The majority of galaxies with current star formation rates (SFRs), $\rm SFR_{\rm o} \ge 10^{-3} \, M_\odot\,yr^{-1}$, in the Local Cosmological Volume, where observations should be reliable, have the property that their observed SFRo is larger than their average SFR. This is in tension with the evolution of galaxies described by delayed-τ models, according to which the opposite would be expected. The tension is apparent in that local galaxies imply the star formation time-scale τ ≈ 6.7 Gyr, much longer than the 3.5–4.5 Gyr obtained using an empirically determined main sequence at several redshifts. Using models where the SFR is a power law in time of the form ∝(t − t1)η for t1 = 1.8 Gyr (with no stars forming prior to t1) implies that η = 0.18 ± 0.03. This suggested near-constancy of a galaxy’s SFR over time raises non-trivial problems for the evolution and formation time of galaxies, but is broadly consistent with the observed decreasing main sequence with increasing age of the Universe.

This document describes the general process of setting up, running, and analysing disc galaxy simulations using the freely available program PHANTOM OF RAMSES (POR). This implements Milgromian Dynamics (MOND) with a patch to the RAMSES grid-based N-body and hydrodynamical code that uses adaptive mesh refinement. We discuss the procedure of setting up isolated and interacting disc galaxy initial conditions for POR, running the simulations, and analysing the results. This manual also concisely documents all previously developed MOND simulation codes and the results obtained with them.

We conduct hydrodynamical MOND simulations of isolated disc galaxies over the stellar mass range M⋆/M⊙ = 107 − 1011 using the adaptive mesh refinement code phantom of ramses (por), an adaptation of the ramses code with a Milgromian gravity solver. The scale lengths and gas fractions are based on observed galaxies, and the simulations are run for 5 Gyr. The main aim is to see whether existing sub-grid physics prescriptions for star formation and stellar feedback reproduce the observed main sequence, and Kennicutt-Schmidt relation that captures how the local and global star formation rates relate to other properties. Star formation in the models starts soon after initialisation and continues as the models evolve. The initialized galaxies indeed evolve to a state which is on the observed main sequence, and the Kennicutt-Schmidt relation. The available formulation of sub-grid physics is therefore adequate and leads to galaxies that largely behave like observed galaxies, grow in radius, and have flat rotation curves − provided we use Milgromian gravitation. Furthermore, the strength of the bars tends to be inversely correlated with the stellar mass of the galaxy, whereas the bar length strongly correlates with the stellar mass. Irrespective of the mass, the bar pattern speed stays constant with time, indicating that dynamical friction does not affect the bar dynamics. The models demonstrate Renzo’s rule and form structures at large radii, much as in real galaxies. In this framework, baryonic physics is thus sufficiently understood to not pose major uncertainties in our modelling of global galaxy properties.

Context. The lenticular galaxy NGC 474 hosts a rich system of tidal shells and streams, some of which are exceptionally bright. Two teams recently presented spectroscopic observations of the brightest shells. These were the first shell spectra ever observed in integrated starlight. The authors studied the stellar populations of the shell, of the center of the galaxy, and of its globular clusters. The precise formation scenario for the tidal features of this prominent galaxy still remained unclear, however. Aims. Here, we add further clues on their formation from the radii of the shells, and we present a scenario for the formation of the tidal features that seems to be unique and can explain all available data. Methods. Shell radii were analyzed with the shell identification method, and we ran self-consistent simulations of the formation of the tidal features. We considered Newtonian as well as MOND gravity. Results. Observations suggest that the tidal features originate from the accretion of a spiral galaxy. According to the shell identification method, the merging galaxies first collided 1.3 Gyr ago and then again 0.9 Gyr ago, thereby forming the shells in two generations. This would also explain the young ages of stellar populations in the center of the galaxy and the young age of the globular clusters. The analytic models of shell propagation that underlie the shell identification method are verified by a simulation. The simulations reproduce the observed morphology of the tidal features well. The accreted spiral likely reached NGC 474 on the plane of the sky nearly radially from the south, its rotation axis pointing toward us. It probably had a stellar mass of about one-sixth of NGC 474, that is, 109.8 M⊙. Apparently, all tidal features in the galaxy originate from one merger.

We present hydrodynamical star-forming simulations in the Milgromian dynamics (MOND) framework of a gas-rich disc galaxy with properties similar to AGC 114905, which has recently been argued to have a rotation curve (RC) that is inconsistent with the MOND prediction. Our first model considers the galaxy in isolation, while our second model includes an external field of $0.05 \, a_{_0}$, the estimated gravitational field from large-scale structure. We show that isophotes in the face-on view can differ from circular at the 50% level. This could mislead observers into overestimating the inclination i between disc and sky planes. Because RCs require a correction factor of 1/sin i, the actual RC could be much higher than reported by observers. This plausibly reconciles AGC 114905 with MOND expectations.