Motohiro Enoki

Gravitational Waves from Supermassive Black Hole Coalescence in a Hierarchical Galaxy Formation Model

We investigate the expected gravitational wave emission from coalescing supermassive black hole (SMBH) binaries resulting from mergers of their host galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the center of the new merged galaxy and form a binary system. We employ a semianalytic model of galaxy and quasar formation based on the hierarchical clustering scenario to estimate the amplitude of the expected stochastic gravitational wave background due to inspiraling SMBH binaries and bursts due to the SMBH binary coalescence events. We find that the characteristic strain amplitude of the background radiation is hc(f) ~ 10-16(f/1 ?Hz)-2/3 for f 1 ?Hz just below the detection limit from measurements of the pulsar timing provided that SMBHs coalesce simultaneously when host galaxies merge. The main contribution to the total strain amplitude of the background radiation comes from SMBH coalescence events at 0 < z < 1. We also find that a future space-based gravitational wave interferometer such as the planned Laser Interferometer Space Antenna might detect intense gravitational wave bursts associated with coalescence of SMBH binaries with total mass Mtot < 107 M? at z 2 at a rate ~1.0 yr-1. Our model predicts that burst signals with a larger amplitude hburst ~ 10-15 correspond to coalescence events of massive SMBH binary with total mass Mtot ~ 108 M? at low redshift (z 1) at a rate ~0.1 yr-1, whereas those with a smaller amplitude (hburst ~ 10-17) correspond to coalescence events of less massive SMBH binaries with total mass Mtot ~ 106 M? at high redshift (z 3).

Probing the Faint End of the Quasar Luminosity Function at z~4 in the COSMOS Field

We searched for quasars that are ~3 mag fainter than the SDSS quasars in the redshift range 3.7 ≾ z ≾ 4.7 in the COSMOS field to constrain the faint end of the quasar luminosity function (QLF). Using optical photometric data, we selected 31 quasar candidates with 22 < i < 24 at z ~ 4. We obtained optical spectra for most of these candidates using FOCAS on the Subaru telescope and identified eight low-luminosity quasars at z ~ 4. In order to derive the QLF based on our spectroscopic follow-up campaign, we estimated the photometric completeness of our quasar survey through detailed Monte Carlo simulations. Our QLF at z ~ 4 has a much shallower faint-end slope (β = −1.67^(+0.11)_(−0.17)) than that obtained by other recent surveys in the same redshift. Our result is consistent with the scenario of downsizing evolution of active galactic nuclei inferred by recent optical and X-ray quasar surveys at lower redshifts.

The Effect of Orbital Eccentricity on Gravitational Wave Background Radiation from Supermassive Black Hole Binaries

A compact binary in an eccentric orbit radiates gravitational waves (GWs) at all integer harmonics of its orbital frequency. In this study, we investigate the effect of orbital eccentricity on the expected gravitational background radiation (GWBR) from supermassive black hole (SMBH) binaries in the nuclei of galaxies. For this purpose, we formulate a power spectrum of the GWBR from cosmological evolving eccentric binaries. Then, we apply this formulation to the case of the GWBR from SMBH binaries. The key to doing this is to correctly estimate the number density of coalescing SMBH binaries. In this study, we use a semi-analytic model of galaxy and SMBH formation. We find that the power spectrum of the GWBR from SMBH binaries on eccentric orbits is suppressed for frequencies

The ν2GC simulations: Quantifying the dark side of the universe in the Planck cosmology

f \lesssim 1~{\rm nHz}

ANTI-HIERARCHICAL EVOLUTION OF THE ACTIVE GALACTIC NUCLEUS SPACE DENSITY IN A HIERARCHICAL UNIVERSE

if the initial eccentricity,

Relations between Galaxy Formation and the Environments of Quasars

e_0

Is Galactic Star-Formation Activity Increased during Cluster Mergers?

, satisfies

THE QUASAR-LBG TWO-POINT ANGULAR CROSS-CORRELATION FUNCTION AT z ∼4 IN THE COSMOS FIELD

e_0 > 0.2

The impact of dust in host galaxies on quasar luminosity functions

and the initial semi-major axis is 300 times Scwarzschild radius. Our model predicts that while the overall shape and amplitude of the power spectrum depend strongly on the processes of galaxy formation, the eccentricity of binaries can affect the shape of the power spectrum for lower frequencies, i.e.,

The quasar luminosity function at redshift 4 with the Hyper Suprime-Cam Wide Survey

f \lesssim 1~{\rm nHz}