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Daniel J. Eisenstein

Publications and source records attributed to Daniel J. Eisenstein.

387 records · Page 22Linked to original sources
arXiv1997

Small Scale Perturbations in a General MDM Cosmology

For a universe with massive neutrinos, cold dark matter, and baryons, we solve the linear perturbation equations analytically in the small-scale limit and find agreement with numerical codes at the 1-2% level. The inclusion of baryons, a cosmological constant, or spatial curvature reduces the small-scale power and tightens limits on the neutrino density from observations of high redshift objects. Using the asymptotic solution, we investigate neutrino infall into potential wells and show that it can be described on all scales by a growth function that depends on time, wavenumber, and cosmological parameters. The growth function may be used to scale the present-day transfer functions back in redshift. This allows us to construct the time-dependent transfer function for each species from a single master function that is independent of time, cosmological constant, and curvature.

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arXiv1997

Baryonic Features in the Matter Transfer Function

We provide scaling relations and fitting formulae for adiabatic cold dark matter cosmologies that account for all baryon effects in the matter transfer function to better than 10% in the large-scale structure regime. They are based upon a physically well-motivated separation of the effects of acoustic oscillations, Compton drag, velocity overshoot, baryon infall, adiabatic damping, Silk damping, and cold-dark-matter growth suppression. We also find a simpler, more accurate, and better motivated form for the zero baryon transfer function than previous works. These descriptions are employed to quantify the amplitude and location of baryonic features in linear theory. While baryonic oscillations are prominent if the baryon fraction exceeds $Ω_0 h^2 + 0.2$, the main effect in more conventional cosmologies is a sharp suppression in the transfer function below the sound horizon. We provide a simple but accurate description of this effect and stress that it is not well approximated by a change in the shape parameter $Γ$.

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arXiv1997

Comment on the Appropriate Null Hypothesis for Cosmological Birefringence

A recent paper (Nodland and Ralston, PRL 78, 3043, astro-ph/9704196) claims to have detected evidence for birefringence in the propagation of radio waves across cosmological distances. In order to assess the statistical significance of their results, the authors analyze simulated data sets, finding a stronger correlation in the real data than in the simulations. Unfortunately, the procedure used for generating the simulated data sets is based on an incorrect null hypothesis. Furthermore, the correct null hypothesis would lead to a stronger correlation in the simulated data sets, weakening the case for birefringence. We conclude that the paper's analysis does not provide statistically significant evidence of birefringence.

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arXiv1996

Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys

We propose a new method to measure the mass of large-scale filaments in galaxy redshift surveys. The method is based on the fact that the mass per unit length of isothermal filaments depends only on their transverse velocity dispersion. Filaments that lie perpendicular to the line of sight may therefore have their mass per unit length measured from their thickness in redshift space. We present preliminary tests of the method and find that it predicts the mass per unit length of filaments in an N-body simulation to an accuracy of ~35%. Applying the method to a select region of the Perseus-Pisces supercluster yields a mass-to-light ratio M/L_B around 460h in solar units to within a factor of two. The method measures the mass-to-light ratio on length scales of up to 50h^(-1) Mpc and could thereby yield new information on the behavior of the dark matter on mass scales well beyond that of clusters of galaxies.

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arXiv1995

Can the Tully-Fisher Relation be the Result of Initial Conditions?

We use Monte Carlo realizations of halo formation histories and a spherical accretion model to calculate the expected scatter in the velocity dispersions of galactic halos of a given mass due to differences in their formation times. Assuming that the rotational velocity of a spiral galaxy is determined by the velocity dispersion of its halo and that its luminosity is related to its total baryonic mass, this scatter translates to a minimum intrinsic scatter in the Tully-Fisher relation. For popular cosmological models we find that the scatter due to variations in formation histories is by itself greater than allowed by observations. Unless halos of spiral galaxies formed at high redshift (z>1) and did not later accrete any significant amount of mass, the Tully-Fisher relation is not likely to be the direct result of cosmological initial conditions but rather a consequence of a subsequent feedback process.

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arXiv1995

Probing Early Clustering with Lyman-Alpha Absorption Lines Beyond the Quasar Redshift

Groups and clusters of galaxies hosting a quasar can be found through the detection of Lyman-alpha absorption lines beyond the quasar redshift. The effect occurs whenever the distortion to the redshift distribution of Lyman-alpha clouds induced by the cluster potential extends beyond the proximity effect of the quasar. Based on CDM cosmological models for the evolution of structure, we predict the probability for finding lines beyond the quasar redshift ($z_{abs}>z_Q$) under the assumption that the physical properties of Lyman-alpha clouds are not affected by flows on large scales (1 Mpc and more) in the quasi-linear regime. If quasars randomly sample the underlying galaxy distribution, the expected number of lines with $z_{abs}>z_Q$ per quasar can be as high as 0.25[(dN/dz)/350] at $z=2$, where $dN/dz$ is the number of Lyman-alpha lines per unit redshift far from the quasar. The probability is enhanced if quasars typically reside in small groups of galaxies. A statistical excess of Lyman-alpha lines is expected near very dim quasars or around metal absorption systems. Due to clustering, the standard approach to the proximity effect overestimates the ionizing background flux at high redshifts by up to a factor of 3. This result weakens the discrepancy between the deduced background flux and the contribution from known populations of quasars.

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arXiv1994

Origin of Quasar Progenitors from the Collapse of Low-Spin Cosmological Perturbations

We show that seeds for quasar black holes could have originated from the initial cosmological collapse of overdense regions with unusually small rotation. The gas in these rare regions collapses into a compact disk that shrinks on a short viscous time scale. Using an analytical model, we calculate the low-spin tail of the probability distribution of angular momenta for objects that collapse out of a Gaussian random field of initial density perturbations. The population of low-spin systems is significant for any viable power spectrum of primordial density perturbations. Most objects form just above the cosmological Jeans mass (\sim 10^5 M_sun) at high redshifts z>10. In the standard cold dark matter cosmology, the comoving density of 10^{6-7} M_sun objects with viscous evolution times shorter than 10^{6-7} years is about 10^{-3} (h/0.5)^3 Mpc^{-3}, comparable to the local density of bright galaxies. The seed black holes tend to reside within larger mass systems that collapse later and supply the gas needed for the bright quasar activity.

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arXiv1994

An Analytical Model for the Triaxial Collapse of Cosmological Perturbations

We present an analytical model for the non-spherical collapse of overdense regions out of a Gaussian random field of initial cosmological perturbations. The collapsing region is treated as an ellipsoid of constant density, acted upon by the quadrupole tidal shear from the surrounding matter. The dynamics of the ellipsoid is set by the ellipsoid self-gravity and the external quadrupole shear. Both forces are linear in the coordinates and therefore maintain homogeneity of the ellipsoid at all times. The amplitude of the external shear is evolved into the non-linear regime in thin spherical shells that are allowed to move only radially according to the mass interior to them. We describe how the initial conditions can be drawn in the appropriate correlated way from a random field of initial density perturbations. By considering many random realizations of the initial conditions, we calculate the distribution of shapes and angular momenta acquired by objects through the coupling of their quadrupole moment to the tidal shear. The average value of the spin parameter, 0.04, is found to be only weakly dependent on the system mass, the mean cosmological density, or the initial power spectrum of perturbations, in agreement with N-body simulations. For the cold dark matter power spectrum, most objects evolve from a quasi-spherical initial state to a pancake or filament and then to complete virialization. Low-spin objects tend to be more spherical. The evolution history of shapes is primarily induced by the external shear and not by the initial triaxiality of the objects. The statistical distribution of the triaxial shapes of collapsing regions can be used to test cosmological models against galaxy surveys on large scales.

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