E. Ripamonti

Ultra-luminous X-ray sources and remnants of massive metal-poor stars

Massive metal-poor stars might form massive stellar black holes (BHs), with mass 25 ≤ m BH /M ⊙ ≤ 80, via direct collapse. We derive the number of massive BHs (N BH ) that are expected to form per galaxy through this mechanism. Such massive BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We select a sample of 64 galaxies with X-ray coverage, measurements of the star formation rate (SFR) and of the metallicity. We find that N BH correlates with the number of observed ULXs per galaxy (N ULX ) in this sample. We discuss the dependence of our model on the SFR and on the metallicity. The SFR is found to be crucial, consistently with previous studies. The metallicity plays a role in our model, since a lower metallicity enhances the formation of massive BHs. Consistently with our model, the data indicate that there might be an anticorrelation between N ULX , normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our results.

Radiation from the first forming stars

The 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 ∼105–6xa0yr, 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 ∼1036xa0erg 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 (∼0.1xa0M⊙xa0yr−1) produces a very high continuum luminosity of ∼1036xa0erg. 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.

Dark matter annihilation effects on the first stars

We study the effects of weakly interacting massive particles (WIMPs) dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-main-sequence (pre-MS) phase of a grid of metal-free stars with masses in the range 5 ≤M∗ ≤ 600 Mforming in the centre of a 10 6 Mhalo atz = 20. DM particles of the parent halo are accreted in the protostellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of O(10 12 GeV cm −3 ) at radii of the order of 10 au. Energy release from annihilation reactions can effectively counteract the gravitational collapse, in agreement with results from other groups. We find this stalling phase (known as a dark star) is transient and lasts from 2.1 × 10 3 yr (M∗ = 600 M� )t o 1.8× 10 4 yr (M∗ = 9M � ). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, ¯

First star formation with dark matter annihilation

We include an energy term based on dark matter (DM) self-annihilation during the cooling and subsequent collapse of the metal-free gas, in haloes hosting the formation of the first stars in the Universe. We find that the feedback induced on the chemistry of the cloud does modify the properties of the gas throughout the collapse. However, the modifications are not dramatic, and the typical Jeans mass within the halo is conserved throughout the collapse, for all the DM parameters we have considered. This result implies that the presence of DM annihilations does not substantially modify the initial mass function of the first stars, with respect to the standard case in which such additional energy term is not taken into account. We find also that when the rate of energy produced by the DM annihilations and absorbed by the gas equals the chemical cooling (at densities yet far from the actual formation of a protostellar core) the structure does not halt its collapse, although that proceeds more slowly by a factor smaller than few per cent of the total collapse time.

Dynamics of stellar black holes in young star clusters with different metallicities - I. Implications for X-ray binaries

We present N body simulations of intermediate-mass (3000 4000 M⊙) young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z⊙), including metal-dependent stellar evolution recipes and binary evolution. Following recent theoretical models of wind mass loss and core collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. In particular, massive metal-poor stars (Z � 0.3 Z⊙) are enabled to form massive stellar black holes (MSBHs, with mass � 25 M⊙) through direct collapse. We find that three-body encounters, and especially dynamical exchanges, dominate the evolution of the MSBHs formed in our simulations. In SCs with Z = 0.01 and 0.1 Z⊙, about 75 per cent of simulated MSBHs form from single stars and become members of binaries through dynamical exchanges in the first 100 Myr of the SC life. This is a factor of > 3 more efficient than in the case of low-mass ( 25 M⊙) stellar black holes. A small but non-negligible fraction of MSBHs power wind-accreting (10 20 per cent) and Roche lobe overflow (RLO, 5 10 per cent) binary systems. The vast majority of MSBH binaries that undergo wind accretion and/or RLO were born from dynamical exchange. This result indicates that MSBHs can power X-ray binaries in low-metallicity young SCs, and is very promising to explain the association of many ultraluminous X-ray sources with low-metallicity and actively star forming environments.

Radiation from early black holes - I. Effects on the neutral intergalactic medium

In the pre-reionization Universe, the regions of the intergalactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to 10(3)-10(4) K, while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (delta T(b) similar to 20-30mK at z less than or similar to 12) which should be observable with forthcoming experiments (e. g. LOFAR). We also find that at z less than or similar to 10 BH emission strongly increases the critical mass separating star-forming and non-star-forming haloes.

The merger fraction of active and inactive galaxies in the local Universe through an improved non-parametric classification

We investigate the possible link between mergers and the enhanced activity of supermassive black holes (SMBHs) at the centre of galaxies, by comparing the merger fraction of a local sample (0.003 ≤ z< 0.03) of active galaxies – 59 active galactic nuclei host galaxies selected from the All-Sky Swift Burst Alert Telescope (BAT) Survey – with an appropriate control sample (247 sources extracted from the HyperLeda catalogue) that has the same redshift distribution as the BAT sample. We detect the interacting systems in the two samples on the basis of non-parametric structural indexes of concentration (C), asymmetry (A), clumpiness (S), Gini coefficient (G) and second-order momentum of light (M20). In particular, we propose a new morphological criterion, based on a combination of all these indexes, that improves the identification of interacting systems. We also present a new software – PyCASS o( PYTHON CAS software) – for the automatic computation of the structural indexes. After correcting for the completeness and reliability of the method, we find that the fraction of interacting galaxies among the active population (20 +7 per cent) exceeds the merger fraction of the control sample (4 +1.7 −1.2 per cent). Choosing a mass-matched control sample leads to equivalent results, although with slightly lower statistical significance. Our findings support the scenario in which mergers trigger the nuclear activity of SMBHs.

Dynamics of massive stellar black holes in young star clusters and the displacement of ultra-luminous X-ray sources

In low-metallicity environments, massive stars might avoid supernova explosions and directly collapse, forming massive (∼25–80xa0M⊙) stellar black holes (MSBHs), at the end of their life. MSBHs, when hosted in young massive clusters, are expected to form binaries and to strongly interact with stars, mainly via three-body encounters. We simulate various realizations of young star clusters hosting MSBHs in hard binaries with massive stars. We show that a large fraction (∼44xa0per cent) of MSBH binaries are ejected on a short time-scale (≤10xa0Myr). The offset of the ejected MSBHs with respect to the parent cluster is consistent with observations of X-ray binaries and ultra-luminous X-ray sources. Furthermore, three-body encounters change the properties of MSBH binaries: the semimajor axis changes by ≤50xa0per cent and the eccentricity of the system generally increases. We shortly discuss the implications of our simulations on the formation of high-mass X-ray binaries hosting MSBHs.

Adaptive mesh refinement simulations of collisional ring galaxies: effects of the interaction geometry

Collisional ring galaxies are the outcome of nearly axisymmetric high-speed encounters between a disc and an intruder galaxy. We investigate the properties of collisional ring galaxies as a function of the impact parameter, the initial relative velocity and the inclination angle. We employ new adaptive mesh refinement simulations to trace the evolution with time of both stars and gas, taking into account star formation and supernova feedback. Axisymmetric encounters produce circular primary rings followed by smaller secondary rings, while off-centre interactions produce asymmetric rings with displaced nuclei. We propose an analytical treatment of the disc warping induced by an inclination angle greater then zero. The star formation history of our models is mainly influenced by the impact parameter: axisymmetric collisions induce impulsive short-lived starburst episodes, whereas off-centre encounters produce long-lived star formation. We compute synthetic colour maps of our models and we find that rings have a B V colour typically � 0.2 mag bluer than the inner and outer disc, in agreement with observations.

Broad [O iii] in the globular cluster RZ 2109: X-ray ionized nova ejecta

We study the possibility that the very broad (∼1500xa0kmxa0s−1) and luminous (L5007∼ 1.4 × 1037xa0ergxa0s−1) [Oxa0iii] line emission observed in the globular cluster RZxa02109 might be explained with the photoionization of nova ejecta by the bright (LX∼ 4 × 1039xa0ergxa0s−1) X-ray source hosted in the same globular cluster. We find that such scenario is plausible and explains most of the features of the RZxa02109 spectrum (line luminosity, absence of H emission lines, peculiar asymmetry of the line profile); on the other hand, it requires the nova ejecta to be relatively massive (≳0.5 × 10−3xa0M⊙), and the nova to be located at a distance ≲0.1xa0pc from the X-ray source. We also predict the time evolution of the RZxa02109 line emission, so that future observations can be used to test this scenario.