Foreground Contamination and the EoR Window Nithyanandan Thyagarajan N. 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 isotropic Morales & Hewitt (2004) Separation of contamination using symmetries in Fourier space Morales & Hewitt (2004)

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 MHz (5 times confusion noise?)

Mode-mixing in (ideal) infinite bandwidth case Obtaining contamination using confusion noise & source distribution due to mode-mixing

Choice of Frequency Window Functions & Spillover Rectangular WindowBlackman-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. 496T 496T128T

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