  We employ several galaxy formation models, in particular, L-GALAXIES,
IllustrisTNG, and EAGLE, as well as observational samples from SDSS and DESI,
to investigate galactic conformity, the observed large-scale correlation
between the star-formation properties of central (primary) galaxies and those
of their neighbours. To analyse the models and observations uniformly, we
introduce CENSAT, a new algorithm to define whether a galaxy is a central or a
satellite system based on an isolation criterion. We find that the conformity
signal is present, up to at least 5 Mpc from the centres of low- and
intermediate-mass centrals in the latest version of L-GALAXIES (Ayromlou et al.
2021), IllustrisTNG, and EAGLE, as well as in SDSS and DESI observational
samples. In comparison, the conformity signal is substantially weaker in an
older version of L-GALAXIES (Henriques et al. 2020). One of the main
differences between this older model and the other models is its neglect of
ram-pressure stripping of the gas reservoirs of galaxies except within the
boundaries of massive cluster haloes. Our observational comparisons demonstrate
that this difference significantly affects the observed large-scale conformity
signal. Furthermore, by examining the contribution of backsplash, fly-by,
central, and satellite galaxies to the conformity signal, we show that much,
but not all, of it arises from primary galaxies near massive systems. Remaining
tensions between the models and observations may be solved by modifying the
physical prescriptions for how feedback processes affect the distribution and
kinematics of gas and the environment around galaxies out to scales of several
Megaparsecs.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics