1,721,102 research outputs found
THE PROBES AND SOURCES OF COSMIC REIONISATION
No full textThe reionisation of the all-pervading intergalactic medium (IGM) is a landmark event in the cosmic history of structure formation. Still, despite much recent progress, a coherent description of the thermal state and ionisation degree of the IGM, the repository of most of the baryons across the history of the Universe, remains elusive. Most of our understanding of IGM physics, and its implication for galaxy formation and metal enrichment, depends critically on the properties of the cosmic ionising background. Over the last 15 years detailed models of cosmic reionisation have been propsed and adopted in cosmological studies. Yet, many uncertainties still exist, and background models are now facing hard challenges, when confronted to new observations both in the local Universe and at high redshifts. In this contribution, I review our current understanding of the reionisation era, and the many problems still open
X-RAY-POLARIZATION IN THE 2-PHASE MODEL FOR AGN AND X-RAY BINARIES
No full textWe calculate the polarization properties of the two-phase model proposed by Haardt & Maraschi to explain the X-ray emission of AGN and X-ray binaries. In this model, hot electrons in an optically thin corona Comptonize the soft photons emitted by the underlying accretion disc to produce the X-rays. We find that the degree of polarization depends strongly on the inclination angle of the system, on the optical depth of the hot corona and on the assumed law for the soft flux. The main result is the strong dependence of the polarization properties on the energy, and in particular the orthogonality between the UV/soft X-ray and the hard X-ray polarizations
X-Ray Weak Active Galactic Nuclei from Super-Eddington Accretion onto Infant Black Holes
No full textA simple model for the X-ray weakness of James Webb Space Telescope–selected broad-line active galactic nuclei (AGNs) is proposed under the assumption that the majority of these sources are fed at super-Eddington accretion rates. In these conditions, the hot inner corona above the geometrically thin disk that is responsible for the emission of X-rays in “normal” AGNs will be embedded instead in a funnel-like reflection geometry. The coronal plasma will Compton upscatter optical/UV photons from the underlying thick disk as well as the surrounding funnel walls, and the high soft-photon energy density will cool down the plasma to temperatures in the range 30–40 keV. The resulting X-ray spectra are predicted to be extremely soft, with power-law photon indices Γ ≃ 2.8–4.0, making high- z super-Eddington AGNs largely undetectable by Chandra
Exploring the connection between AGN radiative feedback and massive black hole spin
Full text linkWe present a novel implementation for active galactic nucleus (AGN) feedback through ultrafast winds in the code GIZMO. Our feedback recipe accounts for the angular dependence of radiative feedback on black hole spin. We self-consistently evolve in time (i) the gas-Accretion process from resolved scales to a smaller scale unresolved (subgrid) AGN disk, (ii) the evolution of the spin of the massive black hole (MBH), (iii) the injection of AGN-driven winds into the resolved scales, and (iv) the spin-induced anisotropy of the overall feedback process. We tested our implementation by following the propagation of the wind-driven outflow into an homogeneous medium, and here we present a comparison of the results against simple analytical models. We also considered an isolated galaxy setup, where the galaxy is thought to be formed from the collapse of a spinning gaseous halo, and there we studied the impact of the AGN feedback on the evolution of the MBH and of the host galaxy. We find that: (i) AGN feedback limits the gas inflow that powers the MBH, with a consequent weak impact on the host galaxy characterized by a suppression of star formation by about a factor of two in the nuclear (≤kpc) region; (ii) the impact of AGN feedback on the host galaxy and on MBH growth is primarily determined by the AGN luminosity rather than by its angular pattern set by the MBH spin (i.e., more luminous AGNs more efficiently suppress central star formation (SF), clearing wider central cavities and driving outflows with larger semiopening angles); (iii) the imprint of the angular pattern of AGN radiation emission is detected more clearly at high (i.e., Eddington) accretion rates. At such high rates, the more isotropic angular patterns, as occur for high spin values, sweep away gas in the nuclear region more easily, therefore causing a slower MBH mass and spin growths and a higher quenching of SF. We argue that the influence of spin-dependent anisotropy of AGN feedback on MBH and galaxy evolution is likely to be relevant in those scenarios characterized by high and prolonged MBH accretion episodes and by high AGN wind-galaxy coupling. Such conditions are more frequently met in galaxy mergers and/or high-redshift galaxies
Fully general relativistic magnetohydrodynamic simulations of accretion flows onto spinning massive black hole binary mergers
No full textWe perform the first suite of fully general relativistic magnetohydrodynamic simulations of spinning massive black hole binary mergers. We consider binary black holes with spins of different magnitudes aligned to the orbital angular momentum, which are immersed in a hot, magnetized gas cloud. We investigate the effect of the spin and degree of magnetization (defined through the fluid parameter β-1pmag/pfluid) on the properties of the accretion flow. We find that magnetized accretion flows are characterized by more turbulent dynamics, as the magnetic field lines are twisted and compressed during the late inspiral. Postmerger, the polar regions around the spin axis of the remnant Kerr black hole are magnetically dominated, and the magnetic field strength is increased by a factor approximately 102 (independently from the initial value of β-1). The magnetized gas in the equatorial plane acquires higher angular momentum and settles in a thin circular structure around the black hole. We find that mass accretion rates of magnetized configurations are generally smaller than in the unmagnetized cases by up to a factor approximately 3. Black hole spins have also a suppressing effect on the accretion rate, as large as approximately 48%. As a potential driver for electromagnetic emission, we follow the evolution of the Poynting luminosity, which increases after merger up to a factor approximately 2 with increasing spin, regardless of the initial level of magnetization of the fluid. Our results stress the importance of taking into account both spins and magnetic fields when studying accretion processes onto merging massive black holes
GRMHD simulations of accretion flows onto unequal-mass, precessing massive binary black hole mergers
No full textIn this work, we use general relativistic magnetohydrodynamics simulations to explore the effect of spin orientation on the dynamics of gas in the vicinity of merging black holes. We present a suite of eight simulations of unequal-mass, spinning black hole binaries embedded in magnetized clouds of matter. Each binary evolution covers approximately 15 orbits before the coalescence. The geometry of the accretion flows in the vicinity of the black holes is significantly altered by the orientation of the individual spins with respect to the orbital angular momentum, with the primary black hole dominating the mass accretion rate M ̇. We observe quasiperiodic modulations of M ̇ in most of the configurations, whose amplitude is dependent on the orientation of the black hole spins. We find the presence of a relation between the average amplitude of M ̇ and the spin precession parameter χp showing that spin misalignment systematically leads to stronger modulation, whereas configurations with spins aligned to the orbital angular momentum damp out the quasiperiodicity. This finding suggests a possible signature imprinted in the accretion luminosity of precessing binaries approaching merger and has possible consequences on future multimessenger observations of massive binary black hole systems
Radiation from the first forming stars
Full text linkThe evolution of radiation emitted during the dynamical collapse of metal-free protostellar clouds is investigated within a spherically symmetric hydrodynamical scheme that includes the transfer of radiation and the chemistry of the primordial gas. The cloud centre collapses on a time-scale of ∼105–6 yr, thanks to line cooling from molecular hydrogen (H2). For most of the collapse time, when the evolution proceeds self-similarly, the luminosity slowly rises up to ∼1036 erg and is essentially a result of H2 infrared (IR) line emission. Later, continuum IR radiation provides an additional contribution, which is mostly a result of the accretion of an infalling envelope upon a small hydrostatic protostellar core that develops in the centre. We follow the beginning of the accretion phase, when the enormous accretion rate (∼0.1 M⊙ yr−1) produces a very high continuum luminosity of ∼1036 erg. Despite the high luminosities, the radiation field is unable to affect the gas dynamics during the collapse and the first phases of accretion, because the opacity of the infalling gas is too small; this is quite different from present-day star formation. We also find that the protostellar evolution is similar among clouds with different initial configurations, including those resulting from three-dimensional cosmological simulations of primordial objects; in particular, the shape of the molecular spectra is quite universal. Finally, we briefly discuss the detectability of this initial cosmic star formation activity
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