We performed a series of high-resolution collisionless N-body simulations designed to study the substructure of Milky Way-size galactic halos (host halos) and the density profiles of halos in a warm dark matter (WDM) scenario with a non-vanishing cosmological constant. The virial masses of the host halos range from 3.5 × 10 12 h −1 M ⊙ to 1.7 × 10 12 h −1 M ⊙ and they have more than 10 5 particles each. A key feature of the WDM power spectrum is the free-streaming length R f,W DM which fixes an additional parameter for the model of structure formation. We analyze the substructure of host halos using three R f,W DM values: 0.2, 0.1, and 0.05 Mpc and compare results to the predictions of the cold dark matter (CDM) model. We find that guest halos (satellites) do form in the WDM scenario but are more easily destroyed by dynamical friction and tidal disruption than their counterparts in a CDM model. The small number of guest halos that we find in the WDM models with respect to the CDM one is the result of a lower guest halo accretion and a higher satellite destruction rate. These two phenomena operate almost with the same intensity in delivering a reduced number of guest halos at z = 0. For the model with R f,W DM = 0.1 Mpc the number of accreted small halos is a factor 2.5 below that of the CDM model while the fraction of destroyed satellites is almost twice larger than that of the CDM model. The larger the R f,W DM value the greater the size of these two effects and the smaller the abundance of satellites. Under the assumption that each guest halo hosts a luminous galaxy, we find that the observed circular velocity function of satellites around the Milky Way and Andromeda is well described by the R f,W DM = 0.1 Mpc WDM model. In the R f,W DM = 0.1 − 0.2 Mpc models, the surviving guest halos at z = 0 -whose masses are in the range M h ≈ 10 9 − 10 11 h −1 M ⊙ -have an average concentration parameter c 1/5 (= r(M h )/r(M h /5)) which is approximately twice smaller than that of the corresponding CDM guest halos. This difference, very likely, produces the higher satellite destruction rate found in the WDM models. The density profile of host halos is well described by the NFW fit whereas guest halos show a wide variety of density profiles. A tendency to form shallow cores is not evident; the profiles, however, are limited by a poor mass resolution in the innermost regions were shallow cores could be expected.
We study the evolution of the halo-halo correlation function and bias in four cosmological models (ΛCDM, OCDM, τ CDM, and SCDM) using very high-resolution N -body simulations with dynamical range of ∼ 10, 000 − 32, 000 (force resolution of ≈ 2 − 4h −1 kpc and particle mass of ≈ 10 9 h −1 M ⊙ ). The high force and mass resolution allows dark matter (DM) halos to survive in the tidal fields of highdensity regions and thus prevents the ambiguities related with the "overmerging problem." This allows us to estimate for the first time the evolution of the correlation function and bias at small (down to ∼ 100h −1 kpc) scales.We find that at all epochs the 2-point correlation function of galaxy-size halos ξ hh is well approximated by a power-law with slope ≈ 1.6 − 1.8. The difference between the shape of ξ hh and the shape of the correlation function of matter results in the scale-dependent bias at scales ∼ < 7h −1 Mpc, which we find to be a generic prediction of the hierarchical models, independent of the epoch and of the model details. The bias evolves rapidly from a high value of ∼ 2 − 5 at z ∼ 3 − 7 to the anti-bias of b ∼ 0.5 − 1 at small ∼ < 5h −1 Mpc scales at z = 0. Another generic prediction is that the comoving amplitude of the correlation function for halos above a certain mass evolves non-monotonically: it decreases from an initially high value at z ∼ 3 − 7, and very slowly increases at z ∼ < 1. We find that our results agree well with existing clustering data at different redshifts, indicating the general success of the hierarchical models of structure formation in which galaxies form inside the host DM halos. Particularly, we find an excellent agreement in both slope and the amplitude between ξ hh (z = 0) in our ΛCDM 60 simulation and the galaxy correlation function measured using the APM galaxy survey. At high redshifts, the observed clustering of the Lyman-break galaxies is also well reproduced by the models. We find good agreement at z ∼ > 2 between our results and predictions of the analytical models of bias evolution. This indicates that we have a solid understanding of the nature of the bias and of the processes that drive its evolution at these epochs. We argue, however, that at lower redshifts the evolution of the bias is driven by dynamical processes inside the nonlinear high-density regions such as galaxy clusters and groups. These processes do not depend on cosmology and tend to erase the differences in clustering properties of halos that exist between cosmological models at high z. Subject headings: cosmology: theory -large-scale structure of universe -methods: numerical
A series of high-resolution ΛCDM cosmological N-body simulations are used to study the properties of galaxy-size dark halos as a function of global environment. We analyse halos in three types of environment: "cluster" (cluster halos and their surroundings), "void" (large regions with density contrasts −0.85), and "field" (halos not contained within larger halos). We find that halos in clusters have a median spin parameter ∼ 1.3 times lower, a minor-to-major axial ratio ∼ 1.2 times lower (more spherical), and a less aligned internal angular momentum than halos in voids and the field. For masses 5 × 10 11 h −1 M ⊙ , halos in cluster regions are on average ∼ 30 − 40% more concentrated and have ∼ 2 times higher central densities than halos in voids. While for halos in cluster regions the concentration parameters decrease on average with mass with a slope of ∼ 0.1, for halos in voids these concentrations do not seem to change with mass. When comparing only parent halos from the samples, the differences are less pronounced but they are still significant. We obtain also the maximum circular velocity-and rms velocity-mass relations. These relations are shallower and more scattered for halos in clusters than in voids, and for a given circular velocity or rms velocity, the mass is smaller at z = 1 than at z = 0 for all environments. At z = 1, the differences in the halo properties with environment almost dissapear, suggesting this that the differences were stablished mainly after z ∼ 1. The halos in the cluster regions undergo more dramatic changes than those in the field or the voids. The differences in halo properties with environment are owing to (i) the dependence of halo formation time on global environment, and (ii) local effects as tidal stripping and the tumultuos histories that halos suffer in high-density regions.We calculate seminumerical models of disk galaxy evolution using halos with the concentrations and spin parameters found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger maximum circular velocity and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from cluster to void environments. Although all these trends agree with observations, the latter tend to show more differences, suggesting this that physical ingredients not considered here as missalignment of angular momentum, halo triaxility, merging, ram pressure stripping, harassment, etc. play an important role for galaxy evolution, specially in high-density environments.
It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (N‐PDF). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develop a power‐law tail at high column densities. The lognormal behaviour of the N‐PDF has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power‐law behaviour occurs when the cloud is dominated by gravity. In the present contribution, we use thermally bi‐stable numerical simulations of cloud formation and evolution to show that, indeed, these two regimes can be understood in terms of the formation and evolution of molecular clouds: a very narrow lognormal regime appears when the cloud is being assembled. However, as the global gravitational contraction occurs, the initial density fluctuations are enhanced, resulting, first, in a wider lognormal N‐PDF, and later, in a power‐law N‐PDF. We thus suggest that the observed N‐PDF of molecular clouds are a manifestation of their global gravitationally contracting state. We also show that, contrary to recent suggestions, the exact value of the power‐law slope is not unique, as it depends on the projection in which the cloud is being observed.
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