1,721,115 research outputs found
Radiative transfer in a clumpy universe: IV. New synthesis models of the cosmic UV/X-ray background
No full text
Low-frequency gravitational radiation from coalescing massive black hole binaries in hierarchical cosmologies
No full textWe compute the expected low-frequency gravitational wave signal from coalescing massive black hole (MBH) binaries at the center of galaxies in a hierarchical structure formation scenario in which seed holes of intermediate mass form far up in the dark halo "merger tree." The merger history of dark matter halos and associated MBHs is followed via cosmological Monte Carlo realizations of the merger hierarchy from redshift z = 20 to the present in a ΛCDM cosmology. MBHs get incorporated through halo mergers into larger and larger structures, sink to the center because of dynamical friction against the dark matter background, accrete cold material in the merger remnant, and form MBH binary systems. Stellar dynamical (three-body) interactions cause the hardening of the binary at large separations, while gravitational wave emission takes over at small radii and leads to the final coalescence of the pair. A simple scheme is applied in which the "loss cone" is constantly refilled and a constant stellar density core forms because of the ejection of stars by the shrinking binary. The integrated emission from inspiraling MBH binaries at all redshifts is computed in the quadrupole approximation and results in a gravitational wave background (GWB) with a well-defined shape that reflects the different mechanisms driving the late orbital evolution. The characteristic strain spectrum has the standard hc(f) f-2/3 behavior only in the range f = 10-9 to 10-6 Hz. At lower frequencies the orbital decay of MBH binaries is driven by the ejection of background stars ("gravitational slingshot"), and the strain amplitude increases with frequency, hc(f) f. In this range the GWB is dominated by 109-1010 M MBH pairs coalescing at 0 z 2. At higher frequencies, f > 10-6 Hz, the strain amplitude, as steep as hc(f) f-1.3, is shaped by the convolution of last stable circular orbit emission by lighter binaries (102-107 M) populating galaxy halos at all redshifts. We discuss the observability of inspiraling MBH binaries by a low-frequency gravitational wave experiment such as the planned Laser Interferometer Space Antenna (LISA). Over a 3 yr observing period LISA should resolve this GWB into discrete sources, detecting 60 (250) individual events above an S/N = 5 (S/N = 1) confidence level
The formation of galaxy stellar cores by the hierarchical merging of supermassive black holes
No full textInteraction of Massive Black Hole Binaries with Their Stellar Environment. III. Scattering of Bound Stars
No full textRADIATIVE TRANSFER INA CLUMPY UNIVERSE: III. THE NATURE OF COSMOLOGICAL IONIZING SOURCES
No full textThe assembly and merging history of supermassive black holes in hierarchical models of galaxy formation
No full textCompound Gravitational Lensing as a Probe of Dark Matter Substructure within Galaxy Halos
No full textWe show how observations of multiply imaged quasars at high redshift can be used as a probe of dark matter clumps (subhalos with masses ≲109 M⊙) within the virialized extent of more massive lensing halos. A large abundance of such satellites is predicted by numerical simulations of galaxy formation in cold dark matter (CDM) cosmologies. Small-scale structure within galaxy halos affects the flux ratios of the images without appreciably changing their positions. We use numerical simulations to quantify the effect of dark matter substructure on the distribution of magnifications and find that the magnification ratio of a typical image pair will deviate significantly from the value predicted by a smooth lensing potential if, near the Einstein radius, only a few percent of the lens surface density is contained in subhalos. The angular size of the continuum source dictates the range of subclump masses that can have a detectable effect: to avoid confusion with gravitational microlensing caused by stars in the lens galaxy, the background source must be larger than the optical-continuum-emitting region of a QSO. We also find that substructure will cause distortions to images on milliarcsecond scales and bias the distribution of QSO magnification ratios - two other possible methods of detection
- …
