58 research outputs found
How Stochastic is the Relative Bias Between Galaxy Types?
Examining the nature of the relative clustering of different galaxy types can
help tell us how galaxies formed. To measure this relative clustering, I
perform a joint counts-in-cells analysis of galaxies of different spectral
types in the Las Campanas Redshift Survey (LCRS). I develop a
maximum-likelihood technique to fit for the relationship between the density
fields of early- and late-type galaxies. This technique can directly measure
nonlinearity and stochasticity in the biasing relation. At high significance, a
small amount of stochasticity is measured, corresponding to a correlation
coefficient of about 0.87 on scales corresponding to 15 Mpc/h spheres. A large
proportion of this signal appears to derive from errors in the selection
function, and a more realistic estimate finds a correlation coefficient of
about 0.95. These selection function errors probably account for the large
stochasticity measured by Tegmark & Bromley (1999), and may have affected
measurements of very large-scale structure in the LCRS. Analysis of the data
and of mock catalogs shows that the peculiar geometry, variable flux limits,
and central surface-brightness selection effects of the LCRS do not seem to
cause the effect.Comment: 38 pages, 14 figures. Submitted to Apj. Modified from a chapter of my
Ph.D. Thesis at Princeton University, available at
http://www-astro-theory.fnal.gov/Personal/blanton/thesis/index.htm
Stochasticity of Bias and Nonlocality of Galaxy Formation: Linear Scales
If one wants to represent the galaxy number density at some point in terms of
only the mass density at the same point, there appears the stochasticity in
such a relation, which is referred to as ``stochastic bias''. The stochasticity
is there because the galaxy number density is not merely a local function of a
mass density field, but it is a nonlocal functional, instead. Thus, the
phenomenological stochasticity of the bias should be accounted for by nonlocal
features of galaxy formation processes. Based on mathematical arguments, we
show that there are simple relations between biasing and nonlocality on linear
scales of density fluctuations, and that the stochasticity in Fourier space
does not exist on linear scales under a certain condition, even if the galaxy
formation itself is a complex nonlinear and nonlocal precess. The stochasticity
in real space, however, arise from the scale-dependence of bias parameter, .
As examples, we derive the stochastic bias parameters of simple nonlocal models
of galaxy formation, i.e., the local Lagrangian bias models, the cooperative
model, and the peak model. We show that the stochasticity in real space is also
weak, except on the scales of nonlocality of the galaxy formation. Therefore,
we do not have to worry too much about the stochasticity on linear scales,
especially in Fourier space, even if we do not know the details of galaxy
formation process.Comment: 24 pages, latex, including 2 figures, ApJ, in pres
The scale-dependence of relative galaxy bias: encouragement for the halo model description
We investigate the relationship between the colors, luminosities, and
environments of galaxies in the Sloan Digital Sky Survey spectroscopic sample,
using environmental measurements on scales ranging from 0.2 to 6 Mpc/h. We
find: (1) that the relationship between color and environment persists even to
the lowest luminosities we probe (absolute magnitude in the r band of about -14
for h=1); (2) at luminosities and colors for which the galaxy correlation
function has a large amplitude, it also has a steep slope; and (3) in regions
of a given overdensity on small scales (1 Mpc/h), the overdensity on large
scales (6 Mpc/h) does not appear to relate to the recent star formation history
of the galaxies. Of these results, the last has the most immediate application
to galaxy formation theory. In particular, it lends support to the notion that
a galaxy's properties are related only to the mass of its host dark matter
halo, and not to the larger scale environment.Comment: submitted to ApJ; full resolution figures and slide material
available at http://cosmo.nyu.edu/blanton/scale_density.htm
Southern Sky Redshift Survey: Clustering of Local Galaxies
We use the two-point correlation function to calculate the clustering
properties of the recently completed SSRS2 survey. The redshift space
correlation function for the magnitude-limited SSRS2 is given by xi(s)=(s/5.85
h-1 Mpc)^{-1.60} for separations between 2 < s < 11 h-1 Mpc, while our best
estimate for the real space correlation function is xi(r) = (r/5.36 h-1
Mpc)^{-1.86}. Both are comparable to previous measurements using surveys of
optical galaxies over much larger and independent volumes. By comparing the
correlation function calculated in redshift and real space we find that the
redshift distortion on intermediate scales is small. This result implies that
the observed redshift-space distribution of galaxies is close to that in real
space, and that beta = Omega^{0.6}/b < 1, where Omega is the cosmological
density parameter and b is the linear biasing factor for optical galaxies. We
also use the SSRS2 to study the dependence of xi on the internal properties of
galaxies. We confirm earlier results that luminous galaxies (L>L*) are more
clustered than sub-L* galaxies and that the luminosity segregation is
scale-independent. We find that early types are more clustered than late types,
but that in the absence of rich clusters, the relative bias between early and
late types in real space, is not as strong as previously estimated.
Furthermore, both morphologies present a luminosity-dependent bias, with the
early types showing a slightly stronger dependence on luminosity. We also find
that red galaxies are significantly more clustered than blue ones, with a mean
relative bias stronger than that seen for morphology. Finally, we find that the
relative bias between optical and iras galaxies in real space is b_o/b_I
1.4.Comment: 43 pages, uses AASTeX 4.0 macros. Includes 8 tables and 16 Postscript
figures, updated reference
Redshift-Space Distortions and the Real-Space Clustering of Different Galaxy Types
We study the distortions induced by peculiar velocities on the redshift-space
correlation function of galaxies of different morphological types in the
Pisces-Perseus redshift survey. Redshift-space distortions affect early- and
late-type galaxies in different ways. In particular, at small separations, the
dominant effect comes from virialized cluster cores, where ellipticals are the
dominant population. The net result is that a meaningful comparison of the
clustering strength of different morphological types can be performed only in
real space, i.e., after projecting out the redshift distortions on the
two-point correlation function xi(r_p,pi). A power-law fit to the projected
function w_p(r_p) on scales smaller than 10/h Mpc gives r_o =
8.35_{-0.76}^{+0.75} /h Mpc, \gamma = 2.05_{-0.08}^{+0.10} for the early-type
population, and r_o = 5.55_{-0.45}^{+0.40} /h Mpc, \gamma =
1.73_{-0.08}^{+0.07} for spirals and irregulars. These values are derived for a
sample luminosity brighter than M_{Zw} = -19.5. We detect a 25% increase of r_o
with luminosity for all types combined, from M_{Zw} = -19 to -20. In the
framework of a simple stable-clustering model for the mean streaming of pairs,
we estimate sigma_12(1), the one-dimensional pairwise velocity dispersion
between 0 and 1 /h Mpc, to be 865^{+250}_{-165} km/s for early-type galaxies
and 345^{+95}_{-65} km/s for late types. This latter value should be a fair
estimate of the pairwise dispersion for ``field'' galaxies; it is stable with
respect to the presence or absence of clusters in the sample, and is consistent
with the values found for non-cluster galaxies and IRAS galaxies at similar
separations.Comment: 17 LaTeX pages including 3 tables, plus 11 PS figures. Uses AASTeX
macro package (aaspp4.sty) and epsf.sty. To appear on ApJ, 489, Nov 199
Bias and Hierarchical Clustering
It is now well established that galaxies are biased tracers of the
distribution of matter, although it is still not known what form this bias
takes. In local bias models the propensity for a galaxy to form at a point
depends only on the overall density of matter at that point. Hierarchical
scaling arguments allow one to build a fully-specified model of the underlying
distribution of matter and to explore the effects of local bias in the regime
of strong clustering. Using a generating-function method developed by
Bernardeau & Schaeffer (1992), we show that hierarchical models lead one
directly to the conclusion that a local bias does not alter the shape of the
galaxy correlation function relative to the matter correlation function on
large scales. This provides an elegant extension of a result first obtained by
Coles (1993) for Gaussian underlying fields and confirms the conclusions of
Scherrer & Weinberg (1998) obtained using a different approach. We also argue
that particularly dense regions in a hierarchical density field display a form
of bias that is different from that obtained by selecting such peaks in
Gaussian fields: they are themselves hierarchically distributed with scaling
parameters . This kind of bias is also factorizable, thus in
principle furnishing a simple test of this class of models.Comment: Latex, accepted for publication in ApJL; moderate revision
Relationship between environment and the broad-band optical properties of galaxies in the SDSS
We examine the relationship between environment and the luminosities, surface
brightnesses, colors, and profile shapes of luminous galaxies in the Sloan
Digital Sky Survey (SDSS). For the SDSS sample, galaxy color is the galaxy
property most predictive of the local environment. Galaxy color and luminosity
jointly comprise the most predictive pair of properties. At fixed luminosity
and color, density is not closely related to surface brightness or to Sersic
index -- the parameter in this study that astronomers most often associate with
morphology. In the text, we discuss what measureable residual relationships
exist, generally finding that at red colors and fixed luminosity, the mean
density decreases at the highest surface brightnesses and Sersic indices. In
general, these results suggest that the morphological properties of galaxies
are less closely related to galaxy environment than are their masses and
star-formation histories.Comment: submitted to ApJ, pedagogy and bitmapped figures for presentations
available at http://cosmo.nyu.edu/blanton/full_density.htm
Measuring the Nonlinear Biasing Function from a Galaxy Redshift Survey
We present a simple method for evaluating the nonlinear biasing function of
galaxies from a redshift survey. The nonlinear biasing is characterized by the
conditional mean of the galaxy density fluctuation given the underlying mass
density fluctuation, or by the associated parameters of mean biasing and
nonlinearity (following Dekel & Lahav 1999). Using the distribution of galaxies
in cosmological simulations, at smoothing of a few Mpc, we find that the mean
biasing can be recovered to a good accuracy from the cumulative distribution
functions (CDFs) of galaxies and mass, despite the biasing scatter. Then, using
a suite of simulations of different cosmological models, we demonstrate that
the matter CDF is robust compared to the difference between it and the galaxy
CDF, and can be approximated for our purpose by a cumulative log-normal
distribution of 1+\delta with a single parameter \sigma. Finally, we show how
the nonlinear biasing function can be obtained with adequate accuracy directly
from the observed galaxy CDF in redshift space. Thus, the biasing function can
be obtained from counts in cells once the rms mass fluctuation at the
appropriate scale is assumed a priori. The relative biasing function between
different galaxy types is measurable in a similar way. The main source of error
is sparse sampling, which requires that the mean galaxy separation be smaller
than the smoothing scale. Once applied to redshift surveys such as PSCz, 2dF,
SDSS, or DEEP, the biasing function can provide valuable constraints on galaxy
formation and structure evolution.Comment: 23 pages, 7 figures, revised version, accepted for publication in Ap
The Masses, Ancestors and Descendents of Extremely Red Objects: Constraints from Spatial Clustering
Wide field near-infrared (IR) surveys have revealed a population of galaxies
with very red opticalIR colors, which have been termed ``Extremely Red
Objects'' (EROs). Modeling suggests that such red colors (R-K > 5) could be
produced by galaxies at z>~1 with either very old stellar populations or very
high dust extinction. Recently it has been discovered that EROs are strongly
clustered. Are these objects the high-redshift progenitors of present day giant
ellipticals (gEs)? Are they already massive at this epoch? Are they the
descendents of the Lyman Break Galaxies (LBG), which have also been
identified as possible high redshift progenitors of giant ellipticals? We
address these questions within the framework of the Cold Dark Matter paradigm
using an analytic model that connects the number density and clustering or bias
of an observed population with the halo occupation function (the number of
observed galaxies per halo of a given mass). We find that EROs reside in
massive dark matter halos, with average mass > 1E13/h100 Msun. The
occupation function that we derive for EROs is very similar to the one we
derive for z=0, L>L* early type galaxies, whereas the occupation function for
LBGs is skewed towards much smaller host halo masses ( ~ 1E11 to 1E12/h100
Msun. We then use the derived occupation function parameters to explore the
possible evolutionary connections between these three populations.Comment: Replaced to match version accepted for publication in ApJ. New model
added; appendix added with dark matter correlation function fits. 12 pages,
uses emulateapj5.st
Galaxy Clustering Evolution in the UH8K Weak Lensing Fields
We present measurements of the two-point galaxy angular correlation function
as a function of apparent magnitude, color, and morphology. We present new
galaxy number counts to limiting magnitudes of I=24.0 and V=25.0. We find
to be well described by a power-law of slope -0.8. We find the
amplitude of the correlation function to decrease monotonically with
increasingly faint apparent magnitude. We compare with predictions utilizing
redshift distributions based on deep spectroscopic observations. We conclude
that simple redshift-dependent models which characterize evolution by means of
the epsilon parameter inadequately describe the observations. We find a strong
clustering dependence on V-I color because galaxies of extreme color lie at
similar redshifts and the angular correlation functions for these samples are
minimally diluted by chance projections.
We then present the first attempt to investigate the redshift evolution of
clustering, utilizing a population of galaxies of the same morphological type
and absolute luminosity. We study the dependence of on
redshift for Lstar early-type galaxies in the redshift range 0.2<z<0.9.
Although uncertainties are large, we find the evolution in the clustering of
these galaxies to be consistent with stable clustering [epsilon=0]. We find
Lstar early-type galaxies to cluster slightly more strongly (rnought =
5.25\pm0.28 \hMpc assuming epsilon=0) than the local full field population.
This is in good agreement with the 2dFGRS value for Lstar early-type galaxies
in the local universe (abridged).Comment: 41 pages, including 12 figs, 10 tables, to appear in Ap
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