Pedro Colín

Substructure and Halo Density Profiles in a Warm Dark Matter Cosmology

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 nonvanishing cosmological constant. The virial masses of the host halos range from 3.5 × 1012 to 1.7 × 1012 h-1 M☉, and they have more than 105 particles each. A key feature of the WDM power spectrum is the free-streaming length Rf,WDM, which fixes an additional parameter for the model of structure formation. We analyze the substructure of host halos using three Rf,WDM 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 Rf,WDM = 0.1 Mpc, the number of accreted small halos is a factor of 2.5 below that of the CDM model, while the fraction of destroyed satellites is almost twice as large as that of the CDM model. The larger the Rf,WDM 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 Rf,WDM = 0.1 Mpc WDM model. In the Rf,WDM = 0.1-0.2 Mpc models, the surviving guest halos at z = 0, whose masses are in the range Mh ≈ 109-1011 h-1 M☉, have an average concentration parameter c1/5 = r(Mh)/r(Mh/5), which is approximately twice as small as 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 Navarro, Frenk, & White (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 where shallow cores could be expected.

Formation and Structure of Halos in a Warm Dark Matter Cosmology

Using high-resolution cosmological N-body simulations, we study how the density profiles of dark matter halos are affected by the filtering of the density power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, Rf, and the velocity dispersion, v, are determined by the mass, mW, of the WDM particle. We found that v is too small to affect the density profiles of WDM halos. Down to the resolution attained in our simulations (~0.01 virial radii), there is not any significant difference in the density profiles and concentrations of halos obtained in simulations with and without the inclusion of v. Resolved soft cores appear only when we artificially increase the thermal velocity dispersion to a value that is much higher than v. We show that the size of soft cores in a monolithic collapse is related to the tangential velocity dispersion. The density profiles of the studied halos with masses down to ~0.01 the filtering mass Mf can be described by the Navarro-Frenk-White shape; soft cores are not formed. Nevertheless, the concentrations of these halos are lower than those of the CDM counterparts and are approximately independent of mass. The cosmogony of halos with masses Mf is not hierarchical: they form through monolithic collapse and by fragmentation of larger structures. The formation epoch of these halos is slightly later than that of halos with masses ≈Mf. The lower concentrations of WDM halos with respect to their CDM counterparts can be accounted for by their late formation epoch. Overall, our results point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with Rf ~ 0.16 Mpc (mW ≈ 0.75 keV; flat cosmology with ΩΛ = h = 0.7) also predicts concentrations, a Tully-Fisher relation, and formation epochs for small halos, which seems to be in better agreement with observations than CDM predictions.

Evolution of bias in different cosmological models

We study the evolution of the halo-halo correlation function and small-scale (?0.2-7 h-1 Mpc) bias in four cosmological models (?CDM, OCDM, ?CDM, and SCDM) using very high resolution n-body simulations with a dynamical range of ~10,000-32,000 (force resolution of ?2-4 h-1 kpc and particle mass of ?109 h-1 M?). The high force and mass resolution allows dark matter halos to survive in the tidal fields of high-density 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 ~100 h-1 kpc) scales. We find that at all epochs the two-point correlation function of galaxy-size halos ?hh is well approximated by a power law with slope ?1.6-1.8. The difference between the shape of ?hh and the shape of the correlation function of matter results in the scale-dependent bias at scales 7 h-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 antibias of b ~ 0.5-1 at small 5 h-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 nonmonotonically: 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 ?hh(z = 0) in our ?CDM60 simulation and the galaxy correlation function measured using the Automatic Plate Measuring Facility 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.

The Dependence of the Mass Assembly History of Cold Dark Matter Halos on Environment

We show by means of a high-resolution N-body simulation how the mass assembly histories of galaxy-sized cold dark matter (CDM) halos depend on environment. Halos in high-density environments form earlier than those in low-density environments, and a higher fraction of their mass is assembled in major mergers. The distribution of the present-day specific mass aggregation rate is strongly dependent on environment. While in low-density environments only ~20% of the halos are not accreting mass at the present epoch, this fraction rises to ~80% at high densities. At z = 1 the median of the specific aggregation rate is ~4 times larger than at z = 0 and almost independent of environment. All the dependences on environment found here are critically enhanced by local processes associated with subhalos because the fraction of subhalos increases as the environment gets denser. The distribution of the halo specific mass aggregation rate and its dependence on environment resemble the relations for the specific star formation rate distribution of galaxies. An analog of the morphology-density relation is also present at the level of CDM halos, being driven by the halo major-merging history. Nevertheless, baryonic processes are necessary in order to explain further details and the evolution of the relations of star formation rate, color, and morphology to environment.

Gravity or turbulence? – II. Evolving column density probability distribution functions in molecular clouds

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior 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 Npdf, and later, in a power-law Npdf. We thus suggest that the observed Npdf 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.

Gravity or turbulence? II. Evolving column density PDFs in molecular clouds

It has been recently shown that molecular clouds do not exhibit a unique shape for the column density probability distribution function (Npdf). Instead, clouds without star formation seem to possess a lognormal distribution, while clouds with active star formation develope a power-law tail at high column densities. The lognormal behavior of the Npdf has been interpreted in terms of turbulent motions dominating the dynamics of the clouds, while the power-law behavior 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 Npdf, and later, in a power-law Npdf. We thus suggest that the observed Npdf 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.

Dwarf Dark Matter Halos

We study properties of dark matter halos at high redshifts z ¼ 2-10 for a vast range of masses with the emphasis on dwarf halos with masses of 10 7 -10 9 h � 1 M� . We find that the density profiles of relaxed dwarf halos are well fitted by the Navarro, Frenk, & White (NFW) profile and do not hav ec ores. We compute the halo mass function and the halo spin parameter distribution and find that the former is very well reproduced by the Sheth & Tormen model, while the latter is well fitted by a lognormal distribution with k0 ¼ 0:042 andk ¼ 0: 63. We estimate the distribution of concentrations for halos in a mass range that covers 6 orders of magnitude, from 10 7 to 10 13 h � 1 M� , and find that the data are well reproduced by the model of Bullock et al. The extrapolation of our results to z ¼ 0 predicts that present-day isolated dwarf halos should have a very large median concentration of � 35. We measure the subhalo circular velocity functions for halos with masses that range from 4:6 ;10 9 to 10 13 h � 1 Mand find that they are similar when normalized to the circular velocity of the parent halo. Dwarf halos studied in this paper are many orders of magnitude smaller than well-studied cluster- and Milky Way-sized halos. Yet, in all respects the dwarfs are just downscaled versions of the large halos. They are cuspy and, as expected, more concentrated. They have the same spin parameter distribution and follow the same mass function that was measured for large halos. Subject headingg cosmology: theory — dark matter — galaxies: formation — galaxies: halos — methods: numerical

Structure and Subhalo Population of Halos in a Self-interacting Dark Matter Cosmology

A series of high-resolution numerical simulations were performed to study the structure and substructure of Milky Way-sized (MW-sized) and cluster-sized halos in a ΛCDM cosmology with self-interacting (SI) dark matter particles. The cross section per unit of particle mass has the form σDM = σ0(1/v100)α, where σ0 is a constant in units of cm2 g-1 and v100 is the relative velocity in units of 100 km s-1. Different values for σ0 with α = 0 or 1 were used. For small values of σDM = const (0.5, α = 0), the core density of the halos at z = 0 is typically higher at a given mass for lower values of σ0 or, at a given σ0, for lower masses. For values of σ0 as high as 3.0, both cluster- and MW-sized halos may undergo the gravothermal catastrophe before z = 0. The core expansion occurs in a stable regime because the heat capacity C is positive in the center. After the maximum expansion, the isothermal core is hotter than the periphery and C < 0. Then the gravothermal catastrophe is triggered. The instability onset can be delayed by both the dynamical heating of the halo by major mergers and the interaction of cool particles with the hot environment of a host halo. When α = 1, the core density of cluster- and MW-sized halos is similar. Using σDM = 0.5-1.0(1/v100), our predictions agree with the central densities and the core scaling laws of halos inferred from the observations of both dwarf and low surface brightness galaxies and clusters of galaxies. Regarding the cumulative vmax function of subhalos within MW-sized halos, when (σ0, α) = (0.1, 0.0), (0.5, 0.0), or (0.5, 1.0) it agrees roughly with observations (luminous satellites) for vmax 30 km s-1, while at vmax = 20 km s-1 the functions are already a factor of 5-8 higher, similar to the CDM predictions. For (σ0, α) = (1.0, 1.0), this function lies above the corresponding CDM function. The structure and number of subhalos are affected by the scattering properties of the host halo rather than by those of the subhalos. The halos with SI have more specific angular momentum at a given mass shell and are rounder than their CDM counterparts. However, the angular momentum excess with regard to CDM is small. We conclude that the introduction of SI particles with σDM 1/v100 may remedy the cuspy core problem of the CDM cosmogony, at the same time keeping a subhalo population similar to that of the CDM halos.

Low-mass galaxy assembly in simulations: regulation of early star formation by radiation from massive stars

Despite recent success in forming realistic present-day galaxies, simulations still form the bulk of their stars earlier than observations indicate. We investigate the process of stellar mass assembly in low-mass field galaxies, a dwarf and a typical spiral, focusing on the effects of radiation from young stellar clusters on the star formation (SF) histories. We implement a novel model of SF with a deterministic low efficiency per free-fall time, as observed in molecular clouds. Stellar feedback is based on observations of star-forming regions, and includes radiation pressure from massive stars, photoheating in H II regions, supernovae and stellar winds. We find that stellar radiation has a strong effect on the formation of low-mass galaxies, especially at z 〉 1, where it efficiently suppresses SF by dispersing cold and dense gas, preventing runaway growth of the stellar component. This behaviour is evident in a variety of observations but had so far eluded analytical and numerical models without radiation feedback. Compared to supernovae alone, radiation feedback reduces the SF rate by a factor of 100 at z ≲ 2, yielding rising SF histories which reproduce recent observations of Local Group dwarfs. Stellar radiation also produces bulgeless spiral galaxies and may be responsible for excess thickening of the stellar disc. The galaxies also feature rotation curves and baryon fractions in excellent agreement with current data. Lastly, the dwarf galaxy shows a very slow reduction of the central dark matter density caused by radiation feedback over the last 7 Gyr of cosmic evolution.

AN EVOLUTIONARY MODEL FOR COLLAPSING MOLECULAR CLOUDS AND THEIR STAR FORMATION ACTIVITY

We present an idealized, semi-empirical model for the evolution of gravitationally contracting molecular clouds (MCs) and their star formation rate (SFR) and efficiency (SFE). The model assumes that the instantaneous SFR is given by the mass above a certain density threshold divided by its free-fall time. The instantaneous number of massive stars is computed assuming a Kroupa initial mass function. These stars feed back on the cloud through ionizing radiation, eroding it. The main controlling parameter of the evolution turns out to be the maximum cloud mass, M max. This allows us to compare various properties of the model clouds against their observational counterparts. A giant molecular cloud (GMC) model (M max ~ 105 M ☉) adheres very well to the evolutionary scenario recently inferred by Kawamura et al. for GMCs in the Large Magellanic Cloud. A model cloud with M max ≈ 2000 M ☉ evolves in the Kennicutt-Schmidt diagram, first passing through the locus of typical low-to-intermediate-mass star-forming clouds, and then moving toward the locus of high-mass star-forming ones over the course of ~10 Myr. Also, the stellar age histograms for this cloud a few Myr before its destruction agree very well with those observed in the ρ-Oph stellar association, whose parent cloud has a similar mass, and imply that the SFR of the clouds increases with time. Our model thus agrees well with various observed properties of star-forming MCs, suggesting that the scenario of gravitationally collapsing MCs, with their SFR regulated by stellar feedback, is entirely feasible and in agreement with key observed properties of MCs.