Supermassive black holes as sources for LISA

Some issues relevant for the formation of supermassive black holes are discused and estimates of the event rates for the emission of gravitational waves by coalescing supermassive black hole binaries are given. The models take into account recent improvements in our knowledge of galaxy and star formation in the high-redshift universe. Estimated event rates range from a few to a hundred per year. Typical events will occur at redshift three or larger in galaxies lying at the (very) faint end of the luminosity function at these redshifts.Comment: 5 pages, LaTeX, 2 postscript figures included; invited talk at the Second International LISA Symposium, Pasadena, July 1998 (ed. W. Folkner, American Institute of Physics

The Clustering of Galaxies around Quasars

We study the cross-correlation between quasars and galaxies by embedding models for the formation and evolution of the two populations in cosmological N-body simulations. We adopt the quasar evolution model of Kauffmann & Haehnelt (2000), in which supermassive black holes are formed and fuelled during major mergers. We define the `bias' parameter b_QG as the ratio of the cross-correlation function xi_QG to the galaxy auto-correlation function xi_GG. On scales larger than 1 h^{-1} Mpc, the values of b_QG predicted by our models at low redshift depend very little on galaxy selection. They measure the characteristic mass of the dark matter halos that host quasars and can be used to estimate the typical quasar lifetime. In current redshift surveys, such measurements will constrain the lifetimes of low z quasars more accurately than measurements of the quasar auto-correlation function, because galaxies have much higher space densities than quasars. On scales smaller than 1 h^{-1} Mpc, the main contribution to xi_QG comes from quasar/galaxy pairs in the same dark matter halo. The amplitude of xi_QG depends both on the location of the host galaxy and on the density profile of other galaxies within the halo. As a result, measurements on these scales yield information about the processes responsible for fuelling super- massive black holes. At high redshifts, our models predict that quasars of fixed luminosity are located in less massive halos than at low redshift. They are therefore less biased relative to galaxies of given luminosity or stellar mass. We have used the simulations to calculate the evolution of the quasar auto- correlation function. We find that models with quasar lifetimes in the range 10^6-10^7 years provide a good match to the results of the 2dF QSO survey.Comment: 14 pages, 9 figures, submitted to MNRA

A first step towards a direct inversion of the Lyman forest in QSO spectra

A method for the recovery of the real space line-of-sight mass density field from Lyman absorption in QSO spectra is presented. The method makes use of a Lucy-type algorithm for the recovery of the HI density. The matter density is inferred from the HI density assuming that the absorption is due to a photoionized intergalactic medium which traces the mass distribution as suggested by recent numerical simulations. Redshift distortions are corrected iteratively from a simultaneous estimate of the peculiar velocity. The method is tested with mock spectra obtained from N-body simulations. The density field is recovered reasonably well up to densities where the absorption features become strongly saturated. The method is an excellent tool to study the density probability distribution and clustering properties of the mass density in the (mildly) non-linear regime. Combined with redshift surveys along QSO sightlines the method will make it possible to relate the clustering of high-redshift galaxies to the clustering of the underlying mass density. We further show that accurate estimates for \Omega_{bar}h^2)^2 J^{-1} H(z)^{-1} and higher order moments of the density probability function can be obtained despite the missing high density tail of the density distribution if a parametric form for the probability distribution of the mass density is assumed.Comment: 10 pages, 6 figure

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

The amplitude of mass density fluctuations at z≈3.25z\approx 3.25 from the Ly-alpha forest of Q1422+231

The real-space optical depth distribution along the line of sight to the QSO Q1422+231 is recovered from two HIRES spectra. The first two moments of the truncated optical depth distribution are used to constrain the density fluctuation amplitude of the intergalactic medium (IGM). The $rms$ of the IGM density at $z\approx 3.25$ estimated from the first spectrum is $\sigma = \sqrt{\exp{[(1.8 \pm 0.27)^2/\alpha^2]}-1}$, with $1.56 <\alpha <2$ for plausible reionization histories. This corresponds to 0.9 \la \sigma \la 2.1 with $\sigma(\alpha =1.7)= 1.44\pm 0.3$. The values obtained from the second spectrum are higher by $\approx 20$. If the IGM density traces the dark matter (DM) as suggested by numerical simulations we have measured the fluctuation amplitude of the DM density at an effective Jeans scale of about a hundred to two hundred (comoving) kpc. For CDM-like power spectra the amplitude of dark matter fluctuations on these small scales depends on the cosmological density parameter $\Omega$. For power spectra normalized to reproduce the space density of present-day clusters and with a slope parameter of $\Gamma=0.21$ consistent with the observed galaxy power spectrum, the inferred $\Omega$ can be expressed as: $\Omega= 0.61 (\alpha/1.7)^{1.3}(x_{_{\rm J}} /0.62)^{-0.6}$ for a flat universe, and $\Omega= 0.91(\alpha/1.7)^{1.3} (x_{_{\rm J}}/0.62)^{-0.7}$ for a $\lambda=0$ universe. $x_{_{\rm J}}$ is the effective Jeans scale in (comoving) \mpc. Based on a suit of detailed mock spectra the 1-$\sigma$ error is $\approx 25$. The estimates increase with increasing $\Gamma$. For the second spectrum we obtain 15% lower values.Comment: 14 pages, submitted to MNRA

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

The nature and evolution of the highly ionized near-zones in the absorption spectra of z~6 quasars

We use state-of-the-art hydrodynamical simulations combined with a 1D radiative transfer code to assess the extent to which the highly ionized regions observed close to z~6 quasars, which we refer to as near-zones, can constrain the ionization state of the surrounding IGM. We find the appearance in Lya absorption of a quasar HII ionization front expanding into a neutral IGM can be very similar to a classical proximity zone, produced by the enhancement in ionizing flux close to a quasar embedded in a highly ionized IGM. The observed sizes of these highly ionized near-zones and their redshift evolution can be reproduced for a wide range of IGM neutral hydrogen fractions for plausible values of the luminosity and lifetime of the quasars. The observed near-zone sizes at the highest observed redshifts are equally consistent with a significantly neutral and a highly ionized surrounding IGM. Stronger constraints on the IGM neutral hydrogen fraction can be obtained by considering the relative size of the near-zones in the Lya and Lyb regions of a quasar spectrum. A large sample of high quality quasar absorption spectra with accurate determinations of near-zone sizes and their redshift evolution in both the Lya and Lyb regions should confirm or exclude the possibility that the Universe is predominantly neutral at the highest observed redshifts. The width of the discrete absorption features in these near-zones will contain important additional information on the ionization state and the previous thermal history of the IGM at these redshifts.Comment: 25 pages, 11 figures, accepted for publication in MNRA

Using the kinematic Sunyaev-Zeldovich effect to determine the peculiar velocities of clusters of galaxies

We have investigated the possibility of inferring peculiar velocities for clusters of galaxies from the Doppler shift of scattered cosmic microwave background (CMB) photons. We find that if the core radius of the gas distribution or the beam size of the instrument is larger than 3-7 arcminutes, then the maximum attainable signal-to-noise ratio is determined by confusion with primary fluctuations. For smaller angular scales, ``cosmic confusion'' is less important and instrumental noise and/or foreground emission will be the limiting factor. For a cluster with the optical depth of the Coma cluster and for an optimal filtering technique, typical one-sigma errors span the wide range from 400 to 1600 km/s, depending on the cosmological model, the resolution of the instrument and the core radius of the cluster. The results have important implications for the design of future high-resolution surveys of the CMB. Individual peculiar velocities will be measurable only for a few fast moving clusters at intermediate redshift unless cosmic fluctuations are smaller than most standard cosmological scenarios predict. However, a reliable measurement of bulk velocities of ensembles of X-ray bright clusters will be possible on very large scales (100-500 Mpc/h).Comment: 34 pages, with 11 figures included. Postscript. Submitted to MNRAS. Latest version (recommended) at http://www.mpa-garching.mpg.de/~max/sz.html or from [email protected]

The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization

We use a large set of hydrodynamical simulations, combined with measurements of the Lyman alpha opacity of the IGM taken from the literature, to obtain robust estimates for the photoionization rate per hydrogen atom at z=5 and 6. We find the photoionization rate drops by a factor of two and four, respectively, compared to our recent measurements at z = 2 - 4. The number of ionizing photons emitted by known sources at z=5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 Angstroms, are in reasonable agreement with the photoionization rates inferred from the Lyman alpha forest if the escape fraction of ionizing photons from galaxies is large (>= 20 per cent). Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z=6. This strongly suggests that the epoch of reionization was photon-starved and extended. [Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA

On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a in the spectrum of the bright z=7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z~7. Current theoretical models imply HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z~6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Ly-a can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.Comment: 11 pages, 6 figures, accepted to MNRA