  Motivated by the need for realistic, dynamically self-consistent, evolving
galaxy models that avoid the inherent complexity of full, and zoom-in,
cosmological simulations, we have developed Nexus, an integral, flexible
framework to create synthetic galaxies made of both collisionless and gaseous
components. Nexus leverages the power of publicly available, tried-and-tested
packages: i) the stellar-dynamics, action-based library AGAMA; and ii) the
Adaptive Mesh Refinement, N-body/hydrodynamical code Ramses, modified to meet
our needs. In addition, we make use of a proprietary module to account for
realistic galaxy formation (sub-grid) physics, including star formation,
stellar feedback, and chemical enrichment. As a framework to perform controlled
experiments of idealised galaxies, Nexus' basic functionality consists in the
generation of bespoke initial conditions (ICs) for any desired galaxy model,
which are advanced in time to simulate the system's evolution. The fully
self-consistent ICs are generated with a distribution-function based approach,
as implemented in the galaxy modelling module of AGAMA -- up to now restricted
to collisionless components, extended in this work to treat two types of
gaseous configurations: (i) hot halos; and (ii) gas disks. For the first time,
we are able to construct equilibrium models with disc gas fractions in the
range $0 < f_{\rm gas} < 1$, needed to model high-redshift galaxies. The
framework is ideally suited to the study of galactic ecology, specifically how
stars and gas work together over billions of years. As a validation of our
framework, we reproduce - and improve upon - several isolated galaxy model
setups reported in earlier studies. Finally, we showcase Nexus by presenting an
interesting type of `nested bar' galaxy class. Future upgrades of Nexus will
include magneto-hydrodynamics and highly energetic particle (`cosmic ray')
heating.
