Jeroen Stil Department of Physics & Astronomy

University of Calgary

Stacking of Radio Surveys

Stacking: piling up different sources

Statistical properties of radio emission of a sample of faint sources below the detection limit of a survey. Special case: Polarization of most sources not directly detectable.Stil et al. (2014) showed sample median polarization can be recovered to the detection limit in Stokes I.

Root-N improvement of noise up to N=105 demonstrated in NVSS.

Requires a catalog of target positions from another survey.Special case: Stacking polarized intensity uses targets from the same radio survey.

Stack continuum or radio spectral lines

Information from stacking:Astrophysical modeling of trends/correlations revealed by stacking related subsamples.Example: Infrared-radio correlation for galaxies as a function of redshift.

Stacking Galaxies as a Function of Inclination

1.0 < R25 < 1.4 1.4 < R25 < 1.8 1.8 < R25 < 2.5 R25 > 2.5

Axial Ratio R25

Flux

Den

sity

(mJy

)

74 MHz

325 MHz

1400 MHz

17,000 targets14

10

2

4Inclination-dependent se

lection effect

in optical input catalog

Stacking AGN Polarization in NVSS

Preliminary

Spectral Index:Steep IntermediateFlat

Differentiate the sample by observable parameters to reveal correlations that astrophysical models must reproduce. Beware of unintended selection effects that may also correlate with signal strength.

Cannot be done with present deep fields

Fractional polarization as afunction of 1.4 GHz flux densityStil et al. (2014)

Requirements for Stacking• Image cubes including “empty sky” (compromise on frequency resolution).• Uniform angular resolution and sensitivity • Flexible and efficient access to complete image archive.• Computing resources with access to data (small footprint but enduring). • Large input catalogs and advanced sample selection.• Alignment with sub-pixel accuracy for Nyquist-sampled images

Technical Challenges

• Aperture-integration of intensity before stacking mitigates position errors, resolved target sources and discretization of data

• Seeding of artificial sources to understand systematics in the data• Stacking offset positions• In-situ noise statistics and coordinates of peak intensity in postage stamp• Sample statistics other than mean or median

• Discretization of data values by design (NVSS) or by nature (X-ray photon statistics)

• Tiling of survey images (overlap, gridding, sorting of images)• Copyright messages and missing data in the images• Access to metadata (survey images and target catalog)• Clustering of target sources

Solutions

Future of Stacking Radio SurveysIntegrate Stacking in the Archive?

Increase in resolution and bandwidth boost survey data volume (EVLA, WSRT, ASKAP, MeerKAT, Square Kilometre Array).Solutions to limit the cost of data storage can create significant hurdles.

Archives are not designed to retrieve millions of target sources and access thousands of survey images simultaneously.

Use distributed science computing platforms such as CyberSKA.

ConclusionsStacking radio surveys provides astrophysical information for large samples that is otherwise inaccessible.Broad-band surveys create new opportunities for stacking.

Current radio surveys can be downloaded and analyzed locally (NVSS, FIRST, WENSS, ATLAS). Stacking future surveys faces challenges in data transport and storage, unless science computing capability is integrated with the archive.