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