Low-metallicity (Z≲ 0.05 Z⊙) massive (≳40 M⊙) stars might end their life by directly collapsing into massive black holes (BHs, 30 ≲mBH/M⊙≲ 80). More than ∼105 massive BHs might have been generated ...via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ∼107 yr. We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel. We also consider a sample of ULX-rich galaxies and we find a possible anticorrelation between the number of ULXs per galaxy and the metallicity in these galaxies. However, the data are not sufficient to draw any robust conclusions about this anticorrelation, and further studies are required.
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.
Massive black hole binaries (BHBs) are expected to be one of the most powerful sources of gravitational waves in the frequency range of the pulsar timing array and of forthcoming space-borne ...detectors. They are believed to form in the final stages of galaxy mergers, and then harden by slingshot ejections of passing stars. However, evolution via the slingshot mechanism may be ineffective if the reservoir of interacting stars is not readily replenished, and the binary shrinking may come to a halt at roughly a parsec separation. Recent simulations suggest that the departure from spherical symmetry, naturally produced in merger remnants, leads to efficient loss cone refilling, preventing the binary from stalling. However, current N-body simulations able to accurately follow the evolution of BHBs are limited to very modest particle numbers. Brownian motion may artificially enhance the loss cone refilling rate in low-N simulations, where the binary encounters a larger population of stars due its random motion. Here we study the significance of Brownian motion of BHBs in merger remnants in the context of the final parsec problem. We simulate mergers with various particle numbers (from 8k to 1M) and with several density profiles. Moreover, we compare simulations where the BHB is fixed at the centre of the merger remnant with simulations where the BHB is free to random walk. We find that Brownian motion does not significantly affect the evolution of BHBs in simulations with particle numbers in excess of one million, and that the hardening measured in merger simulations is due to collisionless loss cone refilling.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source ...powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.
We investigate the effects of minor mergers between an S0 galaxy and a gas-rich satellite galaxy, by means of N-body/smoothed particle hydrodynamics simulations. The satellite galaxy is initially on ...a nearly parabolic orbit and undergoes several periapsis passages before being completely stripped. In most simulations, a portion of the stripped gas forms a warm dense gas ring in the S0 galaxy, with a radius of ~6–13 kpc and a mass of ~107M⊙. The ring is generally short-lived (≲3 Gyr) if it forms from prograde encounters, while it can live for more than 6 Gyr if it is born from counter-rotating or non-coplanar interactions. The gas ring keeps memory of the initial orbit of the satellite galaxy: it is corotating (counter-rotating) with the stars of the disc of the S0 galaxy, if it originates from prograde (retrograde) satellite orbits. Furthermore, the ring is coplanar with the disc of the S0 galaxy only if the satellite’s orbit was coplanar, while it lies on a plane that is inclined with respect to the disc of the S0 galaxy by the same inclination angle as the orbital plane of the satellite galaxy. The fact that we form polar rings as long-lived and as massive as co-planar rings suggests that rings can form in S0 galaxies even without strong bar resonances. Star formation up to 0.01 M⊙ yr-1 occurs for >6 Gyr in the central parts of the S0 galaxy as a consequence of the interaction. We discuss the implications of our simulations for the rejuvenation of S0 galaxies in the local Universe.
Massive metal-poor stars might form massive stellar black holes (BHs), with mass 25 ≤mBH/M⊙≤ 80, via direct collapse. We derive the number of massive BHs (NBH) 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 NBH correlates with the number of observed ULXs per galaxy (NULX) 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 NULX, normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our results.
We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source (ULX) M51 ULX-7 within the UNSEeN project, which was designed to hunt for new pulsating ULXs (PULXs) with ...XMM-Newton. The pulse shape is sinusoidal, and large variations of its amplitude were observed even within single exposures (pulsed fraction from less than 5% to 20%). Source M51 ULX-7 is variable, generally observed at an X-ray luminosity between 1039 and 1040 erg s−1, located in the outskirts of the spiral galaxy M51a at a distance of 8.6 Mpc. According to our analysis, the X-ray pulsar orbits in a 2 day binary with a projected semimajor axis 28 lt-s. For a neutron star (NS) of 1.4 M , this implies a lower limit on the companion mass of 8 M , placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by 0.4 ms in the 31 days spanned by our 2018 May-June observations, corresponding to a spin-up rate . In an archival 2005 XMM-Newton exposure, we measured a spin period of ∼3.3 s, indicating a secular spin-up of , a value in the range of other known PULXs. Our findings suggest that the system consists of a massive donor, possibly an OB giant or supergiant, and a moderately magnetic (dipole field component in the range 1012 G G) accreting NS with weakly beamed emission ( ).
Abstract
Through an ongoing MUSE program dedicated to study gas removal processes in galaxies (GAs Stripping Phenomena in galaxies with MUSE, GASP), we have obtained deep and wide integral field ...spectroscopy of the galaxy JO171. This galaxy resembles the Hoag's galaxy, one of the most spectacular examples of ring galaxies, characterized by a completely detached ring of young stars surrounding a central old spheroid. At odds with the isolated Hoag's galaxy, JO171 is part of a dense environment, the cluster Abell 3667, which is causing gas stripping along tentacles. Moreover, its ring counter-rotates with respect to the central spheroid. The joint analysis of the stellar populations and the gas/stellar kinematics shows that the origin of the ring was not due to an internal mechanism, but was related to a gas accretion event that happened in the distant past, prior to accretion on to Abell 3667, most probably within a filament. More recently, since infall in the cluster, the gas in the ring has been stripped by ram pressure, causing the quenching of star formation in the stripped half of the ring. This is the first observed case of ram-pressure stripping in action in a ring galaxy, and MUSE observations are able to reveal both of the events (accretion and stripping) that caused dramatic transformations in this galaxy.
One of the possible methods to distinguish among various dark matter (DM) candidates is to study the effects of DM decays. We consider four different DM candidates light dark matter (LDM), ...gravitinos, neutralinos and sterile neutrinos, for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and cosmic microwave background (CMB) spectra. We find that LDM particles (1–10 MeV) and sterile neutrinos (2–8 keV) can be sources of partial early reionization (z≲ 100). However, their integrated contribution to Thomson optical depth is small (≲ 0.01) with respect to the 3-yr WMAP results (τe= 0.09 ± 0.03). Finally, they can significantly affect the behaviour of matter temperature. On the contrary, effects of heavy DM candidates (gravitinos and neutralinos) on reionization and heating are minimal. All the considered DM particles have completely negligible effects on the CMB spectra.
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