We compute the effect of the galactic absorption on AGN emission in a cosmological context by including a physical model for AGN feeding and feedback in a semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers for AGN accretion, and on expanding blast waves as a mechanism to propagate outwards the AGN energy injected into the interstellar medium at the center of galaxies. We first test our model against the observed number density of AGNs with different intrinsic luminosity as a function of redshift. The model yields a ''downsizing'' behavior in close agreement with the observed one for z<2. At higher redshifts, the model predicts an overall abundance of AGNs (including Compton-thick sources) larger than the observed Compton-thin sources by a factor around 2 for z>2 and L_X < 10^{44} erg/s. Thus, we expect that at such luminosities and redshifts about 1/2 of the total AGN population is contributed by Compton-thick sources. We then investigate the dependence of the absorbing column density N_H associated to cold galactic gas (and responsible for the Compton-thin component of the overall obscuration) on the AGN luminosity and redshift. We find that the absorbed fraction of AGNs with N_H>10^{22} cm^{-2} decreases with luminosity for z<1; in addition, the total (integrated over luminosity) absorbed fraction increases with redshift up to z around 2, and saturates to the value around 0.8 at higher redshifts. Finally, we predict the luminosity dependence of the absorbed fraction of AGNs with L_X< 3 10^{44} erg/s to weaken with increasing redshift. We compare our results with recent observations, and discuss their implications in the context of cosmological models of galaxy formation.Comment: 13 pages, accepted for publication in Astrophysical Journal; added 2 reference

Context. BL Lac objects undergo strong flux variations involving considerable changes in their spectral shapes. We specifically investigate the X-ray spectral evolution of Mrk 421 over a time span of about nine years. Aims. We aim at statistically describing and physically understanding the large spectral changes in X rays observed in Mrk 421 over this time span. Methods. We perform a homogeneous spectral analysis of a wide data set including archived observations with ASCA, BeppoSAX, RXTE, as well as published and unpublished XMM-Newton data. The presence of uncertainties is taken into account in our correlation analysis. The significance of the correlations found and possible spurious effects are studied with Monte Carlo simulations. Results. We find that the Mrk 421 spectral energy distribution (SED) has a lower peak at energies that vary in the range, 0.1-10 keV while its X-ray spectrum is definitely curved. Parameterizing the X-ray spectra with a log-parabolic model, we find a positive correlation between the position and the height of the SED peak. In addition, we find a negative trend of the spectral curvature parameter vs. the SED peak energy. Conclusions. We show that these relations between the spectral parameters are consistent with statistical or stochastic acceleration of the emitting particles, and provide insight into the physical processes occurring in BL Lac nuclei.

We investigate how the hierarchical merging of dark matter halos, the radiative cooling of baryons, and the energy feedback from supernovae and active galactic nuclei or quasars combine to govern the amount and the thermal state of the hot plasma pervading groups and clusters of galaxies. We show that, by itself, supernova preheating of the external gas flowing into clusters falls short of explaining the observed X-ray scaling relations of the plasma luminosity L_X or the plasma entropy K versus the X-ray temperature T. To account for the scaling laws from rich to poor clusters takes preheating enhanced by the energy input from active galactic nuclei. In groups, on the other hand, the internal impacts of powerful quasars going off in member galaxies can blow some plasma out of the structure. So they depress L_X and raise K to the observed average levels; meanwhile, the sporadic nature of such impulsive events generates the intrinsic component of the wide scatter apparent in the data. The same quasar feedback gives rise in groups to entropy profiles as steep as observed, a feature hard to explain with simple preheating schemes. Finally, we argue a close connection of the L_X-T or the K-T relation with the M_{\bullet}-\sigma correlation between the host velocity dispersion and the masses of the black holes, relics of the quasar activity.Comment: 13 pages, 7 figures, uses REVTeX4 + emulateapj.cls and apjfonts.sty. Accepted by Ap

Context. Many of the extragalactic sources detected in γ rays at TeV energies are BL Lac objects. In particular, they belong to the subclass of "high frequency peaked BL Lacs" (HBLs), as their spectral energy distributions exhibit a first peak in the X-ray band. At a closer look, their X-ray spectra appear to be generally curved into a log-parabolic shape. In a previous investigation of Mrk 421, two correlations were found between the spectral parameters. One involves the height S p increasing with the position E p of the first peak; this was interpreted as a signature of synchrotron emission from relativistic electrons. The other involves the curvature parameter b decreasing as E p increases; this points toward statistical/stochastic acceleration processes for the emitting electrons. Aims. We analyse X-ray spectra of several TeV HBLs to pinpoint their behaviours in the E p − S p and E p − b planes and to compare them with Mrk 421. Methods. We perfom X-ray spectral analyses of a sample of 15 BL Lacs. We report the whole set of observations obtained with the BeppoSAX, XMM-Newton and Swift satellites between 29/06/96 and 07/04/07. We focus on five sources (PKS 0548-322, 1H 1426+418, Mrk 501, 1ES 1959+650, PKS 2155-304) whose X-ray observations warrant detailed searching of correlations or trends.Results. Within our database, we find that four out of five sources, namely PKS 0548-322, 1H 1426+418, Mrk 501 and 1ES 1959+650, follow similar trends as Mrk 421 in the E p − S p plane, while PKS 2155-304 differs. As for the E p − b plane, all TeV HBLs follow a similar behaviour. Conclusions. The trends exhibited by Mrk 421 appear to be shared by several TeV HBLs, such as to warrant discussing predictions from the X-ray spectral evolution to that of TeV emissions.

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.Comment: 26 pages, 2 eps figures, revised version #2, accepted for pubblication in Ap

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. These deplete small to intermediate mass objects, affecting the slope of the luminosity function at its faint end, with significant observable consequences. We model the effect of two-body aggregations using the kinetic Smoluchowski equation. This flattens the mass function by an amount which depends on the histories of the host haloes as they grow by hierarchical clustering. 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 < z < 3, thus flattening the slope of the luminosity function at the faint end. At z ≈ 0 the flattening occurs for −20 < M B < −18, but an upturn is obtained at the very faint end for M B > −16. We compare our predicted luminosity functions with those obtained from deep multicolor surveys in the HDF-N, HDF-S, NTT-DF in the rest-frame B and UV bands for the redshift ranges 0 < z < 1 and 2.5 < z < 3.5, respectively. The comparison shows that the discrepancy of the predictions of other semi-analytic models with the observations is considerably reduced at z > 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 (additional starburst recipes, alternative sources of feedback, different mass distribution of the dark matter haloes) which may conspire in affecting the shape of the luminosity function.

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.Comment: 12 pages Latex + 3 EPS figures, ApJ in press, we have corrected the previous printing problems with the style \ca

We model the effects of shocks on the diffuse, X-ray emitting baryons in clusters of galaxies. Shocks separate the infalling from the inner gas nearly at equilibrium, and dominate the compression and the density gradients of the latter in the dark-matter potential of the cluster. We find that, independently of the detailed shape of the potential, the density gradient is steeper and the compression factor larger for the richer clusters. We show, considering the different merging histories, that in the hierarchical cosmogony the above effects lead, in X-rays, to a luminosity-temperature relation L \propto T^5 at the scale of groups which flattens down to L \propto T^{3} for rich clusters in accord with the observations, and then saturates toward L \propto T^2 for higher temperatures. From the merging histories we also compute statistical fluctuations of the L-T correlation.Comment: aastex, 14 pages, 3 postscript figures, accepted for publication in ApJ