Publications

1998
Eisenstein DJ, Hut P. HOP: A New Group-Finding Algorithm for N-Body Simulations. The Astrophysical Journal [Internet]. 1998;498 :137. Publisher's VersionAbstract

We describe a new method (HOP) for identifying groups of particles inN-body simulations. Having assigned to every particle an estimate of itslocal density, we associate each particle with the densest of the Nhopparticles nearest to it. Repeating this process allows us to trace apath, within the particle set itself, from each particle in thedirection of increasing density. The path ends when it reaches aparticle that is its own densest neighbor; all particles reaching thesame such particle are identified as a group. Combined with an adaptivesmoothing kernel for finding the densities, this method is spatiallyadaptive, coordinate-free, and numerically straightforward. One canproceed to process the output by truncating groups at a particulardensity contour and combining groups that share a (possibly different)density contour. While the resulting algorithm has several user-chosenparameters, we show that the results are insensitive to most of these,the exception being the outer density cutoff of the groups.

Hu W, Eisenstein DJ. Small-Scale Perturbations in a General Mixed Dark Matter Cosmology. The Astrophysical Journal [Internet]. 1998;498 :497. Publisher's VersionAbstract

For a universe with massive neutrinos, cold dark matter, and baryons, wesolve the linear perturbation equations analytically in the small-scale

Eisenstein DJ, Hu W, Tegmark M. Cosmic Complementarity: H 0 and Omega M from Combining Cosmic Microwave Background Experiments and Redshift Surveys. The Astrophysical Journal Letters [Internet]. 1998;504 :L57. Publisher's VersionAbstract

We show that the detection of acoustic oscillations in both upcomingcosmic microwave background (CMB) satellite experiments and

Hu W, Eisenstein DJ, Tegmark M. Weighing Neutrinos with Galaxy Surveys. Physical Review Letters [Internet]. 1998;80 :5255-5258. Publisher's VersionAbstract

We show that galaxy redshift surveys sensitively probe the neutrinomass, with eV mass neutrinos suppressing power by a factor of 2. TheSloan Digital Sky Survey can potentially detect N nearly degeneratemassive neutrino species with massmν>~0.65\(Ωmh2/0.1N\)0.8eV at better than 2σ once microwave background experiments measuretwo other cosmological parameters. Significant overlap exists betweenthis region and that implied by the Liquid Scintillator NeutrinoDetector experiment, and even mν~0.01-0.1 eV, as impliedby the atmospheric anomaly, can affect cosmological measurements.

1997
Eisenstein DJ, Loeb A, Turner EL. Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys. The Astrophysical Journal [Internet]. 1997;475 :421. Publisher's VersionAbstract

We propose a new method to measure the mass of large-scale filaments ingalaxy redshift surveys. The method is based on the fact that the massper unit length of isothermal filaments depends only on their transversevelocity dispersion. Filaments that lie perpendicular to the line ofsight may therefore have their mass per unit length measured from theirthickness in redshift space. We present preliminary tests of the methodand find that it predicts the mass per unit length of filaments in an

Eisenstein DJ, Bunn EF. Appropriate Null Hypothesis for Cosmological Birefringence. Physical Review Letters [Internet]. 1997;79 :1957. Publisher's VersionAbstract

A Comment on the Letter by Borge Nodland and John P. Ralston, Phys. Rev.Lett. 78, 3043 (1997). The authors of the Letter offer a Reply.

1996
Eisenstein DJ. Analytic Models for the Gravitational Formation of Objects in the Universe. Ph.D. Thesis [Internet]. 1996 :1. Publisher's VersionAbstract

In this thesis, we use analytic approximate models of cosmologicalgravitational dynamics to study a variety of astrophysical systems. Webegin with the development of a new analytic collapse model in which thecollapsing region is treated as a homogeneous triaxial ellipsoid actedon by cosmological tidal shear. By performing a Monte Carlo sampling ofthe initial conditions, the ellipsoid model can predict thedistributions of angular momenta and axis ratios of collapsing regions.We then apply this and other analytic tools to certain astrophysicalsituations. In considering the origin of the black holes that powerquasars, we consider the likelihood of objects to be formed withsufficiently little angular momentum as to proceed to collapse to ablack hole; using the ellipsoid model, we estimate the number density ofblack holes formed in this manner to be ~ 10^{-3} Mpc ^{-3}. Next, westudy the clustering of Lyalpha clouds around the massive host of thequasar and show that this can produce clouds with redshifts exceedingthat of the quasar and alter the inferred ultraviolet background fromthe analysis of the proximity effect. We then consider the variation inthe relation between halo masses and velocity dispersions caused byvariations in their merger histories and show that the resulting scatteris larger than that observed for the Tully-Fisher relation. This arguesthat the Tully-Fisher relation does not result simply from theuniversality of halo properties but rather from feedback processesduring galaxy formation. Finally, we apply an exact solution for thesteady-state, self-gravitating, isothermal cylinder to construct amethod for estimating the mass per unit length of linear structures inredshift surveys. Tests of the method against N-body simulations suggest

Eisenstein DJ, Loeb A. Can the Tully-Fisher Relation Be the Result of Initial Conditions?. The Astrophysical Journal [Internet]. 1996;459 :432. Publisher's VersionAbstract

We use Monte Carlo realizations of halo formation histories and aspherical accretion model to calculate the expected scatter in thevelocity dispersions of galactic halos of a given mass due todifferences in their formation times. Assuming that the rotationalvelocity of a spiral galaxy is determined by the velocity dispersion ofits halo and that its luminosity is related to its total baryonic mass,this scatter translates to a minimum intrinsic scatter in theTully-Fisher relation. For popular cosmological models we find that thescatter due to variations in formation histories is by itself greaterthan allowed by observations. Unless halos of spiral galaxies formed athigh redshift (z 1) and did not later accrete any significant amount ofmass, the Tully-Fisher relation is not likely to be the direct result ofcosmological initial conditions but rather a consequence of a subsequentfeedback process.

1995
Eisenstein DJ, Loeb A. An analytical model for the triaxial collapse of cosmological perturbations. The Astrophysical Journal [Internet]. 1995;439 :520-541. Publisher's VersionAbstract

We present an analytical model for the nonspherical collapse ofoverdense regions out of a Gaussian random field of initial cosmologicalperturbations. The collapsing region is treated as an ellipsoid ofconstant density, acted upon by the quadrupole tidal shear from thesurrounding matter. The dynamics of the ellipsoid is set by theellipsoid self-gravity and the external quadrupole shear. Both forcesare linear in the coordinates and therefore maintain homogeneity of theellipsoid at all times. The amplitude of the external shear is evolvedinto the nonlinear regime in thin spherical shells that are allowed tomove only radially according to mass interior to them. The fulldynamical equations then reduce to a set of nine second-order ordinarydifferential equations, which reproduce the linear regime behavior butcan be evolved past turnaround, well into the nonlinear regime. Wedescribe how the initial conditions can be drawn in the appropriatecorrelated way from a random field of initial density perturbations. Themodel is applied to a restricted set of initial conditions that are moresuitable to the above approximations; most notable we focus on theproperties of rare high-density peaks (greater than of approximately = 2sigma). By considering many random realization of the initialconditions, we calculate the distributions of shapes and angular momentaacquired by objects through the coupling of their quadruole moment tothe tidal shear. The average value of the spin parameter, (mean value ofa lambda) approximately = 0.04, is found to be only weakly dependent onthe system mass, the mean cosmological density, or the initial powerspectrum of perturbations, in agreement with N-body simulations. For thecold dark matter power spectrum, most objects evolve from aquasi-spherical initial state to a pancake or filament and then tocomplete virialization. Low-spin objects tend to be more spherical. Theevolution history of shapes is primarily induced by the external shearand not by the initial triaxially of the objects. The statisticaldistribution of the triaxial shapes of collapsing region can be used totest cosmological models against galaxy surveys on large scales.

Eisenstein DJ, Loeb A. Origin of quasar progenitors from the collapse of low-spin cosmological perturbations. The Astrophysical Journal [Internet]. 1995;443 :11-17. Publisher's VersionAbstract

We show that seeds for quasar black holes could have originated from theinitial cosmological collapse of overdense regions with unusually smallrotation. The gas in these rare regions collapses into a compact diskthat shrinks on a short viscous timescale. Using an analytical model, wecalculate the low-pin tail of the probability distribution of angularmomenta for objects that collapse out of a Gaussian random field ofinitial density peturbations. The population of low-spin systems issignificant for any viable power spectrum of primordial densityperturbations. Most objects from just above the cosmological Jeans massapproximately 105 solar mass at high redshifts z greater thanand approximately 10. In the standard cold dark matter cosmology, thecomoving density of 106-7 solar mass objects with viscousevolution times shorter than approximately 106-7 years isapproximately 10-3(h/0.5)3/cubic Mpc, comparableto the local density of bright galaxies. The seed black holes tend toreside within larger mass systems that collapse later and supply the gasneeded for the bright quasar activity.

Loeb A, Eisenstein DJ. Probing Early Clustering with LY alpha Absorption Lines beyond the Quasar Redshift. The Astrophysical Journal [Internet]. 1995;448 :17. Publisher's VersionAbstract

Groups and clusters of galaxies hosting a quasar can be found throughthe detection of Lyα absorption lines beyond the quasar redshift.The effect occurs whenever the distortion to the redshift distributionof Lyα clouds induced by the cluster potential extends beyond theproximity effect of the quasar. Based on cold dark matter (CDM)cosmological models for the evolution of structure, we predict theprobability for finding lines beyond the quasar redshift(Ζabs > ΖQ) under the assumption thatthe physical properties of Lyman alpha clouds are not affected by flowson large scales (≳Mpc) in the quasi-linear regime. If quasarsrandomly sample the underlying galaxy distribution, the expected numberof lines with Ζabs > ΖQ per quasarcan be as high as ˜0.5 × [(dN/dΖ)/350] at Ζ = 2,where dN/dΖ is the number of Lyα lines per unit redshift farfrom the quasar. The probability is enhanced if quasars typically residein small groups of galaxies. A statistical excess of Lyα lines isexpected near very dim quasars or around metal absorption systems. Dueto clustering, the standard approach to the proximity effectoverestimates the ionizing background flux at high redshifts by up to afactor of ˜3. This result weakens the discrepancy between thededuced background flux and the contribution from known populations ofquasars.

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