103 research outputs found
On the Spatial Correlations of Lyman Break Galaxies
Motivated by the observed discrepancy between the strong spatial correlations
of Lyman break galaxies (LBGs) and their velocity dispersions, we consider a
theoretical model in which these starbursting galaxies are associated with dark
matter halos that experience appreciable infall of material. We show using
numerical simulation that selecting halos that substantially increase in mass
within a fixed time interval introduces a ``temporal bias'' which boosts their
clustering above that of the underlying population. If time intervals
consistent with the observed LBGs star formation rates of 50 solar masses per
year are chosen, then spatial correlations are enhanced by up to a factor of
two. These values roughly correspond to the geometrical bias of objects three
times as massive. Thus, it is clear that temporal biasing must be taken into
account when interpreting the properties of Lyman break galaxies.Comment: 5 Pages, 2 Figures, Accepted for Publication in ApJ Letter
Toward an Improved Analytical Description of Lagrangian Bias
We carry out a detailed numerical investigation of the spatial correlation
function of the initial positions of cosmological dark matter halos. In this
Lagrangian coordinate system, which is especially useful for analytic studies
of cosmological feedback, we are able to construct cross-correlation functions
of objects with varying masses and formation redshifts and compare them with a
variety of analytical approaches. For the case in which both formation
redshifts are equal, we find good agreement between our numerical results and
the bivariate model of Scannapieco & Barkana (2002; SB02) at all masses,
redshifts, and separations, while the model of Porciani et al. (1998) does well
for all parameters except for objects with different masses at small
separations. We find that the standard mapping between Lagrangian and Eulerian
bias performs well for rare objects at all separations, but fails if the
objects are highly-nonlinear (low-sigma) peaks. In the Lagrangian case in which
the formation redshifts differ, the SB02 model does well for all separations
and combinations of masses, apart from a discrepancy at small separations in
situations in which the smaller object is formed earlier and the difference
between redshifts or masses is large. As this same limitation arises in the
standard approach to the single-point progenitor distribution developed by
Lacey & Cole (1993), we conclude that a more complete understanding of the
progenitor distribution is the most important outstanding issue in the analytic
modeling of Lagrangian bias.Comment: 22 pages, 8 figures, ApJ, in pres
A Physical Model of Lyman Alpha Emitters
We present a simple physical model for populating dark matter halos with
Lyman Alpha Emiiters(LAEs) and predict the physical properties of LAEs at
z~3-7. The central tenet of this model is that the Ly-alpha luminosity is
proportional to the star formation rate (SFR) which is directly related to the
halo mass accretion rate. The only free parameter in our model is then the
star-formation efficiency (SFE). An efficiency of 2.5% provides the best-fit to
the Ly-alpha luminosity function (LF) at redshift z=3.1, and we use this SFE to
construct Ly-alpha LFs at other redshifts. Our model reproduce the Ly-alpha
LFs, stellar ages, SFR ~1-10; Msun/yr, stellar masses ~ 10^7-10^8 Msun and the
clustering properties of LAEs at z~3-7. We find the spatial correlation lengths
ro ~ 3-6 Mpc/h, in agreement with the observations. Finally, we estimate the
field-to-field variation ~ 30% for current volume and flux limited surveys,
again consistent with observations. Our results suggest that the star
formation, and hence Ly-alpha emission in LAEs is powered by the accretion of
new material, and that the physical properties of LAEs do not evolve
significantly over a wide range of redshifts. Relating the accreted mass,
rather than the total mass of halos, to the Ly-alpha luminosity of LAEs
naturally gives rise to the duty cycle of LAEs.Comment: Published in Ap
Comparing Simulations of AGN Feedback
We perform adaptive mesh refinement (AMR) and smoothed particle hydrodynamics
(SPH) cosmological zoom simulations of a region around a forming galaxy
cluster, comparing the ability of the methods to handle successively more
complex baryonic physics. In the simplest, non-radiative case, the two methods
are in good agreement with each other, but the SPH simulations generate central
cores with slightly lower entropies and virial shocks at slightly larger radii,
consistent with what has been seen in previous studies. The inclusion of
radiative cooling, star formation, and stellar feedback leads to much larger
differences between the two methods. Most dramatically, at z=5, rapid cooling
in the AMR case moves the accretion shock well within the virial radius, while
this shock remains near the virial radius in the SPH case, due to excess
heating, coupled with poorer capturing of the shock width. On the other hand,
the addition of feedback from active galactic nuclei (AGN) to the simulations
results in much better agreement between the methods. In this case both
simulations display halo gas entropies of 100 keV cm^2, similar decrements in
the star-formation rate, and a drop in the halo baryon content of roughly 30%.
This is consistent with AGN growth being self-regulated, regardless of the
numerical method. However, the simulations with AGN feedback continue to differ
in aspects that are not self-regulated, such that in SPH a larger volume of gas
is impacted by feedback, and the cluster still has a lower entropy central
core.Comment: 22 pages, 20 figures, 3 tables, Accepted to ApJ, comments welcom
Power spectrum for the small-scale Universe
The first objects to arise in a cold dark matter universe present a daunting
challenge for models of structure formation. In the ultra small-scale limit,
CDM structures form nearly simultaneously across a wide range of scales.
Hierarchical clustering no longer provides a guiding principle for theoretical
analyses and the computation time required to carry out credible simulations
becomes prohibitively high. To gain insight into this problem, we perform
high-resolution (N=720^3 - 1584^3) simulations of an Einstein-de Sitter
cosmology where the initial power spectrum is P(k) propto k^n, with -2.5 < n <
-1. Self-similar scaling is established for n=-1 and n=-2 more convincingly
than in previous, lower-resolution simulations and for the first time,
self-similar scaling is established for an n=-2.25 simulation. However, finite
box-size effects induce departures from self-similar scaling in our n=-2.5
simulation. We compare our results with the predictions for the power spectrum
from (one-loop) perturbation theory and demonstrate that the renormalization
group approach suggested by McDonald improves perturbation theory's ability to
predict the power spectrum in the quasilinear regime. In the nonlinear regime,
our power spectra differ significantly from the widely used fitting formulae of
Peacock & Dodds and Smith et al. and a new fitting formula is presented.
Implications of our results for the stable clustering hypothesis vs. halo model
debate are discussed. Our power spectra are inconsistent with predictions of
the stable clustering hypothesis in the high-k limit and lend credence to the
halo model. Nevertheless, the fitting formula advocated in this paper is purely
empirical and not derived from a specific formulation of the halo model.Comment: 30 pages including 10 figures; accepted for publication in MNRA
Hybrid cosmological simulations with stream velocities
Publisher's Version/PDFIn the early universe, substantial relative “stream” velocities between the gas and dark matter arise due to radiation pressure and persist after recombination. To assess the impact of these velocities on high-redshift structure formation, we carry out a suite of high-resolution adaptive mesh refinement (AMR) cosmological simulations, which use smoothed particle hydrodynamic data sets as initial conditions, converted using a new tool developed for this work. These simulations resolve structures with masses as small as a few 100 M[subscript circled dot], and we focus on the 10[superscript 6] M[subscript circled dot] “mini-halos” in which the first stars formed. At z ≈ 17, the presence of stream velocities has only a minor effect on the number density of halos below 10[superscript 6] M[subscript circled dot], but it greatly suppresses gas accretion onto all halos and the dark matter structures around them. Stream velocities lead to significantly lower halo gas fractions, especially for ≈10[superscript 5] M[subscript circled dot] objects, an effect that is likely to depend on the orientation of a halo’s accretion lanes. This reduction in gas density leads to colder, more compact radial profiles, and it substantially delays the redshift of collapse of the largest halos, leading to delayed star formation and possibly delayed reionization. These many differences suggest that future simulations of early cosmological structure formation should include stream velocities to properly predict gas evolution, star formation, and the epoch of reionization
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