arXiv subjects

A. Merloni

Publications and source records attributed to A. Merloni.

At least 199 records · Page 11Linked to original sources

arXiv2008

Ongoing and co-evolving star formation in zCOSMOS galaxies hosting Active Galactic Nuclei

We present a study of the host galaxies of AGN selected from the zCOSMOS survey to establish if accretion onto supermassive black holes and star formation are explicitly linked up to z~1. We identify 152 galaxies that harbor AGN, based on XMM observations of 7543 galaxies (i<22.5). Star formation rates (SFRs), including those weighted by stellar mass, are determined using the [OII]3727 emission-line, corrected for an AGN contribution. We find that the majority of AGN hosts have significant levels of star formation with a distribution spanning ~1-100 Msun yr^-1. The close association between AGN activity and star formation is further substantiated by an increase in the AGN fraction with the youthfulness of their stars as indicated by the rest-frame color (U-V) and spectral index Dn(4000); we demonstrate that mass-selection alleviates an artifical peak falling in the transition region when using luminosity-limited samples. We also find that the SFRs of AGN hosts evolve with cosmic time in a manner that closely mirrors the overall galaxy population and explains the low SFRs in AGNs (z<0.3) from the SDSS. We conclude that the conditions most conducive for AGN activity are a massive host galaxy and a large reservoir of gas. Furthermore, a direct correlation between mass accretion rate onto SMBHs and SFR is shown to be weak although the average ratio is constant with redshift, effectively shifting the evidence for a co-evolution scenario in a statistical manner to smaller physical scales. Our findings illustrate an intermittent scenario with an AGN lifetime substantially shorter than that of star formation and underlying complexities regarding fueling over vastly different physical scales yet to be determined [Abridged].

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arXiv2008

Chasing highly obscured QSOs in the COSMOS field

(abridged) We take advantage of the deep Chandra and Spitzer coverage of a large area (more than 10 times the area covered by the Chandra deep fields, CDFs in the COSMOS field, to extend the search of highly obscured, Compton-thick active nuclei to higher luminosity. These sources have low surface density and large samples can be provided only through large area surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS sources with 24$μ$m fluxes higher than 550$μ$Jy. For the MIPS sources not directly detected in the Chandra images we produce stacked images in soft and hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS source population we compare the observed stacked count rates and hardness ratios to those predicted by detailed Monte Carlo simulations including both obscured AGN and star-forming galaxies. The density of lower luminosity Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7--1.2 is $(3.7\pm1.1) \times10^{-5}$ Mpc$^{-3}$, corresponding to $\sim67%$ of that of X-ray selected AGN. The comparison between the fraction of infrared selected, Compton thick AGN to the X-ray selected, unobscured and moderately obscured AGN at high and low luminosity suggests that Compton-thick AGN follow a luminosity dependence similar to that discovered for Compton-thin AGN, becoming relatively rarer at high luminosities. We estimate that the fraction of AGN (unobscured, moderately obscured and Compton thick) to the total MIPS source population is $49\pm10%$, a value significantly higher than that previously estimated at similar 24$μ$m fluxes. We discuss how our findings can constrain AGN feedback models.

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arXiv2008

High redshift quasars in the COSMOS survey: the space density of z>3 X-ray selected QSOs

We present a new measurement of the space density of high redshift (3.0 10^{-15} cgs), homogeneous sample of z>3 QSOs for which spectroscopic (22) or photometric (18) redshifts are available. We present the optical (color-color diagrams) and X-ray properties, the number counts and space densities of the z>3 X-ray selected quasars population and compare our findings with previous works and model predictions. We find that the optical properties of X-ray selected quasars are not significantly different from those of optically selected samples. There is evidence for substantial X-ray absorption (logN_H>23 cm^{-2}) in about 20% of the sources in the sample. The comoving space density of luminous (L_X >10^{44} erg s^-1) QSOs declines exponentially (by an e--folding per unit redshift) in the z=3.0-4.5 range, with a behavior similar to that observed for optically bright unobscured QSOs selected in large area optical surveys. Prospects for future, large and deep X-ray surveys are also discussed.

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arXiv2006

Cosmological evolution of the AGN Kinetic Luminosity Function

We present a first attempt to derive the cosmological evolution of the kinetic luminosity function of AGN based on the joint evolution of the flat spectrum radio and hard X-ray selected AGN luminosity functions. An empirical correlation between jet power and radio core luminosity is found, which is consistent with the theoretical assumption that, below a certain Eddington ratio, SMBH accrete in a radiatively inefficient way, while most of the energy output is in the form of kinetic energy. We show how the redshift evolution of the kinetic power density from such a low-mdot mode of accretion makes it a good candidate to explain the so-called ``radio mode'' of AGN feedback as postulated in many galaxy formation schemes.

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arXiv2006

On the limit cycle instability in magnetized accretion discs

Observational evidence accumulated over the past decade indicates that accretion discs in X-ray binaries are viscously stable unless they accrete very close to the Eddington limit. This is at odds with the most basic standard accretion disc theory, but could be explained by either having the discs to be much cooler whereby they are not radiation pressure dominated, or by a more sophisticated viscosity law. Here we argue that the latter is taking place in practice, on the basis of a stability analysis that assumes that the magneto-rotational-instability (MRI) responsible for generating the turbulent stresses inside the discs is also the source for a magnetically dominated corona. We show that observations of stable discs in the high/soft states of black hole binaries, on the one hand, and of the strongly variable microquasar GRS 1915+105 on the other, can all be explained if the magnetic turbulent stresses inside the disc scale proportionally to the geometric mean of gas and total pressure with a constant of proportionality (viscosity parameter) having a value of a few times 10^{-2}. Implications for bright AGN are also briefly discussed

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arXiv2006

On the X-ray spectra of luminous, inhomogeneous accretion flows

We discuss the expected X-ray spectral and variability properties of black hole accretion discs at high luminosity, under the hypothesis that radiation pressure dominated discs are subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two-phase structure. After deriving the full accretion disc solutions explicitly in terms of the parameters of the model, we study their radiative properties both with a simple two-zones model, treatable analytically, and with radiative transfer simulations which account simultaneously for energy balance and Comptonisation in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. We show that, if not only the density, but also the heating rate within these flows is inhomogeneous, then complex reflection-dominated spectra can be obtained for a high enough covering fraction of the cold phase. In general, large reflection components in the observed X-ray spectra should be associated with strong soft excesses, resulting from the combined emission of ionized atomic emission lines. The variability properties of such systems are such that, even when contributing to a large fraction of the hard X-ray spectrum, the reflection component is less variable than the power-law like emission originating from the hot Comptonising phase, in agreement with what is observed in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls within the family of those trying to explain the complex X-ray spectra of bright AGN with ionized reflection, but presents an alternative, specific, physically motivated, geometrical setup for the complex multi-phase structure of the inner regions of near-Eddington accretion flows.

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arXiv2006

A radio-emitting outflow in the quiescent state of A0620-00: implications for modelling low-luminosity black hole binaries

Deep observations with the Very Large Array of A0620-00, performed in 2005 August, resulted in the first detection of radio emission from a black hole binary at X-ray luminosities as low as 10^{-8.5} times the Eddington limit. The measured radio flux density, of 51 \pm 7 microJy at 8.5 GHz, is the lowest reported for an X-ray binary system so far, and is interpreted in terms of partially self-absorbed synchrotron emission from outflowing plasma. Making use of the estimated outer accretion rate of A0620-00 in quiescence, we demonstrate that the outflow kinetic power must be energetically comparable to the total accretion power associated with such rate, if it was to reach the black hole with the standard radiative efficiency of 10 per cent. This favours a model for quiescence in which a radiatively inefficient outflow accounts for a sizable fraction of the missing energy, and, in turn, substantially affects the overall dynamics of the accretion flow. Simultaneous observations in the X-ray band, with Chandra, confirm the validity of a non-linear radio/X-ray correlation for hard state black hole binaries down to low quiescent luminosities, thereby contradicting some theoretical expectations. Taking the mass term into account, the A0620-00 data lie on the extrapolation of the so called Fundamental Plane of black hole activity, which has thus been extended by more than 2 orders of magnitude in radio and X-ray luminosity. With the addition of the A0620-00 point, the plane relation provides an empirical proof for the scale-invariance of the jet-accretion coupling in accreting black holes over the entire parameter space observable with current instrumentation.

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arXiv2006

The parallel lives of supermassive black holes and their host galaxies

We compare all the available observational data on the redshift evolution of the total stellar mass and star formation rate density in the Universe with the mass and accretion rate density evolution of supermassive black holes, estimated from the hard X-ray selected luminosity function of quasars and active galactic nuclei. We find that on average black hole mass must have been higher at higher redshift for given spheroid stellar mass. Moreover, we find negative redshift evolution of the disk/irregulars to spheroid mass ratio. The total accretion efficiency is constrained to be between 0.06 and 0.12, depending on the exact value of the local SMBH mass density, and on the critical accretion rate below which radiatively inefficient accretion may take place.

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arXiv2006

Why the fundamental plane of black hole activity is not simply a distance driven artifact

The fundamental plane of black hole activity is a non-linear correlation among radio core luminosity, X-ray luminosity and mass of all accreting black holes, both of stellar mass and supermassive, found by Merloni, Heinz and Di Matteo (2003) and, independently, by Falcke, Koerding and Markoff (2004). Here we further examine a number of statistical issues related to this correlation. In particular, we discuss the issue of sample selection and quantify the bias introduced by the effect of distance in two of the correlated quantities. We demonstrate that the fundamental plane relation cannot be a distance artifact, and that its non-linearity must represent an intrinsic characteristic of accreting black holes. We also discuss possible future observational strategies to improve our understanding of this correlation.

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arXiv2004

Constraints on relativistic beaming from estimators of the unbeamed flux

We review the statistical properties of relativistic Doppler boosting relevant for studies of relativistic jets from compact objects based on radio--X-ray(--mass) correlations, such as that found in black-hole X-ray binaries in the low/hard state, or the ``fundamental plane'' of Merloni, Heinz, & DiMatteo. We show that the presence of only moderate scatter in such relations does not necessarily imply low Lorentz factors of the jets producing the radio emission in the samples under consideration. Applying Doppler beaming statistics to a large sample of XRBs and AGN, we derive a limit on the width of the Lorentz factor distribution of black holes with relativistic jets: If the X-rays are unbeamed (e.g., if they originate in the accretion disk or in the slower, innermost part of the jet), the width of the βΓdistribution should be about one order of magnitude or less. If the scatter about the ``fundamental plane'' is entirely dominated by relativistic beaming, a lower limit on the mean Lorentz factor <βΓ> > 5 can be derived. On the other hand, if the X-rays are boosted by the same factor as the radio emission, we show that the observed scatter cannot be reasonably explained by Doppler boosting alone.

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arXiv2003

Iron K alpha line profiles and the inner boundary condition of accretion flows

Recent X-ray observations have shown evidence for exceptionally broad and skewed iron K alpha emission lines from several accreting black hole systems. The lines are assumed to be due to fluorescence of the accretion disk illuminated by a surrounding corona and require a steep emissivity profile increasing in to the innermost radius. This appears to question both standard accretion disc theory and the zero torque assumption for the inner boundary condition, both of which predict a much less extreme profile. Instead it argues that a torque may be present due to magnetic coupling with matter in the plunging region or even to the spinning black hole itself. Discussion so far has centered on the torque acting on the disc. However the crucial determinant of the iron line profile is the radial variation of the power radiated in the corona. Here we study the effects of different inner boundary conditions on the coronal emissivity and on the profiles of the observable Fe K alpha lines. We argue that in the extreme case where a prominent highly redshifted component of the iron line is detected, requiring a steep emissivity profile in the innermost part and a flatter one outside, energy from the gas plunging into the black hole is being fed directly to the corona.

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arXiv2003

Beyond the standard accretion disc model: coupled magnetic disc--corona solutions with a physically motivated viscosity law

We present a systematic, analytical study of geometrically thin, optically thick accretion disc solutions for magnetized turbulent flows, with an alpha-like viscosity prescription. Under the only assumptions that (1) Magneto-Rotational instability (MRI) generates the turbulence that produces the anomalous viscosity needed for accretion to proceed, and that (2) the magnetic field amplified by the instability saturates due to buoyant vertical escape, we are able to self-consistently solve the disc structure equations including the fraction of power, f, that is carried off by vertical Poynting flux (and likely dissipated outside the optically thick disc). For low-viscosity discs, we obtain stable high-f solutions at low accretion rates, when gas pressure dominates, and unstable, low-f, radiation pressure dominated solutions at high accretion rates. For high viscosity discs, instead, a new thermally and viscously stable, radiation pressure dominated solution is found, characterized by f~1 and appearing only above a critical accretion rate (of the order of few tenths of the Eddington one). We discuss the regimes of validity of our assumptions, and the astrophysical relevance of our solutions. We conclude that our newly discovered thin disc solutions, possibly accompanied by powerful, magnetically dominated coronae and outflows, should be seriously considered as models for black holes accreting at super-Eddington rates.

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arXiv2002

Modes of disc accretion onto black holes

I review our current theoretical understanding of the different possible rotating modes of accretion onto a black hole. I discuss both thick adiabatic flows and radiatively efficient thin disc solutions. I present a new self-consistent unified analytical description of two-phase thin disc--corona systems in which magneto-rotational instability is responsible for angular momentum transport in the disc and for the corona generation. Finally, I briefly discuss the role of magnetic fields in bridging the gap between accretion discs theory and jet production mechanisms

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arXiv2002

Microquasars in the low/hard state: strong coronae, compact jets, and the high frequency variability

We apply a model of magnetically dominated coronae above standard accretion discs to the low/hard state of galactic black holes. When the disc-corona coupling is accounted for self-consistently assuming that magneto-rotational instability is at work in the disc, and that the corona is generated by buoyant escape of disc magnetic structures, then the model predicts powerful, X-ray emitting coronae at low accretion rates. A main consequence is discussed: the possibility that the corona itself is the launching site of powerful, MHD driven jets/outflows. This depends crucially of the coronal scaleheight. Finally, we present the first radial profiles of a corona a different accretion rates, and discuss their implications for high frequency variability.

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arXiv2002

How the X-ray spectrum of a Narrow-Line Seyfert 1 galaxy may be reflection dominated

A model for the inner regions of accretion flows is presented where, due to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure dominated, and so may apply to Narrow-Line Seyfert~1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H 0707-495 between 0.5 and 11 keV with reflection dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same Γ=2.35 power-law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in Γ> 2 and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG-6-30-15 Fe K line. The idea needs to be tested with further broadband XMM-Newton observations of NLS1s.

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arXiv2002

Magnetic flares and outflows from structured accretion disc coronae

We present a model for magnetic structured coronae above accretion discs. On the shortest timescales, spatially and temporally correlated coronal flares can explain X-ray temporal and spectral variability observed in Seyfert galaxies. In particular, power density spectra, flux-spectral index and flux-variance correlations are naturally accounted for by the model. More dramatic spectral variations (i.e. state transitions in GBHC) are associated with parameters varying on longer timescales, such as accretion rate, coronal strength or geometry of the inner disc. In the framework of the standard Shakura--Sunyaev accretion disc theory, here we discuss why energetically dominant coronae at low accretion rates are ideal sites for launching powerful MHD driven outflows. Then, if the outflow is radiatively inefficient, then so is the source overall, even without advection being relevant for the dynamics of the accretion flow. This could be an alternative scenario for LLAGN and GBHC in their low/hard state, and may have consequences for our understanding of the accretion history of the universe.

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arXiv2001

Coronal outflow dominated accretion discs: a new possibility for low luminosity black holes?

The spectral energy distributions of galactic black holes in the low/hard state and of low-luminosity AGN possess many common features, the most prominent being: compact, flat (or inverted) spectrum radio cores with high brightness temperatures; excess red and infrared emission, often correlated with the radio flux; an extremely weak (or absent) quasi-thermal hump and a hard X-ray power-law with high energy cut-off. These sources are thought to be accreting at low rates and advection (or convection) dominated accretion flows are usually considered the best candidates to explain them. Here we present an alternative possibility, involving strong, unbound, magnetic coronae generated by geometrically thin, optically thick accretion discs at low accretion rates. First we show that, if angular momentum transport in the disc is due to magnetic turbulent stresses, the magnetic energy density and effective viscous stresses inside the disc are proportional to the geometric mean of the total (gas plus radiation) and gas pressure. Therefore the corona is less powerful in a radiation pressure dominated disc, and the relative fraction of the power liberated in the corona increases as the accretion rate decreases. Furthermore, we discuss reasons why energetically dominant coronae are ideal sites for launching powerful jets/outflows, both MHD and thermally driven. In analysing the spectral properties of such coronal outflow dominated accretion discs, we reach the important conclusion that if the jet/outflow is, as is likely, radiatively inefficient, then so is the source overall, even without advection of energy into the black hole being relevant for the dynamics of the accretion flow.

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arXiv2001

Thunderclouds and accretion discs: a model for the spectral and temporal variability of Seyfert 1 galaxies

X-ray observations of Seyfert 1 galaxies offer the unique possibility of observing spectral variability on timescales comparable to the dynamical time of the inner accretion flow. They typically show highly variable lightcurves, with Power Density Spectra characterized by `red noise' and a break at low frequencies. Time resolved spectral analysis have established that spectral variability on the shortest timescales is important in all these sources, with the spectra getting softer at high fluxes. Here we present a model that is able to explain a number of the above mentioned properties in terms of magnetic flares shining above a standard accretion disc and producing the X-ray spectrum via inverse Compton scattering soft photons (both intrinsic and reprocessed thermal emission from the accretion disc and locally produced synchrotron radiation). We show that the fundamental heating event, likely caused by magnetic reconnection, must be compact, with typical size comparable to the accretion disc thickness and must be triggered at a height at least an order of magnitude larger than its size; the spatial and temporal distribution of flares are not random: the heating of the corona proceeds in correlated trains of events in an avalanche fashion. The amplitude of the avalanches obeys a power-law distribution and determines the size of the active regions where the spectrum is produced. With our model we simulate X-ray lightcurves that reproduce the main observational properties of the Power Density Spectra and of the X-ray continuum short-term variability of Seyfert 1 galaxies. By comparing them with observations of MGC--6-30-15, we are able to infer that the corona in this source must have a large optical depth (tau >1.5) and small average covering fraction.

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