1,721,736 research outputs found
A Stochastic Theory of the Hierarchical Clustering. I. Halo Mass Function
No full textWe present a new theory for the hierarchical clustering of dark matter (DM) halos, based on stochastic differential equations, that constitutes a change of perspective with respect to existing frameworks (e.g., the excursion set approach); this work is specifically focused on the halo mass function. First, we present a stochastic differential equation that describes fluctuations in the mass growth of DM halos, as driven by a multiplicative white (Gaussian) noise dependent on the spherical collapse threshold and on the power spectrum of DM perturbations. We demonstrate that such a noise yields an average drift of the halo population toward larger masses, that quantitatively renders the standard hierarchical clustering. Then, we solve the Fokker–Planck equation associated to the stochastic dynamics, and obtain the Press & Schechter mass function as a (stationary) solution. Moreover, generalizing our treatment to a mass-dependent collapse threshold, we obtain an exact analytic solution capable of fitting remarkably well the N-body mass function over a wide range in mass and redshift. All in all, the new perspective offered by the theory presented here can contribute to a better understanding of the gravitational dynamics leading to the formation, evolution, and statistics of DM halos across cosmic times
A SMIL player for any web browser.
No full textSynchonized Multimedia Integration Language, is a W3C markup language for the definition of complex
multimedia presentations. SMIL documents need a specific player
for its playback and cannot be rendered by modern browsers. In
this paper we present SmilingWeb, a first tentative to implement
a cross-platform player for SMIL presentations contained in
web pages. We implement a JavaScript library, based on the
web standards, which allows to solve the synchronization of
media items contained in multimedia presentations through the
use of any available browser. The proposed solution is crossplatform and cross-browser, therefore it can be potentially used
by any user. The player has been tested with the SMIL Testsuite,
provided by W3C and with a set of very complex multimedia
presentations in order to check its support to the standard and
its scalability. All the tests reported positive results
A Stochastic Theory of the Hierarchical Clustering. II. Halo Progenitor Mass Function and Large-scale Bias
No full textWe generalize the stochastic theory of hierarchical clustering presented in Paper I by Lapi & Danese to derive the (conditional) halo progenitor mass function and the related large-scale bias. Specifically, we present a stochastic differential equation that describes fluctuations in the mass growth of progenitor halos of given descendant mass and redshift, as driven by a multiplicative Gaussian white noise involving the power spectrum and the spherical collapse threshold of density perturbations. We demonstrate that, as cosmic time passes, the noise yields an average drift of the progenitors toward larger masses, which quantitatively renders the expectation from the standard extended Press and Schechter (EPS) theory. We solve the Fokker-Planck equation associated with the stochastic dynamics, and obtain as an exact, stationary solution, the EPS progenitor mass function. Then we introduce a modification of the stochastic equation in terms of a mass-dependent collapse threshold modulating the noise, and solve analytically the associated Fokker-Planck equation for the progenitor mass function. The latter is found to be in excellent agreement with the outcomes of N-body simulations; even more remarkably, this is achieved with the same shape of the collapse threshold used in Paper I to reproduce the halo mass function. Finally, we exploit the above results to compute the large-scale halo bias, and find it in pleasing agreement with the N-body outcomes. All in all, the present paper illustrates that the stochastic theory of hierarchical clustering introduced in Paper I can describe effectively not only halos' abundance, but also their progenitor distribution and their correlation with the large-scale environment across cosmic times
Statistics of dark matter halos in the excursion set peak framework
No full textWe derive approximated, yet very accurate analytical expressions for the abundance and clustering properties of dark matter halos in the excursion set peak framework; the latter relies on the standard excursion set approach, but also includes the effects of a realistic filtering of the density field, a mass-dependent threshold for collapse, and the prescription from peak theory that halos tend to form around density maxima. We find that our approximations work excellently for diverse power spectra, collapse thresholds and density filters. Moreover, when adopting a cold dark matter power spectra, a tophat filtering and a mass-dependent collapse threshold (supplemented with conceivable scatter), our approximated halo mass function and halo bias represent very well the outcomes of cosmological N-body simulations
Growth of Supermassive Black Hole Seeds in ETG Star-forming Progenitors: Multiple Merging of Stellar Compact Remnants via Gaseous Dynamical Friction and Gravitational-wave Emission
No full textWe propose a new mechanism for the growth of supermassive black hole (BH) seeds in the star-forming
progenitors of local early-type galaxies (ETGs) at z1. This envisages the migration and merging of stellar
compact remnants (neutron stars and stellar-mass BHs) via gaseous dynamical friction toward the central highdensity
regions of such galaxies. We show that, under reasonable assumptions and initial conditions, the process
can build up central BH masses of the order of 104–106Me within some 107 yr, so effectively providing heavy
seeds before standard disk (Eddington-like) accretion takes over to become the dominant process for further BH
growth. Remarkably, such a mechanism may provide an explanation, alternative to super-Eddington accretion
rates, for the buildup of billion-solar-massed BHs in quasar hosts at z7, when the age of the universe 0.8 Gyr
constitutes a demanding constraint; moreover, in more common ETG progenitors at redshift z∼2–6, it can concur
with disk accretion to build such large BH masses even at moderate Eddington ratios 0.3 within the short star
formation duration Gyr of these systems. Finally, we investigate the perspectives to detect the merger events
between the migrating stellar remnants and the accumulating central supermassive BH via gravitational-wave
emission with future ground- and space-based detectors such as the Einstein Telescope and the Laser
Interferometer Space Antenna
The masses of black holes in the nuclei of spirals
No full textWe use the innermost kinematics of spirals to investigate whether these galaxies could host the massive black hole remnants that once powered the quasi-stellar object (QSO) phenomenon. Hundreds of rotation curves of early- and late-type spirals are used to place upper limits on the central black hole (BH) masses. We find that (i) in late-type spirals, the central massive dark objects (MDOs) are about 10-100 times smaller than the MDOs detected in ellipticals, and (ii) in early-type spirals, the central bodies are likely to be in the same mass range as the elliptical MDOs. As a consequence, the contribution to the QSO/active galactic nuclei (AGN) phenomenon by the BH remnants eventually hosted in spirals is negligible: rho(BH)(Sb-Im) < 6 x 10(4) M. Mpc(-3). We find several hints that the MDO mass versus bulge mass relationship is significantly steeper in spirals than in ellipticals, although the very issue of the existence of such a relation for late Hubble type objects remains open. The upper limits on the masses of the BHs resident in late-type spirals are stringent: M-BH less than or equal to 10(6)-10(7) M., indicating that only low-luminosity activity could possibly have occurred in these objects
The dramatic size evolution of elliptical galaxies and the quasar feedback
No full textObservations have shown that passively evolving massive galaxies at high redshift are much more compact than local galaxies with the same stellar mass. We argue that the observed strong evolution in size is directly related to the quasar feedback, which removes huge amounts of cold gas from the central regions in a Salpeter time, inducing an expansion of the stellar distribution. The new equilibrium configuration, with a size increased by a factor >~3, is attained after ~40 dynamical times, corresponding to ~2 Gyr. This means that massive galaxies observed at z>=1 will settle on the fundamental plane by z~0.8-1. In less massive galaxies (M*<~2×1010 Msolar), the nuclear feedback is subdominant, and the mass loss is mainly due to stellar winds. In this case, the mass-loss timescale is longer than the dynamical time and results in adiabatic expansion that may increase the effective radius by a factor of up to ~2 in 10 Gyr, although a growth by a factor of ~=1.6 occurs within the first 0.5 Gyr. Since observations are focused on relatively old galaxies, with ages >~1 Gyr, the evolution for smaller galaxies is more difficult to perceive. Significant evolution of velocity dispersion is predicted for both small and large galaxies
The H I Content of Local Late-type Galaxies
No full textWe present a solid relationship between the neutral hydrogen (H I) disk mass and the stellar disk mass of late-type galaxies in the local universe. This relationship is derived by comparing the stellar disk mass function from the Sloan Digital Sky Survey and the H I mass function from the H I Parkes All Sky Survey (HIPASS). We find that the H I mass in late-type galaxies tightly correlates with the stellar mass over three orders of magnitude in stellar disk mass. We cross-check our result with that obtained from a sample of HIPASS objects for which the stellar mass has been obtained by inner kinematics. In addition, we derive the H I versus halo mass relationship and the dependence of all the baryonic components in spirals on the host halo mass. These relationships bear the imprint of the processes ruling galaxy formation, and highlight the inefficiency of galaxies both in forming stars and in retaining their pristine H I gas
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