[Show abstract][Hide abstract] ABSTRACT: We report the discovery of eight new ultra-faint dwarf galaxy candidates in
the second year of optical imaging data from the Dark Energy Survey (DES). Six
of these candidates are detected at high confidence, while two additional
lower-confidence candidates are identified in regions of incomplete or
non-uniform survey coverage. The new stellar systems are found using three
independent automated search techniques, and are identified as statistically
significant overdensities of individually resolved stars consistent with the
isochrone and luminosity function of an old and metal-poor simple stellar
population. The new systems are faint (Mv > -4.7 mag) and span a broad range of
physical sizes (17 pc < $r_{1/2}$ < 181 pc) and heliocentric distances (25 kpc
< D < 214 kpc). All of the new systems have central surface brightnesses (\mu >
27.5 mag arcsec$^2$) consistent with known ultra-faint dwarf galaxies. Roughly
half of the DES candidates are more distant, less luminous, and/or have lower
surface brightnesses than previously known Milky Way satellite galaxies, and
would have had a low probability of detection if observed by the Sloan Digital
Sky Survey. A large fraction of satellite candidates are found in the southern
half of the DES footprint in proximity to the Magellanic Clouds. We find that
the DES data alone exclude (p < 0.001) a spatially isotropic distribution of
Milky Way satellites, and that this distribution can be well, although not
uniquely, explained by a model in which several of the observed DES satellites
are associated with the Magellanic system. Including the current sample, our
model predicts that ~100 ultra-faint galaxies with physical properties
comparable to the DES satellites might exist over the full sky and that 20-30%
of these would be spatially associated with the Magellanic Clouds.
[Show abstract][Hide abstract] ABSTRACT: We measure the cross-correlation between the galaxy density in the Dark
Energy Survey (DES) Science Verification data and the lensing of the cosmic
microwave background (CMB) as reconstructed with the Planck satellite and the
South Pole Telescope (SPT). When using the DES main galaxy sample over the full
redshift range $0.2 < z < 1.2$, a cross-correlation signal is detected at $6
\sigma$ and $4\sigma$ with SPT and Planck respectively. We then divide the DES
galaxies into five photometric redshift bins, finding significant ($>$$2
\sigma$) detections in all bins. Comparing to the fiducial Planck cosmology, we
find the redshift evolution of the signal matches expectations, although the
amplitude is consistently lower than predicted across redshift bins. We test
for possible systematics that could affect our result and find no evidence for
significant contamination. Finally, we demonstrate how these measurements can
be used to constrain the growth of structure across cosmic time. We find the
data are fit by a model in which the amplitude of structure in the $z<1.2$
universe is $0.73 \pm 0.16$ times as large as predicted in the LCDM Planck
cosmology, a $1.7\sigma$ deviation.
[Show abstract][Hide abstract] ABSTRACT: We present the first constraints on cosmology from the Dark Energy Survey
(DES), using weak lensing measurements from the preliminary Science
Verification (SV) data. We use 139 square degrees of SV data, which is less
than 3\% of the full DES survey area. Using cosmic shear 2-point measurements
over three redshift bins we find $\sigma_8 (\Omega_{\rm m}/0.3)^{0.5} = 0.81
\pm 0.06$ (68\% confidence), after marginalising over 7 systematics parameters
and 3 other cosmological parameters. We examine the robustness of our results
to the choice of data vector and systematics assumed, and find them to be
stable. About $20$\% of our error bar comes from marginalising over shear and
photometric redshift calibration uncertainties. The current state-of-the-art
cosmic shear measurements from CFHTLenS are mildly discrepant with the
cosmological constraints from Planck CMB data; our results are consistent with
both datasets. Our uncertainties are $\sim$30\% larger than those from CFHTLenS
when we carry out a comparable analysis of the two datasets, which we attribute
largely to the lower number density of our shear catalogue. We investigate
constraints on dark energy and find that, with this small fraction of the full
survey, the DES SV constraints make negligible impact on the Planck
constraints. The moderate disagreement between the CFHTLenS and Planck values
of $\sigma_8 (\Omega_{\rm m}/0.3)^{0.5}$ is present regardless of the value of
$w$.
[Show abstract][Hide abstract] ABSTRACT: We present measurements of weak gravitational lensing cosmic shear two-point
statistics using Dark Energy Survey Science Verification data. We demonstrate
that our results are robust to the choice of shear measurement pipeline, either
ngmix or im3shape, and robust to the choice of two-point statistic, including
both real and Fourier-space statistics. Our results pass a suite of null tests
including tests for B-mode contamination and direct tests for any dependence of
the two-point functions on a set of 16 observing conditions and galaxy
properties, such as seeing, airmass, galaxy color, galaxy magnitude, etc. We
furthermore use a large suite of simulations to compute the covariance matrix
of the cosmic shear measurements and assign statistical significance to our
null tests. We find that our covariance matrix is consistent with the halo
model prediction, indicating that it has the appropriate level of halo sample
variance. We compare the same jackknife procedure applied to the data and the
simulations in order to search for additional sources of noise not captured by
the simulations. We find no statistically significant extra sources of noise in
the data. The overall detection significance with tomography for our highest
source density catalog is 9.7sigma. Cosmological constraints from the
measurements in this work are presented in a companion paper (DES et al. 2015).
[Show abstract][Hide abstract] ABSTRACT: The joint analysis of galaxy-galaxy lensing and galaxy clustering is a
promising method for inferring the growth function of large scale structure.
This analysis will be carried out on data from the Dark Energy Survey (DES),
with its measurements of both the distribution of galaxies and the tangential
shears of background galaxies induced by these foreground lenses. We develop a
practical approach to modeling the assumptions and systematic effects affecting
small scale lensing, which provides halo masses, and large scale galaxy
clustering. Introducing parameters that characterize the halo occupation
distribution (HOD), photometric redshift uncertainties, and shear measurement
errors, we study how external priors on different subsets of these parameters
affect our growth constraints. Degeneracies within the HOD model, as well as
between the HOD and the growth function, are identified as the dominant source
of complication, with other systematic effects sub-dominant. The impact of HOD
parameters and their degeneracies necessitate the detailed joint modeling of
the galaxy sample that we employ. We conclude that DES data will provide
powerful constraints on the evolution of structure growth in the universe,
conservatively/optimistically constraining the growth function to 7.9\%/4.8\%
with its first-year data that covered over 1000 square degrees, and to
3.9\%/2.3\% with its full five-year data that will survey 5000 square degrees,
including both statistical and systematic uncertainties.
[Show abstract][Hide abstract] ABSTRACT: We cross-match galaxy cluster candidates selected via their
Sunyaev-Zel'dovich effect (SZE) signatures in 129.1 deg$^2$ of the South Pole
Telescope 2500d SPT-SZ survey with optically identified clusters selected from
the Dark Energy Survey (DES) science verification data. We identify 25 clusters
between $0.1\lesssim z\lesssim 0.8$ in the union of the SPT-SZ and redMaPPer
(RM) samples. RM is an optical cluster finding algorithm that also returns a
richness estimate for each cluster. We model the richness $\lambda$-mass
relation with the following function $\langle\ln\lambda|M_{500}\rangle\propto
B_\lambda\ln M_{500}+C_\lambda\ln E(z)$ and use SPT-SZ cluster masses and RM
richnesses $\lambda$ to constrain the parameters. We find $B_\lambda=
1.14^{+0.21}_{-0.18}$ and $C_\lambda=0.73^{+0.77}_{-0.75}$. The associated
scatter in mass at fixed richness is $\sigma_{\ln M|\lambda} =
0.18^{+0.08}_{-0.05}$ at a characteristic richness $\lambda=70$. We demonstrate
that our model provides an adequate description of the matched sample, showing
that the fraction of SPT-SZ selected clusters with RM counterparts is
consistent with expectations and that the fraction of RM selected clusters with
SPT-SZ counterparts is in mild tension with expectation. We model the
optical-SZE cluster positional offset distribution with the sum of two
Gaussians, showing that it is consistent with a dominant, centrally peaked
population and a sub-dominant population characterized by larger offsets. We
also cross-match the RM catalog with SPT-SZ candidates below the official
catalog threshold significance $\xi=4.5$, using the RM catalog to provide
optical confirmation and redshifts for additional low-$\xi$ SPT-SZ
candidates.In this way, we identify 15 additional clusters with $\xi\in
[4,4.5]$ over the redshift regime explored by RM in the overlapping region
between DES science verification data and the SPT-SZ survey.
[Show abstract][Hide abstract] ABSTRACT: We present Magellan/M2FS, VLT/GIRAFFE, and Gemini South/GMOS spectroscopy of
the newly discovered Milky Way satellite Reticulum II. Based on the spectra of
25 Ret II member stars selected from Dark Energy Survey imaging, we measure a
mean heliocentric velocity of 62.8 +/- 0.5 km/s and a velocity dispersion of
3.3 +/- 0.7 km/s. The mass-to-light ratio of Ret II within its half-light
radius is 470 +/- 210 Msun/Lsun, demonstrating that it is a strongly dark
matter-dominated system. Despite its spatial proximity to the Magellanic
Clouds, the radial velocity of Ret II differs from that of the LMC and SMC by
199 and 83 km/s, respectively, suggesting that it is not gravitationally bound
to the Magellanic system. The likely member stars of Ret II span 1.3 dex in
metallicity, with a dispersion of 0.28 +/- 0.09 dex, and we identify several
extremely metal-poor stars with [Fe/H] < -3. In combination with its
luminosity, size, and ellipticity, these results confirm that Ret II is an
ultra-faint dwarf galaxy. With a mean metallicity of [Fe/H] = -2.65 +/- 0.07,
Ret II matches Segue~1 as the most metal-poor galaxy known. Although Ret II is
the third-closest dwarf galaxy to the Milky Way, the line-of-sight integral of
the dark matter density squared is log J = 18.8 +/- 0.6 Gev^2/cm^5 within 0.2
degrees, indicating that the predicted gamma-ray flux from dark matter
annihilation in Ret II is lower than that of several other dwarf galaxies.
The Astrophysical Journal 04/2015; 808(1). DOI:10.1088/0004-637X/808/1/95 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Clusters of galaxies are expected to gravitationally lens the cosmic
microwave background (CMB) and thereby generate a distinct signal in the CMB on
arcminute scales. Measurements of this effect can be used to constrain the
masses of galaxy clusters using CMB data alone. Here we present a measurement
of lensing of the CMB by galaxy clusters using data from the South Pole
Telescope (SPT). We develop a maximum likelihood approach to extract the CMB
cluster lensing signal and validate the method on mock data. We quantify the
effects of several potential sources of systematic error and find that they
generally act to reduce the best-fit cluster mass. The net magnitude of the
systematic shift to lower cluster mass is approximately the size of our
statistical error bar, and we do not attempt to correct for it. We apply the
maximum likelihood technique to 513 clusters selected via their SZ signatures
in SPT data, and rule out the null hypothesis of no lensing at 3.0$\sigma$. The
lensing-derived mass estimate for the full cluster sample is consistent with
that inferred from the SZ flux: $M_{200,\rm{lens}} = 0.76^{+0.37}_{-0.36}
M_{200,\rm{SZ}}$ (68% C.L., statistical error only).
The Astrophysical Journal 12/2014; 806(2). DOI:10.1088/0004-637X/806/2/247 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present details of the construction and characterization of the coaddition of the Sloan Digital Sky Survey (SDSS) Stripe 82 ugriz imaging data. This survey consists of 275 deg2 of repeated scanning by the SDSS camera over –50° ≤ α ≤ 60° and –125 ≤ δ ≤ +125 centered on the Celestial Equator. Each piece of sky has ~20 runs contributing and thus reaches ~2 mag fainter than the SDSS single pass data, i.e., to r ~ 23.5 for galaxies. We discuss the image processing of the coaddition, the modeling of the point-spread function (PSF), the calibration, and the production of standard SDSS catalogs. The data have an r-band median seeing of 1.''1 and are calibrated to ≤1%. Star color-color, number counts, and PSF size versus modeled size plots show that the modeling of the PSF is good enough for precision five-band photometry. Structure in the PSF model versus magnitude plot indicates minor PSF modeling errors, leading to misclassification of stars as galaxies, as verified using VVDS spectroscopy. There are a variety of uses for this wide-angle deep imaging data, including galactic structure, photometric redshift computation, cluster finding and cross wavelength measurements, weak lensing cluster mass calibrations, and cosmic shear measurements.
The Astrophysical Journal 09/2014; 794(2):120. DOI:10.1088/0004-637X/794/2/120 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cosmological parameter estimation is entering a new era. Large collaborations
need to coordinate high-stakes analyses using multiple methods; furthermore
such analyses have grown in complexity due to sophisticated models of cosmology
and systematic uncertainties. In this paper we argue that modularity is the key
to addressing these challenges: calculations should be broken up into
interchangeable modular units with inputs and outputs clearly defined. We
present a new framework for cosmological parameter estimation, CosmoSIS,
designed to connect together, share, and advance development of inference tools
across the community. We describe the modules already available in CosmoSIS,
including CAMB, Planck, cosmic shear calculations, and a suite of samplers. We
illustrate it using demonstration code that you can run out-of-the-box with the
installer available at http://bitbucket.org/joezuntz/cosmosis
Astronomy and Computing 09/2014; 12. DOI:10.1016/j.ascom.2015.05.005
[Show abstract][Hide abstract] ABSTRACT: On large scales, the anisotropies in the cosmic microwave background (CMB)
reflect not only the primordial field but also the energy gain when photons
traverse decaying gravitational potentials of large scales structure, the
Integrated Sachs-Wolfe (ISW) effect. Decomposing the anisotropy signal into a
primordial piece and an ISW component is more urgent than ever as cosmologists
strive to understand the Universe on the largest of scales. Here we present a
likelihood technique for extracting the ISW signal from measurements of the
CMB, the distribution of galaxies, and maps of gravitational lensing. We test
this technique first to simulated data and then we apply it to the combination
of temperature anisotropies, the lensing map made by the Planck satellite, and
the NVSS galaxy survey. We also show projections for upcoming surveys.
Physical Review D 07/2014; 90(12). DOI:10.1103/PhysRevD.90.123009 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Systematic uncertainties that have been subdominant in past large-scale
structure (LSS) surveys are likely to exceed statistical uncertainties of
current and future LSS data sets, potentially limiting the extraction of
cosmological information. Here we present a general framework (PCA
marginalization) to consistently incorporate systematic effects into a
likelihood analysis. This technique naturally accounts for degeneracies between
nuisance parameters and can substantially reduce the dimension of the parameter
space that needs to be sampled. As a practical application, we apply PCA
marginalization to account for baryonic physics as an uncertainty in cosmic
shear tomography. Specifically, we use CosmoLike to run simulated likelihood
analyses on three independent sets of numerical simulations, each covering a
wide range of baryonic scenarios differing in cooling, star formation, and
feedback mechanisms. We simulate a Stage III (Dark Energy Survey) and Stage IV
(Large Synoptic Survey Telescope/Euclid) survey and find a substantial bias in
cosmological constraints if baryonic physics is not accounted for. We then show
that PCA marginalization (employing at most 3 to 4 nuisance parameters) removes
this bias. Our study demonstrates that it is possible to obtain robust, precise
constraints on the dark energy equation of state even in the presence of large
levels of systematic uncertainty in astrophysical processes. We conclude that
the PCA marginalization technique is a powerful, general tool for addressing
many of the challenges facing the precision cosmology program.
[Show abstract][Hide abstract] ABSTRACT: The recent BICEP2 measurement of B-modes in the polarization of the cosmic
microwave background suggests that inflation was driven by a field at an energy
scale of $2\times 10^{16}$ GeV. I explore the potential of upcoming CMB
polarization experiments to further constrain the physics underlying inflation.
If the signal is confirmed, then two sets of experiments covering larger area
will shed light on inflation. Low resolution measurements can pin down the
tensor to scalar ratio at the percent level, thereby distinguishing models from
one another. A high angular resolution experiment will be necessary to measure
the tilt of the tensor spectrum, testing the consistency relation that relates
the tilt to the amplitude.
[Show abstract][Hide abstract] ABSTRACT: These reports present the results of the 2013 Community Summer Study of the
APS Division of Particles and Fields ("Snowmass 2013") on the future program of
particle physics in the U.S. Chapter 4, on the Cosmic Frontier, discusses the
program of research relevant to cosmology and the early universe. This area
includes the study of dark matter and the search for its particle nature, the
study of dark energy and inflation, and cosmic probes of fundamental
symmetries.
[Show abstract][Hide abstract] ABSTRACT: This document represents the response of the Intensity Frontier Neutrino
Working Group to the Snowmass charge. We summarize the current status of
neutrino physics and identify many exciting future opportunities for studying
the properties of neutrinos and for addressing important physics and
astrophysics questions with neutrinos.
[Show abstract][Hide abstract] ABSTRACT: The observed acceleration of the Universe can be explained by modifying
general relativity. One such attempt is the nonlocal model of Deser and
Woodard. Here we fix the background cosmology using results from the Planck
satellite and examine the predictions of nonlocal gravity for the evolution of
structure in the universe, confronting the model with three tests:
gravitational lensing, redshift space distortions, and the estimator of gravity
$E_G$. Current data favor general relativity (GR) over nonlocal gravity: fixing
primordial cosmology with the best fit parameters from Planck leads to weak
lensing results favoring GR by 5.9 sigma; redshift space distortions
measurements of the growth rate preferring GR by 7.8 sigma; and the single
measurement of $E_G$ favoring GR, but by less than 1-sigma. The significance
holds up even after the parameters are allowed to vary within Planck limits.
The larger lesson is that a successful modified gravity model will likely have
to suppress the growth of structure compared to general relativity.
Physical Review D 10/2013; 90(4). DOI:10.1103/PhysRevD.90.043535 · 4.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This is a report on the status and prospects of the quantification of
neutrino properties through the cosmological neutrino background for the Cosmic
Frontier of the Division of Particles and Fields Community Summer Study
long-term planning exercise. Experiments planned and underway are prepared to
study the cosmological neutrino background in detail via its influence on
distance-redshift relations and the growth of structure. The program for the
next decade described in this document, including upcoming spectroscopic galaxy
surveys eBOSS and DESI and a new Stage-IV CMB polarization experiment CMB-S4,
will achieve sigma(sum m_nu) = 16 meV and sigma(N_eff) = 0.020. Such a mass
measurement will produce a high significance detection of non-zero sum m_nu,
whose lower bound derived from atmospheric and solar neutrino oscillation data
is about 58 meV. If neutrinos have a minimal normal mass hierarchy, this
measurement will definitively rule out the inverted neutrino mass hierarchy,
shedding light on one of the most puzzling aspects of the Standard Model of
particle physics --- the origin of mass. This precise a measurement of N_eff
will allow for high sensitivity to any light and dark degrees of freedom
produced in the big bang and a precision test of the standard cosmological
model prediction that N_eff = 3.046.
[Show abstract][Hide abstract] ABSTRACT: The quantity and quality of cosmic structure observations have greatly
accelerated in recent years. Further leaps forward will be facilitated by
imminent projects, which will enable us to map the evolution of dark and
baryonic matter density fluctuations over cosmic history. The way that these
fluctuations vary over space and time is sensitive to the nature of dark matter
and dark energy. Dark energy and gravity both affect how rapidly structure
grows; the greater the acceleration, the more suppressed the growth of
structure, while the greater the gravity, the more enhanced the growth. While
distance measurements also constrain dark energy, the comparison of growth and
distance data tests whether General Relativity describes the laws of physics
accurately on large scales. Modified gravity models are able to reproduce the
distance measurements but at the cost of altering the growth of structure
(these signatures are described in more detail in the accompanying paper on
Novel Probes of Gravity and Dark Energy). Upcoming surveys will exploit these
differences to determine whether the acceleration of the Universe is due to
dark energy or to modified gravity. To realize this potential, both wide field
imaging and spectroscopic redshift surveys play crucial roles. Projects
including DES, eBOSS, DESI, PFS, LSST, Euclid, and WFIRST are in line to map
more than a 1000 cubic-billion-light-year volume of the Universe. These will
map the cosmic structure growth rate to 1% in the redshift range 0<z<2, over
the last 3/4 of the age of the Universe.
[Show abstract][Hide abstract] ABSTRACT: The American Physical Society's Division of Particles and Fields initiated a
long-term planning exercise over 2012-13, with the goal of developing the
community's long term aspirations. The sub-group "Dark Energy and CMB" prepared
a series of papers explaining and highlighting the physics that will be studied
with large galaxy surveys and cosmic microwave background experiments. This
paper summarizes the findings of the other papers, all of which have been
submitted jointly to the arXiv.