the seed perturbation shape or the details of the relaxation process. The profile has a central core; the multiplication of the central density by the core radius is almost independent of the halo mass, in accordance with observations. In the core area the density distribution behaves as an Einasto profile with low index ($n\sim 0.5$); it has an extensive region with $ρ\propto r^{-2}$ at larger distances. This is exactly the shape that observations suggest for the central region of galaxies. On the other hand, this shape does not fit the galaxy cluster profiles. A possible explanation of this fact is that the relaxation is violent in the case of galaxy clusters; however, it is not violent enough when galaxies or smaller dark matter structures are considered. We discuss the reasons for this.

astro-ph.CO↗
arXiv2014

The galaxy halo formation in the absence of violent relaxation and a universal density profile of the halo center

While N-body simulations testify for a cuspy profile of the central region of the dark matter haloes, observations favor a shallow, cored density profile of the central region of, at least, some spiral galaxies and dwarf spheroidals. We show that a central profile, very close to the observed one, inevitably forms in the center of dark matter haloes if we make a supposition about a moderate energy relaxation of the system during the halo formation. If we assume the energy exchange between dark matter particles during the halo collapse to be not too intensive, the profile is universal: it depends almost not at all on the properties of the initial perturbation and is very akin, but not identical, to the Einasto profile with small Einasto index $n\sim 0.5$. We estimate the size of the 'central core' of the distribution, i.e., the extent of the very central region with a respectively gentle profile, and show that the cusp formation is unlikely, even if the dark matter is cold. The obtained profile is in a good agreement with observational data for, at least, some types of galaxies, but clearly disagrees with N-body simulations.

astro-ph.HE↗
arXiv2012

Extragalactic dark matter and direct detection experiments

Recent astronomical data strongly suggest that a significant part of the dark matter, composing the Local Group and Virgo Supercluster, is not incorporated into the galaxy haloes and forms diffuse components of these galaxy clusters. Apparently, a portion of the particles from these components may penetrate into the Milky Way and make an extragalactic contribution to the total dark matter containment of our Galaxy. We find that the particles of the diffuse component of the Local Group are apt to contribute $\sim 12%$ to the total dark matter density near the Earth. The particles of the extragalactic dark matter stand out because of their high speed ($\sim 600$ {km/s}), i.e. they are much faster than the galactic dark matter. In addition, their speed distribution is very narrow ($\sim 20$ {km/s}). The particles have isotropic velocity distribution (perhaps, in contrast to the galactic dark matter). The extragalactic dark matter should give a significant contribution to the direct detection signal. If the detector is sensitive only to the fast particles ($v<450$ {km/s}), the signal may even dominate. The density of other possible types of the extragalactic dark matter (for instance, of the diffuse component of the Virgo Supercluster) should be relatively small and comparable with the average dark matter density of the Universe. However, these particles can generate anomaly high energy collisions in direct dark matter detectors.

astro-ph.CO↗
arXiv2011

Interaction of clumpy dark matter with interstellar medium in astrophysical systems

Contemporary cosmological conceptions suggest that the dark matter in haloes of galaxies and galaxy clusters has most likely a clumpy structure. If a stream of gas penetrates through it, a small-scale gravitational field created by the clumps disturbs the flow resulting in momentum exchange between the stream and the dark matter. In this article, we perform an analysis of this effect, based on the hierarchial halo model of the dark matter structure and Navarro-Frenk-White density profiles. We consider the clumps of various masses, from the smallest up to the highest ones $M\ge 10^{9} M_\odot$. It has been found that in any event the effect grows with the mass of the clump: not only the drag force $\mathfrak F$ acting on the clump, but also its acceleration $w=\mathfrak F/M$ increases. We discuss various astrophysical systems. The mechanism proved to be ineffective in the case of galaxy or galaxy cluster collisions. On the other hand, it played an important role during the process of galaxy formation. As a result, the dark matter should have formed a more compact, oblate, and faster rotating substructure in the halo of our Galaxy. We have shown that this thick disk should be more clumpy than the halo. This fact is very important for the indirect detection experiments since it is the clumps that give the main contribution to the annihilation signal. Our calculations show that the mechanism of momentum exchange between the dark and baryon matter is ineffective on the outskirts of the galactic halo. It means that the clumps from there were not transported to the thick disk, and this regions should be more clumpy than the halo on the average.

astro-ph.CO↗
arXiv2011

Principal properties of the velocity distribution of dark matter particles on the outskirts of the Solar System

The velocity distribution of the dark matter particles on the outskirts of the Solar System remains unclear. We suggest to determine it using experimentally found properties of the oldest halo objects. Indeed, the oldest halo stars and globular clusters form a collisionless system, as well as dark matter particles do, and they evolved in the same gravitational field. If we accept this analogy, we can show that the velocity distribution of the dark matter particles should be highly anisotropic and have a sharp maximum near $\upsilon\sim 500$ {km/s}. The distribution is totally different from the Maxwell one. We analyze the influence of the distribution function on the results of dark matter detection experiments. It is found that the direct detection signal should differ noticeably from the one calculated from the Maxwell distribution with $<\upsilon>\simeq 220$ {km/s}, which is conventional for direct detection experiments (the ratio depends on the detector properties and typically falls within the range between 6 and 0.2). Moreover, the sharp distinction from the Maxwell distribution can be very essential to the observations of dark matter annihilation.

astro-ph.HE↗
arXiv2009

Dark matter annihilation at cosmological redshifts: possible relic signal from annihilation of weakly interacting massive particles

We discuss the possibility to observe the products of dark matter annihilation that was going on in the early Universe. Of all the particles that could be generated by this process we consider only photons, as they are both uncharged and easily detectable. The earlier the Universe was, the higher the dark matter concentration $n$ and the annihilation rate (proportional to $n^2$) were. However, the emission from the very early Universe cannot reach us because of the opacity. The main part of the signal was generated at the moment the Universe had just become transparent for the photons produced by the annihilation. Thus, the dark matter annihilation in the early Universe should have created a sort of relic emission. We obtain its flux and the spectrum. If weakly interacting massive particles (WIMPs) constitute dark matter, it is shown that we may expect an extragalactic gamma-ray signal in the energy range 0.5 - 20 {MeV} with a maximum near 8 {MeV}. We show that an experimentally observed excess in the gamma-ray background at 0.5 - 20 {MeV} could be created by the relic WIMPs annihilation only if the dark matter structures in the universe had appeared before the universe became transparent for the annihilation products ($z \simeq 300$). We discuss in more detail physical conditions whereby this interpretation could be possible.

astro-ph↗
arXiv2009

Accretion of a massive magnetized torus on a rotating black hole

We present numerical simulations of the axisymmetric accretion of a massive magnetized plasma torus on a rotating black hole. We use a realistic equation of state, which takes into account neutrino cooling and energy loss due to nucleus dissociations. We simulated various magnetic field configurations and torus models, both optically thick and thin for neutrinos. It is shown that the neutrino cooling does not significantly change either the structure of the accretion flow or the total energy release of the system. The calculations evidence heating of the wind surrounding the collapsar by the shock waves generated at the jet-wind border. This mechanism can give rise to a hot corona around the binary system like SS433. Angular momentum of the accreting matter defines the time scale of the accretion. Due to the absence of the magnetic dynamo in our calculations, the initial strength and topology of the magnetic field determines magnetization of the black hole, jet formation properties and the total energy yield. We estimated the total energy transformed to jets as $1.3\times 10^{52}$ {ergs} which was sufficient to explain hypernova explosions like GRB 980425 or GRB 030329.

astro-ph.HE↗