The EoR/Dark-Ages & SKA in phase 1&2

Léon Koopmans (Kapteyn Astronomical Institute)

Co-PI LOFAR EoR Key Science Project

SKA1&2: Going from statistical detections to image data-cubes.

a.k.a: “The 50MHz Talk”

General Observational Objectives

• Absorption again high-z radio source (small scale power)• Global signal (total power) of HI (absorption/emission) • RMS fluctuations of HI emission with redshift• Spatial power-spectra (1,2,3D) and higher-order statistics with redshift• HI Image data-cubes (spatial-frequency) for reionization topology

Epoch of Reionization [~ 100 to ~200 MHz]

Late Dark-Ages [~50 to ~100MHz]

Com

plex

ity

• Absorption again high-z radio source (small scale power)• Global signal (total power) of HI (absorption/emission) • RMS fluctuations of HI emission with redshift• Spatial power-spectra (1,2,3D) and higher-order statistics with redshift

• HI Image data-cubes (spatial-frequency) for cosmologySKA2

SKA1

SKA1

LOFARMWAPAPER

See also the excellent related talks by Rawlings, Greenhill, Carilli, Briggs, Pritchard & Santos

Pritchard & Loeb 2009

Spatial Intensity Fluctuations (integrated over non-zero baselines)

Total Intensity (“zero-baseline”)

EoREoR

LateDark Ages

LateDark Ages

Theoretical Motivation (I)

θ~5o

Brightness Temperature

θ~2’

Pritchard & Loeb 2009

Spatial Power-Spectrum

Theoretical Motivation (II)

SKA1: Acore=250,000 m2

Acore=600,000 m2 (P&L09)

SKA2: Acore=2.5 km2

z=9 z=8

z=6z=7

Trac & Chen 2007

Theoretical Motivation (III)

Pritchard & Loeb 2009

k=0.1 Mpc-1

k=1 Mpc-1

k=10 Mpc-1

SKA1&2:

Sufficient S/N to image HIon scales from arcmin to a degree over ν=50-200MHz

(tint=1000hr/BW=1MHz/Acore=600,000m2)

SKA

EoRLateDark Ages

Theoretical Motivation (IVa)

Tseliakhovich & Hirata 2010; Dalal, Pen & Seljak 2010; Maio, Koopmans & Ciardi 2010

z=20 k=0.1

ν = 50 MHzν = 70 MHzν = 150 MHzFluctuations can be off the chart!

Supersonic bulk flows suppresses SF to z~13 in ~5 Mpc patches: increase spin-temp fluctuationsand substantially enhance brightness temp. fluctuations in the late Dark Ages

The shaded area indicates the 1−σ errors per unit redshift expected for 1000 days of observations using a 500m dish like FAST.

Maio, Koopmans & Ciardi (2010)

Theoretical Motivation (IVb)Supersonic bulk flows suppresses SF to z~13 in ~5 Mpc patches: increase spin-temp fluctuations

and substantially enhance brightness temp. fluctuations in the late Dark Ages

Suppression of SFR by order(s) of magnitudecould delay coupling of the spin temperatureto the gas via radiative processes and create cold patches (~5 Mpc scale) seen in absorption next to heated patches.

This can cause much stronger Tb fluctuations around redshifts of 15-30 than thought before!

General EoR/DA features that drivethe technical requirements

General Features from EoR & Dark Ages:

Most of reionization & late DA features occur at 6<z<30 or ν=50-200 MHz.

Substantial power (rms of several mK) on scales ~2 arcmin to ~5 degrees over all those freq.

Total intensity in emission & absorption is predicted to be between +20 mK and -100mK

Major Challenges to detect HI at z>6 (ν<200MHz):

Diffuse polarized Galactic foregrounds (Tsky,I=100K - 4000K from 200MHz - 50MHz)

Extragalactic (compact) sources: confusion noise, variability, polarization, etc.

Ionosphere: diffraction & refraction, strongly wavelength and time dependent

Instrumental Stability (e.g. beam, gains)

SKA Phase 1: Imaging >100MHz

(1) The capability of SKA1 to image ~3.0 mk fluctuations of neutral hydrogen at 3-sigma level in tint=1000 hrs at 150 MHz with BW=1 MHz, covering z~6-15.

(2) Capability of SKA1 to cover all EoR & Dark Ages (z=6-30) features currently expected in the HI power-spectrum and total intensity both in emission and absorption.

(3) Allow HI absorption at sub-KHz level against high-z radio sources.

SKA Phase 1I: Imaging 50-100MHz

(4) The capability of SKA1 to image ~0.3 mk fluctuations of neutral hydrogen at 3-sigma level in tint=1000 hrs at 150 MHz with BW=1 MHz, covering z~6-30. But imaging is feasible to z=30 (see three slides back).

EoR+DA Science Goals for SKA phase 1 & 2

Transition from statistical to imaging capabilities.LOFAR, MWA, PAPER SKA 1&2

See also talk by Steve Rawlings

Frequency Range, Bandwidth & Spectral Resolution

• Capabilities from 50 MHz (z=30) to 200 MHz (z=6) is important to fully cover the expected redshift range over which the EoR occurs, probe the late Dark Ages and cover total intensity features in emission and absorption.

• Full instantaneous frequency coverage helps dramatically in constraining the ionosphere, and reduce # unknowns and integration time, and (polarized) foreground removal: Δν/ν ~1

• High spectral resolution (dν<1 KHz) is critical for HI absorption studies using high-z radio sources (vHI ~ km/s) and reduces bandwidth smearing and RFI

Sensitivity for Imaging Capabilities

• Sensitivity is needed on scales of k = 0.05 - 10 Mpc-1; At z>6 this translates to angular scales of 1.5’ < θ < 4o. (DA decreases beyond z=6.)

• Detection (~3-sigma) of Tb =3 mK fluctuations, BW=1MHz, @3’ FWHM, @150MHz in tint = 1000 hrs requires A/T>600 or Acore>250,000 m2 (SKA1) for Tsys=400K

• Imaging thus requires e.g. 10x current sensitivity of HBA collecting area of LOFAR.

• FoV > 5o (per beam) is needed to sample all scales; larger FoV requires longer baselines as well (ionospheric diffraction). A FoV ~ 20o is approximately optimal but requires outer baselines (Aouter~100,000 m2) up to 200 km to fully control the ionosphere.

UV-coverage & Spatial Resolution

• Fluctuations are on scales of several arcmin: full PS coverage requires ~1.5’ resolution or baselines of up to ~5 km in the core at 150MHz for z>6. Higher-z observations require shorter baselines for fixed k.

• A fully filled UV plane with a relatively uniform (TBD) # of visibilities per UV-annulus is needed to guarantee uniform sensitivity to all scales/modes and no loss of information.

• Instantaneous UV-coverage requires Nant> (½πb2max/Acore)=1/(2η) [minimal redundancy]. Tracking will create redundancy and build S/N. Needed to handle ionosphere and time- varying gains, etc.

• Outer baselines (<200 km) are needed for: (i) reduction ionospheric effects [i.e. bouter~[FoV/5d]x50 km] (see Koopmans 2010), (ii) reduction of confusion noise to the level of the EoR signal (requires ~200 km baselines), (iii) improvement of calibration on compact sources,(iv) cross-correlation/stacking studies.

Field-of-View

• Min/Max FoV is set by several requirements: (i) sampling sufficient volume (i.e. S/N and cosmic variance), (ii) control over the antenna beam (harder for larger FoV)(iii) sampling a large EM field being affected by the ionosphere (re/diffraction).

• A reasonable FoV @150MHz that limits cosmic variance is FoV>5o and a maximum FoV that would have an imprint of <200 km at h=300 km (of ionosphere) is FoV<20o. Larger FoV will require single dipole antennas (>>106 elements) leading to substantial computational costs and harder to control beam shapes. But note that multi-beaming can be done.

Number of Antennas

• Minimum # of antennas for instantaneous UV coverage depends on the filling factor of the core: Max antenna diameter then becomes Dmax = (23/2/π) (Acore/bmax). Beyond this diameter, no instantaneous UV can be achieved.

(i) For Acore = 250,000 m2 (SKA1) → Dant < 50m (Fov=3o) minimal redundancy limit, (ii) For Acore = 2.5 km2 (SKA2) → Dant < 500m (Fov=0.3o) minimal redundancy limit.

• Hence, to have a good instantaneous UV coverage and 5o<FoV150MHz<20o for SKA1, one needs 7m < Dant < 30m. This requires Acore > 250,000 m2 giving perfect snapshot UV coverage already for SKA in phase I. Larger stations (up to 500m for Acore > 250,000 m2) can be allowed in phase II to have full instantaneous UV coverage, but would requires substantial multi-beaming to build a FoV larger than 5 degrees.

Requirements Table (1)These requirements are set by science drivers and not necessarily feasibility.

Main driver for SKA1 is the capability to image the EoR and obtain power-spectra of the Dark Ages: Requires 10x LOFAR HBA core area

Specification Requirement (phase 1) Requirements or Upgrade (phase II)Sensitivity Core (1) A/Tsys >600 (*) A/Tsys >6000 (*)Sensitivity Core (2) σTb ~1 mK (@3’; ~1μJy); tint=1000hr, BW=1MHz (*) σTb ~0.1 mK (@3’; ~1μJy); tint=1000hr, BW=1MHz (*)Tsys Tinst << 200K [v150MHz]-2.55 (sky) over 50-200 MHzAcore(<5 km) >250,000 m2 (>10x LOFAR-core HBA) >2.5 km2 (>100x LOFAR-core HBA)Aouter(<200 km) >100,000 m2; (Assumes FoV=20o, see below) >1.0 km2

Antenna FoV 5o < FoV < 20o [Multi-beam for FoV<20o] (*) From single to multi-beam Polarization Full StokesCore diameter bmax=5 km; FWHMPSF of ~1.5’/3’ @150/75MHzOuter Antennas bout ~ [FoV/5o] x 50 km ~ 200 km for FoV = 20o (*)

Core UV-coverage Instantaneous: Nant > (½πb2max/Acore) ~ 160Antenna Layout TBD Frequency/BW ν = 50 - 200 MHz; Δν/ν ~1 Option? 100-400MHz (SKAI) + 50-100MHz (SKA2)

Spectral resolution <1 kHz incoming data; 100kHz for scienceCritical Frequency νcrit=100MHz If two arrays (νcrit=50 & 100MHz)Correlator tcorr = 1 sec; dν <1KHz

(*) Reference frequency is 150 MHz (if not specified otherwise)

Specification DriversSensitivity Core (1) Imaging of EoR & P.S. of DASensitivity Core (2) Imaging of EoR & P.S. of DATsys Instrument dominated above 150 MHz Acore(<5 km) EoR Imaging @150MHz in tint=1000hrsAouter(<200 km) Ionosphere, FG removal, calibration, confusion noiseAntenna FoV Full coverage of PS, Cosmic Variance, Ionosphere, reduction sidelobesPolarization Foreground (FG) removalCore diameter Spatial power-spectrumOuter Antennas Ionosphere, calibration, reduce confusion noise

Core UV-coverage Reduce information “leakage”; Requires minimum # of stations for fixed Acore and bmax.Antenna Layout PS sensitivity, Spatial & frequency coverage, FG, calibrationFrequency/BW Ionosphere, FGs, information criteriumSpectral resolution RFI, HI Absorption with few km/s velocity resolution Critical Frequency Optimize collecting area and SNR (Tsky dominates below 150MHz)Correlator Reduce RFI & BW/time-smearing

Requirements Table (2)These requirements are set by science drivers and not necessarily feasibility.

Main driver for SKA1 is the capability to image the EoR and obtain power-spectra of the Dark Ages: Requires e.g. 10x LOFAR HBA core area

(*) Reference frequency is 150 MHz (if not specified)

Conclusions

Change the DRM to allow going down to 50 MHz!

(see also talks by Greenhill, Pritchard, Santos, etc.)