arXiv subjects

Emilio Romano-Diaz

Publications and source records attributed to Emilio Romano-Diaz.

27 records · Page 2Linked to original sources

arXiv2009

Dissecting Galaxy Formation: I. Comparison Between Pure Dark Matter and Baryonic Models

We compare assembly of DM halos with and without baryons, within the context of cosmological evolution in the LCDM WMAP3 Universe (baryons+DM, BDM model, and pure DM, PDM model). In representative PDM and BDM models we find that baryons contribute decisively to the evolution of the central region, leading to an isothermal DM cusp, and to a flat DM density core -- the result of heating by dynamical friction of the substructure during a quiescent evolution epoch. This process ablates the cold gas from an embedded disk, cutting the star formation rate by ~10, and heats up the spheroidal gas and stellar components, triggering their expansion. The substructure is more resilient in the presence of baryons. The disk which formed from inside-out as gas dominated, is transformed into an intermediate Hubble type by z ~ 2 and to an early type by z ~ 0.5, based on its gas contents and spheroidal-to-disk stellar mass ratio. Only a relatively small ~20% fraction of DM particles in PDM and BDM models are bound within the radius of maximal circular velocity in the halo -- most of the DM particles perform larger radial excursions. We also find that the fraction of baryons within the halo virial radius somewhat increases during the major mergers and decreases during the minor mergers. The net effect appears to be negligible. While the substructure is being tidally-disrupted, mixing of its debris in the halo is not efficient and becomes even less so with z. The streamers formed after z ~ 1 survive largely to the present time -- an important implication for embedded disk evolution.

astro-ph.CO↗

arXiv2008

Disk Evolution and Bar Triggering Driven by Interactions with Dark Matter Substructure

We study formation and evolution of bar-disk systems in fully self-consistent cosmological simulations of galaxy formation in the LCDM WMAP3 Universe. In a representative model we find that the first generation of bars form in response to the asymmetric dark matter (DM) distribution (i.e., DM filament) and quickly decay. Subsequent bar generations form and are destroyed during the major merger epoch permeated by interactions with a DM substructure (subhalos). A long-lived bar is triggered by a tide from a subhalo and survives for ~10 Gyr. The evolution of this bar is followed during the subsequent numerous minor mergers and interactions with the substructure. Together with intrinsic factors, these interactions largely determine the stellar bar evolution. The bar strength and its pattern speed anticorrelate, except during interactions and when the secondary (nuclear) bar is present. For about 5 Gyr bar pattern speed increases substantially despite the loss of angular momentum to stars and cuspy DM halo. We analyze the evolution of stellar populations in the bar-disk and relate them to the underlying dynamics. While the bar is made mainly of an intermediate age, ~5-6 Gyr, disk stars at z=0, a secondary nuclear bar which surfaces at z~0.1 is made of younger, ~1-3 Gyr stars.

astro-ph↗

arXiv2008

Erasing Dark Matter Cusps in Cosmological Galactic Halos with Baryons

We study the central dark matter (DM) cusp evolution in cosmological galactic halos. Models with and without baryons (baryons+DM, hereafter BDM model, and pure DM, PDM model, respectively) are advanced from identical initial conditions. The DM cusp properties are contrasted by a direct comparison of pure DM and baryonic models. We find a divergent evolution between the PDM and BDM models within the inner ~10 kpc region. The PDM model forms a R^{-1} cusp as expected, while the DM in the BDM model forms a larger isothermal cusp R^{-2} instead. The isothermal cusp is stable until z~1 when it gradually levels off. This leveling proceeds from inside out and the final density slope is shallower than -1 within the central 3 kpc (i.e., expected size of the R^{-1} cusp), tending to a flat core within ~2 kpc. This effect cannot be explained by a finite resolution of our code which produces only a 5% difference between the gravitationally softened force and the exact Newtonian force of point masses at 1 kpc from the center. Neither is it related to the energy feedback from stellar evolution or angular momentum transfer from the bar. Instead it can be associated with the action of DM+baryon subhalos heating up the cusp region via dynamical friction and forcing the DM in the cusp to flow out and to `cool' down. The process described here is not limited to low z and can be efficient at intermediate and even high z.

astro-ph↗

arXiv2007

Evolution of Phase-Space Density in Dark Matter Halos

The evolution of the phase-space density profile in dark matter (DM) halos is investigated by means of constrained simulations, designed to control the merging history of a given DM halo. Halos evolve through a series of quiescent phases of a slow accretion intermitted by violent events of major mergers. In the quiescent phases the density of the halo closely follows the NFW profile and the phase-space density profile, Q(r), is given by the Taylor & Navarro power law, r^{-beta}, where beta ~ 1.9 and stays remarkably stable over the Hubble time. Expressing the phase-space density by the NFW parameters, Q(r)=Qs (r/Rs)^{-beta}, the evolution of Q is determined by Qs. We have found that the effective mass surface density within Rs, Sigma_s = rhos Rs, remains constant throughout the evolution of a given DM halo along the main branch of its merging tree. This invariance entails that Qs ~ Rs^{-5/2} and Q(r) ~ Sigma_s^{-1/2} Rs^{-5/2} (r/ Rs)^{-beta}. It follows that the phase-space density remains constant, in the sense of Qs=const., in the quiescent phases and it decreases as Rs^{-5/2} in the violent ones. The physical origin of the NFW density profile and the phase-space density power law is still unknown. Yet, the numerical experiments show that halos recover these relations after the violent phases. The major mergers drive Rs to increase and Qs to decrease discontinuously while keeping Qs Rs^{5/2} = const. The virial equilibrium in the quiescent phases implies that a DM halos evolves along a sequence of NFW profiles with constant energy per unit volume (i.e., pressure) within Rs.

astro-ph↗

arXiv2007

DTFE analysis of the PSCz local Universe: Density Field and Cosmic Flow

We apply the Delaunay Tessellation Field Estimator (DTFE) to reconstruct and analyze the matter distribution and cosmic velocity flows in the Local Universe on the basis of the PSCz galaxy survey. The prime objective of this study is the production of optimal resolution three-dimensional maps fully volume-covering of the volume-weighted velocity and density fields throughout the nearby Universe, out to a distance of 150 Mpc/h. Based on the Voronoi and Delaunay tessellation defined by the spatial galaxy sample, DTFE involves the estimate of density values on the basis of the volume of the related Delaunay tetrahedra and the subsequent use of theDelaunay tessellation as natural multidimensional (linear) interpolation grid for the corresponding density and velocity fields throughout the sample volume. The linearized model of the spatial galaxy distribution and the corresponding peculiar velocities of the PSCz galaxy sample, produced by Brachini et al. (1999), forms the input sample for the DTFE study. The DTFE maps reproduce the high-density supercluster regions in optimal detail, both their internal structure as well as their elongated or flattened shape. The corresponding velocity flows trace the bulk and shear flows marking the region extending from the Pisces-Perseus supercluster, via the Local superclusters, towards the Hydra-Centaurus and the Shapley concentration. The most outstanding and unique feature of the DTFE maps is the sharply defined radial outflow regions in and around underdense voids, marking the dynamical importance of voids in the Local Universe. The maximum expansion rate of voids defines a sharp cutoff in the DTFE velocity divergence pdf. We found that on the basis of this cutoff DTFE manages to consistently reproduce the value of O_m ~ 0.35 underlying the linearized velocity dataset.

astro-ph↗

arXiv2006

Evolution of Characteristic Quantities for Dark Matter Halo Density Profiles

We investigate the effect of an assembly history on the evolution of dark matter (DM) halos of 10^{12} Msun/h using Constrained Realizations of random Gaussian fields. Five different realizations of a DM halo with distinct merging histories were constructed and evolved. Our main results are: A halo evolves via a sequence of quiescent phases of a slow mass accretion intermitted by violent episodes of major mergers. In the quiescent phases, the density is well fitted by an NFW profile, the inner scale radius Rs and the mass enclosed within it remain constant, and the virial radius (Rvir) grows linearly with the expansion parameter "a". Within each quiescent phase the concentration parameter ("c") scales as "a", and the mass accretion history (Mvir) is well described by the Tasitsiomi etal. fitting formula. In the violent phases the halos are not in a virial equilibrium and both Rs and Rvir grow discontinuously. The violent episodes drive the halos from one NFW dynamical equilibrium to another. The final structure of a halo, including "c", depends on the degree of violence of the major mergers and on their number. Next, we find a distinct difference between the behavior of various NFW parameters taken as averages over an ensemble of halos and those of individual halos. Moreover, the simple scaling relations c--Mvir do not apply to the entire evolution of individual halos, and so is the common notion that late forming halos are less concentrated than early forming ones. The entire evolution of the halo cannot be fitted by single analytical expressions.

astro-ph↗

arXiv2005

Constrained Simulations of Dark Matter Halos

The formation and structure of dark matter halos is studied by constrained simulations. A series of experiments of the formation of a 10^12 Msun/h halo is designed to study the dependence of the density profile on its merging history. We find that the halo growth consist of several quiescent phases intermitted by violent events, with the density well approximated by the NFW profile during the former phases. We find that (1) the NFW scale radius R_s stays constant during the quiescent phase and grows abruptly during the violent one. In contrast, the virial radius grows linearly during the quiescent and abruptly during the violent phases. (2) The central density stays unchanged during the quiescent phase while dropping abruptly during the violent phase, and it does not reflect the formation time of the halo. (3) The clear separation of the evolution of an individual halo into quiescent and violent phases implies that its entire evolution cannot be fitted by simple scaling relations.

astro-ph↗

arXiv2005

Constrained Cosmological Simulations of Dark Matter Halos

The formation and structure of dark matter (DM) halos is studied by means of constrained realizations of Gaussian fields using N-body simulations. A series of experiments of the formation of a 10^{12} Msun halo is designed to study the dependence of the density profile on its merging history. We confirm that the halo growth consists of violent and quiescent phases, with the density well approximated by the Navarro-Frenk-White (NFW) profile during the latter phases. We find that (1) the NFW scale radius R_s stays constant during the quiescent phase and grows abruptly during the violent one. In contrast, the virial radius grows linearly during the quiescent and abruptly during the violent phases. (2) The central density stays unchanged during the quiescent phase while dropping abruptly during the violent phase. (3) The value of \rs reflects the violent merging history of the halo, and depends on the number of violent events and their fractional magnitudes, independent of the time and order of these events. It does not reflect the formation time of the halo. (4) The fractional change in R_s is a nonlinear function of the fractional absorbed kinetic energy within R_s in a violent event.

astro-ph↗

arXiv2005

Local Supercluster Dynamics: External Tidal Impact of the PSCz sample traced by Optimized Numerical Least Action Method

We assess the extent to which the PSCz survey encapsulates the complete or major share of matter inhomogeneities responsible for the external tidal forces affecting the peculiar velocity flow within the Local Supercluster [LS]. We investigate this issue on the basis of mock galaxy catalogs drawn from two large Nbody simulations of cosmic structure formation in two different cosmological scenarios. From these a variety of datasets is selected imitating the observational conditions of either the volume-limited LS mimicking NBG catalog or the deeper magnitude-limited PSCz catalog. The mildly nonlinear dynamics in the mock NBG and PSCz velocities are analyzed by means of an optimized implementation of the Least Action Principle, the Fast Action Method [FAM]. By comparing the modeled and true velocities of the corresponding mock catalogs we assess the extent and nature of the external tidal influence on the LS volume. We find that the dynamics in the inner 30Mpc/h volume is strongly affected by the external forces. Most of the external forces can be traced back to a depth of no more than 100Mpc/h. This is concluded from the fact that the FAM reconstructions of the 100Mpc/h PSCz volume appear to have included most gravitational influences. In addition, we demonstrate that for all considered cosmological models the bulk flow and shear components of the tidal velocity field generated by the external distribution of PSCz galaxies provides sufficient information for representing the full external tidal force field.

astro-ph↗