Research Highlights, 2013
Size-virial relation of galaxies
April 10, 2013 | Read more
Figure: Relation between the half-mass radius of stellar distribution.
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Kravtsov used the halo abundance matching approach and halo mass function extracted from cosmological simulations to estimate virial radii for a representative sample of galaxies spanning nine orders of magnitude and stellar mass. He showed that the radius within which half of the stellar mass of each galaxy scales linearly with halo radius for both elliptical and disk galaxies (see Fig.). Such scaling was expected for disk galaxies in models in which disk has acquired its specific angular momentum during halo collapse via tidal torques, but existence of similar scaling for spheroidal galaxies is a surprise. The results of this study imply that galaxy sizes and radial distribution of baryons are shaped primarily by properties of their parent halos and that the sizes of both late-type disks and early-type spheroids are controlled by halo angular momentum. These results show a remarkable agreement of properties of observed galaxy population with the basic expectations of how galaxies and their host dark matter halos are expected to form in the standard Cold Dark Matter scenario of structure formation.Learn more >>

COUPP-60 installed and taking data at SNOLAB
June 25, 2013
Figure: Installation of the COUPP-60 vessel at SNOLAB.
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The COUPP-60 bubble chamber has been fully installed at the depth of SNOLAB. It is presently taking physics data, with first dark matter limits expected during fall of 2013. The active volume presently contains 40 kg of CF3I, a superheated compound selected to provide maximum sensitivity to both spin-dependent and -independent WIMP-nucleus couplings. It will be soon upgraded to a target mass of 80 kg, using either the same target fluid or C3F8, better matched to detect low-mass WIMPs in the region ~10GeV/c2. COUPP chambers feature the best discrimination against minimum ionizing particles in the field (10-10 rejection factor, to be compared to 10-3 for two-phase xenon detectors). The acoustic rejection of alpha particles is demonstrated to be better than 10-3, and expected to cap at 10-5. The chamber is surrounded by a large water tank serving a triple function, temperature control, neutron moderation and Cerenkov light muon-veto. The design of a next chamber, housing 500 kg of target mass, is well-advanced. The COUPP and PICASSO collaborations have recently merged, with the goal of starting installation of the COUPP 500 detector at SNOLAB during 2015.

Dark Energy Camera sees "First Light"
August 3, 2013
Figure: 4-ton DECam mounted at the prime focus of the 4-m Blanco telescope at CTIO.
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The Dark Energy Survey (DES) is a 5000 deg^2 optical imaging survey in the grizY-bands to a depth of 24th magnitude for galaxies, and a time-domain survey of 10 supernova fields visited on a weekly cadence with a total area of 30 deg^2. DES involves 525 observing nights on the Blanco 4-m Telescope at the Cerro Tololo Inter-American Observatory (CTIO) spread over 5 years, beginning in September 2013. A new 570 megapixel, 3 deg^2 field-of-view camera was built specifically for the project. The DES aims to better understand the mechanisms responsible for the apparent accelerated expansion of the Universe using a suite of complimentary methods which trace the evolution of the cosmic scale factor and the growth of structure over the last 8-10 billion years.

The Dark Energy Survey seeks to increase our understanding of the mysterious Dark Energy by using four separate probes:
  • Galaxy Angular Clustering (Baryon Acoustic Oscillations) A map of 300 million galaxies to z>1
  • Galaxy Cluster Abundances by Mass and Redshift ~100,000 galaxy clusters to z>1
  • Weak Gravitational Lensing Tomography Shape measurements for ~200 million galaxies to z>1
  • Type Ia Supernova Distances Light curves for ~4000 supernovae to z~1

A new imager for the Blanco telescope, the Dark Energy Camera (DECam), was designed to meet the ambitious DES science goals (see Figure). "First light" images with the newly installed camera were taken on 12 September 2012, and extensive "Science Verification" studies were conducted through 22 February 2013. During that time, a contiguous region of 160 deg^2 was imaged to full survey depth. Several smaller patches were observed to varying depths, including a sample of prominent galaxy clusters, as well as calibration fields overlapping with SDSS Stripe 82, Chandra Deep Field South, and the VVDS, COSMOS, and CFHTLS surveys, which will be used to train photometric redshift estimators and evaluate the DECam photometric, astrometric, and object detection performance.

Primordial perturbations in SPT
November 21, 2013 | Read more
Figure: The constraints on the inflationary models in the ns - r plane, where ns is the slope of the primordial perturbations and the ratio of the amplitude of tensor to scalar perturbations. We show the two-dimensional constraints on r and ns as colored contours at the 68% and 95% confidence levels for three datasets: WMAP7(grey contours), WMAP7 + SPT (red contours), and WMAP7+SPT+H0+BAO (blue contours). Adding the SPT bandpowers partially breaks the degeneracy between ns and r in the WMAP7 constraint, which can be seen clearly moving between the grey and red contours.
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Primordial perturbations in SPT. Inflation explains many of the puzzles of the standard cosmology and provides a natural mechanism for producing the seeds of structure in the universe. Most inflationary models predict a nearly scale-invariant spectrum, so that the amplitude of the primordial scalar perturbations that evolve into large scale structure is nearly the same on all length scales. This is characterized by a power spectrum proportional to kns where k is the wavenumber of the Fourier modes. Every inflationary model makes predictions for both ns and r, the relative amplitude of the tensor and scalar perturbations. For most models, ns is not exactly equal to one, an important distinction since the phenomenological choice before inflation was the Harrison-Zel'dovich spectrum with ns=1. So a small deviation from unity is another indication of the success of inflation.

Detailed measurements of the tail of the spectrum of anisotropies in the cosmic microwave background by the South Pole Telescope (SPT) provide the most precise measurement yet of ns. As depicted in Figure the slope of the scalar spectrum is now constrained to deviate from unity at 4-sigma when combining SPT data with the larger scale experiment WMAP. When combined with other cosmological measurements, the significance increases to over 5-sigma.Learn more >>

The SPT Cluster Catalog and Cosmological Constraints
November 28, 2013 | Read more
Mass estimates vs. redshift for four cluster samples: (1) optically confirmed SZ-selected galaxy clusters from the SPT-SZ 2500 deg^2 survey; (2) optically confirmed SZ-selected galaxy clusters from the 950 deg^2 ACT survey; (3) SZ-selected galaxy clusters from the all-sky Planck survey (Planck Collaboration et al. 2013); and (4) X-ray-selected galaxy clusters from the ROSAT all-sky survey (Piffaretti et al. 2011). High-resolution SZ surveys such as the SPT survey, uniquely have a nearly redshift independent selection function. The redshift dependent selection in the Planck survey is due to beam dilution; the redshift dependence of the ROSAT catalog is due to cosmological dimming. The SPT points in the figure therefore graphically illustrate the abundance evolution of massive galaxy clusters.
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The abundance evolution of clusters of galaxies is sensitive to multiple cosmological parameters, including the dark energy density and its equation of state. The SPT finds clusters through the small spectral distortion they impart on the cosmic microwave background (CMB), commonly called the Sunyaev-Zel'dovich (SZ) effect. The SZ effect offers an effective way to find the most massive, distant clusters in the universe because the brightness of the effect is independent of redshift. The SPT-SZ 2500 deg^2 survey was completed in November 2011, and the collaboration is in the process of finalizing the cluster catalog and completing a large multi-wavelength observational program to measure the cluster redshifts and improve the cluster mass calibration. The catalog consists of 450 clusters, 75% of which are newly discovered clusters. The SPT-SZ data is expected to produce constraints on the dark energy equation of state comparable to current constraints from the combination of CMB+BAO+SNe data. This will be an important achievement since the cluster based constraint provides a completely independent systematic test of the standard dark energy paradigm by measuring the effect of dark energy on the growth of structure. The combination of the growth and geometrical based constraints is a test of the underlying framework of dark energy vs. modifications to gravity.Learn more >>