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 $10\mu K (\sigma_v/1400 kms^{-1})^2$ where $\sigma_v$ is the velocity dispersion of the cluster and several $\mu K$ 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

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

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

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