1,721,352 research outputs found
Intermediate-mass black holes in dwarf galaxies: The case of Holmberg II
Full text linkIn order to constrain the density of intermediate mass black holes (IMBHs) in galaxies, we run smoothed particle hydrodynamics (SPH) simulations of a gas-rich disc dwarf galaxy, where different halo and disc populations of IMBHs are embedded. IMBHs, when passing through dense gas regions, can accrete gas and switch on as X-ray sources. We derive the luminosity distribution of simulated IMBHs, by assuming that they accrete at the Bondi-Hoyle rate. The X-ray distribution of simulated IMBHs has been compared with that of observed sources in the dwarf galaxy Holmberg II, chosen for its richness in gas, its small mass (compared to spiral galaxies) and the accuracy of the available X-ray measurements. Holmberg II also hosts one of the strongest IMBH candidates. From this comparison, we find that the density parameter of disc (halo) IMBHs must be Ω•≲ 10−5 Ωb (Ω•≲ 10−2 Ωb, where Ωb is the density parameter of baryons), for a radiative efficiency 10−3 and an IMBH mass of 104 M⊙. These constraints imply that a dwarf galaxy like Holmberg II cannot host more than 1 (1000) disc (halo) 104 M⊙ IMBH
Astrophysics of stellar black holes
No full textOn September 14, 2015, the LIGO interferometers captured a gravitational wave (GW) signal from two merging black holes (BHs), opening the era of GW astrophysics. Five BH mergers have been reported so far, three of them involving massive BHs (>30MaS). According to stellar evolution models, such massive BHs can originate from massive relatively metal-poor stars. Alternatively, gravitational instabilities in the early Universe were claimed to produce BHs in this mass range. The formation channels of merging BH binaries are still an open question: A plethora of uncertainties affect the evolution of massive stellar binaries (e.g.The process of common envelope) and their dynamics. This review is intended to discuss the open questions about BH binaries, and to present the state-of-The-Art knowledge about the astrophysics of black holes for non-specialists, in light of the first LIGO detection
Binary Black Hole Mergers: Formation and Populations
Full text linkWe review the main physical processes that lead to the formation of stellar binary black holes (BBHs) and to their merger. BBHs can form from the isolated evolution of massive binary stars. The physics of core-collapse supernovae and the process of common envelope are two of the main sources of uncertainty about this formation channel. Alternatively, two black holes can form a binary by dynamical encounters in a dense star cluster. The dynamical formation channel leaves several imprints on the mass, spin and orbital properties of BBHs
Collisions versus stellar winds in the runaway merger scenario: Place your bets
No full textThe runaway merger scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters (SCs). On the other hand, the massive stars that participate in the runaway merger lose mass by stellar winds. This effect is tremendously important, especially at high metallicity. We discuss Nbody simulations of massive (∼ 6 × 104 M) SCs, in which we added new recipes for stellar winds and supernova explosion at different metallicity. At solar metallicity, the mass of the final merger product spans from few solar masses up to ∼ 30 M. At low metallicity (0:01-0:1 Z) the maximum remnant mass is ∼ 250 M, in the range of IMBHs. A large fraction (∼ 0:6) of the massive remnants are not ejected from the parent SC and acquire stellar or black hole companions. Finally, I discuss the importance of this result for gravitational wave detection
Rotation in young massive star clusters
No full textHydrodynamical simulations of turbulent molecular clouds show that star clusters form from the hierarchical merger of several sub-clumps. We run smoothed-particle hydrodynamics simulations of turbulence-supported molecular clouds with mass ranging from 1700 to 43 000 M. We study the kinematic evolution of the main cluster that forms in each cloud. We find that the parent gas acquires significant rotation, because of large-scale torques during the process of hierarchical assembly. The stellar component of the embedded star cluster inherits the rotation signature from the parent gas. Only star clusters with final mass < few × 100 M do not show any clear indication of rotation. Our simulated star clusters have high ellipticity (~0.4-0.5 at t = 4 Myr) and are subvirial (Qvir 0.4). The signature of rotation is stronger than radial motions due to subvirial collapse. Our results suggest that rotation is common in embedded massive (1000 M) star clusters. This might provide a key observational test for the hierarchical assembly scenario
Back to the Green Valley: How to Rejuvenate an S0 Galaxy through Minor Mergers
Full text linkAbout half of the S0 galaxies in the nearby Universe show signatures of recent or ongoing star formation. Whether these S0 galaxies were rejuvenated by the accretion of fresh gas is still controversial. We study minor mergers of a gas-rich dwarf galaxy with an S0 galaxy, by means of N-body smoothed-particle hydrodynamics simulations. We find that minor mergers trigger episodes of star formation in the S0 galaxy, lasting for 10 Gyr. One of the most important fingerprints of the merger is the formation of a gas ring in the S0 galaxy. The ring is reminiscent of the orbit of the satellite galaxy, and its lifetime depends on the merger properties: polar and counter-rotating satellite galaxies induce the formation of long-lived smooth gas rings
Impact of metallicity on the evolution of young star clusters
No full textWe discuss the results of N-body simulations of intermediate-mass young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z(circle dot)), including metallicity-dependent stellar evolution recipes and metallicity-dependent prescriptions for stellar winds and remnant formation. The initial half-mass relaxation time of the simulated young SCs (similar to 10 Myr) is comparable to the lifetime of massive stars. We show that mass-loss by stellar winds influences the reversal of core collapse and the expansion of the half-mass radius. In particular, the post-collapse re-expansion of the core is weaker for metal-poor SCs than for metal-rich SCs, because the former lose less mass (through stellar winds) than the latter. As a consequence, the half-mass radius expands faster in metal-poor SCs. The difference in the half-light radius between metal-poor SCs and metal-rich SCs is (up to a factor of 2) larger than the difference in the half-mass radius
A crooked spinning black hole
No full text: New observations challenge the current understanding of black hole formation
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