481 research outputs found
How to measure redshift-space distortions without sample variance
We show how to use multiple tracers of large-scale density with different
biases to measure the redshift-space distortion parameter
beta=f/b=(dlnD/dlna)/b (where D is the growth rate and a the expansion factor),
to a much better precision than one could achieve with a single tracer, to an
arbitrary precision in the low noise limit. In combination with the power
spectrum of the tracers this allows a much more precise measurement of the
bias-free velocity divergence power spectrum, f^2 P_m - in fact, in the low
noise limit f^2 P_m can be measured as well as would be possible if velocity
divergence was observed directly, with rms improvement factor ~[5.2(beta^2+2
beta+2)/beta^2]^0.5 (e.g., ~10 times better than a single tracer for beta=0.4).
This would allow a high precision determination of f D as a function of
redshift with an error as low as 0.1%. We find up to two orders of magnitude
improvement in Figure of Merit for the Dark Energy equation of state relative
to Stage II, a factor of several better than other proposed Stage IV Dark
Energy surveys. The ratio b_2/b_1 will be determined with an even greater
precision than beta, producing, when measured as a function of scale, an
exquisitely sensitive probe of the onset of non-linear bias. We also extend in
more detail previous work on the use of the same technique to measure
non-Gaussianity. Currently planned redshift surveys are typically designed with
signal to noise of unity on scales of interest, and are not optimized for this
technique. Our results suggest that this strategy may need to be revisited as
there are large gains to be achieved from surveys with higher number densities
of galaxies.Comment: 22 pages, 13 figure
Sterile neutrinos as subdominant warm dark matter
In light of recent findings which seem to disfavor a scenario with (warm)
dark matter entirely constituted of sterile neutrinos produced via the
Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for
this mechanism by relaxing the usual hypothesis that the relic neutrino
abundance must necessarily account for all of the dark matter. We first study
how to reinterpret the limits attainable from X-ray non-detection and
Lyman-alpha forest measurements in the case that sterile neutrinos constitute
only a fraction fs of the total amount of dark matter. Then, assuming that
sterile neutrinos are generated in the early universe solely through the DW
mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the
mass-mixing parameters governing their production. Furthermore, we show how the
same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level,
rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.Comment: Minor changes; added references; version accepted for publication in
Phys. Rev.
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
Relative velocity of dark matter and baryonic fluids and the formation of the first structures
At the time of recombination, baryons and photons decoupled and the sound
speed in the baryonic fluid dropped from relativistic to the thermal velocities
of the hydrogen atoms. This is less than the relative velocities of baryons and
dark matter computed via linear perturbation theory, so we infer that there are
supersonic coherent flows of the baryons relative to the underlying potential
wells created by the dark matter. As a result, the advection of small-scale
perturbations (near the baryonic Jeans scale) by large-scale velocity flows is
important for the formation of the first baryonic structures. This effect
involves a quadratic term in the cosmological perturbation theory equations and
hence has not been included in studies based on linear perturbation theory. We
show that the relative motion suppresses the abundance of the first bound
objects, even if one only investigates dark matter haloes, and leads to
qualitative changes in their spatial distribution, such as introducing
scale-dependent bias and stochasticity. We discuss the possible observable
implications for high-redshift galaxy clustering and reionization
Scale-dependent bias from primordial non-Gaussianity in general relativity
In this note we examine the derivation of scale-dependent bias due to
primordial non-Gaussianity of the local type in the context of general
relativity. We justify the use of the Poisson equation in general relativistic
perturbation theory and thus the derivation of scale-dependent bias as a test
of primordial non-Gaussianity, using the spherical collapse model. The
corollary is that the form of scale-dependent bias does not receive general
relativistic corrections on scales larger than the Hubble radius. This leads to
a formally divergent correlation function for biased tracers of the mass
distribution which we discuss.Comment: 6 pages; v2: added discussion of bias and spherical collapse in the
longitudinal gauge; matches version accepted to PR
Galaxy Groups in the SDSS DR4: II. halo occupation statistics
We investigate various halo occupation statistics using a large galaxy group
catalogue constructed from the SDSS DR4 with an adaptive halo-based group
finder. The conditional luminosity function (CLF) is measured separately for
all, red and blue galaxies, as well as in terms of central and satellite
galaxies. The CLFs for central and satellite galaxies can be well modelled with
a log-normal distribution and a modified Schechter form, respectively. About
85% of the central galaxies and about 80% of the satellite galaxies in halos
with masses M_h\ga 10^{14}\msunh are red galaxies. These numbers decrease to
50% and 40%, respectively, in halos with M_h \sim 10^{12}\msunh. For halos of
a given mass, the distribution of the luminosities of central galaxies, ,
has a dispersion of about 0.15 dex. The mean luminosity (stellar mass) of the
central galaxies scales with halo mass as
() for halos with masses M\gg 10^{12.5}\msunh, and
both relations are significantly steeper for less massive halos. We also
measure the luminosity (stellar mass) gap between the first and second
brightest (most massive) member galaxies, (). These gap statistics, especially in halos with M_h \la
10^{14.0}\msunh, indicate that the luminosities of central galaxies are
clearly distinct from those of their satellites. The fraction of fossil groups,
defined as those groups with , ranges from for groups with M_h\sim 10^{14}\msunh to 18-60% for groups with
M_h\sim 10^{13}\msunh. Finally, we measure the fraction of satellites, which
changes from for galaxies with \rmag\sim -22.0 to for
galaxies with \rmag\sim -17.0. (abridged)Comment: 16 pages, 11 figures. Accepted for publication in Ap
Detection of Baryon Acoustic Oscillation Features in the Large-Scale 3-Point Correlation Function of SDSS BOSS DR12 CMASS Galaxies
We present the large-scale 3-point correlation function (3PCF) of the SDSS
DR12 CMASS sample of Luminous Red Galaxies, the largest-ever sample
used for a 3PCF or bispectrum measurement. We make the first high-significance
() detection of Baryon Acoustic Oscillations (BAO) in the 3PCF.
Using these acoustic features in the 3PCF as a standard ruler, we measure the
distance to to precision (statistical plus systematic). We
find for our
fiducial cosmology (consistent with Planck 2015) and bias model. This
measurement extends the use of the BAO technique from the 2-point correlation
function (2PCF) and power spectrum to the 3PCF and opens an avenue for deriving
additional cosmological distance information from future large-scale structure
redshift surveys such as DESI. Our measured distance scale from the 3PCF is
fairly independent from that derived from the pre-reconstruction 2PCF and is
equivalent to increasing the length of BOSS by roughly 10\%; reconstruction
appears to lower the independence of the distance measurements. Fitting a model
including tidal tensor bias yields a moderate significance (
detection of this bias with a value in agreement with the prediction from local
Lagrangian biasing.Comment: 15 pages, 7 figures, submitted MNRA
Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies
‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.15383.xWe investigate a class of rapidly growing emission line galaxies, known as 'Green Peas', first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in Sloan Digital Sky Survey imaging. Their appearance is due to very strong optical emission lines, namely [O iii]λ5007 Å, with an unusually large equivalent width of up to ∼1000 Å. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some active galactic nuclei interlopers including eight newly discovered narrow-line Seyfert 1 galaxies. The star-forming Peas are low-mass galaxies (M∼ 108.5–1010 M⊙) with high star formation rates (∼10 M⊙ yr−1) , low metallicities (log[O/H]+ 12 ∼ 8.7) and low reddening [ E(B−V) ≤ 0.25 ] and they reside in low-density environments. They have some of the highest specific star formation rates (up to ∼10−8 yr−1 ) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myr. The few star-forming Peas with Hubble Space Telescope imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to luminous blue compact galaxies. They are also similar to high-redshift ultraviolet-luminous galaxies, e.g. Lyman-break galaxies and Lyα emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe.Peer reviewe
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