arXiv subjects

M. Dotti

Publications and source records attributed to M. Dotti.

At least 37 records · Page 2Linked to original sources

arXiv2012

On the orientation and magnitude of the black hole spin in galactic nuclei

Massive black holes in galactic nuclei vary their mass M and spin vector J due to accretion. In this study we relax, for the first time, the assumption that accretion can be either chaotic, i.e. when the accretion episodes are randomly and isotropically oriented, or coherent, i.e. when they occur all in a preferred plane. Instead, we consider different degrees of anisotropy in the fueling, never confining to accretion events on a fixed direction. We follow the black hole growth evolving contemporarily mass, spin modulus a and spin direction. We discover the occurrence of two regimes. An early phase (M <~ 10 million solar masses) in which rapid alignment of the black hole spin direction to the disk angular momentum in each single episode leads to erratic changes in the black hole spin orientation and at the same time to large spins (a ~ 0.8). A second phase starts when the black hole mass increases above >~ 10 million solar masses and the accretion disks carry less mass and angular momentum relatively to the hole. In the absence of a preferential direction the black holes tend to spin-down in this phase. However, when a modest degree of anisotropy in the fueling process (still far from being coherent) is present, the black hole spin can increase up to a ~ 1 for very massive black holes (M >~ 100 million solar masses), and its direction is stable over the many accretion cycles. We discuss the implications that our results have in the realm of the observations of black hole spin and jet orientations.

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arXiv2012

Search of sub-parsec massive binary black holes through line diagnosis II

Massive black hole binaries at sub-parsec separations may display in their spectra anomalously small flux ratios between the MgII and CIV broad emission lines, i.e. F_MgII/F_CIV <~ 0.1, due to the erosion of the broad line region around the active, secondary black hole, by the tidal field of the primary. In Paper I by Montuori et al. (2011), we focussed on broad lines emitted by gas bound to the lighter accreting member of a binary when the binary is at the center of a hollow density region (the gap) inside a circum-binary disc. The main aim of this new study is at exploring the potential contribution to the broad line emission by the circum-binary disc and by gaseous streams flowing toward the black hole through the gap. We carry out a post-process analysis of data extracted from a SPH simulation of a circum-binary disc around a black hole binary. Our main result is that the MgII to CIV flux ratio can be reduced to ~ 0.1 within an interval of sub-pc binary separations of the order of a ~ (0.01-0.2)(f_Edd/0.1)^(1/2) pc corresponding to orbital periods of ~ (20-200) (f_Edd/0.1)^(3/4) years for a secondary BH mass in the range M_2 ~ 10^7-10^9 M_sun and a binary mass ratio of 0.3. At even closer separations this ratio returns to increase to values that are indistinguishable from the case of a single AGN (typically F_MgII/F_CIV ~ 0.3-0.4) because of the contribution to the MgII line from gas in the circum-binary disc.

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arXiv2012

B2 0954+25A: a typical Fermi blazar or a gamma-loud Narrow Line Seyfert 1

B2 0954+25A, detected by the {\it Fermi} satellite, is a blazar with interesting observational properties: it has been observed to transit from a jet dominated to a disk dominated state; its radio spectrum appears flat at all observing frequencies (down to 74 MHz); optically, the H$β$ line profile is asymmetric. The flatness of radio spectrum suggests that the isotropic emission from radio lobes is very weak, despite the large size of its jet ($\gtrsim$ 500 kpc). Its broad--band spectral energy distribution is surprisingly similar to that of the prototypical $γ$--ray, radio loud, Narrow Line Seyfert 1 ($γ$--NLS1) galaxy PMN J0948+0022. In this work we revisit the mass estimates of B2 0954+25A considering only the symmetric component of the H$β$ line and find (1--3) $\times 10^8$ M$_{\sun}$. In light of our composite analysis, we propose to classify the source as a transition object between the class of Flat Spectrum Radio Quasar and $γ$--ray, radio loud NLS1. A comparison with two members of each class (3C 273 and PMN J0948+0022) is discussed.

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arXiv2011

Massive black hole binaries: dynamical evolution and observational signatures

The study of the dynamical evolution of massive black hole pairs in mergers is crucial in the context of a hierarchical galaxy formation scenario. The timescales for the formation and the coalescence of black hole binaries are still poorly constrained, resulting in large uncertainties in the expected rate of massive black hole binaries detectable in the electromagnetic and gravitational wave spectra. Here we review the current theoretical understanding of the black hole pairing in galaxy mergers, with a particular attention to recent developments and open issues. We conclude with a review of the expected observational signatures of massive binaries, and of the candidates discussed in literature to date.

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arXiv2011

Multimessenger astronomy with pulsar timing and X-ray observations of massive black hole binaries

We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1) black hole binaries (MBHBs), which are primary targets for ongoing and upcoming pulsar timing arrays (PTAs), are particularly appealing multimessenger carriers. According to current models for massive black hole formation and evolution, the planned Square Kilometer Array (SKA) will collect gravitational wave signals from thousands of such massive systems, being able to individually resolve and locate in the sky several of them (maybe up to a hundred). By employing a standard model for the evolution of MBHBs in circumbinary discs, with the aid of dedicated numerical simulations, we characterize the gas-binary interplay, identifying possible electromagnetic signatures of the PTA sources. We concentrate our investigation on two particularly promising scenarios in the high energy domain, namely, the detection of X-ray periodic variability and of double broad Kαiron lines. Up to several hundreds of periodic X-ray sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming X-ray observatories. Double relativistic Kαlines may be observable in a handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as Athena. (Abridged)

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arXiv2011

BH masses in NLS1: the role of the broad-line region geometry

Narrow Line Seyfert 1 galaxies (NLS1) are generally believed to host "under-massive" black holes with respect to the predictions from the host galaxy -- black hole mass scale relations. Black hole masses in NLS1 are estimated from the continuum luminosity and the width of broad emission lines. Here we show that the "mass deficit" can be canceled out if we assume that the broad line region (BLR) in type-1 AGN has a flat geometry, which is seen face-on in NLS1. The detection of relativistic jets aligned along the line of sight in a number of NLS1 supports this picture. Moreover, a flat geometry of the BLR is also suggested by a general trend of the mass deficit as a function of the line width observed in other type-1 AGN, from quasars to BL Lac objects, and is consistent with a simple extension of the Unified Model of AGN to the BLR geometry.

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arXiv2011

A systematic search for massive black hole binaries in SDSS spectroscopic sample

We present the results of a systematic search for massive black hole binaries in the Sloan Digital Sky Survey spectroscopic database. We focus on bound binaries, under the assumption that one of the black holes is active. In this framework, the broad lines associated to the accreting black hole are expected to show systematic velocity shifts with respect to the narrow lines, which trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929 galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture of two quasars at two different redshifts. The spectral model is a data-driven dimensionality reduction of the SDSS quasar spectra based on a matrix factorization. We identified 32 objects with peculiar spectra. Nine of them can be interpreted as black hole binaries. This doubles the number of known black hole binary candidates. We also report on the discovery of a new class of extreme double-peaked emitters with exceptionally broad and faint Balmer lines. For all the interesting sources, we present detailed analysis of the spectra, and discuss possible interpretations.

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arXiv2010

Search of sub-parsec massive binary black holes through line diagnosis

We investigate on the spectral properties of an active black hole, member of a massive (10^7 - 10^9 Msun) sub-parsec black hole binary. We work under the hypothesis that the binary, surrounded by a circum-binary disc, has cleared a gap, and that accretion occurs onto the secondary black hole fed by material closer to the inner edge of the disc. Broad line emission clouds orbit around the active black hole and suffer erosion due to tidal truncation at the Roche Lobe surface, following gap opening and orbital decay. We consider three of the most prominent broad emission lines observed in the spectra of AGNs, i.e. CIV, MgII and Hβ, and compute the flux ratios between the lines of MgII and CIV (FMgII/FCIV) and those of MgII and Hβ (FMgII/FHβ). We find that close black hole binaries have FMgII/FCIV up to one order of magnitude smaller than single black holes. By contrast FMgII/FHβ may be significantly reduced only at the shortest separations. Peculiarly low values of line flux ratios together with large velocity offsets between the broad and narrow emission lines and/or periodic variability in the continuum (on timescales >= years) would identify genuine sub-pc binary candidates.

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arXiv2010

A path to radio-loudness through gas-poor galaxy mergers and the role of retrograde accretion

In this proceeding we explore a pathway to radio-loudness under the hypothesis that retrograde accretion onto giant spinning black holes leads to the launch of powerful jets, as seen in radio loud QSOs and recently in LAT/Fermi and BAT/Swift Blazars. Counter-rotation of the accretion disc relative to the BH spin is here associated to gas-poor galaxy mergers progenitors of giant (missing-light) ellipticals. The occurrence of retrograde accretion enters as unifying element that may account for the radio-loudness/galaxy morphology dichotomy observed in AGN.

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arXiv2010

A Census of Nuclear Stellar Disks in Early-type Galaxies

Nuclear Stellar Disks (NSDs), of a few tens to hundreds of parsec across, are a common and yet poorly studied feature of early-type galaxies. Still, such small disks represent a powerful tool to constrain the assembling history of galaxies, since they can be used to trace to the epoch when galaxies experienced their last major merger event. By studying the fraction and stellar age of NSDs it is thus possible to test the predictions for the assembly history of early-type galaxies according the current hierarchical paradigm for galaxy formation. In this paper we have produced the most comprehensive census of NSDs in nearby early-type galaxies by searching for such disks in objects within 100 Mpc and by using archival images from the Hubble Space Telescope. We found that NSDs are present in approximately 20% of early-type galaxies, and that the fraction of galaxies with NSDs does not depend on their Hubble type nor on their galactic environment, whereas the incidence of NSDs appears to decline in the most massive systems. Furthermore, we have separated the light contribution of twelve such disks from that of their surrounding stellar bulge in order to extract their physical properties. This doubles the number of decomposed NSDs and although the derived values for their central surface brightness and scale-length are consistent with previous studies they also give a hint of possible different characteristics due to different formation scenario between nuclear disks and other kinds of large galactic disks.

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arXiv2009

AGN pairs: chance superpositions or black hole binaries?

Several active galactic nuclei (AGN) with multiple sets of emission lines separated by over 2000 km/s have been observed recently. These have been interpreted as being due to massive black hole (MBH) recoil following a black hole merger, MBH binaries, or chance superpositions of AGN in galaxy clusters. Moreover, a number of double-peaked AGN with velocity offsets of ~ a few 100 km/s have also been detected and interpreted as being due to the internal kinematics of the narrow line regions or MBH binary systems. Here we reexamine the superposition model. Using the Millennium Run we estimate the total number of detectable AGN pairs as a function of the emission line offset. We show that AGN pairs with high velocity line separations up to ~2000 km/s are very likely to be chance superpositions of two AGN in clusters of galaxies for reasonable assumptions about the relative fraction of AGN. No superimposed AGN pairs are predicted for velocity offsets in excess of ~3000 km/s as the required AGN fractions would violate observational constraints. The high velocity AGN pair numbers predicted here are competitive with those predicted from the models relying on MBH recoil or MBH binaries. However, the model fails to account for the largest emission line velocity offsets that require the presence of MBH binaries.

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arXiv2009

Dual black holes in merger remnants. II: spin evolution and gravitational recoil

Using high resolution hydrodynamical simulations, we explore the spin evolution of massive dual black holes orbiting inside a circumnuclear disc, relic of a gas-rich galaxy merger. The black holes spiral inwards from initially eccentric co or counter-rotating coplanar orbits relative to the disc's rotation, and accrete gas that is carrying a net angular momentum. As the black hole mass grows, its spin changes in strength and direction due to its gravito-magnetic coupling with the small-scale accretion disc. We find that the black hole spins loose memory of their initial orientation, as accretion torques suffice to align the spins with the angular momentum of their orbit on a short timescale (<1-2 Myr). A residual off-set in the spin direction relative to the orbital angular momentum remains, at the level of <10 degrees for the case of a cold disc, and <30 degrees for a warmer disc. Alignment in a cooler disc is more effective due to the higher coherence of the accretion flow near each black hole that reflects the large-scale coherence of the disc's rotation. If the massive black holes coalesce preserving the spin directions set after formation of a Keplerian binary, the relic black hole resulting from their coalescence receives a relatively small gravitational recoil. The distribution of recoil velocities inferred from a simulated sample of massive black hole binaries has median <70 km/s much smaller than the median resulting from an isotropic distribution of spins.

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arXiv2009

Sub-parsec supermassive Binary Quasars: expectations at z<1

We investigate the theoretical expectations for detections of supermassive binary black holes that can be identified as sub-parsec luminous quasars. To-date, only two candidates have been selected in a sample comprising 17,500 sources selected from the Sloan Digital Sky Survey (SDSS) Quasar Catalog at z<0.70 (Boroson & Lauer 2009) In this Letter, we use models of assembly and growth of supermassive black holes (SMBHs) in hierarchical cosmologies to study the statistics and observability of binary quasars at sub-parsec separations. Our goal is twofold: (1) test if such a scarce number of binaries is consistent with theoretical prediction of SMBH merger rates, and (2) provide additional predictions at higher redshifts, and at lower flux levels. We determine the cumulative number of expected binaries in a complete, volume limited sample. Motivated by Boroson & Lauer (2009), we apply the SDSS Quasar luminosity cut (M_i<-22) to our theoretical sample, deriving an upper limit to the observable binary fraction. We find that sub-parsec quasar binaries are intrinsically rare. Our best models predict ~0.01 deg^-2 sub-parsec binary quasars with separations below ~10^4 Schwarzschild radii (v_orb>2000 km/s) at z<0.7, which represent a fraction ~6x10^-4 of unabsorbed quasars in our theoretical sample. In a complete sample of ~10,000 sources, we therefore predict an upper limit of ~10 sub-parsec binary quasars. The number of binaries increases rapidly with increasing redshift. The decreasing lifetime with SMBH binary mass suggests that lowering the luminosity threshold does not lead to a significant increase in the number of detectable sub-parsec binary quasars.

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arXiv2009

Probing the nature of the massive black hole binary candidate SDSS J1536+0441

We present an imaging study of the black hole binary candidate SDSS J1536+0441 (z=0.3893), based on deep, high resolution VzK images collected at the ESO/VLT. The images clearly show an asymmetric elongation, indicating the presence of a companion source at ~1" (~5 kpc projected distance) East from the quasar. The host galaxy of the quasar is marginally resolved. We find that the companion source is a luminous galaxy, the light profile of which suggests the presence of an unresolved, faint nucleus (either an obscured AGN or a compact stellar bulge). The study of the environment around the quasar indicates the occurrence of a significant over-density of galaxies with a redshift compatible with z~0.4. This suggests that it resides in a moderately rich cluster of galaxies.

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arXiv2009

Mass and spin coevolution during the alignment of a black hole in a warped accretion disc

In this paper, we explore the gravitomagnetic interaction of a black hole (BH) with a misaligned accretion disc to study BH spin precession and alignment jointly with BH mass and spin parameter evolution, under the assumption that the disc is continually fed, in its outer region, by matter with angular momentum fixed on a given direction. We develop an iterative scheme based on the adiabatic approximation to study the BH-disc coevolution: in this approach, the accretion disc transits through a sequence of quasi-steady warped states (Bardeen-Petterson effect) and interacts with the BH until the BH spin aligns with the outer angular momentum direction. For a BH aligning with a co-rotating disc, the fractional increase in mass is typically less than a few percent, while the spin modulus can increase up to a few tens of percent. The alignment timescale is between ~ 100 thousands and ~ 1 millions years for a maximally rotating BH accreting at the Eddington rate. BH-disc alignment from an initially counter-rotating disc tends to be more efficient compared to the specular co-rotating case due to the asymmetry seeded in the Kerr metric: counter-rotating matter carries a larger and opposite angular momentum when crossing the innermost stable orbit, so that the spin modulus decreases faster and so the relative inclination angle.

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arXiv2009

SDSSJ092712.65+294344.0: a candidate massive black hole binary

In this Letter we explore the hypothesis that the quasar SDSSJ092712.65+294344.0 is hosting a massive black hole binary embedded in a circumbinary disc. The lightest, secondary black hole is active, and gas orbiting around it is responsible for the blue-shifted broad emission lines with velocity off-set of 2650 km/s, relative to the galaxy rest frame. As the tidal interaction of the binary with the outer disc is expected to excavate a gap, the blue-shifted narrow emission lines are consistent with being emitted from the low-density inhomogeneous gas of the hollow region. From the observations we infer a binary mass ratio q ~ 0.3, a mass for the primary of M1 ~ 2 billion Msun and a semi-major axis of 0.34 pc, corresponding to an orbital period of 370 years. We use the results of cosmological merger trees to estimate the likely-hood of observing SDSSJ092712.65+294344.0 as recoiling black hole or as a binary. We find that the binary hypothesis is preferred being one hundred times more probable than the ejection hypothesis. If SDSSJ092712.65+294344.0 hosts a binary, it would be the one closest massive black hole binary system ever discovered.

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arXiv2009

Massive Binary Black Holes in the Cosmic Landscape

Binary black holes occupy a special place in our quest for understanding the evolution of galaxies along cosmic history. If massive black holes grow at the center of (pre-)galactic structures that experience a sequence of merger episodes, then dual black holes form as inescapable outcome of galaxy assembly. But, if the black holes reach coalescence, then they become the loudest sources of gravitational waves ever in the universe. Nature seems to provide a pathway for the formation of these exotic binaries, and a number of key questions need to be addressed: How do massive black holes pair in a merger? Depending on the properties of the underlying galaxies, do black holes always form a close Keplerian binary? If a binary forms, does hardening proceed down to the domain controlled by gravitational wave back reaction? What is the role played by gas and/or stars in braking the black holes, and on which timescale does coalescence occur? Can the black holes accrete on flight and shine during their pathway to coalescence? N-Body/hydrodynamical codes have proven to be vital tools for studying their evolution, and progress in this field is expected to grow rapidly in the effort to describe, in full realism, the physics of stars and gas around the black holes, starting from the cosmological large scale of a merger. If detected in the new window provided by the upcoming gravitational wave experiments, binary black holes will provide a deep view into the process of hierarchical clustering which is at the heart of the current paradigm of galaxy formation. They will also be exquisite probes for testing General Relativity, as the theory of gravity. The waveforms emitted during the inspiral, coalescence and ring-down phase carry in their shape the sign of a dynamically evolving space-time and the proof of the existence of an horizon.

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arXiv2009

A photometric study of the field around the candidate recoiling/binary black hole SDSS J092712.65+294344.0

We present a photometric FUV to Ks-band study of the field around quasar SDSS J092712.65+294344.0. The SDSS spectrum of this object shows various emission lines with two distinct redshifts, at z=0.699 and z=0.712. Because of this peculiar spectroscopic feature this source has been proposed as a candidate recoiling or binary black hole. A third alternative model involves two galaxies moving in the centre of a rich galaxy cluster. Here we present a study addressing the possible presence of such a rich cluster of galaxies in the SDSS J092712.65+294344.0 field. We observed the 3.6x2.6 square arcmin field in the Ks-band and matched the NIR data with the FUV and NUV images in the GALEX archive and the ugriz observations in the SDSS. From various colour-colour diagrams we were able to classify the nature of 32 sources, only 6-11 of which have colours consistent with galaxies at z~0.7. We compare these numbers with the surface density of galaxies, stars & quasars, and the expectations for typical galaxy clusters both at low and high redshift. Our study shows that the galaxy cluster scenario is in clear disagreement with the new observations.

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