187 research outputs found
A Detection of the Baryon Acoustic Oscillation Features in the SDSS BOSS DR12 Galaxy Bispectrum
We present the first high significance detection () of the Baryon
Acoustic Oscillations (BAO) feature in the galaxy bispectrum of the twelfth
data release (DR12) of the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS
sample (). We measured the scale dilation parameter,
, using the power spectrum, bispectrum, and both simultaneously for
DR12, plus 2048 MultiDark-PATCHY mocks in the North and South Galactic Caps
(NGC and SGC, respectively), and the volume weighted averages of those two
samples (N+SGC). The fitting to the mocks validated our analysis pipeline,
yielding values consistent with the mock cosmology. By fitting to the power
spectrum and bispectrum separately, we tested the robustness of our results,
finding consistent values from the NGC, SGC and N+SGC in all cases. We found
Mpc,
Mpc, and
Mpc from
the N+SGC power spectrum, bispectrum and simultaneous fitting, respectively.Comment: Submitted to Monthly Notices of the Royal Astronomical Society. 13
pages, 11 figure
Estimating the power spectrum covariance matrix with fewer mock samples
The covariance matrices of power-spectrum (P(k)) measurements from galaxy
surveys are difficult to compute theoretically. The current best practice is to
estimate covariance matrices by computing a sample covariance of a large number
of mock catalogues. The next generation of galaxy surveys will require
thousands of large volume mocks to determine the covariance matrices to desired
accuracy. The errors in the inverse covariance matrix are larger and scale with
the number of P(k) bins, making the problem even more acute. We develop a
method of estimating covariance matrices using a theoretically justified,
few-parameter model, calibrated with mock catalogues. Using a set of 600 BOSS
DR11 mock catalogues, we show that a seven parameter model is sufficient to fit
the covariance matrix of BOSS DR11 P(k) measurements. The covariance computed
with this method is better than the sample covariance at any number of mocks
and only ~100 mocks are required for it to fully converge and the inverse
covariance matrix converges at the same rate. This method should work equally
well for the next generation of galaxy surveys, although a demand for higher
accuracy may require adding extra parameters to the fitting function.Comment: 7 pages, 7 figure
Simulating redshift-space distortions for galaxy pairs with wide angular separation
The analysis of redshift-space distortions (RSD) within galaxy surveys provides constraints on the amplitude of peculiar velocities induced by structure growth, thereby allowing tests of General Relativity on extremely large scales. The next generation of galaxy redshift surveys, such as the Baryon Oscillation Spectroscopic Survey and the Euclid experiment, will survey galaxies out to z= 2, over 10-000-20-000-deg2. In such surveys, galaxy pairs with large comoving separation will preferentially have a wide angular separation. In standard plane-parallel theory the displacements of galaxy positions due to RSD are assumed to be parallel for all galaxies, but this assumption will break down for wide-angle pairs. Szalay, Matsubara & Landy, Szapudi, and Papai & Szapudi provided a methodology, based on tripolar spherical harmonics expansion, for computing the redshift-space correlation function for all angular galaxy pair separations. In this paper, we introduce a new procedure for analysing wide-angle effects in numerical simulations. We are able to separate, demonstrate and fit each of the effects described by the wide-angle RSD theory. Our analysis highlights some of the nuances of dealing with wide-angle pairs and shows that the effects are not negligible even for relatively small angles. This analysis will help to ensure the full exploitation of future surveys for RSD measurements, which are currently confined to pair separations less than -80-h-1-Mpc out to z- 0.5. \ua9 2010 The Authors. Journal compilation \ua9 2010 RAS
Forecasting cosmological parameter constraints from near-future space-based galaxy surveys
The next generation of space-based galaxy surveys are expected to measure the
growth rate of structure to about a percent level over a range of redshifts.
The rate of growth of structure as a function of redshift depends on the
behaviour of dark energy and so can be used to constrain parameters of dark
energy models. In this work we investigate how well these future data will be
able to constrain the time dependence of the dark energy density. We consider
parameterizations of the dark energy equation of state, such as XCDM and wCDM,
as well as a consistent physical model of time-evolving scalar field dark
energy, \phi CDM. We show that if the standard, specially-flat cosmological
model is taken as a fiducial model of the Universe, these near-future
measurements of structure growth will be able to constrain the time-dependence
of scalar field dark energy density to a precision of about 10%, which is
almost an order of magnitude better than what can be achieved from a
compilation of currently available data sets.Comment: 27 pages, 7 figures and 3 table
Information content of the angular multipoles of redshift-space galaxy bispectrum
Citation: Gagrani, P., & Samushia, L. (2017). Information content of the angular multipoles of redshift-space galaxy bispectrum. Monthly Notices of the Royal Astronomical Society, 467(1), 928-934. doi:10.1093/mnras/stx135The redshift-space bispectrum (three point statistics) of galaxies depends on the expansion rate, the growth rate and the geometry of the Universe, and hence can be used to measure key cosmological parameters. In a homogeneous Universe, the bispectrum is a function of five variables and unlike its two point statistics counterpart - the power spectrum - which is a function of only two variables - is difficult to analyse unless the information is somehow reduced. The most commonly considered reduction schemes rely on computing angular integrals over possible orientations of the bispectrum triangle, thus reducing it to sets of function of only three variables describing the triangle shape. We use Fisher information formalism to study the information loss associated with this angular integration. Without any reduction, the bispectrum alone can deliver constraints on the growth rate parameter f that are better by a factor of 2.5 compared with the power spectrum, for a sample of luminous red galaxies expected from near future galaxy surveys at a redshift of z similar to 0.65 if we consider all the wavenumbers up to k <= 0.2 h Mpc (1). At lower redshifts the improvement could be up to a factor of 3. We find that most of the information is in the azimuthal averages of the first three even multipoles. This suggests that the bispectrum of every configuration can be reduced to just three numbers (instead of a 2D function) without significant loss of cosmologically relevant information
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