1,735 research outputs found
Faint counts as a function of morphological type in a hierarchical merger model
The unprecedented resolution of the refurbished Wide Field and Planetary
Camera 2 (WFPC2) on the Hubble Space Telescope (HST) has led to major advances
in our understanding of galaxy formation. The high image quality in the Medium
Deep Survey and Hubble Deep Field has made it possible, for the first time, to
classify faint distant galaxies according to morphological type. These
observations have revealed a large population of galaxies classed as irregulars
or which show signs of recent merger activity. Their abundance rises steeply
with apparent magnitude, providing a likely explanation for the large number of
blue galaxies seen at faint magnitudes. We demonstrate that such a population
arises naturally in a model in which structure forms hierarchically and which
is dynamically dominated by cold dark matter. The number counts of irregular,
spiral and elliptical galaxies as a function of magnitude seen in the HST data
are well reproduced in this model.We present detailed predictions for the
outcome of spectroscopic follow-up observations of the HST surveys. By
measuring the redshift distributions of faint galaxies of different
morphological types, these programmes will provide a test of the hierarchical
galaxy formation paradigm and might distinguish between models with different
cosmological parameters.Comment: 5 pages, 3 postscript figures included. To be published as a Letter
in Monthly Notices of the RAS. Postscript version available at
http://star-www.dur.ac.uk/~cmb/counts.htm
Correlations in the Spatial Power Spectrum Inferred from Angular Clustering: Methods and Application to APM
We reconsider the inference of spatial power spectra from angular clustering
data and show how to include correlations in both the angular correlation
function and the spatial power spectrum. Inclusion of the full covariance
matrices loosens the constraints on large-scale structure inferred from the APM
survey by over a factor of two. We present a new inversion technique based on
singular value decomposition that allows one to propagate the covariance matrix
on the angular correlation function through to that of the spatial power
spectrum and to reconstruct smooth power spectra without underestimating the
errors. Within a parameter space of the CDM shape Gamma and the amplitude
sigma_8, we find that the angular correlations in the APM survey constrain
Gamma to be 0.19-0.37 at 68% confidence when fit to scales larger than k=0.2h
Mpc^-1. A downturn in power at k<0.04h Mpc^-1 is significant at only 1-sigma.
These results are optimistic as we include only Gaussian statistical errors and
neglect any boundary effects.Comment: 37 pages, LaTex, 9 figures. Submitted to Ap
Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?
In the six billion years between redshifts z=1 and z=0.1, galaxies change due
to the aging of their stellar populations, the formation of new stars, and
mergers with other galaxies. Here I explore the relative importance of these
various effects, finding that while mergers are likely to be important for the
red galaxy sequence they are unlikely to affect more than 10% of the blue
galaxy sequence. I compare the galaxy population at redshift z=0.1 from the
Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic
Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3
mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I
evaluate the change in color and in the luminosity functions of the two
sequences using some simplistic stellar population synthesis models. These
models indicate that the luminous end of the red sequence fades less than
passive evolution allows by about 0.2 mag. Due to a lack of luminous blue
progenitors, ``dry'' mergers betweeen red galaxies then must create the
luminous red population at z=0.1, if stellar population models are correct. The
blue sequence colors and luminosity function are consistent with a reduction in
the star-formation rate since redshift z=1 by a factor of about three, with no
change in the number density to within 10%. These results restrict the number
of blue galaxies that can fall onto the red sequence by any process, and in
particular suggest that if mergers are catastrophic events they must be rare
for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at
http://cosmo.nyu.edu/blanton/deep2sdss
Estimation of Primordial Spectrum with post-WMAP 3 year data
In this paper we implement an improved (error sensitive) Richardson-Lucy
deconvolution algorithm on the measured angular power spectrum from the WMAP 3
year data to determine the primordial power spectrum assuming different points
in the cosmological parameter space for a flat LCDM cosmological model. We also
present the preliminary results of the cosmological parameter estimation by
assuming a free form of the primordial spectrum, for a reasonably large volume
of the parameter space. The recovered spectrum for a considerably large number
of the points in the cosmological parameter space has a likelihood far better
than a `best fit' power law spectrum up to \Delta \chi^2_{eff} \approx -30. We
use Discrete Wavelet Transform (DWT) for smoothing the raw recovered spectrum
from the binned data. The results obtained here reconfirm and sharpen the
conclusion drawn from our previous analysis of the WMAP 1st year data. A sharp
cut off around the horizon scale and a bump after the horizon scale seem to be
a common feature for all of these reconstructed primordial spectra. We have
shown that although the WMAP 3 year data prefers a lower value of matter
density for a power law form of the primordial spectrum, for a free form of the
spectrum, we can get a very good likelihood to the data for higher values of
matter density. We have also shown that even a flat CDM model, allowing a free
form of the primordial spectrum, can give a very high likelihood fit to the
data. Theoretical interpretation of the results is open to the cosmology
community. However, this work provides strong evidence that the data retains
discriminatory power in the cosmological parameter space even when there is
full freedom in choosing the primordial spectrum.Comment: 13 pages, 4 figures, uses Revtex4, new analysis and results,
references added, matches version accepted to Phys. Rev.
Gravity's smoking gun?
We present a new constraint on the biased galaxy formation picture.
Gravitational instability theory predicts that the two-point mass density
correlation function, \xi(r), has an inflection point at the separation r=r_0,
corresponding to the boundary between the linear and nonlinear regime of
clustering, \xi = 1. We show how this feature can be used to constrain the
square of the biasing parameter, b^2 = \xi_g / \xi on scales r = r_0, where
\xi_g is the galaxy-galaxy correlation function, allowed to differ from \xi. We
apply our method to real data: the \xi_g(r), estimated from the APM galaxy
survey. Our results suggest that the APM galaxies trace the mass at separations
r > 5 Mpc/h, where h is the Hubble constant in units of 100 km/s Mpc. The
present results agree with earlier studies, based on comparing higher order
correlations in the APM with weakly non-linear perturbation theory. Both
approaches constrain the "b" factor to be within 20% of unity. If the existence
of the feature we identified in the APM \xi_g(r) -- the inflection point near
\xi_g = 1 -- is confirmed by more accurate surveys, we may have discovered
gravity's smoking gun: the long awaited ``shoulder'' in \xi, predicted by Gott
and Rees 25 years ago.Comment: 4 pages, 2 figures, minor changes and references added, matches
version published in ApJ letter
Biased-estimations of the Variance and Skewness
Nonlinear combinations of direct observables are often used to estimate
quantities of theoretical interest. Without sufficient caution, this could lead
to biased estimations. An example of great interest is the skewness of
the galaxy distribution, defined as the ratio of the third moment \xibar_3
and the variance squared \xibar_2^2. Suppose one is given unbiased estimators
for \xibar_3 and \xibar_2^2 respectively, taking a ratio of the two does
not necessarily result in an unbiased estimator of . Exactly such an
estimation-bias affects most existing measurements of . Furthermore,
common estimators for \xibar_3 and \xibar_2 suffer also from this kind of
estimation-bias themselves: for \xibar_2, it is equivalent to what is
commonly known as the integral constraint. We present a unifying treatment
allowing all these estimation-biases to be calculated analytically. They are in
general negative, and decrease in significance as the survey volume increases,
for a given smoothing scale. We present a re-analysis of some existing
measurements of the variance and skewness and show that most of the well-known
systematic discrepancies between surveys with similar selection criteria, but
different sizes, can be attributed to the volume-dependent estimation-biases.
This affects the inference of the galaxy-bias(es) from these surveys. Our
methodology can be adapted to measurements of analogous quantities in quasar
spectra and weak-lensing maps. We suggest methods to reduce the above
estimation-biases, and point out other examples in LSS studies which might
suffer from the same type of a nonlinear-estimation-bias.Comment: 28 pages of text, 9 ps figures, submitted to Ap
Features in the Primordial Spectrum from WMAP: A Wavelet Analysis
Precise measurements of the anisotropies in the cosmic microwave background
enable us to do an accurate study on the form of the primordial power spectrum
for a given set of cosmological parameters. In a previous paper (Shafieloo and
Souradeep 2004), we implemented an improved (error sensitive) Richardson-Lucy
deconvolution algorithm on the measured angular power spectrum from the first
year of WMAP data to determine the primordial power spectrum assuming a
concordance cosmological model. This recovered spectrum has a likelihood far
better than a scale invariant, or, `best fit' scale free spectra (\Delta ln L =
25 w.r.t. Harrison Zeldovich, and, \Delta ln L = 11 w.r.t. power law with
n_s=0.95). In this paper we use Discrete Wavelet Transform (DWT) to decompose
the local features of the recovered spectrum individually to study their effect
and significance on the recovered angular power spectrum and hence the
likelihood. We show that besides the infra-red cut off at the horizon scale,
the associated features of the primordial power spectrum around the horizon
have a significant effect on improving the likelihood. The strong features are
localised at the horizon scale.Comment: 8 pages, 4 figures, uses Revtex4, matches version accepted to Phys.
Rev. D, main results and conclusions unchanged, references adde
Assumptions of the primordial spectrum and cosmological parameter estimation
The observables of the perturbed universe, CMB anisotropy and large
structures, depend on a set of cosmological parameters, as well as, the assumed
nature of primordial perturbations. In particular, the shape of the primordial
power spectrum (PPS) is, at best, a well motivated assumption. It is known that
the assumed functional form of the PPS in cosmological parameter estimation can
affect the best fit parameters and their relative confidence limits. In this
paper, we demonstrate that a specific assumed form actually drives the best fit
parameters into distinct basins of likelihood in the space of cosmological
parameters where the likelihood resists improvement via modifications to the
PPS. The regions where considerably better likelihoods are obtained allowing
free form PPS lie outside these basins. In the absence of a preferred model of
inflation, this raises a concern that current cosmological parameters estimates
are strongly prejudiced by the assumed form of PPS. Our results strongly
motivate approaches toward simultaneous estimation of the cosmological
parameters and the shape of the primordial spectrum from upcoming cosmological
data. It is equally important for theorists to keep an open mind towards early
universe scenarios that produce features in the PPS.Comment: 11 pages, 2 figures, discussions extended, main results unchanged,
matches published versio
How BAO measurements can fail to detect quintessence
We model the nonlinear growth of cosmic structure in different dark energy
models, using large volume N-body simulations. We consider a range of
quintessence models which feature both rapidly and slowly varying dark energy
equations of state, and compare the growth of structure to that in a universe
with a cosmological constant. The adoption of a quintessence model changes the
expansion history of the universe, the form of the linear theory power spectrum
and can alter key observables, such as the horizon scale and the distance to
last scattering. The difference in structure formation can be explained to
first order by the difference in growth factor at a given epoch; this scaling
also accounts for the nonlinear growth at the 15% level. We find that
quintessence models which feature late , rapid transitions towards
in the equation of state, can have identical baryonic acoustic
oscillation (BAO) peak positions to those in CDM, despite being very
different from CDM both today and at high redshifts .
We find that a second class of models which feature non-negligible amounts of
dark energy at early times cannot be distinguished from CDM using
measurements of the mass function or the BAO. These results highlight the need
to accurately model quintessence dark energy in N-body simulations when testing
cosmological probes of dynamical dark energy.Comment: 10 pages, 7 figures, to appear in the Invisible Univers International
Conference AIP proceedings serie
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