Central SMBH Research Articles

AT2020ohl: its nature and probable implications

ABSTRACT ASASSN-20hx, a.k.a AT2020ohl, is an ambiguous nuclear transient, which was discovered in the nearby galaxy NGC6297 by the All-Sky Automated Survey for Supernovae. We have investigated the evolution of AT2020ohl using a multiwavelength data set to explain the geometry of the system and the energy radiated by it between X-ray and radio wavelengths. Our X-ray, UV/optical, and radio observations of the object jointly clarify the association of AT2020ohl with the nuclear activity of NGC6297. We detected radio counterpart of AT2020ohl 111 and 313 d after the discovery in Jansky Very Large Array X-band with flux densities 47 ± 14 and 34 ± 3 $\mu$Jy, respectively. Using multiwavelength data analysis, we nullify the possibility of associating any stellar disruption process with this event. We found some evidence showing that the host galaxy is a merger remnant, so the possibility of a binary supermassive black hole (SMBH) system cannot be ruled out. The central SMBH has a mass of ∼1.2 × 107 M⊙. We propose the accretion disc activity as the origin of AT2020ohl – it is either due to disc accretion event on to the central SMBH or due to the sudden accretion activity in a pre-existing accretion disc of the system during the interaction of two SMBHs which became gravitationally bound during a merger process. However, we also admit that with the existing data set, it is impossible to say definitively, among these two probabilities, which one is the origin of this nuclear transient.

MILKY WAY GLOBULAR CLUSTERS: CLOSE ENCOUNTER RATES WITH EACH OTHER AND WITH THE CENTRAL SUPERMASSIVE BLACK HOLE

Using the data from Gaia (ESA) Data Release 2 we performed the orbital calculations of globular clusters (GCs) of the Milky Way. To explore possible collisions between the GCs, using our developed highorder {\phi}-GRAPE code, we integrated (backwards and forward) the orbits of 119 objects with reliable positions and proper motions. In calculations, we adopted a realistic axisymmetric Galactic potential (bulge + disk + halo). Using different impact conditions, we found five pairs of the GCs that likely experienced collisions: Terzan 3 - NGC 6553, Terzan 3 - NGC 6218, Liller 1 - NGC 6522, Djorg 2 - NGC 6552 and NGC 6355 - NGC 6637. We analyzed the GCs interaction rates with the central supermassive black hole. Assuming the maximum 100 pc distance criteria for separation between them we estimated 11 close encounter events. From our numerical simulations, we estimate the close interaction rate as at least one event per Gyr with the impact parameter less than 30 pc; and one event per Myr with the impact parameter less than 60 pc. Our calculations show one very close encounter of NGC 6121 with the central SMBH near 5.5 pc (practically direct collision). Based on the extended literature search for the possible progenitor of our selected 11 GCs, we found that most of them have a Milky Way main bulge origin.

General relativistic rotational energy extraction from black holes-accretion disk systems

The determination of mass and spin parameters of the black holes (BHs) is crucial in the analysis of the merger of BHs and BHs formation and evolution, including accretion. Here we constrain the BH spin with the evaluation of the dimensionless parameter ξ representing the total rotational energy extracted versus the mass of the BH, following procedure introduced by Daly (2009 Astrophys. J. 691 L72–6) that is independent from the details of the specific extraction process. The energy extraction can power an outflow which can be then observed. We relate the energy extraction to the accreting configurations and the accretion processes occurring in a cluster of agglomerate corotating and counter-rotating tori orbiting one central Kerr SMBH, associating ξ to the characteristics of the accretion processes. We relate the regions of tori parameters to features of the energy extraction processes, binding ξ to properties of light surfaces by using the bundles developed by Pugliese and Quevedo (2019 Eur. Phys. J. C 79 209), relating measures in different regions of the spacetimes. We evaluate properties of the BH accretions disks, and correlate spacetimes prior and after their transition due to the energy extraction. Light surfaces are related to the generators of Killing horizons, proving limiting frequency of the stationary observers of the geometries. We consider the photon limiting curves of the stationary observers as constraints for various processes regulated by these frequencies, to relate different BH states, prior and after the energy extraction, investigating regions close to the BH horizons and rotational axis. From methodological view-point we used a naked singularity—BH correspondence defined with metric bundles to predict observational characteristics of the BH—accretion disk system. The analysis points relevant BH spins a ≈ 0.94M, a ≈ 0.7M and a ≈ 0.3M. We show the relation between the rotational law of the tori, the characteristic frequency of the bundle and the relativistic velocity defining the von Zeipel surfaces. The inferior limit on the formation of corotating tori is ℓ/a ⩾ 2, for counter-rotating tori ℓ/a ⩽ −22/5 (ℓ is the fluids specific angular momentum).

Formation of counter-rotating and highly eccentric massive black hole binaries in galaxy mergers

ABSTRACT Supermassive black hole (SMBH) binaries represent the main target for missions such as the Laser Interferometer Space Antenna and Pulsar Timing Arrays. The understanding of their dynamical evolution prior to coalescence is therefore crucial to improving detection strategies and for the astrophysical interpretation of the gravitational wave data. In this paper, we use high-resolution N-body simulations to model the merger of two equal-mass galaxies hosting a central SMBH. In our models, all binaries are initially prograde with respect to the galaxy sense of rotation. But, binaries that form with a high eccentricity, e ≳ 0.7, quickly reverse their sense of rotation and become almost perfectly retrograde at the moment of binary formation. The evolution of these binaries proceeds towards larger eccentricities, as expected for a binary hardening in a counter-rotating stellar distribution. Binaries that form with lower eccentricities remain prograde and at comparatively low eccentricities. We study the origin of the orbital flip by using an analytical model that describes the early stages of binary evolution. This model indicates that the orbital plane flip is due to the torque from the triaxial background mass distribution that naturally arises from the galactic merger process. Our results imply the existence of a population of SMBH binaries with a high eccentricity and could have significant implications for the detection of the gravitational wave signal emitted by these systems.

BOOSTED TIDAL DISRUPTION BY MASSIVE BLACK HOLE BINARIES DURING GALAXY MERGERS FROM THE VIEW OF N-BODY SIMULATION

ABSTRACT Supermassive black hole binaries (SMBHBs) are productions of the hierarchical galaxy formation model. There are many close connections between a central SMBH and its host galaxy because the former plays very important roles on galaxy formation and evolution. For this reason, the evolution of SMBHBs in merging galaxies is a fundamental challenge. Since there are many discussions about SMBHB evolution in a gas-rich environment, we focus on the quiescent galaxy, using tidal disruption (TD) as a diagnostic tool. Our study is based on a series of numerical, large particle number, direct N-body simulations for dry major mergers. According to the simulation results, the evolution can be divided into three phases. In phase I, the TD rate for two well separated SMBHs in a merging system is similar to that for a single SMBH in an isolated galaxy. After two SMBHs approach close enough to form a bound binary in phase II, the disruption rate can be enhanced by ∼2 orders of magnitude within a short time. This “boosted” disruption stage finishes after the SMBHB evolves to a compact binary system in phase III, corresponding to a reduction in disruption rate back to a level of a few times higher than in phase I. We also discuss how to correctly extrapolate our N-body simulation results to reality, and the implications of our results to observations.

Deep Chandra observations of NGC 7457, the X-ray point source populations of a low mass early-type galaxy

We present the X-ray point source population of NGC 7457 based on 124 ks of Chandra observations. Previous deep Chandra observations of low mass X-ray binaries (LMXBs) in early-type galaxies have typically targeted the large populations of massive galaxies. NGC 7457 is a nearby, early-type galaxy with a stellar luminosity of $1.7\times10^{10} L_{K\odot}$, allowing us to investigate the populations in a relatively low mass galaxy. We classify the detected X-ray sources into field LMXBs, globular cluster LMXBs, and background AGN based on identifying optical counterparts in new HST/ACS images. We detect 10 field LMXBs within the $r_{ext}$ ellipse of NGC 7457 (with semi-major axis $\sim$ 9.1 kpc, ellipticity = 0.55). The corresponding number of LMXBs with $L_{x}>2\times10^{37}erg/s$ per stellar luminosity is consistent with that observed in more massive galaxies, $\sim 7$ per $10^{10} L_{K\odot}$. We detect a small globular cluster population in these HST data and show that its colour distribution is likely bimodal and that its specific frequency is similar to that of other early type galaxies. However, no X-ray emission is detected from any of these clusters. Using published data for other galaxies, we show that this non-detection is consistent with the small stellar mass of these clusters. We estimate that 0.11 (and 0.03) LMXBs are expected per $10^{6}M_{\odot}$ in metal-rich (and metal-poor) globular clusters. This corresponds to 1100 (and 330) LMXBs per $10^{10} L_{K\odot}$, highlighting the enhanced formation efficiency of LMXBs in globular clusters. A nuclear X-ray source is detected with $L_{x}$ varying from $2.8-6.8\times10^{38}erg/s$. Combining this $L_{x}$ with a published dynamical mass estimate for the central SMBH in NGC 7457, we find that $L_{x}/L_{Edd}$ varies from $0.5-1.3\times10^{-6}$.

OUP accepted manuscript

We present results based on the observations of the blazar 3C 66A from 2005 November 06 to 2016 February 14 in the BVR and I broadbands using 1.2m telescope of the Mt. Abu InfraRed Observatory (MIRO). The source was observed on 160 nights out of which on 89 nights it was monitored for more than 1 hr to check for the presence of any intra-day variability (IDV). The blazar 3C 66A exhibited significant variations in the optical flux on short and long term time scales. However, unlike highly variable S5 0716+71, it showed IDV duty cycle of about 8% only. Our statistical studies suggest the IDV time scales ranging from $\sim$ 37 min to about 3.12 hours and, in one case, a possibility of the quasi-periodic variations with characteristic timescale of $\sim$ 1.4 hrs. The IDV amplitudes in R$-$band were found to vary from 0.03 mag to as much as 0.6 mag, with larger amplitude of variation when the source was relatively fainter. The typical rate of the flux variation was estimated to be $\sim$0.07 mag hr$^{-1}$ in both, the rising and the falling phases. However, the rates of the brightness variation as high as 1.38 mag/hr were also detected. The shortest timescale of the variation, 37 min, sets an upper limit of $6.92 \times 10^{14}$ cm on the size of the emission region and about $3.7 \times 10^8\ \mathrm{M}_{\odot}$ as an estimate of the mass of the black hole, assuming the origin of the rapid optical variability is in close vicinity of the central SMBH. The long-term study suggests a mild bluer-when-brighter behavior, typical for BL Lacs.

A new VLA/e-MERLIN limit on central images in the gravitational lens system CLASS B1030+074

We present new VLA 22-GHz and e-MERLIN 5-GHz observations of CLASS B1030+074, a two-image strong gravitational lens system whose background source is a compact flat-spectrum radio quasar. In such systems we expect a third image of the background source to form close to the centre of the lensing galaxy. The existence and brightness of such images is important for investigation of the central mass distributions of lensing galaxies, but only one secure detection has been made so far in a galaxy-scale lens system. The noise levels achieved in our new B1030+074 images reach 3 microJy/beam and represent an improvement in central image constraints of nearly an order of magnitude over previous work, with correspondingly better resulting limits on the shape of the central mass profile of the lensing galaxy. Simple models with an isothermal outer power law slope now require either the influence of a central supermassive black hole, or an inner power law slope very close to isothermal, in order to suppress the central image below our detection limit. Using the central mass profiles inferred from light distributions in Virgo galaxies, moved to z=0.5, and matching to the observed Einstein radius, we now find that 45% of such mass profiles should give observable central images, 10% should give central images with a flux density still below our limit, and the remaining systems have extreme demagnification produced by the central SMBH. Further observations of similar objects will therefore allow proper statistical constraints to be placed on the central properties of elliptical galaxies at high redshift.

Unveiling Gargantua: A new search strategy for the most massive central cluster black holes

We aim to unveil the most massive central cluster black holes in the universe. We present a new search strategy which is based on a black hole mass gain sensitive 'calorimeter' and which links the innermost stellar density profile of a galaxy to the adiabatic growth of its central SMBH. In a first step we convert observationally inferred feedback powers into SMBH growth rates by using reasonable energy conversion efficiency parameters, $\epsilon$. In the main part of this paper we use these black hole growth rates, sorted in logarithmically increasing steps encompassing our whole parameter space, to conduct $N$-Body computations of brightest cluster galaxies with the newly developed MUESLI software. For the initial setup of galaxies we use core-Sersic models in order to account for SMBH scouring. We find that adiabatically driven core re-growth is significant at the highest accretion rates. As a result, the most massive black holes should be located in BCGs with less pronounced cores when compared to the predictions of empirical scaling relations which are usually calibrated in less extreme environments. For efficiency parameters $\epsilon<0.1$, BCGs in the most massive, relaxed and X-ray luminous galaxy clusters might even develop steeply rising density cusps. Finally, we discuss several promising candidates for follow up investigations, among them the nuclear black hole in the Phoenix cluster. Based on our results, it might have a mass of the order of $10^{11} M_\odot$.

Does the obscured AGN fraction really depend on luminosity?

We use a sample of 151 local non-blazar AGN selected from the INTEGRAL all-sky hard X-ray survey to investigate if the observed declining trend of the fraction of obscured (i.e. showing X-ray absorption) AGN with increasing luminosity is mostly an intrinsic or selection effect. Using a torus-obscuration model, we demonstrate that in addition to negative bias, due to absorption in the torus, in finding obscured AGN in hard X-ray flux limited surveys, there is also positive bias in finding unobscured AGN, due to Compton reflection in the torus. These biases can be even stronger taking into account plausible intrinsic collimation of hard X-ray emission along the axis of the obscuring torus. Given the AGN luminosity function, which steepens at high luminosities, these observational biases lead to a decreasing observed fraction of obscured AGN with increasing luminosity even if this fraction has no intrinsic luminosity dependence. We find that if the central hard X-ray source in AGN is isotropic, the intrinsic (i.e. corrected for biases) obscured AGN fraction still shows a declining trend with luminosity, although the intrinsic obscured fraction is significantly larger than the observed one: the actual fraction is larger than $\sim 85$% at $L\lesssim 10^{42.5}$ erg/s (17--60 keV), and decreases to $\lesssim 60$% at $L\gtrsim 10^{44}$ erg/s. In terms of the half-opening angle, $\theta$, of an obscuring torus, this implies that $\theta\lesssim 30$ deg in lower-luminosity AGN, and $\theta\gtrsim 45$ deg in higher-luminosity ones. If, however, the emission from the central SMBH is collimated as $dL/d\Omega\propto\cos\alpha$, the intrinsic dependence of the obscured AGN fraction is consistent with a luminosity-independent torus half-opening angle $\theta\sim 30$ deg.

ALMA reveals a warm and compact starburst around a heavily obscured supermassive black hole atz= 4.75

We report ALMA Cycle 0 observations at 1.3mm of LESS J033229.4-275619 (XID403), an Ultraluminous Infrared Galaxy at $z=4.75$ in the Chandra Deep Field South hosting a Compton-thick QSO. The source is not resolved in our data at a resolution of $\sim$0.75 arcsec, placing an upper-limit of 2.5 kpc to the half-light radius of the continuum emission from heated-dust. After deconvolving for the beam size, however, we found a $\sim3\sigma$ indication of an intrinsic source size of $0.27\pm0.08$ arcsec (Gaussian FWHM), which would correspond to $r_{half}\sim0.9\pm0.3$ kpc. We build the far-IR SED of XID403 by combining datapoints from both ALMA and Herschel and fit it with a modified blackbody spectrum. For the first time, we measure the dust temperature $T_d=58.5\pm5.3$ K in this system, which is comparable to what has been observed in other high-z submillimeter galaxies. The measured star formation rate is SFR=$1020\pm150$ $M_{\odot}$ yr$^{-1}$, in agreement with previous estimates at lower S/N. Based on the measured SFR and source size, we constrain the SFR surface density to be $\Sigma_{SFR}>26\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ ($\sim200\;M_{\odot}$yr$^{-1}$kpc$^{-2}$ for $r_{half}\sim0.9$ kpc). The compactness of this starburst is comparable to what has been observed in other local and high-z starburst galaxies. If the gas mass measured from previous [CII] and CO(2-1) observations at low resolution is confined within the same dust region, assuming $r_{half}\sim0.9\pm0.3$ kpc, this would produce a column density of $N_H\sim0.3-1.1\times10^{24}$cm$^{-2}$ towards the central SMBH, similar to the column density of $\approx1.4\times10^{24}$cm$^{-2}$ measured from the X-rays. Then, in principle, if both gas and dust were confined on sub-kpc scales, this would be sufficient to produce the observed X-ray column density without any need of a pc-scale absorber [abridged].

What can we learn from high energy flares in the Fermi sample of FSRQs : from a case study to dozens of objects

In 2008 the flat spectrum radio quasar GB6 J1239+0443 (z=1.762), underwent an high activity period at all wavelengths. In particular, at optical-uv wavelengths, the source underwent a flux enhancement of a factor 15–30 in 6 years, and >10 in half a year, showing a transition from accretion disk to synchrotron jet dominated emission. During the flare, Fermi observed a flat gamma-ray spectrum, extended up to 15 GeV, with no statistically-significant absorption from the broad line region (BLR), suggesting that the blazar-zone is located beyond it. We obtained the same findings from the modeling of the broad-band spectral energy distribution for the flaring activity periods, which is well constrained by the multiwavelength data collected, and from the accretion disk luminosity and black hole mass that we estimated from the archival data. Other authors suggested that 4C +21.35, PMN J2345-1555, and 3C 279 underwent high energy flares originating from outside or just beyond the BLR.We report here the first results obtained in our study, while we will report the results on the complete sample in a forthcoming paper where we will show the multiwavelength spectra on a dozen of FSRQ which we propose radiate gamma-rays far from the central SMBH, and we will discuss the scenarios originating from this result.

Magnetic fields of active galactic nuclei and quasars with regions of polarized broad Hα lines

We present estimates of magnetic field in a number of AGNs from the Spectropolarimetric atlas of Smith, Young & Robinson (2002) from the observed degrees of linear polarization and the positional angles of spectral lines (H-alpha) (broad line regions of AGNs) and nearby continuum. The observed polarization is lower than the Milne value in a non-magnetized atmosphere. We hypothesize that the polarized radiation escapes from optically thick magnetized accretion discs and is weakened by the Faraday rotation effect. This effect is able to explain both the value of the polarization and the position angle. We estimate the required magnetic field in the broad line region by using simple asymptotic analytical formulas for Milne's problem in magnetized atmosphere, which take into account the last scattering of radiation before escaping from the accretion disc. The polarization of a broad spectral line escaping from disc is described by the same mechanism. The characteristic features of polarization of a broad line is the minimum of the degree of polarization in the center of the line and continuous rotation of the position angle from one wing to another. These effects can be explained by existence of clouds in the left (velocity is directed to an observer) and the right (velocity is directed from an observer) parts of the orbit in a rotating keplerian magnetized accretion disc. The base of explanation is existence of azimuthal magnetic field in the orbit. The existence of normal component of magnetic field makes the picture of polarization asymmetric. The existence of clouds in left and right parts of the orbit with different emissions also give the contribution in asymmetry effect. Assuming a power-law dependence of the magnetic field inside the disc, we obtain the estimate of the magnetic field strength at first stable orbit near the central SMBH for a number of AGNs.

SHOCKS AND CAVITIES FROM MULTIPLE OUTBURSTS IN THE GALAXY GROUP NGC 5813: A WINDOW TO ACTIVE GALACTIC NUCLEUS FEEDBACK

We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7 and M~1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from 1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~10^7 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the ICM and the central SMBH.

ON THE POPULATIONS OF RADIO GALAXIES WITH EXTENDED MORPHOLOGY ATz&lt; 0.3

Extended extragalactic radio sources have traditionally been classified into FR I and II types, based on the ratio r of the separation S between the brightest regions on either sides of the host galaxy and the total size T of the radio source. Here we examine the distribution of various physical properties as a function of r of 1040 luminous radio galaxies (RGs) at z<0.3 from the SDSS, NVSS, and FIRST. About 2/3 of the RGs are lobe-dominated (LD), and 1/3 have prominent jets. If we follow the original definition of the FR types (a division based solely on r), FR I and FR II RGs overlap in their host galaxy properties. However, the rare, LD sources with r>0.8 AND OIII5007 line luminosity >10^6 Lsun are markedly different on average from the rest of the RGs, in the sense that they are hosted in lower mass galaxies, live in relatively sparse environments, and have higher accretion rates onto the central SMBH. Thus these objects and the rest of RGs form a well-defined dichotomy. Motivated by the stark differences in the nuclear emission line properties of the RG subsamples, we suggest that the accretion rate onto the SMBH may play the primary role in creating the different morphologies. At relatively high accretion rates, the accretion system may produce powerful jets that create the "classical double" morphology (roughly corresponding to the LD sources with r>0.8 and emission lines); at lower accretion rates the jets from a radiatively inefficient accretion flow generate radio lobes without apparent "hot spots" at the edge (corresponding to the majority of LD sources). At slightly lower accretion rates AND in galaxies with dense galactic structure, sources with prominent jets result. It is possible that while the high accretion rate systems could affect sub-Mpc scale environments, the jets from lower accretion rate systems may efficiently suppress activity within the host galaxies.

FEEDBACK FROM CENTRAL BLACK HOLES IN ELLIPTICAL GALAXIES. III. MODELS WITH BOTH RADIATIVE AND MECHANICAL FEEDBACK

We find, from high-resolution hydro simulations, that winds from AGN effectively heat the inner parts (~100 pc) of elliptical galaxies, reducing infall to the central SMBH; and radiative (photoionization and X-ray) heating reduces cooling flows at the kpc scale. Including both types of feedback with (peak) efficiencies of 3 10^{-4} < epsilon_mech < 10^{-3} and of epsilon_rad ~10^{-1.3} respectively, produces systems having duty-cycles, central SMBH masses, X-ray luminosities, optical light profiles, and E+A spectra in accord with the broad suite of modern observations of massive elliptical systems. Our main conclusion is that mechanical feedback (including all three of energy, momentum and mass) is necessary but the efficiency, based on several independent arguments must be a factor of 10 lower than is commonly assumed. Bursts are frequent at z>1 and decline in frequency towards the present epoch as energy and metal rich gas are expelled from the galaxies into the surrounding medium. For a representative galaxy of final stellar mass ~3 10^{11} Msun, roughly 3 10^{10} Msun of recycled gas has been added to the ISM since z~2 and, of that, roughly 63% has been expelled from the galaxy, 19% has been converted into new metal rich stars in the central few hundred parsecs, and 2% has been added to the central SMBH, with the remaining 16% in the form hot X-ray emitting ISM. The bursts occupy a total time of ~170 Myr, which is roughly 1.4% of the available time. Of this time, the central SMBH would be seen as an UV or optical source for ~45% and ~71$% of the time, respectively. Restricting to the last 8.5 Gyr, the burst occupy ~44 Myr, corresponding to a fiducial duty-cycle of ~5 10^{-3}.

A VERY LARGE ARRAY RADIO SURVEY OF EARLY-TYPE GALAXIES IN THE VIRGO CLUSTER

We present the results of a 8.4 GHz Very Large Array radio survey of early-type galaxies extracted from the ACS Virgo Cluster Survey. The aim of this survey is to investigate the origin of radio emission in early-type galaxies and its link with the host properties in an unexplored territory toward the lowest levels of both radio and optical luminosities. Radio images, available for all 63 galaxies with BT < 14.4, show the presence of a compact radio source in 12 objects, with fluxes spanning from 0.13 to 2700 mJy. The remaining 51 galaxies, undetected at a flux limit of ~0.1 mJy, have radio luminosities L < 4 10E18 W/Hz . The fraction of radio-detected galaxies are a strong function of stellar mass, in agreement with previous results: none of the 30 galaxies with stellar mass M(star) < 1.7 10E10 M(sun) is detected, while 8 of the 11 most massive galaxies have radio cores. There appears to be no simple relation between the presence of a stellar nucleus and radio emission. A multiwavelength analysis of the active galactic nucleus (AGN) emission, combining radio and X-ray data, confirms the link between optical surface brightness profile and radio loudness in the sense that the bright core galaxies are associated with radio-loud AGNs, while non-core galaxies host radio-quiet AGNs. Not all radio-detected galaxies have a X-ray nuclear counter part (and vice-versa). A complete census of AGNs (and supermassive black holes, SMBHs) thus requires observations, at least, in both bands. Nonetheless, there are massive galaxies in the sample, expected to host a large SMBH (M(BH) ~ 10E8 M(sun)), whose nuclear emission eludes detection despite their proximity and the depth and the spatial resolution of the available observations. Most likely this is due to an extremely low level of accretion onto the central SMBH.

Evolution of supermassive black hole spins in the ΛCDM cosmology

Over the last years, several observations suggest that SMBHs likely reside at the centres of all spheroid galaxies. Even more interestingly, their properties seem to correlate with the bulge luminosity – or stellar velocity dispersion – and the bulge mass, suggesting a single mechanism for assembling BHs and forming spheroids in galaxies. We have been investigating the cosmological co-evolution of galaxies and their central SMBH in hierarchical models of galaxy formation, using the semi-analytic model developed in Durham University. We focus on the spin of the SMBHs and study how does it evolve during accretion of gas and mergers with other SMBHs. We conclude that the global SMBH spin distribution in the present universe may be bimodal, provided that the gas is fed into the hole through a self-gravity limited accretion disk.

FEEDBACK FROM CENTRAL BLACK HOLES IN ELLIPTICAL GALAXIES. I. MODELS WITH EITHER RADIATIVE OR MECHANICAL FEEDBACK BUT NOT BOTH

The importance of the radiative feedback from SMBHs at the centers of elliptical galaxies is not in doubt, given the well established relations among electromagnetic output, black hole mass and galaxy optical luminosity. In addition, feedback due to mechanical and thermal deposition of energy from jets and winds emitted by the accretion disk around the central SMBH is also expected to occur. In this paper we improve and extend the accretion and feedback physics explored in our previous papers to include also a physically motivated mechanical feedback. We study the evolution of an isolated elliptical galaxy with the aid of a high-resolution 1-D hydrodynamical code, where the cooling and heating functions include photoionization and Compton effects, and restricting to models which include only radiative or only mechanical feedback. We confirm that for Eddington ratios above 0.01 both the accretion and radiative output are forced by feedback effects to be in burst mode, so that strong intermittencies are expected at early times, while at low redshift the explored models are characterized by smooth, very sub-Eddington mass accretion rates punctuated by rare outbursts. However, the explored models always fail some observational tests. If we assume the high mechanical efficiency of 10^{-2.3}, we find that most of the gas is ejected from the galaxy, the resulting X-ray luminosity is far less than is typically observed and little SMBH growth occurs. But models with low enough mechanical efficiency to accomodate satisfactory SMBH growth tend to allow too strong cooling flows and leave galaxies at z=0 with E+A spectra more frequently than is observed. We conclude that both types of feedback are required. Models with combined feedback are explored in a forthcoming paper [abridged]

The effectiveness of the Kozai mechanism in the Galactic Centre

I examine the effectiveness of Kozai oscillations in the centres of galaxies and in particular the Galactic centre using standard techniques from celestial mechanics. In particular, I study the effects of a stellar bulge potential and general relativity on Kozai oscillations, which are induced by stellar discs. Lockmann et al (2008) recently suggested that Kozai oscillations induced by the two young massive stellar discs in the Galactic centre drives the orbits of the young stars to large eccentricity ($e\approx 1$). If some of these young eccentric stars are in binaries, they would be disrupted near pericentre, leaving one star in a tight orbit around the central SMBH and producing the S-star population. I find that the {\it spherical} stellar bulge suppresses Kozai oscillations, when its enclosed mass inside of a test body is of order the mass in the stellar disc(s). Since the stellar bulge in the Galactic centre is much larger than the stellar discs, Kozai oscillations {\it due to the stellar discs} are likely suppressed. Whether Kozai oscillations are induced from other nonspherical components to the potential (for instance, a flattened stellar bulge) is yet to be determined.