Tiziana Di Matteo

Energy input from quasars regulates the growth and activity of black holes and their host galaxies.

In the early Universe, while galaxies were still forming, black holes as massive as a billion solar masses powered quasars. Supermassive black holes are found at the centres of most galaxies today, where their masses are related to the velocity dispersions of stars in their host galaxies and hence to the mass of the central bulge of the galaxy. This suggests a link between the growth of the black holes and their host galaxies, which has indeed been assumed for a number of years. But the origin of the observed relation between black hole mass and stellar velocity dispersion, and its connection with the evolution of galaxies, have remained unclear. Here we report simulations that simultaneously follow star formation and the growth of black holes during galaxy–galaxy collisions. We find that, in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar. The energy released by the quasar expels enough gas to quench both star formation and further black hole growth. This determines the lifetime of the quasar phase (approaching 100u2009million years) and explains the relationship between the black hole mass and the stellar velocity dispersion.

Modelling feedback from stars and black holes in galaxy mergers

We describe techniques for incorporating feedback from star formation and black hole (BH) accretion into simulations of isolated and merging galaxies. At present, the details of these processes cannot be resolved in simulations on galactic scales. Our basic approach therefore involves forming coarse-grained representations of the properties of the interstellar medium (ISM) and BH accretion starting from basic physical assumptions, so that the impact of these effects can be included on resolved scales. We illustrate our method using a multiphase description of star-forming gas. Feedback from star formation pressurizes highly overdense gas, altering its effective equation of state (EOS). We show that this allows the construction of stable galaxy models with much larger gas fractions than possible in earlier numerical work. We extend the model by including a treatment of gas accretion onto central supermassive BHs in galaxies. Assuming thermal coupling of a small fraction of the bolometric luminosity of accreting BHs to the surrounding gas, we show how this feedback regulates the growth of BHs. In gas-rich mergers of galaxies, we observe a complex interplay between starbursts and central active galactic nuclei (AGN) activity when the tidal interaction triggers intense nuclear inflows of gas. Once an accreting supermassive BH has grown to a critical size, feedback terminates its further growth and expels gas from the central region in a powerful quasar-driven wind. Our simulation methodology is therefore able to address the coupled processes of gas dynamics, star formation and BH accretion during the formation of galaxies.

A Fundamental Plane of black hole activity

We examine the disc-jet connection in stellar mass and supermassive black holes by investigating the properties of their compact emission in the X-ray and radio bands. We compile a sample of ∼100 active galactic nuclei with measured masses, 5-GHz core emission, and 2-10 keV luminosities, together with eight galactic black holes with a total of ∼50 simultaneous observations in the radio and X-ray bands. Using this sample, we study the correlations between the radio (L R ) and the X-ray (L X ) luminosity and the black hole mass (M). We find that the radio luminosity is correlated with both M and L X , at a highly significant level. In particular, we show that the sources define a Fundamental Plane in the three-dimensional (log L R , log L X , log M) space, given by log L R = (0.60 + 0 . 1 1 -0.11) log L X + (0.78 + 0 . 1 1 -0.09) log M + 7.33 + 4 . 0 5 -4.07, with a substantial scatter of σ R = 0.88. We compare our results to the theoretical relations between radio flux, black hole mass, and accretion rate derived by Heinz & Sunyaev. Such relations depend only on the assumed accretion model and on the observed radio spectral index. Therefore, we are able to show that the X-ray emission from black holes accreting at less than a few per cent of the Eddington rate is unlikely to be produced by radiatively efficient accretion, and is marginally consistent with optically thin synchrotron emission from the jet. On the other hand, models for radiatively inefficient accretion flows seem to agree well with the data.

Black Holes in Galaxy Mergers: Evolution of Quasars

Basedonnumericalsimulationsofgas-richgalaxymergers,wediscussamodelinwhichquasaractivityistiedto the self-regulated growth of supermassive black holes in galaxies. The nuclear inflow of gas attending a galaxy collisiontriggersastarburstandfeedsblackholegrowth,butformostofthedurationofthestarburst,theblackhole is ‘‘buried,’’ being heavily obscured by surrounding gas and dust, limiting the visibility of the quasar, especially at optical and ultraviolet wavelengths. As the black hole grows, feedback energy from accretion heats the gas and eventuallyexpelsitinapowerfulwind, leaving behinda‘‘deadquasar.’’Betweenthe buried anddeadphases, there is a window in time during which the galaxy would be seen as a luminous quasar. Because the black hole mass, radiative output, and distribution of obscuring gas and dust all evolve strongly with time, the duration of this phase of observable quasar activity depends on both the waveband and imposed luminosity threshold. We determine the observed and intrinsic lifetimes as a function of luminosity and frequency, and calculate observable lifetimes � 10 Myr for bright quasars in the optical B band, in good agreement with empirical estimates and much smaller than our estimated black hole growth timescales � 100 Myr, naturally producing a substantial population of buried quasars.However,theobservedandintrinsicenergyoutputsconvergeintheIRandhardX-raybandsasattenuation becomes weaker and chances of observation greatly increase. We also obtain the distribution of column densities along sight lines in which the quasar is seen above a given luminosity, and find that our result agrees remarkably well with observed estimates of the column density distribution from the SDSS for the appropriate luminosity thresholds.Ourmodelreproducesawiderangeofquasarphenomena,includingobservedquasarlifetimes,intrinsic lifetimes, column density distributions, and differences between optical and X-ray samples, having properties consistent with observations across more than 5 orders of magnitude in bolometric luminosity from 10 9 to 10 14 L� (� 17PMB P� 30). Subject headingg cosmology: theory — galaxies: active — galaxies: evolution — galaxies: nuclei — quasars: general

Black holes in galaxy mergers: the formation of red elliptical galaxies

We use hydrodynamical simulations to study the color transformations induced by star formation and active galactic nuclei (AGNs) during major mergers of spiral galaxies. Our modeling accounts for radiative cooling, star formation, and supernova feedback. Moreover, we include a treatment of accretion onto supermassive black holes embedded in the nuclei of the merging galaxies. We assume that a small fraction of the bolometric luminosity of an accreting black hole couples thermally to surrounding gas, providing a feedback mechanism that regulates its growth. The encounter and coalescence of the galaxies triggers nuclear gas inflow, which fuels both a powerful starburst and strong black hole accretion. Comparing simulations with and without black holes, we show that AGN feedback can quench star formation and accretion on a short timescale, particularly in large galaxies where the black holes can drive powerful winds once they become sufficiently massive. The color evolution of the remnant differs markedly between mergers with and without central black holes. Without AGNs, gas-rich mergers lead to elliptical galaxies that remain blue owing to residual star formation, even after more than 7 Gyr have elapsed. In contrast, mergers with black holes produce elliptical galaxies that redden much faster, an effect that is more pronounced in massive remnants where a nearly complete termination of star formation occurs, allowing them to redden to u - r 2.3 in less than 1 Gyr. AGN feedback may thus be required to explain the population of extremely red massive early-type galaxies, and it appears to be an important driver in generating the observed bimodal color distribution of galaxies in the local universe.

A physical model for the origin of quasar lifetimes

We propose a model of quasar lifetimes in which longer periods of rapid black hole accretion are distinguished from observationally inferred lifetimes of quasars owing to gas and dust obscuration. In our picture, quasars are powered by gas funneled to galaxy centers, fueling starbursts and feeding black hole growth, but are buried until feedback from the accretion disperses the obscuring material, creating a window in which the black hole is observable as an optical quasar. Eventually, the activity ceases when the accretion rate drops below that required to maintain quasar luminosities. We study this process by simulating galaxy mergers, using the gas density to infer the bolometric luminosity of the black hole and the gas metallicity and column density to determine the B-band attenuation along arbitrary lines of sight. Defining the visible quasar lifetime as the total time with an observed B-band luminosity greater than some lower limit LB, min, we find lifetimes ~10-20 Myr for LB, min = 1011 L? (MB ? -23), in good agreement with observations, but significantly shorter than the intrinsic lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed to intrinsic lifetimes is also a strong function of both the limiting luminosity and observed frequency range.

Radio Foregrounds for the 21 Centimeter Tomography of the Neutral Intergalactic Medium at High Redshifts

Absorption or emission against the cosmic microwave background (CMB) radiation may be observed in the redshifted 21 cm line if the spin temperature of the neutral intergalactic medium (IGM) prior to reionization differs from the CMB temperature. This so-called 21 cm tomography should reveal important information on the physical state of the IGM at high redshifts. The fluctuations in the redshifted 21 cm line, due to gas density inhomogeneities at early times, should be observed at meter wavelengths by the next-generation radio telescopes such as the proposed Square Kilometer Array (SKA). Here we show that the extragalactic radio sources provide a serious contamination to the brightness temperature fluctuations expected in the redshifted 21 cm emission from the IGM at high redshifts. Unless the radio source population cuts off at flux levels above the planned sensitivity of SKA, its clustering noise component will dominate the angular fluctuations in the 21 cm signal. The integrated foreground signal is smooth in frequency space, and it should nonetheless be possible to identify the sharp spectral feature arising from the nonuniformities in the neutral hydrogen density during the epoch when the first UV sources reionize the IGM.

Luminosity-dependent Quasar Lifetimes: A New Interpretation of the Quasar Luminosity Function

We propose a new interpretation of the quasar luminosity function (LF) derived from physically motivated models of quasar lifetimes and light curves. In our picture, quasars evolve rapidly, and their lifetime depends on both their instantaneous and peak luminosities. We study this model using simulations of galaxy mergers that successfully reproduce a wide range of observed quasar phenomena. With lifetimes inferred from the simulations, we deconvolve the observed quasar LF from the distribution of peak luminosities and show that they differ qualitatively, unlike the simple models of quasar lifetimes used previously. We find that the bright end of the LF traces the intrinsic peak quasar activity but that the faint end consists of quasars that are either undergoing exponential growth to much larger masses and higher luminosities, or are in sub-Eddington quiescent states going into or coming out of a period of peak activity. The break in the LF directly corresponds to the maximum in the intrinsic distribution of peak luminosities, which falls off at both brighter and fainter luminosities. Our interpretation of the quasar LF provides a physical basis for the nature and slope of the faint-end distribution, as well as the location of the break luminosity.

Tracing the cosmological assembly of stars and supermassive black holes in galaxies

We examine possible phenomenological constraints for the joint evolution of supermassive black holes (SMBHs) and their host spheroids. 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 SMBHs, estimated from the hard X-ray selected luminosity function of quasars and active galactic nuclei for a given radiative efficiency, ∈. We assume that the ratio of the stellar mass in spheroids to the black hole mass density evolves as (1 + z) -α , while the ratio of the stellar mass in discs + irregulars to that in spheroids evolves as ( 1 + z) -β , and we derive constraints on α, β and ∈. We find that a > 0 at more than the 4a level, implying a larger black hole mass at higher redshift for a given spheroid stellar mass. The favoured values for β are typically negative, suggesting that the fraction of stellar mass in spheroids decreases with increasing redshift. This is consistent with recent determinations that show that the mass density at high redshift is dominated by galaxies with irregular morphology. In agreement with earlier work, we constrain ∈ to be between 0.04 and 0.11, depending on the exact value of the local SMBH mass density, but almost independently of a and β.

Galactic centre stellar winds and Sgr A* accretion

We present a detailed discussion of our new 3D numerical models for the accretion of stellar winds on to Sgr A ∗ . In our most sophisticated models, we put stellar wind sources on realistic orbits around Sgr A ∗ , we include recently discovered slow winds (v w ∼ 300 km s −1 ), and we account for optically thin radiative cooling. We test our approach by first modelling only one-phase fast stellar winds (v w ∼ 1000 km s −1 ). For stellar wind sources fixed in space, the accretion rate is of the order of ˙ M � 10 −5 Myr −1 , fluctuates by 10 per cent, and is in good agreement with previous models. In contrast, ˙ M decreases by an order of magnitude for wind sources following circular orbits, and fluctuates by ∼50 per cent. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively after being shocked, forming cold clumps and filaments immersed into the X-ray-emitting gas. We investigate two orbital configurations for the stars in this scenario, an isotropic distribution and two rotating discs with perpendicular orientation. The morphology of cold gas is quite sensitive to the orbital distribution of the stars. In both cases, however, most of the accreted gas is hot, producing a quasi-steady floor in the accretion rate, of the order of ∼3 × 10 −6 Myr −1 , consistent with the values deduced from Chandra observations. The cold gas accretes in intermittent, short but powerful accretion episodes, which may give rise to large-amplitude variability in the luminosity of Sgr A ∗ on time-scales of tens to hundreds of years. The circularization radii for the flows are about 10 3 and 10 4 Schwarzschild radii, for the one- and two-phase wind simulations, respectively, never forming the quasi-spherical accretion flows suggested in some previous work. Our work suggests that, averaged over time-scales of hundreds to thousands of years, the radiative and mechanical luminosity of Sgr A ∗ may be substantially higher than it is in its current state. Further improvements of the wind accretion modelling of Sgr A ∗ will