I have a question about the treatment of gas mass return in the TNG
simulation. Based on Section 2.3.2 of Pillepich et al. 2018 (wind particles), I
thought that a wind particle was originally ejected from a star-forming gas
particle (or gas cell in precise), which later may cool and become the gas
particle again. This means that in the star-formation process for a star-forming
gas cell, a new gas particle is released (as the form of wind particle).
However, recently I heard a new different story. It says: "gas return is handled
in TNG by semi-continuously returning mass from star particles to their nearby
gas cells, i.e. the mass of a star particle nearly continuously decreases while
the mass in nearby cells nearly continuously increases. However, the method
of mass return used in TNG does not involve directly spawning of new gas cells."
I am getting quite confused for the treatment of gas return: whether it is a
sudden process with producing a wind particle or it is a continuous process
without ejecting a new particle? I really appreciate that someone in TNG group
can answer my question. Thanks very much in advance.
There are two different parts of the physical model here:
(1) Stellar feedback, i.e. the creation of galactic-scale outflows due to energy released by stars. This is what you describe in your first paragraph: wind particles are created by star-forming gas (stochastically, i.e. sometimes), and these flow out of the galaxy.
(2) Stellar mass return, i.e. as stars evolve and age, they synthesize metals. These metals, and mass in general, is expelled from stars due to stellar mass loss processes (e.g. stellar winds). This is what you describe in your second paragraph.