Active Galactic Nuclei - The only known type of astrophysical objects powered by a massive black hole (MBH, masses >10^5 Msun) are Active Galactic Nuclei (AGN). AGNs emit over the entire electromagnetic spectrum. The central MBH accretes matter through the accretion disk generating optical and UV emissions.  X-ray emission is produced by the Comptonization of these seed photons by a corona of hot electrons located in a region closest to the MBH (Haardt et al. 1993).  The UV photons ionize dense clouds (Broad Line Regions), located at sub-pc distance from the MBH, producing strong broad emission lines in the UV/optical AGN spectrum. At larger distance from the central black hole (10^2-10^3 pc), there are clouds less dense and with lower temperatures, called Narrow Line Regions, producing narrow emission lines in the optical spectra. The central engine is surrounded by a toroidal optically thick obscuring medium (extending from ∼1 pc up to a few times 100 pc) that absorbs the UV/optical emission and reprocesses it in the infrared band. Finally, less than 10% of AGN show highly collimated jets composed of relativistic plasma and emitted from the centre of the sources. These jets extend out to  hundreds of kpc and are visible in the radio band. 

Thesis #1 - Optical & X-ray properties of MBH binary candidates 

Direct observations of MBH binaries are still a challenging task since their angular separations are much smaller than the spatial resolutions of current facilities. Indirect method have been pursued to detect them (Severgnini et al. 2018, Serafinelli et al. 2020, Severgnini et al. 2022). Simulations suggest that MBH binaries would excavate a cavity inside a circumbinary disk, and the accretion will occur by means of two minidisks, surrounding each of the MBH (d’Ascoli et al., 2018). Periodic modulation in UV and X-ray light curves is expected with periods comparable to the binary period. Additionally, the two minidisks may produce double-peaked Fe emission lines in the X-rays, with energies Doppler-shifted by the minidisk orbital motion. As binaries enter their gravitational-wave dominated inspiral, they inevitably may induce double-peaks and large periodic shifts also in the broad emission lines. It is expected that these observational features hold until the very last phases of the evolution of the MBH binaries, in which the systems will emit gravitational waves.

The aim of this thesis is to study the optical and X-ray properties of three of the most promising MBH binary candidates discovered so far.  

The selected candidate will reduce and analyze optical and X-ray (photometric and spectroscopic) data already available for these three systems by searching for periodic modulations in the X-ray light curves and significant emission line variations. The results will be compared with the predictions of theoretical models.

Thesis #2 - Build up of samples of MBH with masses <10^7 Msun

The gravitational event from the inspiral and merger of two MBHs with masses <10^7 Msun could be observable by the up-coming GW experiments like the Laser Interferometer Space Antenna, (Amaro-Seoane et al. 2023) and the Lunar Gravitational Wave Antenna (Harms et al. 2021). In spite of their multi-messenger relevance, very few MBH binaries are know so far. 

The aim of this thesis is to build up the first well-defined catalog of active MBH binary candidates detectable with future GW experiments. 

The selected candidate will exploit the public BH archives to select all local MBHs in a proper range of masses and (s)he will cross-correlate this sample with multi-wavelength photometric and spectroscopic catalogues/archives looking for signatures suggesting the binary nature.

Thesis #3 - Are dual AGN the fate of merging galaxies?

Due to the lack of unbiased and systematic follow-ups, the connection between the galaxy merging process and AGN triggering has been highly speculative (see De Rosa et al. 2019, De Rosa et al. 2023).

Our group was recently awarded dedicated optical spectroscopic observations (Large Binocular Telescope data) of well-defined samples of optically selected double-nucleus interacting galaxies with relative projected separations lower than fews kpc (Mezcua et al. 2014). These latter galaxies constitute a new potential sample of sub-kpc dual AGN.

The aim of this thesis is to provide the first fair census of dual AGN (kpc scale separation) for a well-defined and sizeable sample of local galaxies in an advanced-state of merger.

The selected candidate will reduce and analyze the optical spectra in order to 1. assess the presence of AGN in each nucleus of the selected systems; 2. quantify the incidence of single and dual AGN in mergers as a function of the relative separation; 3. investigate the properties of the SMBH and host galaxy  environments in systems hosting AGN.

Thesis #4 - Dual Radio AGN candidates with VLBI

One of the most compelling pieces of evidence for confirming the dual AGN nature comes from radio Very Long Baseline Interferometry (VLBI), owing to an exquisite angular resolution (~10 milli-arcsec at GHz frequencies, e.g. Herrera-Ruiz et al. 2017) which enables to unambiguously pin down dual AGN cores down to parsec separation. Our research group is actively involved in the analysis and exploitation of deep VLBI data (from both archival and proprietary programs) targeting dual AGN candidates over the full sky. 

The aim of this thesis project is the identification and radio morphological classification of the most promising dual radio AGN systems discovered so far. The selected candidate will not only become familiar with cutting-edge VLBI pipelines, but (s)he will also gain experience in interpreting VLBI images and cross-matching them with existing multi-wavelength ancillary data, shedding light on the possible dual AGN nature of those targets with unprecedented detail.