Searcharxiv⌕ Search

arXiv subjects

Adi Nusser

Publications and source records attributed to Adi Nusser.

At least 73 records · Page 4Linked to original sources
arXiv2008

Hydrostatic equilibrium of a porous intracluster medium: implications for mass fraction and X-ray luminosity

The presence of dilute hot cavities in the intracluster medium (ICM) at the cores of clusters of galaxies changes the relation between gas temperature and its X-ray emission properties. Using the hydrostatic equations of a porous medium we solve for the ICM density for a given temperature as a function of the filling factor of dilute bubbles. We find that at a given temperature, the core X-ray luminosity increases with the filling factor. If the frequency of AGNs in clusters were higher in the past, then the filling factor could correspondingly be significant, with implications for the cluster scaling relations at high redshifts. This is especially important for the core properties, including the temperature-luminosity ($L_X-T$) relation and estimates of the core gas mass. The results imply an epoch-dependent sensitivity of the $L_X-T$ relation in the core to the porosity of the ICM. Detection of such an effect would give new insights into AGN feedback.

astro-ph↗
arXiv2008

Searching for Neutrinos from WIMP Annihilations in the Galactic Stellar Disk

Weakly interacting massive particles (WIMPs) are a viable candidate for the relic abundance of dark matter (DM) produced in the early universe. So far WIMPs have eluded direct detection through interactions with baryonic matter. Neutrino emission from accumulated WIMP annihilations in the solar core has been proposed as a signature of DM, but has not yet been detected. These null results may be due to small scale DM density fluctuations in the halo with the density of our local region being lower than the average (around 0.3 GeV/cm^3 ). However, the accumulated neutrino signal from WIMP annihilations in the Galactic stellar disk would be insensitive to local density variations. Inside the disk, dark matter can be captured by stars causing an enhanced annihilation rate and therefore a potentially higher neutrino flux than what would be observed from elsewhere in the halo. We estimate a neutrino flux from the WIMP annihilations in the stellar disk to be enhanced by more than an order of magnitude compared to the neutrino fluxes from the halo. We offer a conservative estimate for this enhanced flux, based on the WIMP-nucleon cross-sections obtained from direct-detection experiments by assuming a density of around 0.3 GeV/cm^3 for the local DM. We also compare the detectability of these fluxes with a signal of diffuse high energy neutrinos produced in the Milky Way by the interaction of cosmic rays (CRs) with the interstellar medium (ISM). These comparative signals should be observable by large neutrino detectors.

astro-ph↗
arXiv2007

Boundary-value problems in cosmological dynamics

The dynamics of cosmological gravitating system is governed by the Euler and the Poisson equations. Tiny fluctuations near the big bang singularity are amplified by gravitational instability into the observed structure today. Given the current distribution of galaxies and assuming initial homogeneity, dynamical reconstruction methods have been developed to derive the cosmic density and velocity fields back in time. The reconstruction method described here is based a least action principle formulation of the dynamics of collisionless particle (representing galaxies). Two observational data sets will be considered. The first is the distribution of galaxies which is assumed to be an honest tracer of the mass density field of the dark matter. The second set is measurements of the peculiar velocities (deviations from pure Hubble flow) of galaxies. Given the first data set, the reconstruction method recovers the associated velocity field which can then be compared with the second data set. This comparison constrains the nature of the dark matter and the relation between mass and light in the Universe.

astro-ph↗
arXiv2007

Neutralino Annihilations and the Gas Temperature in the Dark Ages

Assuming the dark matter is made entirely from neutralinos, we re-visit the role of their annihilation on the temperature of diffuse gas in the high redshift universe. We consider neutralinos of particle mass 36 GeV and 100 GeV, respectively. The former is able to produce ~7 electron/positron particles per annihilation through the fremionic channel, and the latter ~53 particles assuming a purely bosonic channel. High energy electron/positron particles up-scatter the Cosmic Microwave Background (CMB) photons into higher energies via the inverse-Compton scattering. The process produces a power-law electron/positron energy spectrum of index -1 in the energy range of interest, independent of the initial energy distribution. The corresponding energy spectrum of the up-scattered photons is a power-law of index -1/2, if absorption by the gas is not included. The scattered photons photo-heat the gas by releasing electrons which deposit a fraction (14%)of their energy as heat into the ambient medium. For uniformly distributed neutralinos the heating is insignificant. The effect is greatly enhanced by the clumping of neutralinos into dense haloes. We use a time-dependent clumping model which takes into account the damping of density fluctuations on mass scales smaller than ~10^{-6}M_sol. With this clumping model, the heating mechanism boosts the gas temperature above that of the CMB after a redshift of z 30. By z\approx 10 the gas temperature is nearly 100 times its temperature when no heating is invoked. Similar increase is obtained for the two neutralino masses considered.

astro-ph↗
arXiv2007

Back-in-time dynamics of the cluster IE 0657-56 (the Bullet System)

We present a simplified dynamical model of the ``Bullet'' system of two colliding clusters. The model constrains the masses of the system by requiring that the orbits of the main and sub components satisfy the cosmological initial conditions of vanishing physical separation a Hubble time ago. This is also known as the timing argument. The model considers a system embedded in an over-dense region. We argue that a relative speed of $4500 \rm km/s$ between the two components is consistent with cosmological conditions if the system is of a total mass of $2.8\times 10^{15}h^{-1} M_\odot$ is embedded in a region of a (mild) over-density of 10 times the cosmological background density. Combining this with the lensing measurements of the projected mass, the model yields a ratio of 3:1 for the mass of the main relative to that of the subcomponent. The effect of the background weakens as the relative speed between the two components is decreased. For relative speeds lower than $\sim 3700\rm km/s$, the timing argument yields masses which are too low to be consistent with lensing.

astro-ph↗
arXiv2007

Halo Assembly Bias in the Quasi-linear Regime

We address the question of whether or not assembly bias arises in the absence of highly non-linear effects such as tidal stripping of halos near larger mass concentrations. Therefore, we use a simplified dynamical scheme where these effects are not modeled. We choose the punctuated Zel'dovich (PZ) approximation, which prevents orbit mixing by coalescing particles coming within a critical distance of each other. A numerical implementation of this approximation is fast, allowing us to run a large number of simulations to study assembly bias. We measure an assembly bias from 60 PZ simulations, each with 512^3 cold particles in a 128h^-1 Mpc cubic box. The assembly bias estimated from the correlation functions at separations < 5h^-1 Mpc for objects (halos) at z=0 is comparable to that obtained in full N-body simulations. For masses 4x10^11 h^-1 Mo the "oldest" 10% haloes are 3-5 times more correlated than the "youngest" 10%. The bias weakens with increasing mass, also in agreement with full N-body simulations. We find that that halo ages are correlated with the dimensionality of the surrounding linear structures as measured by the parameter (λ_1+λ_2+λ_3)/ (λ_1^2+λ_2^2+λ_3^2)^{1/2} where λ_i are proportional to the eigenvalues of the velocity deformation tensor. Our results suggest that assembly bias may already be encoded in the early stages of the evolution.

astro-ph↗
arXiv2006

Suppressing cluster cooling flows by self-regulated heating from spatially distributed population of AGNs

(Abridged) Existing models invoking AGN activty to resolve the cooling flow conundrum in galaxy clusters focus exclusively on the role of the central galaxy. Such models require fine-tuning of highly uncertain microscopic transport properties to distribute the thermal thermal over the entire cluster cooling core. We propose that the ICM is instead heated by multiple, spatially distributed AGNs. There is mounting observational evidence for multiple AGNs in cluster environments. Active AGNs drive bubbles into the ICM. We identify three distinct interactions between the bubble and the ICM: (1) Upon injection, the bubbles expand rapidly in situ to reach pressure equilibrium with their surroundings, generating shocks and waves whose dissipation is the principal source of ICM heating. (2) Once inflated, the bubbles rise buoyantly at rate determined by a balance with the viscous drag force, which itself results in some additional heating. (3) Rising bubbles expand and compress their surroundings. This process is adiabatic and does not contribute to any additional heating; rather, the increased ICM density due to compression enhances cooling. Our model sidesteps the ``transport'' issue by relying on the spatially distributed galaxies to heat the cluster core. We include self regulation in our model by linking AGN activity in a galaxy to cooling characteristics of the surrounding ICM. We use a spherically symmetric one-dimensional hydrodynamical code to carry out a preliminary study illustrating the efficacy of the model. Our self-regulating scenario predicts that there should be enhanced AGN activity of galaxies inside the cooling regions compared to galaxies in the outer parts of the cluster. This prediction remains to be confirmed or refuted by observations.

astro-ph↗
arXiv2006

General Relativistic Self-Similar Solutions in Cosmology

We present general relativistic solutions for self-similar spherical perturbations in an expanding cosmological background of cold pressure-less gas. We focus on solutions having shock discontinuities propagating in the surrounding cold gas. The pressure, $p$, and energy-density, $μ$, in the shock-heated matter are assumed to obey $p=wμ$, where $w$ is a positive constant. Consistent solutions are found for shocks propagating from the symmetry center of a region of a positive density excess over the background. In these solutions, shocks exist outside the radius marking the event horizon of the black hole which would be present in a shock-less collapse. For large jumps in the energy-density at the shock, a black hole is avoided altogether and the solutions are regular at the center. The shock-heated gas does not contain any sonic points, provided the motion of the cold gas ahead of the shock deviates significantly from the Hubble flow. For shocks propagating in the uniform background, sonic points always appear for small jumps in the energy-density. We also discuss self-similar solutions without shocks in fluids with $w<-1/3$.

astro-ph↗
arXiv2006

The Probability Distribution of the Lya transmitted flux from a sample of SDSS quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lya forest from a sample of 3492 quasars included in the SDSS DR3 data release. Our intention is to investigate the sensitivity of the Lya flux PDF as measured from low resolution and low signal-to-noise data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. The quasar continuum is described by the superposition of a power law and emission lines. We perform a power law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of 0.59 +/- 0.36 in the redshift range 2.5<z<3.5. Taking into account the variation in the continuum indices increases the mean flux by 3 and 7 per cent at z=3 and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lya flux PDF and power spectrum of high resolution data. Using our power law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of 10-15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than 10 per cent.

astro-ph↗
arXiv2006

The morphology of cosmological reionization by means of Minkowski Functionals

The morphology of the total gas and the neutral hydrogen (H I) distributions during the cosmological epoch of reionization can be quantified with Minkowski Functionals (MFs) of isodensity surfaces. We compute the MFs from the output of a high-resolution numerical simulation which includes explicit treatment of transfer of UV ionizing radiation. "Galaxies" identified in the simulation using semi-analytic models of galaxy formation are assumed to be the sole sources of UV photons. The MFs of the total gas distribution are well described by the analytic expressions derived for lognormal random fields. The statistical properties of the diffuse H I depend on the gas distribution and on the way ionized regions propagate in the inter-galactic medium (IGM). The deviations of the MFs of the H I distribution from those of a lognormal random field are, therefore, caused by reionization. We use the MFs to discriminate between the various stages of reionization in the simulation. We suggest a simple model of reionization which reproduces the MFs derived from this simulation. Using random realizations of lognormal density fields, we also assess the ability of MFs to distinguish between different reionization scenarios. Our results are relevant to the analysis of future cosmological twenty-one centimeter maps.

astro-ph↗
arXiv2006

The Epoch of Reionization

We have modelled the process of reionization of the IGM by photoionization by galaxies, in order to predict the epoch of reionization. We use a sophisticated semi-analytic model to track the formation of galaxies. Our study represents a much more complete and physically consistent modelling of reionization than has been conducted in the past, containing significant improvements in the modelling of the collapse of baryons into dark matter halos, and in the model for the cooling and condensation of gas within halos (including photoheating from a self-consistently computed ionizing background and cooling due to H2). We find that reionization can be achieved by z~10-20 in a LCDM cosmological model with sigma8~0.9. However, a cosmological model with a running spectral index is only able to achieve reionization before z~9 if extreme assumptions are made about the physics of feedback at high redshifts. We also consider the galaxy formation model discussed by Baugh et al., and find that it is able to reionize the Universe by z~12. The previous results assume that all of the ionizing photons produced by stars in galaxies are able to escape and ionize the IGM. If this is not the case, then the redshift of reionization could be substantially reduced. We find that extended periods of partial reionization and double reionizations can occur in models in which the first stars formed via cooling by H2 molecules, are very massive, and in which the escape fraction of ionizing photons ~10-30%. Such models do not fully reionize until z~6-7, but predict an electron scattering optical depth as large as 0.15. Models with lower sigma8=0.7-0.8 as suggested by the recent WMAP three year data have reduced redshifts of reionization, but can be consistent with the lower optical depth also suggested by the WMAP three year data.

astro-ph↗
arXiv2005

Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations

We test the ability of the numerical action method (NAM) to recover the individual orbit histories of mass tracers in an expanding universe in a region of radius 26Mpc/h, given the masses and redshift-space coordinates at the present epoch. The mass tracers are represented by dark matter haloes identified in a high resolution N-body simulation of the standard LCDM cosmology. Since previous tests of NAM at this scale have traced the underlying distribution of dark matter particles rather than extended haloes, our study offers an assessment of the accuracy of NAM in a scenario which more closely approximates the complex dynamics of actual galaxy haloes. We show that NAM can recover present-day halo distances with typical errors of less than 3 per cent, compared to 5 per cent errors assuming Hubble flow distances. The total halo mass and the linear bias were both found to be constained at the 50 per cent level. The accuracy of individual orbit reconstructions was limited by the inability of NAM, in some instances, to correctly model the positions of haloes at early times solely on the basis of the redshifts, angular positions, and masses of the haloes at the present epoch. Improvements in the quality of NAM reconstructions may be possible using the present-day three-dimensional halo velocities and distances to further constrain the dynamics. This velocity data is expected to become available for nearby galaxies in the coming generations of observations by SIM and GAIA.

astro-ph↗
arXiv2005

The impact of galactic winds from LBGs on the Intergalactic Medium

An excess of sight-lines close to Lyman-break galaxies (LBGs) with little or no absorption in QSO absorption spectra has been reported and has been interpreted as the effect of galactic winds on the Intergalactic Medium. We use here numerical simulations to investigate the flux probability function close to plausible sites of LBGs. We show that the flux distribution near our LBGs in the simulation depends strongly on redshift, and is very sensitive to the averaging procedure. We show that a model without galactic winds and a model with a wind bubble size of 0.5Mpc/h (comoving) are equally consistent with the new determination of the conditional flux distribution by Adelberger et al. (2005). Models with the larger bubble sizes suggested by the previous observations of Adelberger et al. (2003) based on a much smaller sample at higher redshift are not consistent with the new data. We therefore argue that the volume filling factor of galactic winds driven by LBGs may be much smaller than previously thought, and that most of the metals responsible for the metal absorption associated with the low column density Lya forest are unlikely to have been ejected by LBGs.

astro-ph↗
arXiv2005

Consequences of short range interactions between dark matter and protons in galaxy clusters

Protons gain energy in short range collisions with heavier dark matter particles (DMPs) of comparable velocity dispersion. We examine the conditions under which the heating of baryons by scattering off DMPs can offset radiative cooling in the cores of galaxy clusters. Collisions with a constant cross section independent of the relative velocity of the colliding particles, cannot produce stable thermal balance. In this case, avoiding an unrealistic increase of the central temperatures yields the upper bound on the cross-section, σ_xp<10^-25 cm^2 (m_x/m_p), where m_x and m_p are the DMP and proton mass, respectively. A stable balance, however, can be achieved for a power law dependence on the relative velocity, V, of the form σ_xp \propto V^a with a<-3. An advantage of this heating mechanism is that it preserves the metal gradients observed in clusters.

astro-ph↗
arXiv2005

The Alcock-Paczynski test in redshifted twenty one centimeter maps

We examine the possibility of constraining the cosmological mean mass and dark energy densities by an application of the Alcock-Paczynski test on redshifted 21-cm maps of the epoch of reionization. The 21-cm data will be provided as a function of frequency and angular positions on the sky. The ratio of the frequency to angular distance scales can be determined by inspecting the anisotropy pattern of the correlation function of brightness temperature. We assess the sensitivity of the distance ratio to the cosmological parameters and present a technique for disentangling geometrical distortions from redshift space distortions caused by peculiar motions.

astro-ph↗
arXiv2005

Patchy He II reionization and the physical state of the IGM

We present a Monte-Carlo model of He II reionization by QSOs and its effect on the thermal state of the clumpy intergalactic medium (IGM). The model assumes that patchy reionization develops as a result of the discrete distribution of QSOs. It includes various recipes for the propagation of the ionizing photons, and treats photo-heating self-consistently. The model provides the fraction of He III, the mean temperature in the IGM, and the He II mean optical depth -- all as a function of redshift. It also predicts the evolution of the local temperature versus density relation during reionization. Our findings are as follows: The fraction of He III increases gradually until it becomes close to unity at $z\sim 2.8-3.0$. The He II mean optical depth decreases from $τ\sim 10$ at $z\geq 3.5$ to $τ\leq 0.5$ at $z\leq 2.5$. The mean temperature rises gradually between $z\sim 4$ and $z\sim 3$ and declines slowly at lower redshifts. The model predicts a flattening of the temperature-density relation with significant increase in the scatter during reionization at $z\sim 3$. Towards the end of reionization the scatter is reduced and a tight relation is re-established. This scatter should be incorporated in the analysis of the Ly$α$ forest at $z\leq 3$. Comparison with observational results of the optical depth and the mean temperature at moderate redshifts constrains several key physical parameters.

astro-ph↗
arXiv2005

Joint modeling of the probability distribution and power spectrum of the Lya forest: comparison with observations at z=3

We presents results of joint modeling of the probability distribution function (PDF) and the one-dimensional power spectrum (PS) of the Lya forest flux decrement. The sensitivity of these statistical measures to the shape and amplitude of the linear matter power spectrum is investigated using two variants of the LCDM cosmology. In the first model, the linear power spectrum has a scale-invariant spectral index, whereas in the second, it has a negative running index (RSI). We generate mock catalogs of QSO spectra, and compare their statistical properties to those of the observations at z=3. We perform a joint fit of the power spectrum and PDF. A scale-invariant model with σ_8=0.9 matches well the data if the mean IGM temperature is T\leq 15000K. For higher temperature, it tends to overestimate the flux power spectrum over scales k < 0.01s/km. The discrepancy is less severe when the PS alone is fitted. However, models matching the PS alone do not yield a good fit to the PDF. A joint analysis of the flux PS and PDF tightens the constraints on the model parameters and reduces systematic biases. The RSI model is consistent with the observed PS and PDF only if the temperature is T\geq 20000K. The best fit models reproduce the slope and normalisation of the column density distribution, irrespective of the shape and amplitude of the linear power spectrum. They are also consistent with the observed line-width distribution given the large uncertainties. Our joint analysis suggests that σ_8 is likely to be in the range 0.7 - 0.9 for a temperature 10000K<T<20000K and a reasonable reionization history.

astro-ph↗