Most of the investigations of galaxies and of quasi-stellar objects (QSOs) in the electromagnetic universe feature the occurrence of three main epochs of evolution, along cosmic history.
- COSMIC DAWN
The cosmic dawn is the epoch extending from redshift z ~ 20 when the universe was only a few hundred million years old to redshift z ~ 6, corresponding to about one billion year. During that epoch, dark mater haloes begin to collapse and the first stars, the first black holes and galactic discs start to form and grow, lightening up the universe. Around z ~ 11 - 6 the universe completed the phase of cosmic re-ionisation of gas turning neutral hydrogen and helium, into a hot tenuous intergalactic plasma.
The farthest QSO ULAS J1120+0641, Gamma Ray Burst GRB 090423 and galaxy MACS0647-JD, detected at the limits of current capabilities, were in place when the universe was less than one billion years old, at redshift ~ 7, 8, 9, resepectively. They are the brightest sources probing the tip of an underlying distribution of fainter early objects, the less luminous pre-galactic structures and black holes for which little is known. Even the brightest QSOs fade away in the optical due to the Gunn-Peterson through and the search for the deepest sources may be hindered by confusion due to crowding and the unresolved background light. Thus, unexplored is the entire universe of objects that are the building blocks of the largest at the tip of their distributions.
- COSMIC HIGH NOON
The cosmic high noon is an epoch of critical transformations for galaxies, extending from z ~ 6 to 2. Around redshift 3, the luminous QSOs and the star formation rate (SFR) have their peak. Galactic discs had much higher surface densities and gas fractions than now, and the nature of gravitational instabilities seeded in their amorphous structures and the physics of star's formation may have been different or more extreme than today. The cosmic-integrated star formation rate and the accretion rate of gas feeding black holes and their powerful outflows were probably at maximum strength around z ~ 2. Galaxy mergers during cosmic high noon were likely to be the force driving the process of galaxy assembly, star formation and black hole growth. The role of mergers is still a matter of dispute but it is at the base of our current paradigm of galaxy formation.
- COSMIC AFTERNOON
The cosmic afternoon races the epoch of decline of both the star formation and QSO's activity. It is a phase of relented evolution extending from z ~ 1 to the present. Observations of galaxies and of the less luminous active galactic nuclei (AGN) give a description of this quieter universe. Dormant black holes, as dark massive objects, are now found in near galaxies. Their mass correlates tightly with the mass of the stars in the host galaxy revealing the occurrence of a joint, symbiotic evolution that likely established during cosmic high noon and dawn.
Among the galaxies, the Milky Way, our closest environment, is the perfect habitat for exploring the nature of all stellar populations, and in particular of compact objects, the white dwarfs, neutron stars and stellar black holes that we observe isolated or in binaries. Over the years the study of these sources allowed to unravel key processes of stellar evolution indicating, e.g. pathways for the formation of type Ia supernovae -standard candles for exploring the geometry of cosmic expansion- and evolution tracks for forming neutron star binary systems. Neutron star binaries have been the first cosmic laboratories to test General Relativity, giving unambiguous proof, albeit indirect, that gravitational waves exist in nature.
Surprisingly the Milky Way offers also the closest example of an imminent merger in our Local Group: Andromeda along with a handful of lesser galaxies is falling toward us, and Andromeda and the Milky Way house central black holes that will pair to form an binary before the Sun will expand into a red-giant.