Vladimir Avila-Reese

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.

On the Formation and Evolution of Disk Galaxies: Cosmological Initial Conditions and the Gravitational Collapse

We use a semianalytical approach and the standard σ8 = 1 cold dark matter (SCDM) cosmological model to study the gravitational collapse and virialization, the structure, and the global and statistical properties of isolated dark matter galactic halos that emerge from primordial Gaussian fluctuations. First, from the statistical properties of the primordial density fluctuation field, the possible mass aggregation histories (MAHs) are generated. Second, these histories are used as the initial conditions of the gravitational collapse. To calculate the structure of the virialized systems, we have generalized the secondary infall model to allow arbitrary MAHs and internal thermal motions. The average halo density profiles we obtained agree with the profile derived as a fitting formula to results of N-body cosmological simulations by Navarro, Frenk, & White. The comparison of the density profiles with the observational data is discussed, and some possible solutions to the disagreement found in the inner regions are proposed. The results of our approach, after considering the gravitational dragging of the baryon matter that forms a central disk in centrifugal equilibrium, show that the empirical Tully-Fisher (TF) relation and its scatter can be explained through the initial cosmological conditions, at least for the isolated systems. The σ8 = 1 SCDM model produces galaxies with high velocities when compared with observations, but when the SCDM power spectrum is normalized to σ8 = 0.57, an excellent agreement with the observable TF relation is found, suggesting that this relation is the natural extension to galactic scales of the observed galaxy distribution power spectrum. The theoretical TF scatter is close to the measured one. The slope of the TF relation is practically invariant with respect to the spin parameter λ.

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.

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.

Disc galaxy evolution models in a hierarchical formation scenario: structure and dynamics

We predict the internal structure and dynamics of present-day disc galaxies using galaxy evolution models within a hierarchical formation scenario. The halo mass aggregation histories, for a flat cold dark matter model with cosmological constant, were generated and used to calculate the virialization of dark matter haloes. A diversity of halo density profiles were obtained, the most typical one being close to that suggested by Navarro, Frenk & White. We modelled the way in which discs in centrifugal equilibrium are built within the evolving dark haloes, using gas accretion rates proportional to the halo mass aggregation rates, and assuming detailed angular momentum conservation. We calculated the gravitational interactions between halo and disc – including the adiabatic contraction of the halo due to disc formation – and the hydrodynamics, star formation and evolution of the galaxy discs. We find that the slope and zero-point of the Tully–Fisher (TF) relation in the infrared bands may be explained as a direct consequence of the cosmological initial conditions. This relation is almost independent of the assumed disc mass fraction, when the disc component in the rotation curve decomposition is non-negligible. Thus, the power spectrum of fluctuations can be normalized at galaxy scales through the TF relation independently of the disc mass fraction assumed. The rms scatter of the model TF relation originates mainly from the scatter in the dark halo structure and, to a minor extension, from the dispersion of the primordial spin parameter λ. The scatter obtained from our models does not disagree with the observational estimates. Our models allow us to understand why the residuals of the TF relation do not correlate significantly with disc size or surface brightness. We can also explain why low and high surface brightness galaxies have the same TF relation; the key point is the dependence of the star formation efficiency on the disc surface density. The correlations between gas fraction and surface brightness, and between scalelength and Vmax obtained with our models agree with those observed. Discs formed within the growing haloes, where λ is assumed to be time independent, have nearly exponential surface density distributions. The shape of the rotation curves changes with disc surface brightness and is nearly flat for most cases. The rotation curve decompositions show a dominance of dark matter down to very small radii, in conflict with some observational inferences. The introduction of shallow cores in the dark halo attenuates this difficulty and produces haloes with slightly smaller rotation velocities. Other features of our galaxy models are not strongly influenced by the shallow core.

Formation Rate, Evolving Luminosity Function, Jet Structure, and Progenitors for Long Gamma-Ray Bursts

We constrain the isotropic luminosity function (LF) and formation rate of long γ-ray bursts (GRBs) by fitting models jointly to both the observed differential peak-flux and redshift distributions. We find evidence supporting an evolving LF, where the luminosity scales as (1 + z)δ, with an optimal δ of 1.0 ± 0.2. For a single-power law LF, the best slope is approximately -1.57 with an upper luminosity of 1053.3 ergs s-1, while the best slopes for a double-power law LF are approximately -1.6 and -2.6, with a break luminosity of 1052.7 ergs s-1. Our finding implies a jet model intermediate between the universal structured (θ) ∝ θ-2 model and the quasi-universal Gaussian structured model. For the uniform-jet model our result is compatible with an angle distribution between 2° and 15°. Our best-constrained GRB formation rate histories increase from z = 0 to 2 by a factor of ~30 and then continue increasing slightly. We connect these histories to the cosmic star formation history and compare with observational inferences up to z ~ 6. GRBs could be tracing the cosmic rates of both the normal and obscured star formation regimes. We estimate a current GRB event rate in the Milky Way of ~5 × 10-5 yr-1 and compare it with the birthrate of massive close Wolf-Rayet + black hole binaries with orbital periods of hours. The agreement is rather good, suggesting that these systems could be the progenitors of the long GRBs.

THE STELLAR-SUBHALO MASS RELATION OF SATELLITE GALAXIES

We extend the abundance matching technique (AMT) to infer the satellite-subhalo and central-halo mass relations (MRs) of local galaxies as well as the corresponding satellite conditional mass functions (CSMFs). We use as inputs the observed galaxy stellar mass function (GSMF) decomposed into centrals and satellites and the ΛCDM distinct halo and subhalo mass functions. We explore the effects of defining the subhalo mass, m sub, at the time of (sub)halo accretion (m acc sub) versus defining it at the time of observation (m obs sub); we also test the standard assumption that centrals and satellites follow the same MRs. We show that this assumption leads to predictions in disagreement with observations, especially when m obs sub is used. We find that when the satellite-subhalo MRs are constrained by the satellite GSMF, they are always different from the central-halo MR: The smaller the stellar mass, the less massive the subhalo of satellites as compared to the halo of centrals of the same stellar mass. This difference is more dramatic when m obs sub is used instead of m acc sub. On average, for stellar masses lower than ~2 × 1011 M ☉, the dark mass of satellites decreased by 60%-65% with respect to their masses at accretion time. We find that MRs for both definitions of subhalo mass yield CSMFs in agreement with observations. Also, when these MRs are used in a halo occupation model, the predicted two-point correlation functions at different stellar mass bins agree with observations. The average stellar-halo MR is close to the MR of central galaxies alone, and conceptually this average MR is equivalent to abundance matching the cumulative total GSMF to the halo + subhalo mass function (the standard AMT). We show that the use of m obs sub leads to less uncertain MRs than m acc sub and discuss some implications of the obtained satellite-subhalo MR. For example, we show that the tension between abundance and dynamics of Milky Way satellites in the ΛCDM cosmogony gives a value of ~ – 1.6 in the faint-end slope of the GSMF upturns.

The cooling function of HD molecule revisited

We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for J 8 rotational levels, and for the vibrational levels v = 0, 1, 2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of the kinetic temperatures, T k, from 10 2 to 2 × 10 4 K and the number densities, nH, from 1 to 10 8 cm −3 .W efind that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For n H 10 5 and T k ∼ 10 4 K the cooling function becomes more than an order of magnitude higher than previously reported. We give also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 276 K (redshifts of 10, 30 and 100). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal Pages/anton/DATA/HD cooling/HD cool. html. We discuss the relevance to explore the effects of including our results into models and simulations of galaxy formation, especially in the regime when gas cools down from temperatures above ∼3000 K.

Constraints on dark matter physics from dwarf galaxies through galaxy cluster haloes

One of the predictions of the standard cold dark matter model is that dark haloes have centrally divergent density profiles. An extensive body of rotation curve observations of dwarf and low surface brightness galaxies shows the dark haloes of those systems to be characterized by soft constant density central cores. Several physical processes have been proposed to produce soft cores in dark haloes, each one with different scaling properties. With the aim of discriminating among them we have examined the rotation curves of dark matter dominated dwarf and low surface brightness galaxies and the inner mass profiles of two clusters of galaxies lacking a central cD galaxy and with evidence of soft cores in the centre. The core radii and central densities of these haloes scale in a well defined manner with the depth of their potential wells, as measured through the maximum circular velocity. As a result of our analysis we identify self-interacting cold dark matter as a viable solution to the core problem, where a non-singular isothermal core is formed in the halo center surrounded by a Navarro, Frenk, & White profile in the outer parts. We show that this particular physical situation predicts core radii in agreement with observations. Furthermore, using the observed scalings, we derive an expression for the minimum cross section (�) which has an explicit dependence with the halo dispersion velocity (v). If mx is the mass of the dark matter particle: �/mx � 4 10 25 (100 kms 1 /v) cm 2 /Gev.

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.