Narrow Results

We consider the Kepler two-body problem in presence of the cosmological constant $\mathrm{\Lambda }$. Contrary to the classical case, where finite solutions exist for any angular momentum of the system $L$, in presence of $\mathrm{\Lambda }$ finite solutions exist only in the interval $0<L<L_{\mathrm{\text{lim}}}(\mathrm{\Lambda })$. The qualitative picture of the two-body motion is described, and critical parameters of the problem are found. Application is made to the relative motion of the Local Group and Virgo cluster.

We discuss the existence of an acceleration scale in galaxies and galaxy clusters and its relevance for the nature of dark matter. The presence of the same acceleration scale found at very different length scales, and in very different astrophysical objects, strongly supports the existence of a fundamental acceleration scale governing the observed gravitational physics. We comment on the implications of such a fundamental acceleration scale for constraining cold dark matter models as well as its relevance for structure formation to be explored in future numerical simulations.

In this paper, we find a four-dimensional metric for a large black hole immersed in dark matter. Specifically, we look for and find a static spherically symmetric black hole solution to the Einstein equations which gives, in the Newtonian limit, the rotation curves of galaxies, including the flat region and the baryonic Tully–Fisher relation, and which has a regular horizon. We obtain as well the energy–momentum tensor of the dark matter sourcing this spacetime and it turns, in special, to satisfy the four energy conditions (dominant, weak, null and strong) everywhere outside the horizon. This black-hole-dark-matter system represents a successful simplified model for galaxies, opens a new area for exploring the relativistic regime of dark matter, and shows that the theory of General Relativity together with dark matter can account for the rotation curves of galaxies.

We study the nonlinear structure formation in cosmology accounting for the quantum nature of the dark matter (DM) particles in the initial conditions at decoupling, as well as in the relaxation and stability of the DM halos. Different from cosmological N-body simulations, we use a thermodynamic approach for collisionless systems of self-gravitating fermions in general relativity, in which the halos reach the steady state by maximizing a coarse-grained entropy. We show the ability of this approach to provide answers to crucial open problems in cosmology, among others: the mass and nature of the DM particle, the formation and nature of supermassive black holes in the early Universe, the nature of the intermediate mass black holes in small halos, and the core-cusp problem.

We consider the Kepler two-body problem in the presence of a cosmological constant $\mathrm{\Lambda }$. Several dimensionless parameters characterizing the possible orbit typologies are used to identify open and closed trajectories. The qualitative picture of the two-body motion is described and critical parameters of the problem are found.

The evolution of the baryon distribution in different phases, derived from cosmological simulations, are here reported. These computations indicate that presently most of baryons are in a warm-hot intergalactic (WHIM) medium (about 43%) while at z = 2.5 most of baryons constitute the diffuse medium (about 74%). Stars and the cold gas in galaxies represent only 14% of the baryons at z = 0. For z < 4 about a half of the metals are locked into stars while the fraction present in the WHIM and in the diffuse medium increases with a decreasing redshift. In the redshift range 0 ≤ z ≤ 2.5, the amount of metals in the WHIM increases from 4% to 22% while in the diffuse medium it increases from 0.6% to 4%. This enrichment process is due essentially to a turbulent diffusion mechanism associated to mass motions driven by supernova explosions. At z = 0, simulated blue (late type) galaxies show a correlation of the oxygen abundance present in the cold gas with the luminosity of the considered galaxy that agrees quite well with data derived from HII regions.

We use SPH simulations to investigate the effects of gas on the inspiral and merger of massive black hole binary. This scenario is motivated by the very massive nuclear gas disk observed in the central regions of merging galaxies. We run a variety of models, ranging from simulations with relaty smooth gas to cases in which the gas has a more clumpy spatial distribution. We also vary the Mgas/Mbin ratio identifying the case M_bin/M_gas = 1 as an important limiting test-case for the analytic formulation of the dynamical process that dominates the binary-disk interaction in the late evolution of the system. We find that gas has an important effect in the evolution of interacting massive black hole binary on merging galaxies and that this interaction can be separate in two principal stages. The coalescence timescale that we find for the MBH binary varies between 5 · 10⁶ yr and 2.5 · 10⁷ yr for typical parameters of ULIRGs galaxies. This results supports scenarios of massive black hole evolution and growth in which hierarchical merging plays an important role. The final coalescence of the black hole leads to gravitational radiation emission that would be detected out to high redshift by LISA.

We provide a brief summary of the history of galaxy structure studies. We also introduce several large-scale redshift surveys and summarize the most commonly used methods to identify the groups and clusters of galaxies. We present several catalogs of galaxy groups. These catalogs can be used to study the galaxy groups in different environments. We also consider the properties of superclusters of galaxies in the nearby universe and describe the largest system of galaxies observed — the Sloan Great Wall.

Palatini $f(R)$ gravity is probably the simplest extension of general relativity (GR) and the simplest realization of a metric-affine theory. It has the same number of degrees of freedom as GR and, in vacuum, it is straightforwardly mapped into GR with a cosmological constant. The mapping between GR and Palatini $f(R)$ inside matter is possible but at the expense of reinterpreting the meaning of the matter fields. The physical meaning and consequences of such mapping will depend on the physical context. Here, we consider three such cases within the weak field limit: solar system dynamics, planetary internal dynamics (seismology), and galaxies. After revising our previous results on the solar system and Earth’s seismology, we consider here the possibility of $f(R)$ Palatini as a dark matter candidate. For any $f(R)$ that admits a polynomial approximation in the weak field limit, we show here, using SPARC data and a recent method that we proposed, that the theory cannot be used to replace dark matter in galaxies. We also show that the same result applies to the Eddington-inspired Born–Infeld gravity. Differently from the metric $f(R)$ case, the rotation curve data are sufficient for this conclusion.

The South Pole Telescope (SPT) is a 10-meter telescope designed to survey the millimeter-wave sky, taking advantage of the exceptional observing conditions at the Amundsen-Scott South Pole Station. The telescope and its ground-breaking 960-element bolometric camera finished surveying 2500 square degrees at 95. 150, and 220 GHz in November 2011. We have discovered hundreds of galaxy clusters in the SPT-SZ survey through the Sunyaev-Zel’dovich (SZ) effect. The formation of galaxy clusters the largest bound objects in the universe is highly sensitive to dark energy and the history of structure formation. I will discuss the cosmological constraints from the SPT-SZ galaxy cluster sample as well as future prospects with the soon to-be-installed SPT-3G camera.

The negative hydrogen ion H^- plays an important role in the continuum opacity of late-type stars and in the kinetics of low-metallicity gas. We review the H^- photodetachment cross section along with other processes involving H^-. We address H^- in early Universe chemistry, the role of oscillator-strength sum-rules in constraining its continuum cross section, and the influence of auto-detaching resonances on the efficiency of H^- photodestruction in the reioniza-tion era.

Cosmological and astrophysical surveys from radio to far-infrared, in both temperature and polarization, offer a unique view of the Universe properties and of the formation and evolution of its structures. The last release, close to be finalized, of the Planck mission results sets the scene for cosmological models and parameters, while the comparison with other types of data sets raises the issue of possible tensions about some parameters, first of all the Hubble constant. At the same time, on the extragalactic side, Planck carried out the deepest systematic all-sky survey of SZ galaxy clusters and detected thousands of dusty galaxies and many hundreds of extragalactic radio sources, also allowing us to investigate many specific topics, including molecular hydrogen clouds in galactic halos. The exploitation of future generation of CMB missions and the next radio facilities will allow us to deeply investigate several topics in cosmology and astrophysics, from the existence of primordial gravitational waves to the energy releases in the primeval plasma, from the dawn ages and the epoch of reionization to the formation and evolution of early galaxies and clusters, while a wide set of open astrophysical problems can be studied with future IR missions.

The recent associations of neutrino events with blazars (e.g. TXS 0506+056, 3HSP J095507.9+355101) provided a unique opportunity to study the possible physical connection between the multiwavelength electromagnetic and neutrino emissions. We present SOPRANO, a new conservative implicit kinetic code which follows the time evolution of the isotropic distribution functions of protons, neutrons and the secondaries produced in photo-pion and photo-pair interactions, alongside with the evolution of photon and electron/positron distribution functions. In the current work, we apply SOPRANO to model the broadband spectrum of TXS 0506+056, 3HSP J095507.9+355101 and 3C 279 blazars. It was possible to constrain main physical parameters within both a pure hadronic and lepto-hadronic scenarios.

High-redshift blazars are among the most powerful objects in the Universe. The spectral and temporal properties of 33 distant blazars (z > 2.5) detected in the high-energy γ-ray band are discussed using Fermi-LAT and Swift Ultraviolet and Optical Telescope/X-ray Telescope (UVOT/XRT) data accumulated during 2008-2018. The properties of the jets of these blazar obtained by modeling the multiwavelength spectral energy distributions within a one-zone leptonic scenario are presented and discussed.

We investigate binary lenses with 1/rⁿ potentials in the asymmetric case with two lenses with different indexes n and m. These kinds of potentials have been widely used in several contexts, ranging from galaxies with halos described by different power laws to lensing by wormholes or exotic matter.

The BL Lac object 3C 66A was the target of an intensive multiwavelength monitoring campaign organized in 2003–2004. During the campaign, its spectral energy distribution (SED) was measured and flux measurements from radio to X-ray frequencies as well as upper limits in the very high energy (VHE) γ-ray regime were obtained. Here, we reproduce the SED and optical spectral variability pattern observed during our multiwavelength campaign using a time-dependent leptonic jet model. Our model could successfully simulate the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~ 4 GeV.

Dark matter has been recognized as an essential part of matter for over 70 years now, and many suggestions have been made, what it could be. Most of these ideas have centered on Cold Dark Matter, particles that are expected in extensions of standard particle physics, such as supersymmetry. Here we explore the concept that dark matter is sterile neutrinos, a concept that is commonly referred to as Warm Dark Matter. Such particles have keV masses, and decay over a very long time, much longer than the Hubble time. In their decay they produce X-ray photons which modify the ionization balance in the early universe, increasing the fraction of molecular Hydrogen, and thus help early star formation. Sterile neutrinos may also help to understand the baryon-asymmetry, the pulsar kicks, the early growth of black holes, the minimum mass of dwarf spheroidal galaxies, as well as the shape of dark matter halos. As soon as all these tests have been quantitative in its various parameters, we may focus on the creation mechanism of these particles, and could predict the strength of the sharp X-ray emission line, expected from any large dark matter assembly. A measurement of this X-ray emission line would be definitive proof for the existence of may be called weakly interacting neutrinos, or WINs.

We present results from our recent studies of a class of galaxy groups and clusters known as "Fossils" which are dominated by a giant elliptical galaxy and are arguably the end product of mergers of galaxies within groups. Fossils are the archetypal relaxed systems thus seen as simple laboratories to study formation and evolution of galaxies and haloes in the absence of recent major mergers.

Multi-wavelength study of the largest sample of fossil systems in the observations complemented by cosmological simulations were undertaken to understand their origin and evolution. We study the dark matter distribution in fossils using their X-ray derived mass profiles. Fossils show higher halo concentration, for a given mass of the halo, compared to non-fossil groups and clusters indicating an early formation epoch for fossils. The study of their mass evolution in the Millennium simulations, including hot gas and semi-analytic galaxies, shows that their haloes form earlier than those of non-fossil systems. We estimate the space density of fossils in the simulations which agrees well with the observations.

Dark matter has been recognized as an essential part of matter for over 70 years now, and many suggestions have been made, what it could be. Most of these ideas have centered on Cold Dark Matter, particles that are predicted in extensions of standard particle physics, such as supersymmetry. Here we explore the concept that dark matter is sterile neutrinos, particles that are commonly referred to as Warm Dark Matter. Such particles have keV masses, and decay over a very long time, much longer than the Hubble time. In their decay they produce X-ray photons which modify the ionization balance in the very early universe, increasing the fraction of molecular Hydrogen, and thus help early star formation. Sterile neutrinos may also help to understand the baryon-asymmetry, the pulsar kicks, the early growth of black holes, the minimum mass of dwarf spheroidal galaxies, as well as the shape and smoothness of dark matter halos. As soon as all these tests have been made quantitative in their various parameters, we may focus on the creation mechanism of these particles, and could predict the strength of the sharp X-ray emission line, expected from any large dark matter assembly. A measurement of this X-ray emission line would be definitive proof for the existence of may be called weakly interacting neutrinos, or WINs.