arXiv subjects

A. Cavaliere

Publications and source records attributed to A. Cavaliere.

At least 55 records · Page 3Linked to original sources

arXiv2003

Quasar Evolution Driven by Galaxy Encounters in Hierarchical Structures

We link the evolution of the galaxies in the hierarchical clustering scenario with the changing accretion rates of cold gas onto the central massive black holes that power the quasars. We base on galaxy interactions as main triggers of accretion; the related scaling laws are taken up from Cavaliere & Vittorini (2000), and grafted to a semi-analytic code for galaxy formation. As a result, at high $z$ the protogalaxies grow rapidly by hierarchical merging; meanwhile, much fresh gas is imported and also destabilized, so the holes are fueled at their full Eddington rates. At lower $z$ the galactic dynamical events are mostly encounters in hierarchically growing groups; now the refueling peters out, as the residual gas is exhausted while the destabilizing encounters dwindle. So, with no parameter tuning other than needed for stellar observables, our model uniquely produces at $z>3$ a rise, and at $z\lesssim 2.5 $ a decline of the bright quasar population as steep as observed. In addition, our results closely fit the observed luminosity functions of quasars, their space density at different magnitudes from $z\approx 5$ to $z\approx 0$, and the local $m_{BH}-σ$ relation.

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arXiv2002

Revisiting the Blazar Main Sequence

A discussion of the FSRQ -- BL Lac unification. All distinctive features marking these two Blazar subclasses find an unifying explanation if the sources are powered by central engines constituted by similar Kerr holes, but fueled at high and low accretion rates, respectively. The connection need not be a genetic one, but evidence toward some FSRQs switching into BL Lacs at lower z will be provided by moderately negative BL Lac evolution. Then an extrapolation will be warranted toward ultra-high energy particle accelerators operating at very low accretion rates.

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arXiv2002

AGN Feedback on the Intracluster Medium

Galaxy groups are quite underluminous in X-rays compared to clusters, so the intracluster medium has to be considerably underdense in the former. We consider this to be due to substantial energy fed back into the ICM when the baryons in the member galaxies condense into stars ending up in SNe, or accrete on to central supermassive black holes energizing AGNs. We compute the outflow and the blowout effects driven by the AGNs and the resulting, steep luminosity-temperature correlation L_X-T. We compare this with the SN contribution and with the X-ray data; the latter require the AGN energy to be coupled to the surrounding ICM at fractional levels around 5 10^{-2}. We link the L_X-T behavior with the parallel effects of the AGN feedback on the gas in the host galaxy; we find that these yield a correlation steep up to M_{\bullet}\propto σ^5 between the galactic velocity dispersions and the central BH masses.

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arXiv2002

Binary Aggregations in Hierarchical Galaxy Formation: The Evolution of the Galaxy Luminosity Function

We develop a semi-analytic model of hierarchical galaxy formation with an improved treatment of the evolution of galaxies inside dark matter haloes. We take into account not only dynamical friction processes building up the central dominant galaxy, but also binary aggregations of satellite galaxies inside a common halo described using the kinetic Smoluchowski equation. The description of gas cooling, star formation and evolution, and Supernova feedback follows the standard prescriptions widely used in semi-analytic modelling. We find that binary aggregations are effective in depleting the number of small/intermediate mass galaxies over the redshift range 1 -16. We compare our predicted luminosity functions with those obtained from deep multicolor surveys in the rest-frame B and UV bands for the redshift ranges 0 1 and even more at z ~ 3 by the effect of binary aggregations. The predictions from our dynamical model are discussed and compared with the effects of complementary processes which may conspire in affecting the shape of the luminosity function.

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arXiv2002

Relic Black Holes In Galactic Nuclei

We discuss the link between the observations of distant quasars and those of massive dark objects in the cores of many local galaxies. We show how the formation of early black holes gives rise to the luminosity function of high z quasars, while it imprints into their dark local relics a related shape of the mass-dispersion correlation. We propose that in its lower section the correlation slope will tell the (otherwise uncertain) strenght of the feedback effect from the quasar radiation on the host galaxies.

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arXiv2002

The Blazar Main Sequence

We propose a sequence (the Blazar main sequence, BMS) that links the two main components of the Blazar class, namely, the Flat Spectrum Radio Quasars and the BL Lacertae objects, and yields all their distinctive features in a correlated way. In this view, both type of sources are centered on a supermassive Kerr hole close to maximal spin that produces a relativistic jet observed close to the axis, and on the associated accretion disk that emits more isotropically. But the FSRQs are energized by accretion at rates dm/dt ~ 1, and are dominated by the disk components (thermal and electrodynamic jet-like component) which provide outputs in excess of L ~ 10^46 erg/s. On the other hand, accretion levels dm/dt << 1 are enough to energize the BL Lacs; here the radiation is highly non-thermal and the power partly contributed by the rotational energy of the central Kerr hole, with the latter and the disk together sustaining L ~ 10^44 erg/s in the jet frame for several Gyrs. If so, we expect the BL Lacs to show quite different evolutionary signatures from the FSRQs, and in particular, number counts close to the Euclidean shape, or flatter if the sources make a transition to the BL Lac from a FSRQ mode. In addition, for lower dm/dt along the BMS, we expect the large scale electric fields to be less screened out, and to accelerate fewer particles to higher energies with non-thermal radiations at higher frequencies; so in moving from FSRQs to BL Lacs these non-thermal radiations will peak at frequencies inversely correlated with the disk output. For the BL Lacs such dependence implies increased scatter when one tries a correlation with the total ouput. At its endpoint, the BMS suggests widespread objects that are radiatively silent, but still efficient in accelerating cosmic rays to ultra high energies.

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arXiv2001

The SZ Effect as a Probe of Non-Gravitational Entropy in Groups and Clusters of Galaxies

We investigate how strongly and at what scales the Sunyaev- Zel'dovich effect reflects the shifting balance between the two processes that compete for governing the density and the thermodynamic state of the hot intra-cluster medium pervading clusters and groups of galaxies: the hierarchical clustering of the DM; the non-gravitational energy and momentum fed back into the ICM by the condensing baryons. We base on a SAM of galaxy formation and clustering to describe how the baryons are partitioned among the hot, the cool and the stellar phase; the partition shifts as the galaxies cluster hierarchically, and as the feedback by stellar winds and SN explosions follows the star formation. Their impact is amplified by the same large scale accretion shocks that thermalize the gravitational energy of gas falling into the growing potential wells. We compute the Compton parameter $y$, and find a relation of $y$ with the ICM temperature, the $y-T$ relation, which departs from the self-similar scaling and bends down at temperatures typical of galaxy groups. We model-independently relate this with the analogous behaviour of the L_x - T relation, and discuss to what extent our results are generic of the hierarchical models of galaxy formation and clustering.

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arXiv2001

The Connection between BL Lacs and FSRQs

We discuss the features that mark the Flat Spectrum Radio Quasars from the BL Lacertae objects. We propose that FSRQs exceeding L = 10^{46} erg/s are powered by black hole accreting at rates dm/dt = 1 - 10; their power is dominated by the disk components, thermal and BZ. Instead, sources accreting at rates 10^{-3} < dm/dt < 10^{-2} radiate in the BL Lac mode; here the power is mainly non-thermal, and is driven by the rotational energy stored in a Kerr hole which sustains L < 10^{46} erg/s in the jet frame for Gyrs. The two populations may be even linked if around the same objects dm/dt drops in time; then some negative cosmological evolution is expected for the newborn BL Lacs fed by the dying FSRQs. Further implications are discussed.

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arXiv2000

The Fall of the Quasar Population

We derive quantitative predictions of the optical and X-ray luminosity functions (LF) for QSs in the redshift range $z<3$. Based on BH paradigm, we investigate how the accretion is controlled by the surrounding structures, as these grow hierarchically. We argue that for $z < 3$ efficient black hole fueling is triggered by the encounters of a gas-rich host with its companions in a group. The dispersion of the dynamical parameters in the encounters produces a double power-law LF. Strong luminosity evolution (LE) is produced as these encounters deplete the gas supply in the host; an additional, milder density evolution obtains since the interactions become progressively rarer as the groups grow richer but less dense. From the agreement with the optical and the X-ray data, we conclude that the evolution of the bright quasars is articulated in two ways. Earlier than $z~3$ the gas-rich protogalaxies grow by merging, which also induces parallel growth of central holes accreting at Eddington rates. In the later era of group assemblage the host encounters with companions drive onto already existing holes further but meager accretion; these consume the gas in the hosts, and cause supply-limited emissions which are intermittent, go progressively sub-Eddington and peter out. Then other fueling processes come to the foreground; we discuss the faint emissions, especially noticeable in X-rays, which are expected when hosts in the field cannibalize satellite galaxies with their meager gas contents.

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arXiv1999

X-raying the Star Formation History of the Universe

The current models of early star and galaxy formation are based upon the hierarchical growth of dark matter halos, within which the baryons condense into stars after cooling down from a hot diffuse phase. The latter is replenished by infall of outer gas into the halo potential wells; this includes a fraction previously expelled and preheated, due to momentum and energy fed back by the SNe which follow the star formation. We identify such an implied hot phase with the medium known to radiate powerful X-rays in clusters and in groups of galaxies. We show that the amount of the hot component required by the current star formation models is enough to be observable out to redshifts $z \approx 1.5$ in forthcoming deep surveys from {\it Chandra} and {\it XMM}, especially in case the star formation rate is high at such and earlier $z$. These X-ray emissions constitute a necessary counterpart, and will provide a much wanted probe of the SF process itself (in particular, of the SN feedback), to parallel and complement the currently debated data from optical and IR observations of the young stars.

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arXiv1999

The History of Cosmic Baryons: X-ray Emission vs. Star Formation Rate

We relate the star formation from cold baryons in virialized structures to the X-ray properties of the associated diffuse, hot baryonic component. Our computations use the standard ``semi-analytic'' models to describe i) the evolution of dark matter halos through merging after the hierarchical clustering, ii) the star formation governed by radiative cooling and by supernova feedback, iii) the hydro- and thermodynamics of the hot gas, rendered with our Punctuated Equilibria model. So we relate the X-ray observables concerning the intra-cluster medium to the thermal energy of the gas pre-heated and expelled by supernovae following star formation, and then accreted during the subsequent merging events. We show that at fluxes fainter than $F_X\approx 10^{-15}$ erg/cm$^2 $ s (well within the reach of next generation X-ray observatories) the X-ray counts of extended extragalactic sources (as well as the faint end of the luminosity function, the contribution to the soft X-ray background, and the $L_X-T$ correlation at the group scales) increase considerably when the star formation rate is enhanced for z>1 as indicated by growing optical/infrared evidence. Specifically, the counts in the range 0.5-2 keV are increased by factors $\sim 4$ when the the feedback is decreased and star formation is enhanced as to yield a flat shape of the star formation rate for 2<z<4.

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arXiv1999

X-raying the Cosmic Star Formation Rate

Using current semi-analytic models of structure and star formation based on hierarchical clustering, we connect the cosmic star formation rate (SFR) to the X-ray emission properties of groups and clusters of galaxies. We show that when the baryons cool down from a hot phase to condense into stars with a SFR flat for z>2, then such hot phase yields deep X-ray source counts (at fluxes F=10^{-15} erg/cm^2 s in the energy band 0.5-2 keV) about 5 times larger than in the case of a SFR peaked at z ~ 1.5. We also discuss the effect of the SFR on the shape and evolution of the X-ray L-T relation.

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arXiv1999

The Evolution of the BL Lacertae Objects

The BL Lac population is set apart from the rest of the active galactic nuclei by a number of peculiar features, including remarkably little signs of cosmological evolution. We focus on this feature, and take hint from the observations to reconsider extraction of the primary energy from a Kerr hole via the Blandford-Znajek (BZ) process; in fact, we stress how this can power the emitting jets of the BL Lac objects in a regime of low accretion. We show that the timescale for the BZ process to exhaust the rotational energy of the hole spans many gigayears in all such sources. We compute scales long and uniform enough to yield an intrinsically slow population evolution, giving rise to number counts of ``Euclidean'' or flatter shape; even longer timescales apply to the additional, analogous process involving the inner accretion disk directly. Such slow evolution takes place in ambients and at epochs (z \lsim 1) where interactions of the host galaxies with companions become rare or weak. On the other hand, at higher z, we argue that frequent interactions of the hosts within groups enhanced, each for some 10^{-1} Gyr, the accretion rates and the emissions from the disks, attaining power levels that are typical of the radio-loud quasars. But such interactions were also likely to disorder the large-scale magnetic field in the disk and to decrease angular momentum of the hole, thus limiting the lifetime of all BZ emissions to under a few gigayears. Then the stronger evolution of the flat-spectrum radio-loud quasars was driven, similar to the radio-quiet ones, by the interaction rate decreasing rapidly with z.

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arXiv1999

Hot Gas in Clusters of Galaxies: the Punctuated Equilibria Model

We develop a model to describe the evolution of the intra-cluster X-ray emitting baryons, as they are included in the dark matter potential wells of galaxy clusters evolving through subsequent merging events in the framework of hierarchical clustering. The gas is assumed to re-adjusts to a new hydrostatic equilibrium after each merging event. Before merging it is gravitationally heated at the local virial temperature when bound in subclusters; at early $z$ the gas is preheated by supernova activity following star formation. In detail, we compute analytically the following steps: the dynamic histories of dark matter halos with their merging events; the associated infall of gas into a halo, with compressions and shocks estabilishing the conditions at the cluster boundary; the updated disposition of the gas in the potential well matching such conditions; the statistical convolution of the key quantities over the merging histories. The model predicts the density and surface brightness profiles with no free parameters; the so-called $β$ parameter is itself an outcome of the model, and the polytropic index $γ$ is internally constrained to a narrow range. We obtain declining temperature profiles, and profiles for the density and for the surface brightness shallower in groups compared with clusters. Our model groups also contain a lower baryonic fraction on average, but with a scatter considerably larger than at cluster scales. Various statistics are obtained analytically upon averaging over the merging histories. In particular, we predict in different cosmologies the statistical correlation $L-T$ of luminosity with temperature; similarly we derivethe correlation $R_X-T$ for the size of the X-ray emitting region. The intrinsic scatter in both correlations is also predicted.

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arXiv1998

A Physical Model for Baryons in Clusters of Galaxies

The X-ray emission from clusters of galaxies is one of the best observational probe to investigate the distribution of dark matter at intermediate and high redshifts. Since the disposition of the intracluster plasma (ICP) responsible of the emission is crucial to link X-ray properties to the global properties of the dark matter halos, we propose a semi--analytical approach for the diffuse baryons. This comprises the following blocks: Monte Carlo merging histories to describe the dynamics of dark matter halos; the central hydrostatic disposition for the ICP; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. From our model we predict the $L-T$ correlation, consistent with the data as for shape and scatter.

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arXiv1998

The Rise and Fall of the Quasars

The coherent rise and fall of the quasar population is discussed in terms of gas accretion onto massive black holes, governed by the hierarchically growing environment. The rise is related to plentiful accretion during the assemblage of the host galaxies; the fall to intermittent accretion when these interact with companions in a group. The LFs are computed out to z=6, and are related to the mass distribution of relict BH found in local galaxies. The histories of the QS and of the star light are compared.

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arXiv1998

Diffuse Baryons in Groups and Clusters of Galaxies

To predict the X-ray observables associated to the diffuse baryons in clusters of galaxies, we develop a new physical approach to model such a hot intra-cluster plasma. Such approach is based on punctuated equilibria. and comprises the following blocks: Monte Carlo ``merging histories'' of dark matter potential wells; the central hydrostatic disposition for the ICP, reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. We predict the L-T relation, consistent with the data as for shape and scatter. This we combine with the mass distribution provided by the hierarchical clustering for different COBE-normalized CDM models, to predict the z-resolved luminosity functions, the source counts, the redshift distributions and contribution of the unresolved groups and clusters to the soft X-ray background. When compared with the recent ROSAT surveys, our results confirm that the critical cosmology with Standard CDM is ruled out by its overproduction of local clusters. On account of underproduction, instead, we rule out open cosmologies except for a narrow range around Ω_o=0.5; even there, we find the consistency with the full data base to be hardly marginal. For Ω_o=0.3 in flat geometry, we obtain acceptable fits. For the tilted CDM perturbation spectrum with high baryonic content in the critical universe, we obtain marginal consistency. Finally, we discuss the effective limitations of X-ray clusters and groups as cosmological signposts, and their brighter prospects toward the astrophysics of the ICP and the cosmogony of large, high-contrast structures.

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arXiv1997

The Twofold Quasar-galaxy Connection

The rise and the fall of the QS population are explained in terms of massive black holes forming/accreting during the assemblage of the host galaxies, and of accretion rekindled by interactions of the host with companions in a group. We compute the LFs out to z= 6. We also predict the masses of relict black holes to be found in many galaxies. We compare the histories of the QSO and of the star light.

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