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We investigate the evolution of the tidal field experienced by massive star clusters using cosmological simulations of Milky Way-sized galaxies. Clusters in our simulations experience the strongest tidal force in the first few hundred Myr after formation, when the maximum eigenvalue of the tidal tensor reaches several times 104 Gyr−2. After about 1 Gyr the tidal field plateaus at a lower value, with the median λm ∼ 3 × 103 Gyr−2. The fraction of time clusters spend in high tidal strength (λm > 3 × 104 Gyr−2) regions also decreases with their age from ∼20 per cent immediately after formation to less than 1 per cent after 1 Gyr. At early ages both the in situ and ex situ clusters experience similar tidal fields, while at older ages the in situ clusters in general experience stronger tidal field due to their lower orbits in host galaxy. This difference is reflected in the survival of clusters: we looked into cluster disruption calculated in simulation runtime and found that ex situ star clusters of the same initial mass typically end up with higher bound fraction at the last available simulation snapshot than the in situ ones.

 

ABSTRACT We study the growth of stellar discs of Milky Way-sized galaxies using a suite of cosmological simulations. We calculate the half-mass axis lengths and axis ratios of stellar populations split by age in galaxies with stellar mass $M_{*}=10^7\!-\!10^{10}\, \mathrm{M}_{\odot }$ at redshifts z > 1.5. We find that in our simulations stars always form in relatively thin discs, and at ages below 100 Myr are contained within half-mass height z1/2 ∼ 0.1 kpc and short-to-long axial ratio z1/2/x1/2 ∼ 0.15. Disc thickness increases with the age of stellar population, reaching median z1/2 ∼ 0.8 kpc and z1/2/x1/2 ∼ 0.6 for stars older than 500 Myr. We trace the same group of stars over the simulation snapshots and show explicitly that their intrinsic shape grows more spheroidal over time. We identify a new mechanism that contributes to the observed disc thickness: rapid changes in the orientation of the galactic plane mix the configuration of young stars. The frequently mentioned ‘upside-down’ formation scenario of galactic discs, which posits that young stars form in already thick discs at high redshift, may be missing this additional mechanism of quick disc inflation. The actual formation of stars within a fairly thin plane is consistent with the correspondingly flat configuration of dense molecular gas that fuels star formation. 

ABSTRACT We examine the nature of kpc-scale clumps seen in high-redshift galaxies using a suite of cosmological simulations of galaxy formation. We identify rest-frame UV clumps in mock HST images smoothed to 500 pc resolution, and compare them with the intrinsic 3D clumps of young stars identified in the simulations with 100 pc resolution. According to this comparison for the progenitors of Milky Way-sized galaxies probed by our simulations, we expect that the stellar masses of the observed clumps are overestimated by as much as an order of magnitude, and that the sizes of these clumps are also overestimated by factor of several, due to a combination of spatial resolution and projection. The masses of young stars contributing most of the UV emission can also be overestimated by factor of a few. We find that most clumps of young stars present in a simulation at one time dissolve on a timescale shorter than ∼150 Myr. Some clumps with dense cores can last longer but eventually disperse. Most of the clumps are not bound structures, with virial parameter αvir > 1. We find similar results for clumps identified in mock maps of H α emission measure. We examine the predictions for effective clump sizes from the linear theory of gravitational perturbations and conclude that they are inconsistent with being formed by global disc instabilities. Instead, the observed clumps represent random projections of multiple compact star-forming regions.