1,017 research outputs found
On The Validity of the Streaming Model for the Redshift-Space Correlation Function in the Linear Regime
The relation between the galaxy correlation function in real and
redshift-space is derived in the linear regime by an appropriate averaging of
the joint probability distribution of density and velocity. The derivation
recovers the familiar linear theory result on large scales but has the
advantage of clearly revealing the dependence of the redshift distortions on
the underlying peculiar velocity field; streaming motions give rise to
distortions of while variations in the anisotropic
velocity dispersion yield terms of order . This
probabilistic derivation of the redshift-space correlation function is similar
in spirit to the derivation of the commonly used ``streaming'' model, in which
the distortions are given by a convolution of the real-space correlation
function with a velocity distribution function. The streaming model is often
used to model the redshift-space correlation function on small, highly
non-linear, scales. There have been claims in the literature, however, that the
streaming model is not valid in the linear regime. Our analysis confirms this
claim, but we show that the streaming model can be made consistent with linear
theory {\it provided} that the model for the streaming has the functional form
predicted by linear theory and that velocity distribution is chosen to be a
Gaussian with the correct linear theory dispersion.Comment: 14 pages, no figures, uuencoded compressed postscrip
Galaxy Clustering Around Nearby Luminous Quasars
We examine the clustering of galaxies around a sample of 20 luminous low
redshift (z<0.30) quasars observed with the Wide Field Camera-2 on the Hubble
Space Telescope. The HST resolution makes possible galaxy identification
brighter than V=23.5 and as close as 2'' to the quasar. We find a significant
enhancement of galaxies within a projected separation of < 100 kpc/h of the
quasars. If we model the qso/galaxy correlation function as a power law with a
slope given by the galaxy/galaxy correlation function, we find that the ratio
of the qso/galaxy to galaxy/galaxy correlation functions is . The
galaxy counts within r<15 kpc/h of the quasars are too high for the density
profile to have an appreciable core radius ( > 100 kpc). Our results reinforce
the idea that low redshift quasars are located preferentially in groups of
10-20 galaxies rather than in rich clusters. We see no significant difference
in the clustering amplitudes derived from radio-loud and radio-quiet
subsamples.Comment: 16 pages, 3 figures (included), 2 tables, Apj in pres
Fourier Analysis of Redshift Space Distortions and the Determination of Omega
The peculiar velocities of galaxies distort the pattern of galaxy clustering
in redshift space, making the redshift space power spectrum anisotropic. In the
linear regime, the strength of this distortion depends only on the ratio , where is the cosmological
density parameter and is the bias parameter. We derive a linear theory
estimator for based on the harmonic moments of the redshift space power
spectrum. Using N-body simulations, we examine the impact of non-linear
gravitational clustering on the power spectrum anisotropy and on our
-estimator. Non-linear effects can be important out to wavelengths
Mpc/h or larger; in most cases, they lower the quadrupole
moment of the power spectrum and thereby depress the estimate of below
the true value. With a sufficiently large redshift survey, the scaling of
non-linear effects may allow separate determinations of and .
We describe a practical technique for measuring the anisotropy of the power
spectrum from galaxy redshift surveys, and we test the technique on mock
catalogues drawn from the N-body simulations. Preliminary application of our
methods to the 1.2 Jy IRAS galaxy survey yields at
wavelengths Mpc/h . Non-linear effects remain important at
these scales, so this estimate of is probably lower than the true
value.Comment: uuencoded compressed postscript fil
Size-scaling of clump instabilities in turbulent, feedback regulated disks
We explore the scaling between the size of star-forming clumps and rotational
support in massively star-forming galactic disks. The analysis relies on
simulations of a clumpy galaxy at and the observed DYNAMO sample of rare
clumpy analogs at to test a predictive clump size scaling
proposed by \citet{Fisher2017ApJ...839L...5F} in the context of the Violent
Disk Instability (VDI) theory. We here determine the clump sizes using a
recently presented 2-point estimator, which is robust against resolution/noise
effects, hierarchical clump substructure, clump-clump overlap and other
galactic substructure. After verifying Fisher's clump scaling relation for the
DYNAMO observations, we explore whether this relation remains characteristic of
the VDI theory, even if realistic physical processes, such as local asymetries
and stellar feedback, are included in the model. To this end, we rely on
hydrodynamic zoom-simulations of a Milky Way-mass galaxy with four different
feedback prescriptions. We find that, during its marginally stable epoch at
, this mock galaxy falls on the clump scaling relation, although its
position on this relation depends on the feedback model. This finding implies
that Toomre-like stability considerations approximately apply to large
() instabilities in marginally stable turbulent disks,
irrespective of the feedback model, but also emphasizes that the global clump
distribution of a turbulent disk depends strongly on feedback.Comment: Accepted by ApJ, no changes made. 11 pages, 4 figure
A Discotic Disguised as a Smectic: A Hybrid Columnar Bragg Glass
We show that discotics, lying deep in the columnar phase, can exhibit an
x-ray scattering pattern which mimics that of a somewhat unusual smectic liquid
crystal. This exotic, new glassy phase of columnar liquid crystals, which we
call a ``hybrid columnar Bragg glass'', can be achieved by confining a columnar
liquid crystal in an anisotropic random environment of e.g., strained aerogel.
Long-ranged orientational order in this phase makes {\em single domain} x-ray
scattering possible, from which a wealth of information could be extracted. We
give detailed quantitative predictions for the scattering pattern in addition
to exponents characterizing anomalous elasticity of the system.Comment: 4 RevTeX pgs, 2 eps figures. To appear in PR
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