Galactic Metabolism and Stellar Formation

spiral form composed of younger stars and molecular hydrogen clouds. The entire Galaxy is then immersed in a galactic halo composed of sparse stars, globular clusters, gas and a form of Bose-Einstein condensate called dark matter⁴. We must not think of Galaxies as isolated matter clusters in empty space, but as living systems endowed with their own metabolism and their own breathing. These are enormous organisms that eat hydrogen transforming it into stars and producing helium and metals that are reintroduced into the interstellar medium first and then into the intergalactic one. Galaxies are immersed in a fluid called intergalactic medium from which they draw the gas necessary for their breathing. The flow of this gas inside a Galaxy is at the origin of its stellar formation. Molecular clouds, dynamized by gas coming from outside the Galaxy or by shocks produced by supernovae in the Galaxy itself, cause a condensation and aggregation process to begin which will ultimately lead to the formation of a new star. The stars then, as supernovae, will return a metallically enriched gas, with gas clouds that reach up to $4\text{kpc}$ in the galactic halo before this can cool down and reprecipitate inside the Galaxy according to a process of the order of tens of millions of years[15]. ⁴Regarding this we will soon publish a detailed writing entitled the superfluid Universe. Every Galaxy therefore has a continuous flow and reflux of hot and cold gas which, if interrupted, would radically alter stellar formation, even stopping it completely. Such an event could have occurred 7 billion years ago to the Milky Way due to a series of shocks that overheated its internal gas preventing the external one from entering and thus blocking the formation of new stars. When the internal gas cooled down, stellar production would have resumed again continuing to the present day. An important element in this respiratory cycle of the Galaxy derives from its nucleus, consisting of a compact supermassive region so gravitationally strong as to prevent light itself from escaping its attractive force and which for this reason is called a Supermassive Black Hole⁵. The center of the Galaxy has an important role in stellar formation and in keeping the galactic vital breath alive[10, 12]. The gaseous halo endowed with its own turbulent motion condenses into localized and high-density peaks, leading to the fall of cold clouds and hot filaments. The clouds then rain towards the center of the Galaxy colliding chaotically and inelastically, canceling their angular momentum. Part of the mass continues to precipitate towards the center of the Galaxy while part of the gravitational energy of the gas is converted into mechanical energy generating ultra-rapid flows of ionized gas directed outward at a speed comparable to that of light, before slowing down and cooling in a typical time of about $100\text{Myr}$. These flows of ionized gas keep the galactic halo hot and in thermal equilibrium which, like an immense heartbeat, pulses gently near hydrostatic equilibrium[11]. ⁵We will continue to use the current terminology of "Supermassive Black Hole" even if it would be better to use that of Supermassive Compact Object. The idea of Black Hole automatically implies the existence of an internal singularity beyond the event horizon, which frankly we find difficult both to prove and to reject observationally[71]. The observation of the sky leads to identifying two fundamental populations of Galaxies distinguished by their evolution[17]: the red ones, in which evolution is passive and to which elliptical and lenticular Galaxies generally belong, and the blue ones, in which evolution is characterized by star formation and whose color for this reason is more vivid and blue compared to the previous ones. Between these two populations there exists a small population designated as green transitioning between the two red and blue evolution regimes. In general these Galaxies are very few in number compared to the others, a sign that the transition from one type of evolution to another is, comparably with galactic times, very rapid. Observing very ancient Galaxy clusters we notice a greater presence of blue-type Galaxies according to an effect known as the Butcher-Oemler effect. From this effect it would seem that blue-type Galaxies were more common in the past and that gradually these ceased stellar formation activity and moved to passive evolution. What we know at the moment