the dawn of 21 cm cosmology with edges judd d. bowman caltech alan e. e. rogers haystack observatory
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The Dawn of 21 cm Cosmology with EDGES
Judd D. BowmanCaltech
Alan E. E. RogersHaystack Observatory
21 cm cosmology
Image: Scientific American 2006
75% Hydrogen 75% Hydrogen (by mass)(by mass)
Illingworth
21 cm hyperfine line of hydrogen
• For a cloud of hydrogen gas between us and a radiation source, 21 cm scattering changes the observed brightness temperature:
• Universal radiation source: Cosmic Microwave Background
standard model cosmology
neutral fraction
localover-density
spin temperature
21 cm Background
“Science with the MWA”Greenhill, Bowman, et al. (2010, in prep)
Figure by Matt McQuinn
Science, Vol. 325. no. 5948, pp. 1617 – 1619, 25 September 2009
KineticCMB
Ionized fraction xi = 1 - xHI
Mean brightness temperature
Spin, TS
Pritchard & Loeb 2008
Why global 21 cm?
• Straightforward probe of mean neutral fraction and HI gas temperatures (spin + kinetic)• Star formation history, galaxy evolution, early feedback
mechanisms, etc.• Direct constraint on redshift and duration of reionization
• “Simpler” than imaging/power spectrum– Average over large solid angle– Signal fills aperture of any antenna – a single dipole is sufficient– Ignore ionospheric distortions– Polarized foregrounds reduced
• The only feasible probe of the Dark Ages (z>15) IGM for at least the next decade
Foregrounds for 21 cm cosmology
Milky Way – synchrotron emission100-1000 K @ 200 MHz
Haslam et al. (1982)
Foregrounds
|21 cm|
[MHz]
z
TF()
Experiment to Detect the Global Epoch of Reionization Signature
(EDGES)
RFI trailerAntenna
EDGESSpectral band: 90-205 MHzSpectral resolution: 13 kHz FOV: ~80 deg (FWHM)Dynamic range: >106
EDGES block diagram
Bowman & Rogers (in prep)
Comparison-switched spectrometer
• 3-position switch to measure (cycle every 10s):
• Solve for antenna temperature:
(Tcal > TL 300 K, TA 250 K, TR 20 K)
• Limitations: – Total power differences between TL and TA produce residuals
– Temporal variations: comparing measurements distinct different times
Antenna (p2)Internal load (p0)
Noise source (p1)
p1– p0
p2 – p0“Calibrated” sky spectrum
T_A ~ (p2 – p0) / (p1 – p0)
“Calibrated” sky spectrumw/ RFI filtering and integration
Comparison-switched spectrometer
EDGES Latest Results
Measured spectrum
Murchison Radio-Astronomy Observatory (MRO)
Aug 20 – Oct 20, 2009
1440 wall-clock hours on sky~500 hours after RFI filtering ~50 hours actual integration
Total power in band vs. time (Aug 23, 2009)
Average antenna temperature 90-205 MHz
Systematic free performance level:RMS = 30 mK in 13 kHz channels (thermal)
~5 mK in 2 MHz bins
20 Oct 2009
Integration… rms vs. time w/ baseband removal
Integration time [hours]
RMS
[K]
Model fitting
• Polynomial term:
• Simple step model of reionization:
3 science parameters: T21, , and 0
12 nuisance parameters: an (ACKK!!)
11
0n
nnap
021
2
erfcT
m
“instantaneous” reionization
T21
to account for impedance mismatch + galactic spectrum
Was reionization instantaneous?
Fit for T21
with fixed = ∞Test all 0
Confidence intervals on T21 with fixed =
reionization barrier
October 2009
yellow: 68%gray: 95%
Bowman & Rogers (in prep)
Bowman et al. 2008
Pace of progress
February 2009August 2009September 2009
yellow: 68%gray: 95%
reionization barrier
How long was reionization?
Fit for with fixed T21 = CDM
Test all 0
Confidence intervals on dx/dz with fixed T21
Δz> 0.2Δz> 0.4
Bowman & Rogers (in prep)
21 cm derivative: constraints and forecasts
Feb 2009
anticipated systematic limit (no/low RFI)
Integrate +improve bandpass
Oct 2009
z=13z=6 z=25
NOT reionization…
absorption
Summary
• Concluded 3 month deployment in MRO– Deepest broadband spectrum ever acquired: 5 mK rms
– Instantaneous reionization ruled out: 21 cm rapid step constrained to <30 mK between 6<z<13
– First direct dxi/dz constraints on diffuse IGM
• The next step:– Upgrade digital backend; exploring Berkeley CASPER open
architecture boards for high throughput
– Redesign antenna to improve impedance match (use lower order polynomial for continuum removal)
– Attempt detection of z>15-25 absorption feature to “set clock” for interpreting reionization
The end