227 research outputs found
Alignment of galaxy spins in the vicinity of voids
We provide limits on the alignment of galaxy orientations with the direction
to the void center for galaxies lying near the edges of voids. We locate
spherical voids in volume limited samples of galaxies from the Sloan Digital
Sky Survey using the HB inspired void finder and investigate the orientation of
(color selected) spiral galaxies that are nearly edge-on or face-on. In
contrast with previous literature, we find no statistical evidence for
departure from random orientations. Expressed in terms of the parameter c,
introduced by Lee & Pen to describe the strength of such an alignment, we find
that c<0.11(0.13) at 95% (99.7%) confidence limit within a context of a toy
model that assumes a perfectly spherical voids with sharp boundaries.Comment: 8 pages, 4 figures; v2 discussion expanded, references fixed, matches
version accepted by JCA
A unified pseudo- framework
The pseudo- is an algorithm for estimating the angular power and
cross-power spectra that is very fast and, in realistic cases, also nearly
optimal. The algorithm can be extended to deal with contaminant deprojection
and purification, and can therefore be applied in a wide variety of
scenarios of interest for current and future cosmological observations. This
paper presents NaMaster, a public, validated, accurate and easy-to-use software
package that, for the first time, provides a unified framework to compute
angular cross-power spectra of any pair of spin-0 or spin-2 fields,
contaminated by an arbitrary number of linear systematics and requiring - or
-mode purification, both on the sphere or in the flat-sky approximation. We
describe the mathematical background of the estimator, including all the
features above, and its software implementation in NaMaster. We construct a
validation suite that aims to resemble the types of observations that
next-generation large-scale structure and ground-based CMB experiments will
face, and use it to show that the code is able to recover the input power
spectra in the most complex scenarios with no detectable bias. NaMaster can be
found at https://github.com/LSSTDESC/NaMaster, and is provided with
comprehensive documentation and a number of code examples.Comment: 27 pages, 17 figures, accepted in MNRAS. Code can be found at
https://github.com/LSSTDESC/NaMaste
How to estimate the 3D power spectrum of the Lyman- forest
We derive and numerically implement an algorithm for estimating the 3D power
spectrum of the Lyman- (Ly-) forest flux fluctuations. The
algorithm exploits the unique geometry of Ly- forest data to
efficiently measure the cross-spectrum between lines of sight as a function of
parallel wavenumber, transverse separation and redshift. The key to fast
evaluation is to approximate the global covariance matrix as block-diagonal,
where only pixels from the same spectrum are correlated. We then compute the
eigenvectors of the derivative of the signal covariance with respect to
cross-spectrum parameters, and project the inverse-covariance-weighted spectra
onto them. This acts much like a radial Fourier transform over redshift
windows. The resulting cross-spectrum inference is then converted into our
final product, an approximation of the likelihood for the 3D power spectrum
expressed as second order Taylor expansion around a fiducial model. We
demonstrate the accuracy and scalability of the algorithm and comment on
possible extensions. Our algorithm will allow efficient analysis of the
upcoming Dark Energy Spectroscopic Instrument dataset.Comment: 29 pages, many figures. Minor changes in v2, accepted in JCA
Recovering 21cm Monopole Signals Without Smoothness
We expect the monopole signal at the lowest frequencies below MHz to
be composed to two components: the deep Rayleigh-Jeans tail of the cosmic
microwave background and two distinct features: the dark ages trough at MHz and the cosmic dawn trough at Mhz. These are hidden under
orders of magnitude brighter foregrounds whose emission is approximately a
power-law with a spectral index . It is usually assumed that
monopole signals of interest are separable from foregrounds on the basis of
spectral smoothness. We argue that this is a difficult approach and likely
impossible for the Dark Ages trough. Instead, we suggest that the fluctuations
in the foreground emission around the sky should be used to build a model
distribution of possible shapes of foregrounds, which can be used to constrain
presence of a monopole signal. We implement this idea using normalizing flows
and show that this technique allows for efficient unsupervised detection of the
amplitude, width and center of the Dark Ages trough as well as Rayleigh-Jeans
tail of the cosmic microwave background for a sufficiently sensitive
experiment. We discuss the limitations of the inherent assumptions in this
method and the impact on the design of future low-frequency experiments
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