arXiv subjects

Adi Nusser

Publications and source records attributed to Adi Nusser.

At least 91 records · Page 5Linked to original sources

arXiv2005

Instabilities of Galactic Disks in the Presence of Star Formation

We discuss the stability of galactic disks in which the energy of interstellar clouds is gained in encounters with expanding supernova remnants and lost in inelastic collisions. Energy gain and loss processes introduce a phase difference between the pressure and density perturbations, making disks unstable on small scales for several recipes of star formation. This is in contrast to the standard stability analysis in which small scale perturbations are stabilized by pressure. In the limit of small scales the dispersion relation for the growth rate reduces to that of thermal instabilities in a fluid without gravity. If instabilities lead to star formation, then our results imply a secondary mode of star formation which operates on small scales and feeds on the existence of a primary mode on intermediate scales. This may be interpreted as a positive feedback. Further, the standard stability criterion on intermediate scales is significantly modified.

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arXiv2005

Modeling the formation of galaxy clusters in MOND

We use a one dimensional hydrodynamical code to study the evolution of spherically symmetric perturbations in the framework of Modified Newtonian Dynamics (MOND). The code evolves spherical gaseous shells in an expanding Universe by employing a MOND-type relationship between the fluctuations in the density field and the gravitational force, $g$. We focus on the evolution of initial density perturbations of the form $δ_{i}\sim r_{i}^{-s}$ for $0<s<3$. A shell is initially cold and remains so until it encounters the shock formed by the earlier collapse of shells nearer to the centre. During the early epochs $g$ is sufficiently large and shells move according to Newtonian gravity. As the physical size of the perturbation increases with time, $g$ gets smaller and the evolution eventually becomes MOND dominated. However, the density in the inner collapsed regions is large enough that they re-enter the Newtonian regime. The evolved gas temperature and density profiles tend to a universal form that is independent of the the slope, $s$, and of the initial amplitude. An analytic explanation of this intriguing result is offered. Over a wide range of scales, the temperature, density and entropy profiles in the simulations, depend on radius roughly like $r^{0.5}$, $r^{-1.5}$ and $r^{1.5}$, respectively. We compare our results with XMM-{\it Newton} and {\it Chandra} observations of clusters. The temperature profiles of 16 observed clusters are either flat or show a mild decrease at $R\gtsim 200\rm kpc$. MOND profiles show a significant increase that cannot reconciled with the data. Our simulated MOND clusters are substantially denser than the observed clusters. It remains to be seen whether these difficulties persist in three-dimensional hydrodynamical simulations with generic initial conditions.

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arXiv2005

The spin temperature of neutral hydrogen during cosmic pre-reionization

We re-examine the role of collisions in decoupling the HI 21-cm spin temperature from the cosmic microwave background (CMB). The cross section for de-exciting the 21-cm trasition in collisions with free electrons is more than 10 times larger than it is in collisions with other atoms. If the fraction of free electrons in the diffuse cosmic gas is between 10 and 30 per cent then collisions alone can decouple the spin temperature from the cosmic microwave background (CMB), even in moderately under-dense regions at z >15. This decoupling is especially important during the very early stages of re-ionization when a Ly-alpha continuum background had yet to be established. As a detailed example, we develop a semi-analytic model to quantify 21-cm emission signatures from a diffuse gas which is partially ionized at z>10 by an X-ray background. We find 21-cm differential brightness temperature fluctuations with a mean of ~8 mK and a rms value as large as 5 mK, for a frequency resolution bandwidth of 100 KHz and a beamsize of 3 arcmin. Another example where free electron-atom collisions are important is during the recombination of bubbles ionized by short-lived UV sources. When the ionized fraction in these bubbles drops to 10-20 per cent their differential temperature can be as high as 10 mK. We also consider the correlation of the 21-cm temperature as a function of frequency. By isotropy, a comparison with the angular projected correlation in the plane of the sky should yield an estimate for the matter density parameter, Omega_m, and the dark energy component. This variant of the Alcock-Paczyski test is applicable for any reionization scenario.

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arXiv2005

Structure Formation With a Long-Range Scalar Dark Matter Interaction

Numerical simulations show that a long-range scalar interaction in a single species of massive dark matter particles causes voids between the concentrations of large galaxies to be more nearly empty, suppresses accretion of intergalactic matter onto galaxies at low redshift, and produces an early generation of dense dark matter halos. These three effects, in moderation, seem to be improvements over the Lambda CDM model predictions for cosmic structure formation. Because the scalar interaction in this model has negligible effect on laboratory physics and the classical cosmological tests, it offers an observationally attractive example of cosmology with complicated physics in the dark sector, notably a large violation of the weak equivalence principle.

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arXiv2004

Monitoring lensed starlight emitted close to the Galactic Center

We describe the feasibility of detecting the gravitational deflection of light emitted by stars moving under the influence of the massive object at the Galactic center. Light emitted by a star orbiting behind the central mass has a smaller impact parameter than the star itself, and suffers the effect of gravitational lensing, providing a closer probe of the central mass distribution and hence a stricter test of the black hole hypothesis. A mass of $4.3\times 10^{6} M_{\odot}$ causes a $0.1-2\rm mas$ deviation in the apparent position of orbiting stars projected within $10^{\circ}$ of the line of sight to the galactic center. In addtion, we may uniquely constrain the distance to the center of the galaxy because lensing deflections constrain the ratio $\rg/R_{0}$ of the Schwarzschild radius to the distance to the black hole, $R_{o}$, whereas the ratio $\rg/R_{o}^{3}$ is obtained by fitting the orbit.

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arXiv2004

Redshift distortions in one-dimensional power spectra

We present a model for one-dimensional (1D) matter power spectra in redshift space as estimated from data provided along individual lines of sight. We derive analytic expressions for these power spectra in the linear and nonlinear regimes, focusing on redshift distortions arising from peculiar velocities. In the linear regime, redshift distortions enhance the 1D power spectra only on small scales, and do not affect the power on large scales. This is in contrast to the effect of distortions on three-dimensional (3D) power spectra estimated from data in 3D space, where the enhancement is independent of scale. For CDM cosmologies, the 1D power spectra in redshift and real space are similar for wavenumbers $q<0.1h/Mpc$ where both have a spectral index close to unity, independent of the details of the 3D power spectrum. Nonlinear corrections drive the 1D power spectrum in redshift space into a nearly universal shape over scale $q<10h/Mpc$, and suppress the power on small scales as a result of the strong velocity shear and random motions. The redshift space, 1D power spectrum is mostly sensitive to the amplitude of the initial density perturbations. Our results are useful in particular for power spectra computed from the SDSS quasars sample.

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arXiv2004

Galactic winds and the Lya forest

We study the effect of galactic outflows on the Lya forest. The winds are modelled as fully ionised spherical bubbles centered around galactic haloes. The observed flux probability distribution and flux power spectrum limit the volume filling factor of bubbles to be less than 10%. We have compared the mean flux as a function of distance from haloes with the Adelberger etal (ASSP) measurement. A model with bubbles of constant radius of 1.5Mpc/h surrounding the most massive haloes matches the high transmissivity at separations <0.5Mpc/h, but the increase of the transmissivity at small scales is more gradual than observed. At separations <0.5Mpc/h, our estimate of the cosmic variance error is $ΔF\sim 0.3$, 30% higher than that of ASSP. The difficulty in matching the rise in the transmissivity at small separations is caused by residual absorption of neutral hydrogen lying physically outside the bubbles but having a redshift position similar to the haloes. The flux level is thus sensitive to the amplitude of the coherent velocity shear near haloes which increases with halo mass in the simulation. A model where LBGs are starbursts in small mass haloes matches the observations with smaller bubble size than one where more massive haloes host the LBGs. A starburst model with a bubble radius of 1Mpc/h and a volume filling factor of 2% is consistent with the ASSP measurements at the 1-1.5$σ$ level. If this model is correct the sharp rise of the transmissivity at small separation in the ASSP sample is due to cosmic variance and is expected to become more moderate for a larger sample.

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arXiv2002

Self-similar cosmological expansion of collisional gas

We derive similarity solutions for the expansion of negative initial density perturbations δM/M \propto r^-s (s>0) in an Einstein-de Sitter universe filled with collisional baryonic gas and collisionless matter. The solutions are obtained for planar, cylindrical, and spherical perturbations. For steep perturbations, central cavities surrounded by over-dense shells expanding as t^{2(s+1)/3s} develop in both matter components. We also consider the case when baryonic shells are driven by internal pressure. Before redshift \sim 8-10 baryonic shells cool by inverse Compton scattering with the cosmic background radiation and can form bound objects of mass \sim 10^11Ω_b^{-1/2} M_{\odot} (1+z)^{-3/2}. Hot shocks of radius R cause a Compton y-distortion of order y=10^{-5}Ω_b (R/10 Mpc)^2 f_F, where f_F is the volume filling factor of these shells.

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arXiv2002

Statistics of Neutral Regions During Hydrogen Re-Ionization

We present predictions for two statistical measures of the hydrogen reionization process at high redshift. The first statistic is the number of neutral segments identified in spectra of high redshift QSOs as a function of their length. The second is the cross-correlation of neutral regions with possible sources of ionizing radiation. These independent probes are sensitive to the topology of the ionized regions. If reionization proceeded from high to low density regions then the cross-correlation will be negative, while if voids were ionized first then we expect a positive correlation and a relatively small number of long neutral segments. We test the sensitivity of these statistics for reionization by stars in high redshift galaxies. The flux of ionizing radiation emitted from stars is estimated by identifying galaxies in an N-body simulation using a semi-analytic galaxy formation model. The spatial distribution of ionized gas is traced in various models for the propagation of the ionization fronts. A model with ionization proceeding from high to low density regions is consistent with the observations of Becker et al. (2001), while models in which ionization begins in the lowest density regions appear to be inconsistent with the present data.

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arXiv2002

Superclusters with thermal SZ effect surveys

We use a simple analytic model to compute the angular correlation function of clusters identified in upcoming thermal SZ effect surveys. We then compute the expected fraction of close pairs of clusters on the sky that are also close along the line of sight. We show how the expected number of cluster pairs as a function of redshift is sensitive to the assumed biasing relation between the cluster and the mass distribution. We find that, in a LambdaCDM model, the fraction of physically associated pairs is 70% for angular separations smaller than 20 arcmin and clusters with specific flux difference larger than 200 mJy at 143 GHz. The agreement of our analytic results with the Hubble volume N-body simulations is satisfactory. These results quantify the feasibility of using SZ surveys to compile catalogues of superclusters at any redshifts.

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arXiv2002

Peculiar Velocity Reconstruction with Fast Action Method: Tests on Mock Redshift Surveys

We present extensive tests of the Fast Action Method (FAM) for recovering the past orbits of mass tracers in an expanding universe from their redshift-space coordinates at the present epoch. The tests focus on the reconstruction of present-day peculiar velocities using mock catalogs extracted from high resolution $N$-body simulations. The method allows for a self-consistent treatment of redshift-space distortions by direct minimization of a modified action for a cosmological gravitating system. When applied to ideal, volume limited catalogs, FAM recovers unbiased peculiar velocities with a 1-D, 1σerror of ~220 km/s, if velocities are smoothed on a scale of 5 Mpc/h. Alternatively, when no smoothing is applied, FAM predicts nearly unbiased velocities for objects residing outside the highest density regions. In this second case the 1σ$error decreases to a level of ~150 km/s. The correlation properties of the peculiar velocity fields are also correctly recovered on scales larger than 5 Mpc/h. Similar results are obtained when FAM is applied to flux limited catalogs mimicking the IRAS PSCz survey. In this case FAM reconstructs peculiar velocities with similar intrinsic random errors, while velocity-velocity correlation properties are well reproduced beyond scales of ~8 Mpc/h. We also show that FAM provides better velocity predictions than other, competing methods based on linear theory or Zel'dovich approximation. These results indicate that FAM can be successfully applied to presently available galaxy redshift surveys such as IRAS PSCz.

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arXiv2001

Modified Newtonian Dynamics of Large Scale Structure

We examine the implications of Modified Newtonian Dynamics (MOND) on the large scale structure in a Friedmann-Robertson-Walker universe. We employ a ``Jeans swindle'' to write a MOND-type relationship between the fluctuations in the density and the gravitational force, $\vg$. In linear Newtonian theory, $|\vg|$ decreases with time and eventually becomes $<g_0$, the threshold below which MOND is dominant. If the Newtonian initial density field has a power-law power-spectrum of index $n<-1$, then MOND domination proceeds from small to large scale. At early times MOND tends to drive the density power-spectrum towards $k^{-1}$, independent of its shape in the Newtonian regime. We use N-body simulations to solve the MOND equations of motion starting from initial conditions with a CDM power-spectrum. MOND with the standard value $g_0=10^{-8} cm s^{-2}$, yields a high clustering amplitude that can match the observed galaxy distribution only with strong (anti-) biasing. A value of $g_0 \approx 10^{-9}cm s^{-2}$, however, gives results similar to Newtonian dynamics and can be consistent with the observed large scale structure.

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arXiv2001

Polarization of the Cosmic Microwave Background from Non-Uniform Reionization

We study the signal in the Cosmic Microwave Background (CMB) polarization anisotropy resulting from patchy reionization. It is well known that the primordial polarization of the CMB is very sensitive to the details of reionization. Combining a semi-analytic model of galaxy formation, in which the optical depth to the reionization epoch is in the range 0.014 to 0.048, with a high resolution N-body simulation we find that reionization generates a peak with amplitude 0.05~0.15 μK at large angular scales. The position of this peak reveals the size of the horizon at reionization, whilst its amplitude is a measure of the optical depth to reionization. On small scales, ionized patches prior to full reionization create a second order polarization signal due to the coupling of the free electron density fluctuation with the quadrupole moment of the temperature anisotropy. Careful study reveals that the coupling generates the same power spectra for electric and magnetic modes, whose amplitudes of polarization anisotropies from this process are predicted to be ~10nK. The amplitude depends strongly on the total baryon density $Ω_b$ and on the spatial correlations of the free electron density in the ionized regions, and weakly on the fraction of ionizing photons able to escape their source galaxy, $f_{esc}$. The first- and second-order signals are therefore sensitive to the details of how the reionization occurred. Detection of these signals will place important constraints on the reionization history of the Universe.

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arXiv2000

Non-uniform reionization by galaxies and its effect on the cosmic microwave backgroun

We present predictions for the reionization of the intergalactic medium by stars in high-redshift galaxies. We calculate ionizing luminosities of galaxies, including the effects of absorption by interstellar gas and dust on the escape fraction, in order to calculate the filling factor of ionized hydrogen in the IGM. For a $Λ{\rm CDM}$ cosmology and a physical calculation of the escape fraction, we find that the hydrogen in the IGM will be reionized at redshift z=6.1 if the IGM has uniform density, but only by z=4.5 if the IGM is clumped. We combine our model with an N-body simulation of the distribution of dark matter in the universe in order to calculate the secondary temperature anisotropies induced in the cosmic microwave background (CMB) by scattering off free electrons. The models predict a spectrum of secondary anisotropies covering a broad range of angular scales, with fractional temperature fluctuations $\sim 10^{-7}-10^{-6}$ on arcminute scales. The amplitude depends somewhat on the geometry of reionization, with models in which the regions of highest gas density are reionized first giving larger CMB fluctuations than the case where galaxies ionize surrounding spherical regions, and models where low density regions reionize first giving the smallest fluctuations. Measurement of these anisotropies can therefore put important constraints on the reionization process, in particular, the redshift evolution of the filling factor, and should be a primary objective of a next generation submillimeter telescope such as the Atacama Large Millimeter Array. (abrdiged)

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arXiv2000

Self-similar spherical collapse with non-radial motions

We derive the asymptotic mass profile near the collapse center of an initial spherical density perturbation, $δ\propto M^{-ε}$, of collision-less particles with non-radial motions. We show that angular momenta introduced at the initial time do not affect the mass profile. Alternatively, we consider a scheme in which a particle moves on a radial orbit until it reaches its turnaround radius, r_*. At turnaround the particle acquires an angular momentum $L={\cal L} \sqrt{GM_* r_*}$ per unit mass, where M_* is the mass interior to r_*. In this scheme, the mass profile is $M\propto r^{3/(1+3ε)}$ for all $ε>0$, in the region $r/r_t\ll {\cal L}$, where r_t is the current turnaround radius. If ${\cal L} \ll 1$ then the profile in the region ${\cal L} \ll r/r_t \ll$ is $M\propto r$ for $ε<2/3$. The derivation relies on a general property of non-radial orbits which is that ratio of the pericenter to apocenter is constant in a force field k(t) r^{n} with k(t) varying adiabatically.

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arXiv2000

Self-similar collapse of collisional gas in an expanding Universe

Similarity solutions are found for the adiabatic collapse of density perturbations $δM/M \propto r^{-s}$ $(s>0)$ in a flat universe containing collisional gas only. The solutions are obtained for planar, cylindrical, and spherical perturbations with zero initial pressure. For adiabatic index $γ\ge 4/3$, a shock develops at a fixed fraction of the current turnaround distance. Near the center of a spherical perturbations with $γ>4/3$ and $s>1/2$, the gas is in quasi-hydrostatic equilibrium (pressure supported) and has an asymptotic power law density profile, $ρ\sim r^{-3s/(s+1)}$, independent of $γ$. For $s\le 1/2$, the profile depends on $γ$, the pressure is finite, the temperature decreases inward, and gravity dominates pressure causing a continuous inward flow. Although for $1/2 0$. We also derive the asymptotic behaviour of the gas variables near the center in a universe dominated by collisionless matter. In such a universe, the gas in a spherical perturbation with $s<2$ cannot be pressure supported and the temperature approaches a constant near the center. The solutions and the asymptotic behaviour are relevant for modelling the gas distribution in galaxy clusters and pancake-like superclusters, and determining the structure of haloes of self-interacting dark matter with large interaction cross section.

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arXiv2000

Comparison of the ENEAR Peculiar Velocities with the PSCz Gravity Field

We present a comparison between the peculiar velocity field measured from the ENEAR all-sky $D_n-σ$ catalog and that derived from the galaxy distribution of the IRAS PSCz redshift survey. The analysis is based on a modal expansion of these data in redshift space by means of spherical harmonics and Bessel functions. The effective smoothing scale of the expansion is almost linear with redshift reaching 1500km/s at 3000km/s. The general flow patterns in the filtered ENEAR and PSCz velocity fields agree well within 6000km/s, assuming a linear biasing relation between the mass and the PSCz galaxies. The comparison allows us to determine the parameter $β=Ω^{0.6}/b$, where $Ω$ is the cosmological density parameter and $b$ is the linear biasing factor. A likelihood analysis of the ENEAR and PSCz modes yields $β=0.5 +- 0.1$, in good agreement with values obtained from Tully-Fisher surveys.

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arXiv2000

Large scale motions in superclusters: their imprint in the CMB

We identify high density regions of supercluster size in high resolution N-body simulations of a representative volume of three Cold Dark Matter Universes. By assuming that (1) the density and peculiar velocities of baryons trace those of the dark matter, and (2) the temperature of plasma is proportional to the velocity dispersion of the dark matter particles in regions where the crossing times is smaller than the supercluster free-fall time, we investigate how thermal motions of electrons in the intra-cluster medium and peculiar velocity of clusters can affect the secondary anisotropies in the cosmic microwave background (CMB). We show that the thermal effect dominates the kinematic effect and that the largest thermal decrements are associated with the most massive clusters in superclusters. Thus, searching for the presence of two or more close large CMB decrements represents a viable strategy for identifying superclusters at cosmological distances. Moreover, maps of the kinematic effect in superclusters are characterized by neighboring large peaks of opposite signs. These peaks can be as high as ~ 10 microK at the arcminute angular resolution. Simultaneous pointed observations of superclusters in the millimeter and submillimeter bands with upcoming sensitive CMB experiments can separate between the thermal and kinematic effect contributions and constrain the evolution of the velocity field in large overdense regions.

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