New Bounds for the Mass of Warm Dark Matter Particles Using Results from Fermionic King Model

After reviewing several aspects about the thermodynamics of self-gravitating systems that undergo the evaporation (escape) of their constituents, some recent results obtained in the framework of fermionic King model are applied here to the analysis of galactic halos considering warm dark matter (WDM) particles. According to the present approach, the reported structural parameters of dwarf galaxies are consistent with the existence of a WDM particle with mass in the keV scale. Assuming that the dwarf galaxy Willman 1 belongs to the region III of fermionic King model (whose gravothermal collapse is a continuous phase transition), one obtains the interval 1.2 keV ≤ m ≤ 2.6 keV for the mass of WDM particle. This analysis improves previous estimates by de Vega and co-workers [Astropart. Phys. 46 (2013) 14–22] considering both the quantum degeneration and the incidence of the constituents evaporation. This same analysis evidences that most of galaxies are massive enough to undergo a violent gravothermal collapse (a discontinuous microcanonical phase transition) that leads to the formation of a degenerate core of WDM particles. It is also suggested that quantum-relativistic processes governing the cores of large galaxies (e.g., the formation of supermassive black holes) are somehow related to the gravothermal collapse of the WDM degenerate cores when the total mass of these systems are comparable to the quantum-relativistic characteristic mass Mc=ℏc/G3/2m−2≃1012M⊙ obtained for WDM particles with mass m in the keV scale. The fact that a WDM particle with mass in the keV scale seems to be consistent with the observed properties of dwarf and large galaxies provides a strong support to this dark matter candidate.

The Clustering Dynamics of Primordial Black Boles in N-Body Simulations

We explore the possibility that Dark Matter (DM) may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian N-body code. We compute the cluster rate of evaporation and extract the binary and merged sub-populations along with their parent and merger tree histories, lifetimes and formation rates, the dynamical and orbital parameter profiles, the degree of mass segregation and dynamical friction and power spectrum of close encounters. Overall, we find that PBHs can constitute a viable DM candidate, and that their clustering presents a rich phenomenology throughout the history of the Universe. We show that binary systems constitute about 9.5% of all PBHs at present, with mass ratios of q¯B=0.154, and total masses of m¯T,B=303M⊙. Merged PBHs are rare, about 0.0023% of all PBHs at present, with mass ratios of q¯B=0.965 with total and chirp masses of m¯T,B=1670M⊙ and m¯c,M=642M⊙, respectively. We find that cluster puffing up and evaporation leads to bubbles of these PBHs of order 1 kpc containing at present times about 36% of objects and mass, with one-hundred pc-sized cores. We also find that these PBH sub-haloes are distributed in wider PBH haloes of order hundreds of kpc, containing about 63% of objects and mass, coinciding with the sizes of galactic halos. We find at last high rates of close encounters of massive Black Holes (M∼1000M⊙), with ΓS=(1.2+5.9−0.9)×107yr−1Gpc−3 and mergers with ΓM=1337±41yr−1Gpc−3.

MASS DISTRIBUTION OF DARK MATTER HALO AND SCALE EVOLUTION OF EARLY TYPE GALAXIES

In this paper we use two suites of ultra-high resolution N-body simulations Phoenix and Aquarius Projects to study the assembly history of sub-halos and its dependence on host halo mass. We found that more massive haloes have more progenitors, which is in contrast with former works because they counted dynamical progenitors repeatedly. Less massive halos have larger fraction of dynamical progenitors than more massive ones. The typical accretion time depends strongly on host halo mass. Progenitors of galactic halos are accreted at higher redshift than that of cluster halos. Once these progenitors orbit their primary systems, they rapidly lose their original mass but not their identifiers. Most of the progenitors are able to survive to present day. At given redshift, the survival fraction of accreted sub-halos is independent of host halo mass, while sub-halos in high mass halos lost more mass. In the second part, we use a semi-analytical galaxy formation model compiled on a Millennium Simulation to study the size evolution of massive early-type galaxies from redshift z = 2 to present days. We find that the model we used is able to well reproduce the amplitude and slope of size-mass relation, as well as its evolution. The amplitude of this relation reflects the typical compactness of dark matter halos at the time when most stars are formed. This link between size and star formation epoch is propagated in through galaxy combinations. Minor combinations are increasingly important with increasing present day stellar mass for galaxies more massive than 1011.4M⊙. At lower masses, major combinations are more important. In situ star formation contributes more to the size growth than it does to stellar mass growth. Similar to former works, we find that minor combinations dominate the subsequent growth both in stellar mass and in size for early formed early-type galaxies.

Exploring the Dust Content of Galactic Halos with Herschel*. III. NGC 891

We present deep far-infrared observations of the nearby edge-on galaxy NGC 891 obtained with the Herschel Space Observatory and the Spitzer Space Telescope. The maps confirm the detection of thermal emission from the inner circumgalactic medium (halo) and spatially resolve a dusty superbubble and a dust spur (filament). The dust temperature of the halo component is lower than that of the disk but increases across a region of diameter ≈8.0 kpc extending at least 7.7 kpc vertically from one side of the disk, a region we call a superbubble because of its association with thermal X-ray emission and a minimum in the synchrotron scaleheight. This outflow is breaking through the thick disk and developing into a galactic wind, which is of particular interest because NGC 891 is not considered a starburst galaxy; the star formation rate surface density, 0.03M⊙ yr−1 kpc−2, and gas fraction, just $10\%$ in the inner disk, indicate the threshold for wind formation is lower than previous work has suggested. We conclude that the star formation surface density is sufficient for superbubble blowout into the halo, but the cosmic ray electrons may play a critical role in determining whether this outflow develops into a fountain or escapes from the gravitational potential. The high dust-to-gas ratio in the dust spur suggests the material was pulled out of NGC 891 through the collision of a minihalo with the disk of NGC 891. We conclude that NGC 891 offers an example of both feedback and satellite interactions transporting dust into the halo of a typical galaxy.

Thermodynamic Constraints on the Non-Baryonic Dark Matter Gas Composing Galactic Halos

To explain rotation curves of spiral galaxies through Newtonian orbital models, massive halos of non-baryonic dark matter (NBDM) are commonly invoked. The postulated properties are that NBDM interacts gravitationally with baryonic matter, yet negligibly interacts with photons. Since halos are large, low-density gaseous bodies, their postulated attributes can be tested against classical thermodynamics and the kinetic theory of gas. Macroscopic models are appropriate because these make few assumptions. NBDM–NBDM collisions must be elastic to avoid the generation of light, but this does not permit halo gas temperature to evolve. If no such collisions exist, then the impossible limit of absolute zero would be attainable since the other available energy source, radiation, does not provide energy to NBDM. The alternative possibility, an undefined temperature, is also inconsistent with basic thermodynamic principles. However, a definable temperature could be attained via collisions with baryons in the intergalactic medium since these deliver kinetic energy to NBDM. In this case, light would be produced since some proportion of baryon collisions are inelastic, thereby rendering the halo detectable. Collisions with baryons are unavoidable, even if NBDM particles are essentially point masses. Note that <0.0001 × the size of a proton is needed to avoid scattering with γ-rays, the shortest wavelength used to study halos. If only elastic collisions exist, NBDM gas would collapse to a tiny, dense volume (zero volume for point masses) during a disturbance—e.g., cosmic rays. NBDM gas should occupy central galactic regions, not halos, since self-gravitating objects are density stratified. In summary, properties of NBDM halos as postulated would result in violations of thermodynamic laws and in a universe unlike that observed.

Intrinsic Nature of Dark Matter in the Galactic Halo

We obtain more straightforwardly the main intrinsic features of dark matter distribution in the halos of galaxies by considering the spherically symmetric space-time, which satisfies the flat rotational curve condition, and the geometric equation of state resulting from the modified gravity theory. In order to measure the equation of state for dark matter in the galactic halo, we provide a general formalism taking into account the modified f(X) gravity theories. Here, f(X) is a general function of X∈{R,G,T}, where R,G and T are the Ricci scalar, the Gauss-Bonnet scalar and the torsion scalar, respectively. These theories yield that the flat rotation curves appear as a consequence of the additional geometric structure accommodated by those of modified gravity theories. Constructing a geometric equation of state wX≡pX/ρX and inspiring by some values of the equation of state for the ordinary matter, we infer the properties of dark matter in galactic halos of galaxies.

Gravitational entropy of wormholes with exotic matter and in galactic halos

In this work, we study and compare the features of gravitational entropy near the throat of transversable wormholes formed by exotic matter and wormholes in galactic halos. We have verified that gravitational entropy and entropy density of these wormholes in regions near their throats are indistinguishable for objects of same throat, despite the fact that they are described by different metrics and by distinct energy–momentum tensors. We have found that the gravitational entropy density diverges near the throat for both cases, probably due to a nontrivial topology at this point, however, allowing the interesting interpretation that a maximum flux of information can be carried through the throat of these wormholes. In addition, we have found that both are endowed with an entropic behaviour similar to Hawking–Bekenstein’s entropy of nonrotating and null charge black holes.