On the connection between galactic downsizing and the most fundamental
  galactic scaling relations
  In their evolution, star-forming galaxies are known to follow scaling
relations between some fundamental physical quantities, such as the
mass-metallicity and the main sequence relations. We aim at studying the
evolution of galaxies that, at a given redshift, lie simultaneously on the
mass-metallicity and main sequence relations (MZR, MSR). To this aim, we use
the analytical, 'leaky-box' chemical evolution model of Spitoni et al. (2017),
in which galaxy evolution is described by an infall timescale $\tau$ and a wind
efficiency $\lambda$. We provide a detailed analysis of the temporal evolution
of galactic metallicity, stellar mass, mass-weighted age and gas fraction. The
evolution of the galaxies lying on the MZR and MSR at $z\sim0.1$ suggests that
the average infall time-scale in two different bins of stellar masses
($M_{\star}<10^{10} M_{\odot}$ and $M_{\star}>10^{10} M_{\odot}$) decreases
with decreasing redshift. This means that at each redshift, only the youngest
galaxies can be assembled on the shortest timescales and still belong to the
star-forming MSR. In the lowest mass bin, a decrease of the median $\tau$ is
accompanied by an increase of the median $\lambda$ value. This implies that
systems which have formed at more recent times will need to eject a larger
amount of mass to keep their metallicity at low values. Another important
result is that galactic downsizing, as traced by the age-mass relation, is
naturally recovered by imposing that local galaxies lie on both the MZR and
MSR. Finally, we study the evolution of the hosts of C$_{\rm IV}$ -selected
AGN, which at $z\sim 2$ follow a flat MZR, as found by Mignoli et al. (2019).
If we impose that these systems lie on the MSR, at lower redshifts we find an
'inverted' MZR, meaning that some additional processes must be at play in their
evolution.
