Tracing the formation of the first galaxies in the early Universe is one of the hottest topics in Astrophysics, but also one of the most challenging areas, since such distant (and generally low mass) galaxies are very faint at most wavelengths. I have been pioneering the use of far-IR fine structure lines to identify and characterize primordial galaxies. Far-IR fine structure lines are generally the strongest emission lines in the spectrum of any galaxy, hence the most promising tool to detect and trace galaxies at high redshift. Among these lines the [CII]158um is often the strongest.

The first detection of [CII]158um at high redshift, specifically at z=6.4, was obtained by Maiolino et al. (2005). This discovery has opened the era of using far-IR lines for searching and characterizing distant galaxies, nowadays one of the main science cases of major facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA). Since the first [CII] detection at high redshift, several groups world wide have used far-IR fine structure lines to identify distant galaxies and to trace their evolutionary processes, with steadily growing detection rates and delivering exquisite maps of the interstellar medium and circum-galactic medium in these primordial systems.

Additional highlights obtained by exploiting this technique by members of my team or collaborating groups include:

• the first fully resolved interferometric maps, revealing extended, optically elusive star forming systems (Gallerani, Neri, Maiolino et al. 2012);

• the detection of other far-IR lines at high redshift, used to constrain the physical properties and metal content of distant galaxies (Nagao, Maiolino, et al. 2012, De Carli et al. 2012);

• the discovery of exceptionally strong [CII] emitters (Maiolino et al. 2009, De Breuck et al. 2011);

• the detection of massive outflows and very extended star formation in primordial systems (Maiolino et al. 2012, Cicone, Maiolino et al. 2015, Bischetti, Maiolino et al. 2019, Ginolfi et al. 2020);

• the detection of [CII] and [OIII]88um at z~7, i.e. well within the re-ionization epoch, directly tracing the assembly of primeval galaxies (Maiolino et al. 2015, Carniani, Maiolino et al. 2017, Pentericci et al. 2016, Smit et al. 2017, Carniani, Maiolino et al. 2018, Carniani, Maiolino et al. 2018);

• dynamical maps of massive systems at high redshift (De Breuck et al. 2014, Walter et al. 2009, Carniani et al. 2014, Trakhtenbrot et al. 2017, Smit et al. 2017);

• identification of extremely obscured primordial galaxies (Gallerani, Neri, Maiolino et al. 2012, Walter et al. 2012, Carniani, Maiolino et al. 2018, Carniani, Maiolino et al. 2018)