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Quasars and Jets in the early Universe: Understanding the Formation and Evolution of Supermassive Black-Holes

Representative image for thesis Tutors: Alessandro Caccianiga, Alberto Moretti, Fabrizio Tavecchio - INAF-OAB Milano
Lasting: 3 years. PhD thesis.

Supermassive black holes (SMBH), with masses reaching several billions of solar masses (Msun), are present in the center of nearly all galaxies. Current theoretical models assume that they formed relatively "light" (102-104 Msun) when the Universe was ∼300-500 Myr old and then rapidly accreted matter to reach the huge masses observed today. However, the observation of galaxies in the early Universe (less than 1 Gyr old) hosting already massive SMBH (∼109 Msun) is a challenge for current models, since the time available for the black-hole growth was very short. For this reason, searching and studying SMBH in the early Universe, in particular those actively accreting matter (the so-called quasars), is a lively and hot topic in current astrophysical research.
A general review on this topic can be found here:

Our research group, currently including 4 staff researchers, 1 PhD student and 1 post-doc, has been actively working in this important research field in the last 5 years. In particular we have been focused on the enigmatic sub-set of quasars that are able to produce powerful relativistic jets (the so-called "radio-loud quasars") in order to establish their actual role in the overall picture. The list of publications of our group on this research field in the last 3 years can be found here:

In this thesis we offer to the PhD student the possibility of taking a leading role on some specific aspects of this stimulating field either by exploiting the large amount of data we have collected so far and/or obtaining new data set by writing specific proposals. The PhD student will also have the possibility of spending one or more periods abroad to gain expertise and foster external collaborations.

The (non-exhaustive) list of possible lines of reasearch to be carried out includes:
  • The search and multiwavelength follow-up of new radio-loud QSO at high redshifts by exploiting both the existing and the new generation of radio/optical/infrared surveys. Our group has a very good expertise in this field as demonstrated by the fact that more than half of the radio-loud quasar at redshift above 6 has been discovered by two PhD students in our group. We expect that, with the existing surveys, it will be possible to discover several z>5.5/6 radio-loud quasars in the next couple of years. In addition, this work will soon benefit of the new data-sets produced by the new generation of radio and optical+near/IR (LSST) surveys which will be available in a 2 years timeframe and for which our group has a privileged access. Thanks to these new data it will be possible to push the discovery limit up to z~7, a currently unexplored territory for radio-loud quasars.

  • Studying the innermost part of the relativistic jets of high-z QSO using the Very Long Baseline Interferometry (VLBI) technique. This special technique, that has been recently used to directly image the SMBH shadow in M87 and SgrA*, is based on the combination of the simultaneous observation of different radio telescopes distributed worldwide and it permits to obtain the sharpest view of the innermost part (within a few parsecs) of a jet. We have already obtained VLBI data on about 20 high redshift QSO, using the European VLBI Network (EVN) and this unique data set can be the starting point for a stimulating PhD thesis work.

  • Extended X-ray jets: the relativistic jets departing from radio-loud Quasars, in some cases extend over tens of kpc and are directly observable as resolved structures at different wavelengths. In particular, the X-ray emission of these objects is particular intriguing for, at least, two reasons. First, the physical mechanism responsible for this emission is unclear and has been debated for over two decades. Second, current observations show that their properties evolve with redshift. Starting from the observation of the most distant extended object ever observed (at redshift z=6.1), in a recent work conducted by a PhD student of our group, we interpreted these phenomena as due to the interaction of the jet electrons with the CMB radiation which is much denser in the early Universe. However, since this explanation is contentious, an extensive comparison of the model predictions with larger datasets would be very useful for testing the presented model. This is a suitable PhD thesis in a forefront research field for a student who would like to complement the analysis of observational data with a more theoretical approach.