Arcetri Astrophysical Observatory

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Guidetta Torricelli

Broad Line Region structure

Recent studies on X-ray absorption variability on time scales of hours/days demonstrate that the X-ray "torus" is made of clouds with the same density and distance from the center as the BLR emission line clouds, thus suggesting a close link between the BLR and the X-ray circumnuclear absorber. Until now, this approach has been applied to a small number of cases. However, more long X-ray observations of bright sources with XMM-Newton and Suzaku, suitable for absorption variability analysis, are present in public archives and hence this X-ray absorption analysis can be applied to other sources. The idea is that the results of these observational studies will be fundamental for a better physical description of the absorbers and, therefore,for a better knowledge of the actual structure of the BLR. The analysis is in progress.

Research on AGN


The formation and evolution of Active Galactic Nuclei (AGN), attributed to accreting massive black holes (BHs), are closely related to the formation and evolution of the galaxies which host them. Our group has been studying the co-evolution of AGN, BHs, and their host galaxies for several years now. We rely on a multiwavelength approach, from the millimeter and the IR to the X-ray. Specifically, our group is engaged in studies of:

Research on the LOCAL UNIVERSE


Galaxy evolution is occurring at the present epoch in the Local Universe, through gas accretion and gas consumption via star formation, or through passive evolution. Evolutionary processes can be studied in nearby galaxies with a detail that high-redshift objects do not allow. We rely on a multiwavelength approach and comparison with theoretical predictions; special attention is given to the interstellar medium of star-forming and quiescent galaxies and how its properties vary with galaxy mass, metallicity, and environment. The ultimate aim is to exploit studies of nearby galaxies in the cosmological context to better understand how galaxies form and evolve. Specifically, our group is engaged in studies of:



How cosmic structures formed and how galaxies were assembled remain key open questions in the study of modern cosmology. Theoretical models of primordial star formation and observations of samples faint high-redshift galaxies and clusters are the cornerstone of our group. By combining results from observational and theoretical research, the ultimate aim is to constrain the epoch of structure and galaxy formation, and broaden our understanding of physical processes at work in the early universe. Specifically, our group is engaged in studies of:

  • Formation, evolution, and survival of dust grains in primordial objects, and comparison of theoretical predictions and observations (Simone Bianchi)
  • Early-type galaxy scaling relations at high redshift (Sperello di Serego Alighieri)
  • Chemical evolution and mass-metallicity scaling relations over cosmic epochs (Giovanni Cresci, Anna Gallazzi, Filippo MannucciAlessandro Marconi, Guido Risaliti)
  • Stellar populations, dynamics, morphology, and physical properties of distant galaxy populations (e.g., GEMS followup, LSD, SINS survey, LEGA-C ESO public spectrocopic survey, WEAVE-StePS survey at intermediate redshift using the WEAVE fiber spectrograph, VANDELS, KLEVER, JWST GTO Survey, MOONS GTO) (Giovanni Cresci, Mirko Curti, Anna Gallazzi, Filippo MannucciAlessandro Marconi, Matilde Mingozzi, Emanuele Nardini, Michele Perna, Stefano Zibetti)
  • Supernova rates and progenitor properties as a function of galaxy morphology and redshift (Filippo Mannucci)
  • Clusters of galaxies at high redshift: structure formation and cosmology, thermodynamics of the Intra Cluster Medium and physics of cool cores (Paolo Tozzi)
  • X-ray surveys of galaxy clusters (Paolo Tozzi)
  • QSO absorption systems (Stefano Zibetti)
  • Gamma-ray bursts, afterglows, and their host galaxies, also through the Consorzio Italiano per Burst Ottici (CIBO) (Leslie Hunt, Filippo Mannucci, Sperello di Serego Alighieri)
  • Cosmic Polarization Rotation, an astrophysical test of fundamental physics (Sperello di Serego Alighieri)
  • Quasars as cosmological tools: measurement of cosmological parameters and tests of the cosmological model through a Hubble diagram of quasars, built using the non-linear relation between UV and X-ray emission in quasars (Elisabeta Lusso, Guido Risaliti)
  • Properties and formation of first stars, first galaxies, and first black holes (Stefania Salvadori)
  • Spectroscopic characterization of a high-z galaxy cluster (Viviana Casasola, Laura Magrini)

Guido Risaliti

Main research interests:
1) Structure and geometry of the circumnuclear medium of AGN, particularly through hard X-ray observations of obscured sources.
2) Spectral energy distribution of AGN, X-ray to optical correlations, bolometric corrections and contribution of AGN to the cosmic backgrounds
3) Physics of Ultraluminous Infrared Galaxies (ULIRGs): analysis of the AGN and