latest results from the cosmic background imager

2The Cosmic Background Imager – AAS Denver, June 2004

Latest Results from the Cosmic Background Imager

Steven T. Myers

National Radio Astronomy Observatory

Socorro, NM


A Theoretical Digression


The Cosmic Microwave Background

• Discovered 1965 (Penzias & Wilson)– 2.7 K blackbody– Isotropic– Relic of hot “big bang”– Last scattering “surface” z ~ 1000

• COBE 1992– Blackbody 2.725 K– 3 mK dipole (Doppler)– Anisotropies 10-5


Primary Anisotropies

Courtesy Wayne Hu – http://background.uchicago.edu


Secondary Anisotropies

Courtesy Wayne Hu – http://background.uchicago.edu


CMB Polarization

• Due to quadrupolar intensity field at scattering

Courtesy Hu & White– http://background.uchicago.edu


Polarization Power Spectra

Hu & Dodelson ARAA 2002


CMB State of the Art: WMAP + “ext”

WMAP Satellite


The Cosmic Background Imager


The Cosmic Background Imager

• A collaboration between– Caltech (A.C.S. Readhead PI)– NRAO– CITA– Universidad de Chile– University of Chicago

• With participants also from– U.C. Berkeley, U. Alberta, ESO, IAP-Paris, NASA-MSFC,

Universidad de Concepción

• Funded by– National Science Foundation, the California Institute of

Technology, Maxine and Ronald Linde, Cecil and Sally Drinkward, Barbara and Stanley Rawn Jr., the Kavli Institute, and the Canadian Institute for Advanced Research


The Instrument

• 13 90-cm Cassegrain antennas– 78 baselines

• 6-meter platform– Baselines 1m – 5.51m

• 10 1 GHz channels 26-36 GHz– HEMT amplifiers (NRAO)

– Cryogenic 6K, Tsys 20 K

• Single polarization (R or L)– Polarizers from U. Chicago

• Analog correlators– 780 complex correlators

• Field-of-view 44 arcmin– Image noise 4 mJy/bm 900s

• Resolution 4.5 – 10 arcmin


Site – Northern Chilean Andes


The CMB and Interferometry

• The sky can be uniquely described by spherical harmonics– CMB power spectra are described by multipole l ( the angular

scale in the spherical harmonic transform)

• For small (sub-radian) scales the spherical harmonics can be approximated by Fourier modes– The conjugate variables are (u,v) as in radio interferometry– The uv radius is given by l / 2

• The projected length of the interferometer baseline gives the angular scale – Multipole l = 2 B /

• An interferometer naturally measures the transform of the sky intensity in l space convolved with aperture


3-Axis mount : rotatable platform


CBI Beam and uv coverage

• Over-sampled uv-plane– excellent PSF– allows fast gridded method (Myers et al. 2000)


New: 2000+2001 Extended Mosaics

• CBI field-of-view 45’ → l ~180– narrow CMB peaks, mosaicing required!


New: CBI 2000+2001 Results

Noise Power

Resid. Sources


CBI 2000+2001, WMAP, ACBAR


New: Cosmological Parameters

• Data:– WMAP– CBI + WMAP– CBI + ALL

• Priors:– flat tot=1

– 45 < H0 < 90

– t0 > 10 Gyr

• Reference:– Readhead et al.

ApJ in press (2004) astro-ph/0402359


New: Running Spectral Index?

• Data:– WMAP– CBI + WMAP– CBI + ALL +

LSS prior

• Weak Priors:– flat tot=1

– 45 < H0 < 90

– t0 > 10 Gyr• Combination:

– 8 vs. s


Dawson et al. 2002

SZE Angular Power SpectrumSZE Angular Power Spectrum

•Smooth Particle Hydrodynamics (5123) [Wadsley et al. 2002]

•Moving Mesh Hydrodynamics (5123) [Pen 1998]

•143 Mpc 8=1.0

•200 Mpc 8=1.0

•200 Mpc 8=0.9

•400 Mpc 8=0.9

[Bond et al. 2002]


New: High l excess & ambient SZ?

• Data:– CBI + BIMA– CBI + BIMA +

ACBAR

• Details:– non-Gaussian

correction (F=3)

• References:– BIMA 30GHz

• Dawson et al. 2002

– ACBAR• Goldstein et al.

2003

• Kuo et al. 2004


SZE with CBI: z < 0.1 clusters

Udomprasert 2003, PhD thesis, Caltech


CBI Polarization


Polarization Interferometry

• CBI receivers can observe either R or L circular pol– RR and LL measure CMB intensity (temperature) I– RL and LR measure CMB polarization Q,U


Polarization E and B modes

• A useful decomposition of the polarization signal is into gradient and curl modes – E and B:

The CBI measures E & B “directly” !


Polarization Issues

• Low signal levels– High sensitivity and long integrations needed– Prone to systematics and foreground contamination

• Instrumental polarization– Well-calibrated system necessary– Straightforward for interferometry (leakage R↔L)

• Stray radiation– Sky (atmosphere) ~unpolarized (good!)– Ground highly polarized (bad!)– Scan differencing or projection necessary

• Computationally intensive!– covariances TT, EE, BB, TE, EB, TB plus N and Csrcs

– 6 x 6 mosaics with scan projection Cscan


DASI & WMAP Polarization

Courtesy Wayne Hu – http://background.uchicago.eduhttp://astro.uchicago.edu/dasi/polexpert/


CBI Current Polarization Data

• Observing since Sep 2002 in compact configuration– Data processed through May 2004

• Four mosaics 02h, 08h, 14h, 20h

– 02h, 08h, 14h 6 x 6 fields, 45’ centers– 20h deep strip 6 fields

• Scan subtraction/projection– observe scan of 6 fields, 3m apart = 45’– lose on 1/6 data to differencing (cf. ½ previously)

• Point source projection– list of NVSS sources (extrapolation to 30 GHz unknown)– need 30 GHz GBT measurements to identify brightest srcs


CBI Polarization Projections


CBI Polarization Data Processing

• Massive data processing exercise– 4 mosaics, 300 nights observing– more than 106 visibilities total!– scan projection over 3.5° requires fine gridding

• more than 104 gridded estimators

• Parallel computing critical– both gridding and likelihood now parallelized using MPI

• using 256 node/ 512 proc McKenzie cluster at CITA

• 2.4 GHz Intel Xeons, gigabit ethernet, 1.2 Tflops!

• current limitation 1 GB memory per node

– code development ongoing• currently 1 day per full run needed


CBI Current Polarization Data

• Currently data to May 2004 processed– Preliminary data analysis (still more tests pending)

• if you sign the non-disclosure agreement…


Conclusions

• CMB interferometry competitive– straightforward analysis {RR,RL} → {TT,EE,BB,TE}– polarization systematics minimized– currently only measurements of EE (WMAP pending)– but, hard to scale up due to correlator complexity

• CMB results– large l TT excess still significant

• next gen small-scale experiments (SZA) will nail this!

– possible running index ns(l)• marginal significance (probably overstated)

– polarization observations successful• results very preliminary! still more tests…


The CMB From NRAO HEMTs

OVRO/BIMA


The CBI Collaboration

Caltech Team: Tony Readhead (Principal Investigator), John Cartwright, Alison Farmer, Russ Keeney, Brian Mason, Steve Miller, Steve Padin (Project Scientist), Tim Pearson, Walter Schaal, Martin Shepherd, Jonathan Sievers, Pat Udomprasert, John Yamasaki.Operations in Chile: Pablo Altamirano, Ricardo Bustos, Cristobal Achermann, Tomislav Vucina, Juan Pablo Jacob, José Cortes, Wilson Araya.Collaborators: Dick Bond (CITA), Leonardo Bronfman (University of Chile), John Carlstrom (University of Chicago), Simon Casassus (University of Chile), Carlo Contaldi (CITA), Nils Halverson (University of California, Berkeley), Bill Holzapfel (University of California, Berkeley), Marshall Joy (NASA's Marshall Space Flight Center), John Kovac (University of Chicago), Erik Leitch (University of Chicago), Jorge May (University of Chile), Steven Myers (National Radio Astronomy Observatory), Angel Otarola (European Southern Observatory), Ue-Li Pen (CITA), Dmitry Pogosyan (University of Alberta), Simon Prunet (Institut d'Astrophysique de Paris), Clem Pryke (University of Chicago).

The CBI Project is a collaboration between the California Institute of Technology, the Canadian Institute for Theoretical Astrophysics, the National Radio Astronomy Observatory, the University of Chicago, and the Universidad de Chile. The project has been supported by funds from the National Science Foundation, the California Institute of Technology, Maxine and Ronald Linde, Cecil and Sally Drinkward, Barbara and Stanley Rawn Jr., the Kavli Institute,and the Canadian Institute for Advanced Research.

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