QSO Activity Research Articles

The environment of large equivalent width Lyα emitters at z ∼ 3

AbstractWe present the environmental properties of a sample of 260 Lyα emitters (LAEs) with extremely large equivalent widths (EW0 > 240 Å) at z ∼ 3. Such large EW LAEs may be very low-metal / young starbursts in void regions or Hi gas /galaxies illuminated by ionizing radiation from the QSO. We undertook Subaru/HSC Lyα imaging observations around a hyper luminous QSO at z = 2.84. We identified 3490 LAEs including 260 large EW LAEs. We found that the large EW LAEs tend to lie at galaxy over-dense regions surrounding the QSO, suggesting that most of the large EW LAEs are illuminated by the QSO. From the radial distribution of the large EW LAEs from the QSO, we found that the fraction of large EW LAE shows two excesses in the range of 2 to 5 Mpc and 14 to 16 Mpc from the QSO. If the two peaks are reflected with the QSO activity, the QSO should have episodic lifetimes with ∼10 Myr duration and ∼30 Myr separations. However, we require future spectroscopic observations to investigate if the large EW LAEs far from the QSO are low-metal young starbursts.

Analysis of interacting and isolated quasars

Aims. The main goal of this study was to determine the effects on equivalent widths (EWs) of some spectral lines produced in the quasars by the presence of surrounding galaxies. To carry this out, a sample of 4663 quasars (QSOs) in the redshift range of 0.20 to 0.40 from the Sloan Digital Sky Survey data release 7 was analyzed. Methods. Three QSO sub-samples were defined, taking into account the projected separations and radial velocity differences with neighboring galaxies. In this way, we utilized two sub-samples of QSOs with strong and weak galaxy interactions, with projected separations smaller than 70 kpc, and between 70 and 140 kpc, respectively, and with radial velocity differences less than 5000 km s−1. These sub-samples were compared with isolated QSOs defined as having greater projected separations and radial velocity differences to the galaxies. Results. From a statistical study of the EWs of relevant spectral lines in the QSOs, we show an increment of the EWs of about 20% in the [OIII]λλ4959, 5007 lines and 7% in Hα for QSOs with stronger galaxy interactions relative to the isolated QSOs. These increments were also observed restricting the sub-samples to velocity differences of 3000 km s−1. These results indicate that some line EWs of QSOs could be marginally influenced by the environment and that they are not affected by the emission of the host galaxy, which was estimated to be around 10% of the total emission. Furthermore, in order to gain a better understanding of the origin of the Hα emission line, we performed broad and narrow line decomposition in 100 QSOs in the restricted Sint sub-sample and also 100 randomly selected QSOs in the Iso sub-sample. When these QSOs were compared, the narrow component remained constant whereas the broad component was incremented. Our results, which reveal slight differences in EWs of some emission lines, suggest that galaxy interactions with QSOs may affect the QSO activity.

The spectral energy distributions of K+A galaxies from the UV to the mid-IR: stellar populations, star formation and hot dust

We present spectrum synthesis fits to 808 K+A galaxies selected from the Sloan Digital Sky Survey (SDSS) and population synthesis of their spectral energy distributions, extending from the far UV (0.15 micron) to the mid IR (22 micron), based on the results of STARLIGHT code fitting to the SDSS spectra. Our modelling shows that K+A galaxies have undergone a large starburst, involving a median 50% of their present stellar masses, superposed over an older stellar population. The metal abundance of the intermediate-age stars shows that star formation did not take place in pristine gas, but was part of a dramatic increase in the star formation rates for originally gas-rich objects. We find no evidence for on-going QSO activity in the UV, which is well modeled by the emission of intermediate-age stars. We use K+A galaxies as local counterparts of high redshift objects to test for the presence of Thermally Pulsing AGB stars in similarly-aged populations and find no excess in the infrared due to emission from such stars, arguing that more distant galaxies are indeed old and massive at their redshift. All of our galaxies show significant excesses in the mid-IR compared to the light from their stars. We fit this ad hoc with a 300K blackbody. Possible sources include TP-AGB stars, obscured young star clusters and hidden AGNs, heating a significant dust component.

THE CHANDRA MULTI-WAVELENGTH PROJECT: OPTICAL SPECTROSCOPY AND THE BROADBAND SPECTRAL ENERGY DISTRIBUTIONS OF X-RAY-SELECTED AGNs

From optical spectroscopy of X-ray sources observed as part of ChaMP, we present redshifts and classifications for a total of 1569 Chandra sources from our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO, Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20% stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled extensive photometry from X-ray to radio bands. Together with our spectroscopic information, this enables us to derive detailed SEDs for our extragalactic sources. We fit a variety of templates to determine bolometric luminosities, and to constrain AGN and starburst components where both are present. While ~58% of X-ray Seyferts require a starburst event to fit observed photometry only 26% of the X-ray QSO population appear to have some kind of star formation contribution. This is significantly lower than for the Seyferts, especially if we take into account torus contamination at z>1 where the majority of our X-ray QSOs lie. In addition, we observe a rapid drop of the percentage of starburst contribution as X-ray luminosity increases. This is consistent with the quenching of star formation by powerful QSOs, as predicted by the merger model, or with a time lag between the peak of star formation and QSO activity. We have tested the hypothesis that there should be a strong connection between X-ray obscuration and star-formation but we do not find any association between X-ray column density and star formation rate both in the general population or the star-forming X-ray Seyferts. Our large compilation also allows us to report here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO candidates. Also we have identified the highest redshift (z=5.4135) X-ray selected QSO with optical spectroscopy.

Quasar induced galaxy formation: a new paradigm?

We discuss observational evidence that quasars play a key role in the formation of galaxies starting from the detailed study of the quasar HE0450-2958 and extending the discussion to a series of converging evidence that radio jets may trigger galaxy formation. The direct detection with VISIR at the ESO-VLT of the 7 kpc distant companion galaxy of HE0450-2958 allows us to spatially separate the sites of quasar and star formation activity in this composite system made of two ultra-luminous infrared galaxies (ULIRGs). No host galaxy has yet been detected for this quasar, but the companion galaxy stellar mass would bring HE0450-2958 in the local M(BH)-M(stellar bulge) relation if it were to merge with the QSO. This is bound to happen because of their close distance (7 kpc) and small relative velocity (~60-200 km/s). We conclude that we may be witnessing the building of the M(BH)-M(stellar bulge) relation, or at least of a major event in that process. The star formation rate (~340 Msun/yr), age (40-200 Myr) and stellar mass ([5-6]x10^10 Msun) are consistent with jet-induced formation of the companion galaxy. We suggest that HE0450-2958 may be fueled in fresh material by cold gas accretion from intergalactic filaments. We map the projected galaxy density surrounding the QSO as a potential tracer of intergalactic filaments and discuss a putative detection. Comparison to other systems suggests that inside-out formation of quasar host galaxies and jet-induced galaxy formation may be a common process. Two tests are proposed for this new paradigm: (1) the detection of offset molecular gas or dust emission with respect to the position of distant QSOs, (2) the delayed formation of host galaxies as a result of QSO activity, hence the two step building of the M(BH)/M(stellar bulge) ratio.

A New Twist to an Old Story: HE 0450−2958 and the ULIRG → Optically Bright QSO Transition Hypothesis

We report on interferometric imaging of the CO -->J = 1?0 and -->J = 3?2 line emission from the controversial QSO/galaxy pair HE 0450?2958. The detected CO -->J = 1?0 line emission is found associated with the disturbed companion galaxy, not the luminous QSO, and implies -->Mgal(H2) ~ (1?2) ? 1010 M?, which is 30% of the dynamical mass in its CO-luminous region. Fueled by this large gas reservoir, this galaxy is the site of an intense starburst with -->SFR ~ 370 M? yr ?1, placing it firmly on the upper gas-rich/star-forming end of ultra luminous infrared galaxies (ULIRGs, -->LIR > 1012 L?). This makes HE 0450?2958 the first case of extreme starburst and powerful QSO activity, intimately linked (triggered by a strong interaction) but not coincident. The lack of CO emission toward the QSO itself renews the controversy regarding its host galaxy by making a gas-rich spiral (the typical host of narrow-line Seyfert 1 AGNs) less likely. Finally, given that HE 0450?2958 and similar IR-warm QSOs are considered typical ULIRG ? optically bright QSO transition candidates, our results raise the possibility that some may simply be gas-rich/gas-poor (e.g., spiral/elliptical) galaxy interactions which activate an optically bright unobscured QSO in the gas-poor galaxy, and a starburst in the gas-rich one. We argue that such interactions may have gone largely unnoticed even in the local universe because the combination of tools necessary to disentangle the progenitors (high-resolution and S/N optical and CO imaging) became available only recently.

Evidence for Merger Remnants in Early‐Type Host Galaxies of Low‐Redshift QSOs1

We present results from a pilot HST ACS deep imaging study in broadband V of five low-redshift QSO host galaxies classified in the literature as ellipticals. The aim of our study is to determine whether these early-type hosts formed at high redshift and have since evolved passively, or whether they have undergone relatively recent mergers that may be related to the triggering of the nuclear activity. We perform two-dimensional modeling of the light distributions to analyze the host galaxies' morphology. We find that, while each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the majority of them (4/5) reveal significant fine structure such as shells and tidal tails. These structures contribute between ~5% and 10% to the total V-band luminosity of each host galaxy within a region of r ∼ 3reff and are indicative of merger events that occurred between a few hundred Myr and a Gyr ago. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy. Our results thus support a consistent scenario in which most of the QSO host galaxies suffered mergers with accompanying starbursts that likely also triggered the QSO activity in some way, but we are also left with considerable uncertainty on physical mechanisms that might have delayed this triggering for several hundred Myr after the merger.

Spectacular Shells in the Host Galaxy of the QSO MC2 1635+119

We present deep HST/ACS images and Keck spectroscopy of MC2 1635+119, a QSO hosted by a galaxy previously classified as an undisturbed elliptical. Our new images reveal dramatic shell structure indicative of a merger event in the relatively recent past. The brightest shells in the central regions of the host are distributed alternately in radius, with at least two distinct shells on one side of the nucleus and three on the other, out to a distance of ~13 kpc. The light within the five shells comprises ~6% of the total galaxy light. Lower surface brightness ripples or tails and other debris extend out to a distance of ~65 kpc. A simple N-body model for a merger reproduces the inner shell structure and gives an estimate for the age of the merger between ~30 Myr and ~1.7 Gyr, depending on a range of reasonable assumptions. While the inner shell structure is suggestive of a minor merger, the total light contribution from the shells and extended structures are more indicative of a major merger. The spectrum of the host galaxy is dominated by a population of intermediate age (~1.4 Gyr), indicating a strong starburst episode that may have occurred at the time of the merger event. We speculate that the current QSO activity may have been triggered in the recent past by either a minor merger, or by debris from an older (~Gyr) major merger that is currently ``raining'' back into the central regions of the merger remnant.

Discovery of a Probable Physical Triple Quasar

We report the discovery of the first known probable case of a physical triple quasar. A previously known double system, QQ 1429-008 at z = 2.076, is shown to contain a third, fainter QSO component at the same redshift. Deep optical and IR imaging has failed to reveal a plausible lensing galaxy group or a cluster, and we are unable to construct any viable lensing model that could lead to the observed distribution of source positions and relative intensities of the three QSO image components. There are also hints of differences in the broadband spectral energy distributions of the different components, which are more naturally understood if they are physically distinct active galactic nuclei (AGNs). Therefore, we conclude that this system is most likely a physical triple quasar, the first such close QSO grouping known at any redshift. The projected component separations are ~30-50 kpc, typical of interacting galaxy systems. The existence of this highly unusual system supports the standard picture in which galaxy interactions lead to the onset of QSO activity.

PAH Emission and Star Formation in the Host of the [FORMULA][F]z~2.56[/F][/FORMULA] Cloverleaf QSO

We report the first detection of the 6.2micron and 7.7micron infrared `PAH' emission features in the spectrum of a high redshift QSO, from the Spitzer-IRS spectrum of the Cloverleaf lensed QSO (H1413+117, z~2.56). The ratio of PAH features and rest frame far-infrared emission is the same as in lower luminosity star forming ultraluminous infrared galaxies and in local PG QSOs, supporting a predominantly starburst nature of the Cloverleaf's huge far-infrared luminosity (5.4E12 Lsun, corrected for lensing). The Cloverleaf's period of dominant QSO activity (Lbol ~ 7E13 Lsun) is coincident with an intense (star formation rate ~1000 Msun/yr) and short (gas exhaustion time ~3E7yr) star forming event.

Unveiling the oldest and most massive galaxies at very high redshift

Context. The identification and characterisation of massive galaxies over a wide redshift range allow us to place stringent constraints on the cosmic history of galaxy mass assembly and on current models of galaxy formation and evolution. Aims. This work explores the existence of high-redshift massive galaxies unveiled with Spitzer +IRAC, but missed by conventional selection techniques based on optical and near-infrared observations.Methods. To this end, we used multi-wavelength imaging data available for the GOODS-South field (130 arcmin2 ) and selected a flux-limited sample from the IRAC 3.6 μ m image to Jy (m (AB) -band image ( AB). Our selection unveiled 20 galaxies on which we performed a detailed analysis. For each galaxy, we built a spectral energy distribution (SED) based on optical-to-8μ m photometry. The SEDs were then used to estimate the photometric redshifts and to derive the main galaxies' physical properties. Further constraints were also obtained from the available X-ray and 24 μ m data. Results. The majority of the sample (14 out of 20) sources show degenerate/bimodal solutions for the photometric redshifts. These can either be heavily dust-enshrouded () starbursts at with bolometric luminosities , or massive post-starburst galaxies in the redshift interval with stellar masses of ~. The remaining six galaxies present a less ambiguous photometric redshift: with the exception of one low-z dusty source, these latter objects favour a low-extinction solution, with four of them showing best-fit photo-z solutions at . One galaxy, ID-6, the only source in our sample with both an X-ray and a 24 μ m detection, might be an extremely massive object at detected during a post-starburst phase with concomitant QSO activity responsible for the 24 μ m and X-ray emissions (although a lower-z solution is not excluded). Conclusions. Our investigation of Spitzer -selected galaxies that have very red SEDs and completely undetected in the optical reveals a potential population of massive galaxies at , which appear to include significant AGN emissions. These sources may be the oldest stellar systems at , given that the estimated ages are close to the age of the Universe at that redshift. We found that these, previously unrecognised, optically obscured objects might provide an important contribution to the massive end () of the high-z stellar mass function, and they would almost double it. Our evidence in these mature high-z galaxies of the widespread presence of hidden AGNs may have important implications for galaxy formation, due to their feedback effects on the surrounding ISM.

QSO Absorption Lines from QSO s

We present the results of a search for metal absorption lines in the spectra of background QSOs whose sight lines pass close to foreground QSOs. We detect Mg II λλ2796, 2803 absorption in Sloan Digital Sky Survey (SDSS) spectra of four z > 1.5 QSOs whose lines of sight pass within 26-98 h kpc of lower redshift (z ≃ 0.5-1.5) QSOs. The 100% detection rate (four out of four pairs) of Mg II in the background QSOs is clearly at odds with the incidence of associated zabs ≃ zem systems—absorbers that exist toward only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen toward one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central active galactic nucleus. We discuss possible origins for the Mg II lines, including absorption by gas from the foreground QSO host galaxy, companion galaxies fueling the QSO through gravitational interactions, and tidal debris left by galaxy mergers or interactions that initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena.

Spitzer Observations of High-Redshift QSOs

We have observed 13 z >= 4.5 QSOs using the Multiband Imaging Photometer for Spitzer, nine of which were also observed with the Infrared Array Camera. The observations probe rest wavelengths ~ 0.6-4.3 micron, bracketing the local minimum in QSO spectral energy distributions (SEDs) between strong optical emission associated directly with accretion processes and thermal emission from hot dust heated by the central engine. The new Spitzer photometry combined with existing measurements at other wavelengths shows that the SEDs of high redshift QSOs (z >= 4.5) do not differ significantly from typical QSOs of similar luminosity at lower redshifts (z <~ 2). This behavior supports other indications that all the emission components and physical structures that characterize QSO activity can be established by z = 6.4. The similarity also suggests that some QSOs at high redshift will be very difficult to identify because they are viewed along dust-obscured sight lines.

Looking for obscured QSOs in the X-ray emitting ERO population

We present XMM-Newton data centered on one of the MUNICS Near Infrared Cluster Survey fields (S2F1) and we discuss the X-ray properties of the 6 X-ray emitting EROs found. For one of them we have already obtained the redshift using near-infrared spectroscopic data, while for the remaining 5 EROs the analysis is based on photometric redshifts. We find evidence for the presence of an X-ray obscured QSO in at least 5 out of the 6 X-ray emitting EROs. For these 5 objects we derive intrinsic (2–10 keV) luminosities in excess of 1044 erg s-1 and intrinsic column densities higher than 1022 cm-2. These values have been obtained through a basic X-ray spectral analysis for the three brightest sources and through the analysis of the hardness ratios for the remaining two. All of these 5 X-ray emitting EROs appear extended in the optical/near-infrared bands indicating that the host galaxy emission dominates at these wavelengths. This suggests that the hosted AGNs are likely to be absorbed also in the optical/near-infrared bands: i.e. they are likely X-ray obscured possible type 2 QSOs. For the remaining ERO the presence of an AGN is suggested both by its high 0.5–2 keV luminosity ( erg s-1) and by its X-ray-to-optical flux ratio. In this case the quality of the present data prevents us from placing firm constraints on the AGN type hosted. Finally, the near-IR spectrum obtained for one of the 6 EROs classifies the host galaxy as an elliptical at with a stellar mass well in excess of 1011 . This result corroborates the possible link between the QSO activity and the formation of massive spheroids.

Reasoning From Fossils: Learning from the Local Black Hole Population about the Evolution of Quasars

We discuss a simple model for the growth of supermassive black holes (BHs) at the center of spheroidal stellar systems. In particular, we assess the hypotheses that (1) star formation in spheroids and BH fueling are proportional to one another, and (2) the BH accretion luminosity stays near the Eddington limit during luminous quasar phases. With the aid of this simple model, we are able to interpret many properties of the QSO luminosity function, including the puzzling steep decline of the characteristic luminosity from redshift z=2 to to z=0: indeed the residual star formation in spheroidal systems is today limited to a small number of bulges, characterized by stellar velocity dispersions a factor of 2-3 smaller those of the elliptical galaxies hosting QSOs at z > 2. A simple consequence of our hypotheses is that the redshift evolution of the QSO emissivity and of the star formation history in spheroids should be roughly parallel. We find this result to be broadly consistent with our knowledge of the evolution of both the global star formation rate, and of the evolution of the QSO emissivity, but we identify interesting discrepancies at both low and high redshifts, to which we offer tentative solutions. Finally, our hypotheses allow us to present a robust method to derive the duty cycle of QSO activity, based on the observed QSO luminosity function, and on the present-day relation between the masses of supermassive BHs and those of their spheroidal host stellar systems. The duty cycle is found to be substantially less than unity, with characteristic values in the range (3-6)x10^(-3), and we compute that the average bolometric radiative efficiency is epsilon=0.07. Finally, we find that the growth in mass of individual black holes at high redshift (z>2) can be dominated by mergers, and is therefore not necessarily limited by accretion.

Gemini Imaging of QSO Host Galaxies atz ∼ 2

We present results of a Gemini adaptive optics (AO) imaging program to investigate the host galaxies of typical QSOs at z ~ 2. Our aim is to study the host galaxies of typical L QSOs at the epoch of peak QSO and star formation activity. The large database of faint QSOs provided by the Two-Degree Field QSO Redshift Survey allows us to select a sample of QSOs at z = 1.75-2.5 that have nearby (<12'' separation) bright stars suitable for use as AO guide stars. We have observed a sample of nine QSOs. The images of these sources have AO-corrected FWHM of between 011 and 025. We use multiple observations of point-spread function (PSF) calibration star pairs to quantify any uncertainty in the PSF. We then factored these uncertainties into our modeling of the QSO plus host galaxy. In only one case did we convincingly detect a host (2QZ J133311.4+001949, at z = 1.93). This host galaxy has K = 18.5 ± 0.2 mag with a half-light radius Re = 055 ± 01 equivalent to ~3L, assuming a simple passively evolving model. From detailed simulations of our host galaxy modeling process, we find that for four of our targets we should be sensitive to host galaxies that are equivalent to ~2L (passively evolved). Our nondetections therefore place tight constraints on the properties of L QSO host galaxies, which can be no brighter (after allowing for passive evolution) than the host galaxies of L active galactic nuclei at low redshift, although the QSOs themselves are a factor of ~50 brighter. This implies that either the fueling efficiency is much greater at high redshift or that more massive black holes are active at high redshift.

QSO+Galaxy Association and Discrepant Redshifts in NEQ3

Spectroscopy and deep imaging of the group NEQ3 are presented. This system is formed by three compact objects with relative separations of ~2.6 and ~2.8 arcsec, and a lenticular galaxy at ~17 arcsec from the geometric centre of the group. A diffuse filament is located on a line joining the three compact objects and the main galaxy. Analysis of these observations confirms the redshift previously known for three of the objects (z=0.1239 for the main galaxy, and z=0.1935 and 0.1939 for two of the compact objects). We have also determined the previously unknown redshift of the third compact object as z=0.2229. Using the relative strength and width of the main spectral lines, we have classified the compact objects as two HII galaxies and one QSO (the object at z=0.1935). With cross-correlation techniques, we have tentatively estimated the redshift of the filament as z=0.19 (although a weaker component also appears at z=0.12) so that it is probably associated with the halo of the two compact objects at this redshift. The two objects at redshift ~0.19 represent possibly one of the more clear examples of starburst (and perhaps QSO activity) driven by interaction. These, and the relation between these two objects and the other two at (~0.12 and ~0.22) make the nature of this system intriguing, being difficult to explain the whole association on conventional scenarios.

Galaxy merging, the fundamental plane of elliptical galaxies and the MBH -- 0 relation

We explore the effects of dissipationless merging on the Fundamental Plane of elliptical galaxies by using a N-body code based on a new, high performance numerical scheme (Dehnen 2002). We investigate the two extreme cases of galaxy growth by equal mass merging and accretion of small stellar systems; in a subset of simulations we also consider the presence of dark matter halos around the merging galaxies. Curiously, we found that the Fundamental Plane is preserved by major merging, while in the accretion scenario its edge–on thickness is only marginally reproduced, with substantial thickening in the case of merging with low angular momentum. We also found that both the Faber-Jackson and Kormendy relations are not reproduced by the simulations, in accordance with the results of a preliminary analysis based on a simple application of the virial theorem. Finally, we discuss the implications of our results for the origin of the MBH-�0 and Magorrian relations. We found that dissipationless merging is unable to reproduce the MBH-�0 relation, if the black hole masses add linearly (while the Magorrian relation is nicely reproduced); on the contrary a black hole merging with substantial emission of gravitational waves reproduces the MBH-�0 relation but fails at reproducing the Magorrian relation. We argue that our results strongly point towards a major role of dissipation in the formation of early–type galaxies and in the growth of their central supermassive black holes, thus supporting the idea of a link between galaxy formation and QSO activity.

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 sling-shot 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 vc ≳ 100 km s−1 as postulated in the model of Kauffmann and Haehnelt (2000 Mon. Not. R. Astron. Soc. 311 576) 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.

Multiple supermassive black holes in galactic bulges

We study the number and interaction rates of supermassive black holes in galactic bulges as predicted by hierarchical models of galaxy formation in which the spheroidal components of galaxies are formed by mergers. In bright ellipticals, the number of events that can eject a central supermassive binary black hole is large. Central binaries must therefore merge in less than a Hubble time - otherwise there will be too much scatter in the M_-σ * relation and too many off-centre galactic nuclei. We propose that binary black holes are able to merge during the major gas accretion events that trigger QSO activity in galaxies. If this is the case, less than 10 per cent of faint ellipticals and 40 per cent of bright ellipticals are predicted to harbour binary black holes with near equal masses at their centres. This binary may be ejected away from the centre of the galaxy or even into intergalactic space in up to 20 per cent of the most luminous ellipticals. The number of low-mass black holes that can interact with the central object is predicted to be a strong function of galaxy luminosity. In most faint ellipticals, no black holes fall into the centre of the galaxy after the last major gas accretion event, but in the most luminous ellipticals, an average of 10 low-mass black holes interact with the central supermassive object after this time. It is expected that stars will be ejected from galaxy cores as these low mass ratio binaries harden. Multiple black holes in galactic bulges thus provide a natural explanation for the strong systematic trends in the observed central density profiles of ellipticals as a function of luminosity.