Description: The growth of galaxies through adiabatic accretion of dark matter is one of the main drivers of galaxy evolution. By isolating it from other processes such as mergers, we analyse how it affects the evolution of star clusters. Our study comprises a fast and approximate exploration of the orbital and intrinsic cluster parameter space, and more detailed monitoring of their evolution, through N -body simulations for a handful of cases. We find that the properties of present-day star clusters and their tidal tails differ very little, whether the clusters are embedded in a growing galactic halo for 12 Gyr, or in a static one.

Description: The strong time-dependence of the dynamics of galactic bars yields a complex and rapidly evolving distribution of dense gas and star forming regions. Although bars mainly host regions void of any star formation activity, their extremities can gather the physical conditions for the formation of molecular complexes and mini-starbursts. Using a sub-parsec resolution hydrodynamical simulation of a Milky Way-like galaxy, we probe these conditions to explore how and where bar (hydro-)dynamics favours the formation or destruction of molecular clouds and stars. The interplay between the kpc-scale dynamics (gas flows, shear) and the parsec-scale (turbulence) is key to this problem. We find a strong dichotomy between the leading and trailing sides of the bar, in term of cloud fragmentation and in the age distribution of the young stars. After orbiting along the bar edge, these young structures slow down at the extremities of the bar, where orbital crowding increases the probability of cloud–cloud collision. We find that such events increase the Mach number of the cloud, leading to an enhanced star formation efficiency and finally the formation of massive stellar associations, in a fashion similar to galaxy–galaxy interactions. We highlight the role of bar dynamics in decoupling young stars from the clouds in which they form, and discuss the implications on the injection of feedback into the interstellar medium (ISM), in particular in the context of galaxy formation.

Description: Together with interstellar turbulence, gravitation is one key player in star formation. It acts both at galactic scales in the assembly of gas into dense clouds and inside those structures for their collapse and the formation of pre-stellar cores. To understand to what extent the large-scale dynamics govern the star formation activity of galaxies, we present hydrodynamical simulations in which we generalize the behaviour of gravity to make it differ from Newtonian dynamics in the low-acceleration regime. We focus on the extreme cases of interacting galaxies, and compare the evolution of galaxy pairs in the dark matter paradigm to that in the Milgromian dynamics (MOND) framework. Following up on the seminal work by Tiret & Combes, this paper documents the first simulations of galaxy encounters in MOND with a detailed Eulerian hydrodynamical treatment of baryonic physics, including star formation and stellar feedback. We show that similar morphologies of the interacting systems can be produced by both the dark matter and MOND formalisms, but require a much slower orbital velocity in the MOND case. Furthermore, we find that the star formation activity and history are significantly more extended in space and time in MOND interactions, in particular in the tidal debris. Such differences could be used as observational diagnostics and make interacting galaxies prime objects in the study of the nature of gravitation at galactic scales.

Description: The radii of young (100 Myr) star clusters correlate only weakly with their masses. This shallow relation has been used to argue that impulsive tidal perturbations, or ‘shocks’, by passing giant molecular clouds (GMCs) preferentially disrupt low-mass clusters. We show that this mass–radius relation is in fact the result of the combined effect of two-body relaxation and repeated tidal shocks. Clusters in a broad range of environments including those like the solar neighbourhood evolve towards a typical radius of a few parsecs, as observed, independent of the initial radius. This equilibrium mass–radius relation is the result of a competition between expansion by relaxation and shrinking due to shocks. Interactions with GMCs are more disruptive for low-mass clusters, which helps to evolve the globular cluster mass function (GCMF). However, the properties of the interstellar medium in high-redshift galaxies required to establish a universal GCMF shape are more extreme than previously derived, challenging the idea that all GCs formed with the same power-law mass function.

Description: We present a self-consistent hydrodynamical simulation of a Milky Way-like galaxy at a resolution of 0.05 pc. The model includes star formation and a new implementation of stellar feedback through photoionization, radiative pressure and supernovae. The simulation resolves the structure of the interstellar medium at sub-parsec resolution for a few cloud lifetimes and at 0.05 pc for about a cloud-crossing time. The turbulence cascade and gravitation from kpc scales are de facto included in smaller structures like molecular clouds. We show that the formation of a bar influences the dynamics of the central ~100 pc by creating resonances. At larger radii, the spiral arms host the formation of regularly spaced clouds: beads on a string and spurs. These instabilities pump turbulent energy into the gas, generally in the supersonic regime. Because of asymmetric drift, the supernovae explode outside their gaseous nursery, which diminishes the effect of feedback on the structure of clouds. The evolution of clouds is thus mostly due to fragmentation and gas consumption, regulated mainly by supersonic turbulence. The transition from turbulence-supported to self-gravitating gas is detected in the gas density probability distribution function at ~2000 cm –3 . The power-spectrum density suggests that gravitation governs the hierarchical organization of structures from the galactic scale down to a few pc.

Description: We study the connection between the large-scale dynamics and the gas fuelling towards a central black hole via the analysis of a Milky Way-like simulation at subparsec resolution. This allows us to follow a set of processes at various scales (e.g. the triggering of inward gas motion towards inner resonances via the large-scale bar, the connection to the central black hole via minispirals) in a self-consistent manner. This simulation provides further insights on the role of shear for the inhibition of star formation within the bar in regions with significant amount of gas. We also witness the decoupling of the central gas and nuclear cluster from the large-scale disc, via interactions with the black hole. This break of symmetry in the mass distribution triggers the formation of gas clumps organized in a time-varying 250 pc ring-like structure, the black hole being offset by about 70 pc from its centre. Some clumps form stars, while most get disrupted or merge. Supernovae feedback further create bubbles and filaments, some of the gas being expelled to 100 pc or higher above the galaxy plane. This helps remove angular momentum from the gas, which gets closer to the central dark mass. Part of the gas raining down is being accreted, forming a 10 pc polar disc-like structure around the black hole, leading to an episode of star formation. This gives rise to multiple stellar populations with significantly different angular momentum vectors, and may lead to a natural intermittence in the fuelling of the black hole.

Description: The distinction between globular clusters and dwarf galaxies has been progressively blurred by the recent discoveries of several extended star clusters, with size (20–30 pc) and luminosity (–6 〈  M v  〈 –2) comparable to the one of the faint dwarf spheroidals. In order to explain their sparse structure, it has been suggested that they formed as star clusters in dwarf galaxy satellites that later accreted on to the Milky Way. If these clusters form in the centre of dwarf galaxies, they evolve in a tidally compressive environment where the contribution of the tides to the virial balance can become significant, and lead to a supervirial state and subsequent expansion of the cluster, once removed. Using N -body simulations, we show that a cluster formed in such an extreme environment undergoes a sizable expansion, during the drastic variation of the external tidal field due to the accretion process. However, we show that the expansion due to the removal of the compressive tides is not enough to explain the observed extended structure, since the stellar systems resulting from this process are always more compact than the corresponding clusters that expand in isolation due to two-body relaxation. We conclude that an accreted origin of extended globular clusters is unlikely to explain their large spatial extent, and rather favour the hypothesis that such clusters are already extended at the stage of their formation.

Description: We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multicomponent model for stellar feedback and reaching numerical convergence in the global star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericentre passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated with the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultracompact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic centre. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.

Description: We introduce a numerical method to integrate tidal effects on collisional systems, using any definition of the external potential as a function of space and time. Rather than using a linearization of the tidal field, this new method follows a differential technique to numerically evaluate the tidal acceleration and its time derivative. These are then used to integrate the motions of the components of the collisional systems, like stars in star clusters, using a predictor–corrector scheme. The versatility of this approach allows the study of star clusters, including their tidal tails, in complex, multicomponents, time-evolving external potentials. The method is implemented in the code nbody 6.

Description: Using parsec-resolution simulations of a typical galaxy merger, we study the triggering of starbursts by connecting the (inter-)galactic dynamics to the structure of the interstellar medium. The gravitational encounter between two galaxies enhances tidal compression over large volumes, which increases and modifies the turbulence, in particular its compressive mode with respect to the solenoidal one. This generates an excess of dense gas leading to intense star formation activity. Along the interaction, the compressive turbulence modifies the efficiency of gas-to-star conversion which, in the Schmidt–Kennicutt diagram, drives the galaxies from the sequence of discs to that of starbursts.