Foto del docente

Cristian Vignali

Associate Professor

Department of Physics and Astronomy "Augusto Righi"

Academic discipline: FIS/05 Astronomy and Astrophysics


One of the main topics in modern observational cosmology and, in particular, in X-ray astrophysics is the study of the obscured Active Galactic Nuclei (AGN), in particular those at high redshift, the co-called Type 2 quasars.
This kind of research can be pursued efficiently through X-ray surveys and, over the last few years, with infrared observations, whose complementarity to X-rays has been probed clearly by the Spitzer satellite. The population of obscured AGN is relevant for many cosmological reasons: at first, these sources have a key role in the X-ray background radiation; secondly, most of the energy density in the Universe, associated to accretion phenomena, is due to obscured AGN. Although many investigations have been carried out recently with the Chandra and XMM-Newton satellites, the number and properties of the most heavily obscured AGN, the so-called Compton thick, remain to be defined; for these AGN, the X-ray obscuration is such to prevent the nuclear radiation from being observed directly below 10 keV, where typically only the scattered/reflected component can be observed. A valuable observational strategy to pick up these obscured sources, also at high redshift, consists of a combined X-ray plus infrared selection.
Another relevant topic in modern astrophysics is the study of the first AGN to form in the Universe, in particular of their growth, relation with their environment, and evolution of their properties over cosmic time. At present, only the most luminous AGN (quasars) at high redshift are essentially known, since the discovery of the weaker high-redshift AGN population is observationally, both in current optical and X-ray surveys, more challenging. Several theoretical models predict a large number of AGN at the highest redshifts, when the age of the Universe was less than 1 Gyr. At present, an extensive search for and characterization of high-z AGN is ongoing in Bologna using a multi-wavelength approach. Furthermore, high-redshift AGN are being searched for also in z~6 luminous quasar fields as over-densities of massive dark matter halos in the 'young' Universe.  

High-energy astrophysics has been characterized, over the last decade, by numerous studies with the purpose of finding luminous and obscured AGN at high redshift, the so-called Type 2 quasars. These objects, predicted by the Unified Models, play a key role in modeling the X-ray background radiation (XRB). There are evidences that a significant fraction (50-80%) of the energy density in the Universe associated to accretion phenomena onto super-massive black holes (SMBHs) is linked to obscured AGN. Moreover, comparing the
fraction of resolved XRB below 6 keV (80-90%) and at energies 6-10 keV (50-60%), a further population of heavily obscured, maybe Compton-thick AGN (with column densities above 10^24 cm^-2) can be assumed. These sources are probably not revealed efficiently in current X-ray surveys, even in the deepest ones. The study of these sources cannot be conducted without a proper and large multi-wavelength coverage in order to characterize, from a physical and morphological point of view, the obscured AGN, and to estimate how many of these sources are lost in surveys conducted in only one band (e.g., the X-ray band) because of selection effects.
In this regard, the deep narrow-field X-ray surveys, coupled with the shallower ones on larger fields carried out by Chandra and XMM-Newton, have allowed for a more comprehensive understanding of the issues related to the obscured AGN population at high redshift. Similarly, recent observations in the near and mid-infrared with the Spitzer satellite have played a key role in defining selection criteria to pick up obscured quasars at high redshift in an alternative way wrt those offered by X-ray surveys.
The overall picture is that only a multi-wavelength approach (e.g., by selecting sources with large mid-infrared to optical flux ratio) we will be able to have a more accurate view of all the physical issues related to the obscured AGN population.
In this research field, there is growing evidence that a fraction of sources at high redshift has very red colors; with R-K>5, these sources,
called Extremely Red Objects (EROs), are a mixed bag with a relevant fraction of AGN, whose extinction in the optical/near-infrared is associated to the obscuration observed in X-rays. Given the good quality of the data for many of these sources, it is now possible to perform accurate and systematic studies of these objects, in order to estimate how many EROs have properties consistent with accretion processes. For the faintest X-ray EROs, "stacking analysis" is able to provide the average properties. The cosmological implication of these studies are multiple, in primis because, if they host an AGN, they probably represent a co-evolution phase between the SMBH
(AGN) and the host galaxy, responsible for most of the emission in the optical and near-infrared.
The evolution of AGN is another issue of strong impact in modern astrophysics. Although it is clear that they evolve, many doubts arise when their evolution is connected with their environment. In this regard, the study of luminous quasars at z>4, when the age of the Universe is less than 1 Gyr, has been a widely studied topic over the last decade. X-ray emission is a common property of AGN and can be revealed, thanks to current X-ray detectors on-board Chandra and XMM-Newton, up to very high redshifts. X-ray radiation can probe the innermost regions of AGN, thus providing a detailed picture of the emission mechanisms in act in the process of energy production. The current availability of large samples of quasars, especially thanks to the Sloan Digital Sky Survey, has provided a significant number of sources for X-ray follow-up observations aimed at investigating how their emission evolves over cosmic time and the dependence upon their UV/optical properties. These studies are supported, in the local Universe, by detailed X-ray spectroscopic studies using XMM-Newton and Chandra data. Similar studies at the highest redshifts will be possible only with the next generation of large collecting-area telescopes like the recently approved (by ESA) Athena mission.