Foreground Contamination and the EoR Window

Nithyanandan ThyagarajanN. Udaya Shankar

Ravi Subrahmanyan

(Raman Research Institute, Bangalore)

Outline• Need for EoR studies• Tomography of redshifted neutral hydrogen• HI power spectrum measurements• Challenges due to contamination

– Frequency dependent beams (mode-mixing)• Quantifying mode-mixing

– MWA case study• Conclusions

– Significance of mode-mixing– Implications for the detection of EoR power:

Array configuration

EoR window

Detection strategy

Need for EoR studies• High-z quasars useful up to z~7• CMB probes out to z=1100• Intermediate period (Dark ages)• Process of reionization not well

constrained• Structure formation & galaxy evolution• Redshifted HI studies suggested • Current and planned purpose-built

instruments can reach adequate sensitivities

Redshifted 21-cm: a direct probe of dark ages and EoR

HI Tomography• Slices of neutral hydrogen at

different redshifts• Provide a map of evolution

of the spatial distribution of HI in the EoR

• Extreme sensitivities required

• Achievable only with SKA; not with current instruments

Trac & Cen (2007)

Have to wait for SKA for HI tomography?

HI Power Spectrum• Statistical detections seem

feasible• Forms a key science of SKA

precursors & pathfinders– MWA– LOFAR– PAPER– GMRT– LWA– PAST

Lidz et al. (2008)

Power spectrum measurements of HI at EoR seem feasible

Challenges due to Contamination

• Foreground Galactic emission

• Foreground extragalactic radio continuum sources

• Radio recombination lines in the Galaxy

Expected sources of contamination

Foreground Removal• Knowledge of spectral

information– Galactic modeling– Extragalactic source

spectral index

• Knowledge of power spectrum symmetry– HI power spectrum

isotropic– Foregrounds not isotropicMorales & Hewitt (2004)

Separation of contamination using symmetries in Fourier space

Mor

ales

& H

ewitt

(200

4)

Contamination after Foreground Removal

• Confusion from unresolved unsubtracted/mis-subtracted sources due to poor angular resolution & limited flux sensitivity

• Confusion from frequency dependent beams (mode-mixing)

• Contamination from imaging algorithms (Vedantham et al. 2011)

Our focus on mode-mixing contamination

Mode-mixing Principle

Bowman et al. (2009)

Vedantham et al. (2011)

Transverse structure of contamination translates to a line-of-sight structure due to mode-mixing l-f invariance

Quantifying Mode-mixing

Vedantham et al. (2011) Hopkins et al. (2003)

Present work extends Vedantham et al. from individual sources to source distribution on sky

Instrumental k-space EoR Window

Vedantham et al.(2011)

MWA Case Study

• 496 tiles (courtesy: Adam Beardsley)

• 32 MHz bandwidth• Frequency windows– Rectangular– Blackman-Nutall

• Foreground removal assumed to be done to a flux density limit of 50 mJy @ 150 MHz (5 times confusion noise?)

Mode-mixing in (ideal) infinite bandwidth case

22fixed

2222 ),(),(),()(ml

Czmm mlmlBmlPk

mJy 50

2),(

2max

min

S

SmlC dS

dSdnS

ff

ll

Obtaining contamination using confusion noise & source distribution due to mode-mixing

Choice of Frequency Window Functions & Spillover

Rectangular Window Blackman-Nutall Window

Blackman-Nutall window has lower sidelobe levels in comparison to a rectangular window at the cost of resolution in k-space

Mode-mixing in a Finite Frequency Window & Spillover into EoR window

Contamination structure spillover in the EoR window

What’s the Signal?

Projecting a spherical signal onto a plane in k-space

Lidz et al. (2008)

Transverse k-space Weighting

• Obtain EoR power spectrum as seen through the telescope

Weighting applied on transverse k-space identical to uv-weighting

EoR Signal vs. Mode-mixing effect

EoR line-of-sight power spectrum as seen through MWA compared to mode-mixing contamination

128T vs. 496T496T 128T

Summary

• Detailed study of mode-mixing• Blackman-Nutall window reduces spillover levels• Strategies– Array configuration

• Increase baseline to reduce confusion– Observation

• After peeling with lower levels of confusion noise, use only shorter baselines to:– estimate power spectrum to reduce sidelobe confusion – to extend EoR window