Research interests

Radio-loud (RL) active galactic nuclei (AGN) are those super-massive black holes (SMBHs) that are able to expel part of the accreting matter into two relativistic bipolar jets. They are therefore usually referred to as jetted AGN and represent up to 10% of the total AGN population. Understanding the mechanisms responsible for the launch and the emission of these jets is of crucial importance for studying their role in SMBH accretion and evolution, and for investigating their feedback on the intergalactic medium.

High-redshift RL quasars (QSOs) are indispensable tools for studying the early evolutionary stage of the first jetted SMBHs, their feedback on the host galaxy and the environment, and their contribution to the re-ionization epoch. 

A systematic search for these objects and their subsequent multi-wavelength study is necessary to better constrain the properties of jetted SMBHs in the first giga-years following the Big Bang.

The search for high-z Radio-Loud quasars

In an effort to enlarge the current sample of high-z RL QSOs, I am conducting different projects that combines optical (PanSTARRS, DES, DESI, Decals), infrared (WISE, UKIDSS, UHS, VIKING, VHS), and radio (NVSS, RACS, LOFAR, VLASS) datasets to identify distant radio sources all over the sky, by using the so-called dropout technique. This method is based on color-color selection criteria aimed at searching for the Lyman-α break in broadband photometric catalogs. Then, I ask for spectroscopic follow ups to identify the true high-z sources.

The typical "face" of a high-z quasars. The flux of high-redshift sources at wavelengths shorter than the Ly-α emission line is strongly absorbed by the intervening HI clouds along the line of sight. The absorption feature at the position of the Ly-α line creates a drop (i.e. the Ly-α break), which can be recognised with broad-band photometry. High-z quasars are visibile only in red filters and not in blue bands. They appear point-like. In case I show the Pan-STARRS PS1 g, r, i, z, y cutout images of PSO J191+86 (RL QSO at z=5.3). 

The multi-wavelength characterisation of high-z Radio-Loud quasars

After their identification with spectroscopic follow up, a multi-wavelength analysis is necessary to reveal the properties of high-z RL QSOs and compare then to the lower redshift population. In my research I am particularly interested in radio and X-ray properties, the two spectral bands that trace the emission of the relativistic jet. 

Radio

Analysis of radio images and archival radio data. 

Estimation of radio-loudness (a parameter that described the intensity of the radio emission with respect to the optical one), radio spectral index (that provides information on the jet orientation), and radio luminosity.  

Study the jet structures at different scales, thanks to the collaboration with radio-astronomers at IRA-Bologna (Dr. Cristiana Spingola and Prof. Daniele Dallacasa). 


Figure: Radio jet of PSOJ0309+27, a blazar at z=6.1, observed with the Very Long Baseline Array (VLBA); image credit: Spingola et al. 2020 / Bill Saxton NRAO/AUI/NSF

X-ray

Analysis of the X-ray data. 

Estimation of X-ray jet parameter, like photon index and alpha_ox (that describes the intensity of the X-ray emission with respect to the optical one) 

Study of X-ray kpc jets

Figure: Rest frame spectral energy distribution of PSO J0309+27 from radio to X-rays frequencies. The X-ray emission (green butterfly) is stronger with respect to the emission coming from a X-ray corona (red area).

SMBH mass 

Estimation of the central black hole mass by using the so-called Single Epoch (i.e. virial) method, thanks to the detection and analysis of broad emission lines (like CIV and MgII); estimation of the Eddington ratio, bolometric luminosity, lines parameters

Figure: Double Gaussian fit of the CIV emission line detected for PSOJ0309+27 with LBT/LUCI instrument which allow the estimation of its SMBH mass.

The environment of z>6 Radio-Loud quasars

RL AGN are commonly found in rich environments at different cosmic epochs. Galaxies over-densities around both low-z (z∼0.5-2) and high-z (z∼3-5.8) RL AGN have been found by several authors. Theoretical models strongly support a preferential over-dense environment around RL AGN. All these results suggest that the presence of a relativistic jet may indeed be preferentially triggered in dense environments. 

Therefore luminous radio sources are good beacons for finding protoclusters of galaxies at high redshifts.

With the aim of tracing the earliest stages of protocluster evolution in RL AGN field at the highest redshift, we  started a pilot project to study the environment of z>6 RL QSOs: PSO J0309+27 at z=6.1 and ILTJ1037+40 at z=6.07, a blazar and a blazar candidate, respectively. We use LBT/LBC to imaging the field of these two QSOs in r,i, and z filter, with the aim of selectng candidates Lyman-Break Galaxies through the color-color technique (r-i vs i-z colours). 

This project is in collaboration with M. Mignoli, R. Gilli, F. Vito and R. Decarli at INAF-OAS (Bologna) and with Prof. C. Mazzucchelli at Universidad Diego Portales (Chile)