A. Franceschini

Revisiting the infrared spectra of active galactic nuclei with a new torus emission model

We describe improved modelling of the emission by dust in a toroidal-like structure heated by a central illuminating source within active galactic nuclei (AGNs). We have chosen a simple but realistic torus geometry, a flared disc, and a dust grain distribution function including a full range of grain sizes. The optical depth within the torus is computed in detail taking into account the different sublimation temperatures of the silicate and graphite grains, which solves previously reported inconsistencies in the silicate emission feature in type 1 AGNs. We exploit this model to study the spectral energy distributions (SEDs) of 58 extragalactic (both type 1 and type 2) sources using archival optical and infrared data. We find that both AGN and starburst contributions are often required to reproduce the observed SEDs, although in a few cases they are very well fitted by a pure AGN component. The AGN contribution to the far-infrared luminosity is found to be higher in type 1 sources, with all the type 2 requiring a substantial contribution from a circumnuclear starburst. Our results appear in agreement with the AGN unified scheme, because the distributions of key parameters of the torus models turn out to be compatible for type 1 and type 2 AGNs. Further support to the unification concept comes from comparison with medium-resolution infrared spectra of type 1 AGNs by the Spitzer observatory, showing evidence for a moderate silicate emission around 10 μm, which our code reproduces. From our analysis we infer accretion flows in the inner nucleus of local AGNs characterized by high equatorial optical depths (A v ≃ 100), moderate sizes (R max < 100 pc) and very high covering factors (f ≃ 80 per cent) on average.

A long-wavelength view on galaxy evolution from deep surveys by the Infrared Space Observatory

We discuss the constraints set on galaxy evolution by a variety of data from deep extragalactic surveys performed in the mid-IR and far-IR with the Infrared Space Observatory and with millimetric telescopes at longer wavelengths. These observations indicate extremely high rates of evolution for IR galaxies, exceeding those measured for galaxies at other wavelengths and comparable or larger than the rates observed for quasars. We also match the modelled integrated emission by IR galaxies at any redshifts with the observed spectral intensity of the extragalactic IR background (CIRB), as a further constraint. The multi-wavelength statistics on IR galaxies can be reconciled with each other by assuming for the bulk of the population spectral energy distributions (SED) as typical for starbursts, which we take as an indication that stellar (rather than AGN, see also Fadda et al. [CITE]) activity powers IR emission by faint galaxies. According to our model and following the analysis of Elbaz et al. ([CITE]), the deep ISO surveys at 15 μ m may have already resolved more than 50% of the bolometric CIRB intensity: the faint ISO 15 μ m source samples, relatively easy to identify in deep optical images (Aussel et al. [CITE]), can then allow to investigate the origin of the CIRB background. From our fits to the observed optical-IR SEDs, these objects appear to mostly involve massive galaxies hosting luminous starbursts (

HerMES: deep galaxy number counts from a P(D) fluctuation analysis of SPIRE Science Demonstration Phase observations

SFRsim 100

HerMES: SPIRE Science Demonstration Phase maps

M_odot

Cold dust and young starbursts: spectral energy distributions of Herschel SPIRE sources from the HerMES survey

/yr). The evolutionary scheme we infer from these data considers a bimodal star formation (SF), including a phase of long-lived quiescent SF, and enhanced SF taking place during transient events recurrently triggered by interactions and merging. We interpret the strong observed evolution as an increase with z of the rate of interactions between galaxies ( density evolution ) and an increase of their IR luminosity due to the more abundant fuel available in the past ( luminosity evolution ): both factors enhance the probability to detect a galaxy during the active phase at higher z . Very schematically, we associate the origin of the bulk of the optical/NIR background to the quiescent evolution, while the CIRB is interpreted as mostly due the dusty starburst phase. The latter possibly leads to the formation of galaxy spheroids, when the dynamical events triggering the starburst re-distribute already present stellar populations. The large energy contents in the CIRB and optical backgrounds are not easily explained, considering the moderate efficiency of energy generation by stars: a top-heavy stellar IMF associated with the starburst phase (and compared with a more standard IMF during the quiescent SF) would alleviate the problem. The evolution of the IR emissivity of galaxies from the present time to

Infrared spectroscopy of faint 15 mu m sources in the Hubble Deep Field South : First hints at the properties of the sources of the IR background

zsim 1

Interpretation of deep counts of radio sources

is so strong that the combined set of constraints by the observed z -distributions and the CIRB spectrum impose it to turn-over at

Constraints on large-scale clustering from the autocorrelation properties of the X-ray background

z> 1

ISOCAM observations of the deep IRAS 60 micron sample in the NEP region: II. Comparison of ISO and IRAS galaxy counts☆

: scenarios in which a dominant fraction of stellar formation occurs at very high- z are not supported by our analysis.