336 research outputs found
Hierarchical build-up of galactic bulges and the merging rate of supermassive binary black holes
The hierarchical build-up of galactic bulges should lead to the build-up of
present-day supermassive black holes by a mixture of gas accretion and merging
of supermassive black holes. The tight relation between black hole mass and
stellar velocity dispersion is thereby a strong argument that the supermassive
black holes in merging galactic bulges do indeed merge. Otherwise the ejection
of supermassive black holes by gravitational slingshot would lead to excessive
scatter in this relation. At high redshift the coalescence of massive black
hole binaries is likely to be driven by the accretion of gas in the major
mergers signposted by optically bright QSO activity. If massive black holes
only form efficiently by direct collapse of gas in deep galactic potential
wells with v_c > 100 km/s as postulated in the model of Kauffmann & Haehnelt
(2000) LISA expects to see event rates from the merging of massive binary black
holes of about 0.1-1 yr^{-1} spread over the redshift range 0 < z < 5. If,
however, the hierarchical build-up of supermassive black holes extends to
pre-galactic structures with significantly shallower potential wells event
rates may be as high as 10-100 yr^{-1} and will be dominated by events from
redshift z > 5.Comment: 8 pages, 4 postscript figures. Proceedings of the 4th International
LISA Symposium, Penn State University, 19-24 July 2002, ed. L S Fin
A Unified Model for the Evolution of Galaxies and Quasars
We incorporate a simple scheme for the growth of supermassive black holes
into semi-analytic models that follow the formation and evolution of galaxies
in a cold dark matter dominated Universe. We assume that supermassive black
holes are formed and fuelled during major mergers. If two galaxies of
comparable mass merge, their central black holes coalesce and a few percent of
the gas in the merger remnant is accreted by the new black hole over a
timescale of a few times 10^7 years. With these simple assumptions, our model
not only fits many aspects of the observed evolution of galaxies, but also
reproduces quantitatively the observed relation between bulge luminosity and
black hole mass in nearby galaxies, the strong evolution of the quasar
population with redshift and the relation between the luminosities of nearby
quasars and those of their host galaxies. The strong decline in the number
density of quasars from z=2 to z=0 is due to the combination of three effects:
i) a decrease in the merging rate, ii) a decrease in the amount of cold gas
available to fuel black holes, and iii) an increase in the timescale for gas
accretion. In a LCDM cosmology the predicted decline in the total content of
cold gas in galaxies is consistent with that inferred from observations of
damped Lyman-alpha systems. Our results strongly suggest that the evolution of
supermassive black holes, quasars and starbursts is inextricably linked to the
hierarchical build-up of galaxies.Comment: 30 pages, Latex, 18 figures included, submitted to MNRA
Lensing Induced Cluster Signatures in Cosmic Microwave Background
We show that clusters of galaxies induce step-like wiggles on top of the
cosmic microwave background (CMB). The direction of the wiggle is parallel to
the large scale gradient of CMB allowing one to isolate the effect from other
small scale fluctuations. The effect is sensitive to the deflection angle
rather than its derivative (shear or magnification) and is thus tracing outer
parts of the cluster with higher sensitivity than some other methods. A typical
amplitude of the effect is where
is the velocity dispersion of the cluster and several
signals extend out to a fraction of a degree. We derive the expressions for the
temperature profile for several simple parameterized cluster models and
identify some degeneracies between parameters. Finally, we discuss how to
separate this signal from other imprints on CMB using custom designed filters.
Detection of this effect is within reach of the next generation of small scale
CMB telescopes and could provide information about the cluster density profile
beyond the virial radius.Comment: 10 pages, 3 figures, submitted to Ap
The impact of galactic winds from LBGs on the Intergalactic Medium
An excess of sight-lines close to Lyman-break galaxies (LBGs) with little or
no absorption in QSO absorption spectra has been reported and has been
interpreted as the effect of galactic winds on the Intergalactic Medium. We use
here numerical simulations to investigate the flux probability function close
to plausible sites of LBGs. We show that the flux distribution near our LBGs in
the simulation depends strongly on redshift, and is very sensitive to the
averaging procedure. We show that a model without galactic winds and a model
with a wind bubble size of 0.5Mpc/h (comoving) are equally consistent with the
new determination of the conditional flux distribution by Adelberger et al.
(2005). Models with the larger bubble sizes suggested by the previous
observations of Adelberger et al. (2003) based on a much smaller sample at
higher redshift are not consistent with the new data. We therefore argue that
the volume filling factor of galactic winds driven by LBGs may be much smaller
than previously thought, and that most of the metals responsible for the metal
absorption associated with the low column density Lya forest are unlikely to
have been ejected by LBGs.Comment: 5 pages, 3 figure
Retaining Black Holes with Very Large Recoil Velocities
Recent numerical simulations of binary black hole mergers show the
possibility of producing very large recoil velocities (> 3000 km/s). Kicks of
this magnitude should be sufficient to eject the final black hole from
virtually any galactic potential. This result has been seen as a potential
contradiction with observations of supermassive black holes residing in the
centers of most galaxies in the local universe. Using an extremely simplified
merger tree model, we show that, even in the limit of very large ejection
probability, after a small number of merger generations there should still be
an appreciable fraction (>50%) of galaxies with supermassive black holes today.
We go on to argue that the inclusion of more realistic physics ingredients in
the merger model should systematically increase this retention fraction,
helping to resolve a potential conflict between theory and observation. Lastly,
we develop a more realistic Monte Carlo model to confirm the qualitative
arguments and estimate occupation fractions as a function of the central
galactic velocity dispersion.Comment: 6 pages, 3 figures; Comments welcom
The power spectrum of the flux distribution in the Lyman-alpha forest of a Large sample of UVES QSO Absorption Spectra (LUQAS)
The flux power spectra of the Lyman-alpha forest from a sample of 27 QSOs
taken with the high resolution echelle spectrograph UVES on VLT are presented.
We find a similar fluctuation amplitude at the peak of the ``3D'' flux power
spectrum at k ~ 0.03 (km/sec)^(-1) as the study by Croft et al. (2002), in the
same redshift range. The amplitude of the flux power spectrum increases with
decreasing redshift if corrected for the increase in the mean flux level as
expected if the evolution of the flux power spectrum is sensitive to the
gravitational growth of matter density fluctuations. This is in agreement with
the findings of McDonald et al. (2000) at larger redshift. The logarithmic
slope of the "3D" flux power spectrum, P_F(k), at large scales k < 0.03
(km/sec)^(-1), is 1.4 +- 0.3, i.e. 0.3 shallower than that found by Croft et
al. (2002) but consistent within the errors.Comment: 18 pages, 9 PS figures, 6 tables. Note that the k-values of the 1D
flux power spectrum had been erroneously shifted by half a bin size (in log
k) in the previous version. All the other results are unaffected. New tables
can be found at http://www.ast.cam.ac.uk/~rtnigm/luqas.ht
Massive perturbers and the efficient merger of binary massive black holes
We show that dynamical relaxation in the aftermath of a galactic merger and
the ensuing formation and decay of a binary massive black hole (MBH), are
dominated by massive perturbers (MPs) such as giant molecular clouds or
clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body
stellar relaxation alone, and efficiently scatter stars into the binary MBH's
orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the
point where energy losses from the emission of gravitational waves (GW) lead to
rapid coalescence. We model this process based on observed and simulated MP
distributions and take into account the decreased efficiency of the star-binary
MBH interaction due to acceleration in the galactic potential. We show that
mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within
the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a
few percent of the typical gas mass in a post-merger nucleus to coalesce in a
Hubble time. The fate of a binary MBH in a gas poor galactic merger is less
certain, although massive stellar structures (e.g. clusters, stellar rings)
could likewise lead to efficient coalescence. These coalescence events are
observable by their strong GW emission. MPs thus increase the cosmic rate of
such GW events, lead to a higher mass deficit in the merged galactic core and
suppress the formation of triple MBH systems and the resulting ejection of MBHs
into intergalactic space.Comment: 14 pages, 4 figures, 3 tables. More detailed explanations and changes
in structure. Section on hypervelocity stars moved to another paper (in
preparation). Results and conclusions unchanged. Accepted to Ap
How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
We present constraints on the mass of warm dark matter (WDM) particles
derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES
spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of
mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics
and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital
Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28
keV (2 sigma) for thermal relics and sterile neutrinos. These results improve
previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions
unchanged. Accepted version by PR
Measuring the Mach number of the Universe via the Sunyaev-Zeldovich effect
We introduce a new statistic to measure more accurately the cosmic sound
speed of clusters of galaxies at different redshifts. This statistic is
evaluated by cross-correlating cosmic microwave background (CMB) fluctuations
caused by the Sunyaev-Zel'dovich effect from observed clusters of galaxies with
their redshifts. When clusters are distributed in redshift bins of narrow
width, one could measure the mean squared cluster peculiar velocity with an
error \sigma_{C_S^2}\lsim (300{\rm km/s})^2. This can be done around z>0.3 with
clusters of flux above 200 mJy which will be detected by PLANCK, coupled with
high resolution microwave images to eliminate the cosmological part of the CMB
fluctuations. The latter can be achieved with observations by the planned ALMA
array or the NSF South Pole telescope and other surveys. By measuring the
cosmic sound speed and the bulk flow in, e.g., 4 spheres of ~ 100h^{-1}Mpc at
z=0.3, we could have a direct measurement of the matter density
0.21<\Omega_m<0.47 at 95 % confidence level.Comment: Ap.J.Letters, submitte
The Infrared Afterglow of Supermassive Black Hole Mergers
We model the spectra and light curves of circumbinary accretion disks during
the time after the central black holes merge. The most immediate effect of this
merger is the dissipation of energy in the outer regions of the disk due to the
gravitational wave energy and linear momentum flux released at merger. This has
the effect of perturbing the gas in the disk, which then radiates the
dissipated energy over a cooling timescale, giving a characteristic infrared
signal for tens of thousands of years when the total black hole mass is M~10^8
M_sun. On the basis of a simple cosmological merger model in which a typical
supermassive black hole undergoes a few major mergers during its lifetime, we
predict that ~10^4-10^5 of these IR sources should be observable today and
discuss the possibility of identifying them with multi-wavelength surveys such
as SWIRE/XMM-LSS/XBootes and COSMOS.Comment: v2: expanded discussion of optical depth calculations; ApJ in pres
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