The nuclear region of some galaxies dominates the global energy output at all wavelengths from radio to gamma-rays. They are extremely luminous (up to 1047-48 erg s-1) and variable on short timescales (less than a day). The most efficient mechanism able to explain the observed properties is the conversion of matter to radiation through accretion onto a massive (106-9 Solar masses) black hole. For this reason we believe that Active Galactic Nuclei (AGN) are powered by Super Massive Black Hole (SMBH).
Since more than a decade, it has been realized that SMBH are not rare objects, but are present in the nuclei of all galaxies with a bulge component, implying that most of the galaxies went through an active phase. The correlations between SMBH mass and their host galaxies global properties (i.e. stellar mass, velocity dispersion) strongly indicate that their growth and evolution are tightly linked via mutual feedback mechanisms over the Cosmic time.
The research activity of our group is focused towards a better understanding of the physical mechanisms responsible for the SMBH growth and their effect on the galaxy formation and the ionization history of the intergalactic medium from the epoch of Re-ionization to the present days. Our studies are mainly based on archival and proprietary data collected from X-ray observations (XMM-Newton, Chandra and NuSTAR) complemented by an extensive multi-wavelength coverage with radio (VLA, ALMA), Infrared (Spitzer, Herschel) and optical (VLT, Subaru, LBT, Hubble) ground and space based facilities.
Our research group actively participates to large international collaborations (i.e. COSMOS, E-CDFS/CANDELS, XMM-XXL). Some of us are members of the NuSTAR and eROSITA science teams and deeply involved in the definition of the science case of future missions (i.e. Athena+, WFXT).
Researchers Involved: Andrea Comastri, Roberto Gilli, Marco Mignoli, Eros Vanzella staff at INAF-OABO; Marcella Brusa, Cristian Vignali staff at DIFA-UNIBO; Nico Cappelluti, Giorgio Lanzuisi Post Doc at INAF-OABO, Fabio Vito, Stefano Marchesi and Michele Perna PhD students at DIFA-UNIBO. Several under graduated students and Post Docs in the last 3 years.
Previous Collaborators: Piero Ranalli, Kazushi Iwasawa, Ioannis Georgantopoulos, Francesca Civano, Elisabeta Lusso, Manolis Rovilos, Agnese Del Moro, Serena Falocco, Sara Cazzoli, Enrica Bellocchi
Grants awarded in the last 3 years: PRIN-INAF 2011; PRIN MIUR 2012; PRIN-INAF 2012; FP7-SPACE; FP7-CIG; ASI-NuSTAR
Laurea and PhD thesis Opportunities: Many research projects for both “Tesi di Laurea Magistrale” and PhD thesis are available here. Interested students are encouraged to contact the team members or just visiting us.
Meetings: The team members meet regularly every two weeks on Friday (Friday AGN Meetings or FAME) to discuss recent results, review the status of the various projects and plan for future proposals and science goals. The meeting format is very informal with short talks or communications and ample time for discussion.
X-ray emission offers a unique signature of accretion onto SMBH, being able to penetrate through obscuring material and relatively unaffected by stellar light. Our group pioneered hard X-ray surveys and associated multi-wavelength follow up since the launch of the Italian X-ray satellite BeppoSAX (the HELLAS survey) and later on with the HELLAS2XMM project. The heritage of these projects is a leading role of the Bologna Team in the science exploitation of extragalactic surveys by major high energy satellites such as Chandra, XMM, and more recently, NuSTAR.
The COSMOS survey
COSMOS is an HST treasury project (640 orbits) to survey about 2 square degrees of an equatorial field with the Advanced Camera for Survey (ACS). The project, started in 2003, includes observations from all the major ground and space based facilities over the entire electromagnetic spectrum from the radio band to X-rays. Our team was deeply involved in both the XMM and Chandra surveys (from the planning of the observing strategy to the scientific exploitation of the data). The original Chandra coverage of about 1 square degree (C-COSMOS) is currently expanded to the entire field (COSMOS Legacy).
The deepest X-ray observations were performed in a region with a relatively clear view of the extragalactic sky named CDFS. This region in the southern emisphere includes also the GOODS and Candels Ultra Deep fields with HST and is the subject of deep exposures with most of the current and future observing facilities. In the X-ray band the current 4 Million seconds exposure will be increased to about 7 Megasec in 2014 and will be the deepest field for decades to come. The Chandra exposure is complemented by the ultra deep XMM survey in the CDFS which has been conceived, proposed and is currently lead by our team.
The shallow, wide angle (~50 square degrees) XMM-XXL survey will map two extragalactic regions of 25 deg2, using 10 ks XMM observations to yield a point-source sensitivity of ~ 5 10-15 erg/s/cm2 in the [0.5-2] keV band. It is the largest XMM project approved to date (3Ms in December 2010, >6Ms in total). Some 600 new galaxy clusters are expected out to z~1.5-2 as well as more than 10,000 active galactic nucleus (AGNs) out to z~4. The main goal of the project is to constrain the Dark Energy equation of state using clusters of galaxies. This survey will also have lasting legacy value for cluster scaling laws and studies of AGNs and X-ray background. Our Team is deeply involved in the search for high redshift Quasars and Obscured AGN.
The NuSTAR mission has deployed the first orbiting telescopes to focus light in the hard (3 – 79 keV) X-ray band. Our view of the universe in this spectral window has been limited because previous orbiting telescopes have not employed true focusing optics, but rather have used coded apertures that have intrinsically high backgrounds and limited sensitivity. One of the primary science goal is to take a census of SMBH black holes performing deep observations of the extragalactic sky.
The evolution of early SMBH: demography and physics of high-redshift (z > 3) AGN.
The infancy of SMBHs, i.e. how these unique systems are assembled on the first place, may hold the key to understand the physical processes responsible for the subsequent co-eval galaxy-AGN growth and assembly. The study of the first objects is today a very “hot” topic, as witnessed by the fact that the all new-generation instruments such as ALMA, JWST, eROSITA and Euclid have among their primary goals the study of the early Universe, when the UV radiation from the first objects re-ionised the intergalactic medium. Our group provided a significant improvement in the study of the cosmological evolution of the X-ray emission of high-z (z>3) QSO, sampling XLF to progressively down luminosities from a few times 1044 erg s-1 to a few times 1043 erg s-1 (Brusa et al. 2009, Civano et al. 2011, Vito et al. 2013). Recently, Gilli et al. (2011) reported the discovery of a bona-fide highly obscured QSO at z~5 in the 4 Ms CDFS field; less than 0.1 QSO at z > 4 were expected in the CDFS area if their space density declines as observed in the optical band and at higher Xray luminosities (Brusa et al. 2009). These results highlight the leverage of large area and deep X–ray surveys on our understanding of primeval SMBH evolution. In the last few years we started a new a program to access the epoch during which the first SMBH are expected to form (z > 6) through the study of fluctuations of the Cosmic Infrared and X-ray Backgrounds. Our group is involved in the international collaboration LIBRAE (Looking for Infrared Background Anisotropies for Euclid) that aims to study the properties of the first lights and black holes in the Universe with the forthcoming mission Euclid.
Our group is also pioneering techniques to study the large scale structures around the most distant SMBHs known, i.e. QSOs at z>6. These objects already possess SMBHs of more than 1 billion solar masses when the Universe was less than one billion year old. Current models of early black hole formation suggest this rapid growth may be favored in dense environment and hence that early QSOs may reside within the most massive dark matter halos collapsed at that time, which are surrounded by large scale structures of both dark matter and galaxies, extending for tens of physical Mpc. We have embarked in a large observational program to identify overdensities of galaxies around QSOs at z~6 using the best optical wide-field instrumentation that is available for such studies. We are indeed already exploiting data from the Large Binocular Telescope (LBT), where Italy has 25% guaranteed time, and will obtain more data from both LBT and VLT in the near future.
The (obscured) accretion history of the Universe
The cosmic X-ray background (XRB), which is largely due to accretion SMBH integrated over cosmic history, has been almost completely resolved up to about 5-6 keV by deep X-ray surveys
The large majority of the X-ray sources obscured by large columns of cold gas 1023 cm-2, in good agreement with the predictions of AGN synthesis models (Gilli et al. 2007). The evolution of the AGN luminosity function turns out to be luminosity dependent with a behavior similar to that observed for star formation lending an independent support to the fact that the formation and evolution of SMBHs and their host galaxies are closely related. The very nature of this link is not completely understood, nor is the behavior of the various correlations known at early epochs. A key uncertainties in determining the evolution of SMBH accretion history is the effect of heavy obscuration especially in the Compton thick regime when the optical depth of the obscuring gas exceeds a few units and the source is almost invisible below 10 keV. Our group actively pursue, since more than a decade, an extensive program to search for and characterize the most heavily obscured, accreting SMBH over a large fraction of the Cosmic Time (z~0.5-3). The research activity is mainly X-ray driven and based on the direct search from deep and medium deep survey data (e.g. Comastri et al. 2011). Multi wavelength follow up observations and archival data in the above mentioned survey fields are also fundamental for a proper characterization of the bolometric luminosities (Lusso et al. 2012; 2013) and accretion rates. The Spectral Energy Distribution (SED) approach is also fundamental to separate the obscured accretion driven luminosity from the host galaxy starlight and, in turn, to study how the feedback mechanisms regulate their joint evolution as a function of the Cosmic Time and SMBH and hiost galaxy physical properties.
The joint evolution of SMBH and their host galaxies within the Universe Large Scale Structure
The physical processes that lead to the formation of a SMBH are largely unknown. On the one hand, mergers and encounters of galaxies in dense environments may channel large amounts of gas towards their nuclei, hence triggering efficient accretion and SMBH growth. On the other hand, internal, secular processes within individual galaxies may also produce efficient SMBH growth. The study of the cosmic environment and of the large scale structures within which AGN reside is a powerful tool to understand what are the mechanisms that trigger nuclear accretion. Our group is actively investigating how AGN are distributed, i.e. clustered, in space at different cosmic epochs. These “clustering” studies provide an estimate of the typical mass of the dark matter halos and of the host galaxies in which AGN reside, hence showing whether they are more likely to form in large and dense environments such as galaxy clusters and groups, or instead they mainly form in field galaxies. In addition, by studying the clustering of AGN populations with different physical properties (eg. obscuration, luminosity, BH mass, host mass and morphology, and so on), one can understand the way how SMBH and their hosts have evolved from the early Universe to the present epoch. The COSMOS and CDFS surveys in which we are involved are among the best available fields to perform such clustering studies.
In the near future the survey science will be boosted by eROSITA (extended ROentgen Survey with an Imaging Telescope Array, Predehl et al. 2010): the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission, currently an approved X–ray telescope at energies < 10 keV planned to be launched in 2015. The mission, led by the German Institute MPE, will perform an X-ray all sky survey reaching limiting fluxes about ten times fainter than the ROSAT survey, and will allow to tackle a broad band of scientific issues, among them the census of accreting black holes in the nearby universe out to the very high redshift.
On longer timescales (~ 2030), transformational science is expected by Europe’s next generation X-ray telescope Athena. In particular two key questions in Astrophysics will be addressed: 1) How does ordinary matter assemble into the large scale structures that we see today? and 2) How do black holes grow and shape the Universe?
The Athena mission provides the necessary angular resolution, spectral resolution, throughput, detection sensitivity, and survey grasp to revolutionize our understanding of these issues. These capabilities will also provide a powerful observatory to be used in all areas of astrophysics. The Bologna Team is actively participating to the Science Team activities with a leading role in the definition of the observing strategy for the search for the first (z~6-10) SMBH. On November 28, 2013 the ESA SPC recommended the “Hot and Energetic Universe” as the science theme for the next Large Mission (L2) to be launched in 2028
The Wide Field X-Ray Telescope (WFXT) is a medium-class mission concept designed to be 2-orders-of-magnitude more sensitive than any previous or planned X-ray mission for large area surveys and to match in sensitivity the next generation of wide-area optical, IR and radio surveys (e.g. LSST, Euclid, SKA). The WFXT extragalactic surveys will reach limiting fluxes comparable to the those of the major Chandra and XMM surveys, but over 1000 times larger areas. These surveys are expected to provide an astrophysical dataset of more than 0.5 millionclusters of galaxies to z ~ 2, 10 millions AGN to z > 6, and 0.1 million normal and starburst galaxies at z < 1, that will constitute a vast scientific legacy for decades. Our group is responsible for coordinating the activities related to the definition of AGN science themes and survey strategy.
1. High-redshift obscured AGN in the COSMOS field
The goal of this project is related to the study of the properties of obscured AGN selected from the zCOSMOS-Deep Survey on the basis of the presence of strong and narrow CIV (1549A) emission in the 2-3 redshift range.
The fraction of heavily obscured AGN at high-redshift will be estimate using individual X-ray spectroscopy, X-ray stacking analysis and mid-IR/optical criteria, and accretion and star-formation rates will be derived by SED-fitting analysis.
2. Analysis of the unresolved X-ray background in the 3Ms XMM-CDFS Survey
The goal of this project is the study of the population of AGN which are not individually resolved in the deep 3Ms exposure of the XMM-CDFS Survey and whose emission is what is called X-ray background. Obscured and high-redshift AGN are presumably hidden in this integrated emission which must be properly decoupled from the dominant contribution of the instrumental background.
3. To scrape the bottom of the barrel in the quest for obscured quasars
The project is based on an X-ray spectroscopic analysis of the low signal-to-noise ratio AGN in the 3Ms exposure of the XMM-CDFS, with particular emphasis on those optically classified as obscured quasars.
This X-ray spectral investigation should deal with a proper modeling of the XMM background to derive reliable measurements of source photon index, column density, intrinsic luminosity, and constraints on the iron emission line.