We study the distribution of cold dark matter (CDM) in cosmological simulations from the FIRE (Feedback In Realistic Environments) project, for $M_{ast}sim10^{4-11},M_{odot}$ galaxies in $M_{rm h}sim10^{9-12},M_{odot}$ halos. FIRE incorporates explic
it stellar feedback in the multi-phase ISM, with energetics from stellar population models. We find that stellar feedback, without fine-tuned parameters, greatly alleviates small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile ($alpha$) shows a strong mass dependence: profiles are shallow at $M_{rm h}sim10^{10}-10^{11},M_{odot}$ and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of $alpha$, and relatively late growth of cores. Because the star formation efficiency $M_{ast}/M_{rm h}$ is strongly halo mass dependent, a rapid change in $alpha$ occurs around $M_{rm h}sim 10^{10},M_{odot}$ ($M_{ast}sim10^{6}-10^{7},M_{odot}$), as sufficient feedback energy becomes available to perturb the DM. Large cores are not established during the period of rapid growth of halos because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid buildup has completed. Stellar feedback dramatically reduces circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the Too Big To Fail problem without invoking non-standard DM. Finally, feedback and baryonic contraction in Milky Way-mass halos produce DM profiles slightly shallower than the Navarro-Frenk-White profile, consistent with the normalization of the observed Tully-Fisher relation.
We present a detailed analysis of the local evolution of 206 Lagrangian Volumes (LVs) selected at high redshift around galaxy seeds, identified in a large-volume $Lambda$ cold dark matter ($Lambda$CDM) hydrodynamical simulation. The LVs have a mass r
ange of $1 - 1500 times 10^{10} M_odot$. We follow the dynamical evolution of the density field inside these initially spherical LVs from $z=10$ up to $z_{rm low} = 0.05$, witnessing highly non-linear, anisotropic mass rearrangements within them, leading to the emergence of the local cosmic web (CW). These mass arrangements have been analysed in terms of the reduced inertia tensor $I_{ij}^r$, focusing on the evolution of the principal axes of inertia and their corresponding eigendirections, and paying particular attention to the times when the evolution of these two structural elements declines. In addition, mass and component effects along this process have also been investigated. We have found that deformations are led by dark matter dynamics and they transform most of the initially spherical LVs into prolate shapes, i.e. filamentary structures. An analysis of the individual freezing-out time distributions for shapes and eigendirections shows that first most of the LVs fix their three axes of symmetry (like a skeleton) early on, while accretion flows towards them still continue. Very remarkably, we have found that more massive LVs fix their skeleton earlier on than less massive ones. We briefly discuss the astrophysical implications our findings could have, including the galaxy mass-morphology relation and the effects on the galaxy-galaxy merger parameter space, among others.
Relaxed, massive galactic objects have been identified at redshifts z = 4;5; and 6 in hydrodynamical simulations run in a large cosmological volume. This allowed us to analyze the assembly patterns of the high mass end of the galaxy distribution at t
hese high zs, by focusing on their structural and dynamical properties. Our simulations indicate that massive objects at high redshift already follow certain scaling relations. These relations define virial planes at the halo scale, whereas at the galactic scale they define intrinsic dynamical planes that are, however, tilted relative to the virial plane. Therefore, we predict that massive galaxies must lie on fundamental planes from their formation. We briefly discuss the physical origin of the tilt in terms the physical processes underlying massive galaxy formation at high z, in the context of a two-phase galaxy formation scenario. Specifically, we have found that it lies on the different behavior of the gravitationally heated gas as compared with cold gas previously involved in caustic formation, and the mass dependence of the energy available to heat the gas.
The mass assembly and star formation histories of massive galaxies identified at low redshift z in different cosmological hydrodynamical simulations, have been studied through a detailed follow-up backwards in time of their constituent mass elements
(sampled by particles) of different types. Then, the configurations they depict at progressively higher zs have been analysed. The analyses show that these histories share common generic patterns, irrespective of particular circumstances. In any case, the results we have found are different depending on the particle type. The most outstanding differences follow. We have found that by z ~ 3.5 - 6, mass elements identified as stellar particles at z=0 exhibit a gaseous cosmic-web-like morphology with scales of ~ 1 physical Mpc, where the densest mass elements have already turned into stars by z ~ 6. These settings are in fact the densest pieces of the cosmic web, where no hot particles show up, and dynamically organized as a hierarchy of flow convergence regions, that is, attraction basins for mass flows. On the other hand, mass elements identified at the diffuse hot coronae surrounding massive galaxies at z = 0, do not display a clear web-like morphology at any z. Diffuse gas is heated when flow convergence regions go through contractive deformations, and most of it keeps hot and with low density along the evolution. To shed light on the physical foundations of the behaviour our analyses show up, as well as on their possible observational implications, these patterns have been confronted with some generic properties of singular flows as described by the adhesion model. We have found that these common patterns simulations show can be interpreted as a consequence of flow properties, that, moreover, could explain different generic observational results on massive galaxies or their samples. We briefly discuss some of them.[Abridged]
