Astronomical Surveys

Matthew Colless

Observational Techniques Workshop

April 2001

Properties of Surveys

Type and purpose General-use or highly specific? Targeted or

blind? Imaging or spectroscopic?

Area and spatial resolution All-sky, wide-field, pencil-beam?

Wavelength and spectral resolution Broad-band, narrow-band, spectroscopic?

Depth and quality Faintest detections? Reliability? Precision?

Analysis, storage and use Reductions, data volume, dissemination?

Survey Design Factors -product of telescope aperture ()

and area on sky (). Important figure-of-merit for imaging surveys;

time taken to cover given area to survey depth.

Efficiency - relative system throughput. telescope, instrument, detector throughputs.

Multiplex - number of objects that can be observed simultaneously. Important figure-of-merit for spectroscopic

surveys (in combination with field-of-view).

Overheads - non-survey observing time Field acquisition, calibrations, readout, etc.

Example I

Imaging survey of sky - what combination of telescope + detector is quickest?

Telescope/instrument: AAT/WFI has =15m2, =0.25deg2, =0.2

so speed = 0.75 UKST/film has =1.5m2, =25.0deg2, =0.02

so speed = 0.75 In this case decision would be based on other

components of overall survey speed (overheads, telescope availability) or survey goals (spatial resolution, precision of photometry).

Example II

Redshift survey of sky - what combination of telescope + detector is quickest?

Survey speed /N where N max(1,/M) is the number of times each field must be observed and is surface density of targets.

Telescope/instrument: AAT/2dF - =15m2, =3deg2, =0.1, M=400;

so speed min(4.5,600/) UKST/6dF - =1.5m2, =30deg2, =0.2,

M=150 so speed min(9,45/) Hence the crucial factor here is source density:

• if > 10deg-2 then use 2dF • if < 10deg-2 then use 6dF

The 2dF Galaxy Redshift Survey250,000 galaxies over 2000sq.deg magnitude-limited at bJ=19.45

Figures of Merit for 2dFGRS = 15 sq. metres x 3 sq. deg.

Multiplex: 400 fibres

Efficiency: robotic operation

2dGRS

Selection Effects

Selection effects - the eternal bane of observational astronomy: spurious trends and correlations biases (e.g. Malmquist bias and its relatives)

Good survey design - demands minimizing and controlling selection effects: well-defined target selection criteria well-determined errors on measurements simulated data to determine selection

effects on the quantities or relations of interest

Redshift completeness is a function of magnitude - harder to identify redshift for fainter objects.

In fact, completeness depends on both magnitude and S/N of each set of data - lower S/N gives stronger magnitude effect on completeness

Example I - redshift incompleteness in

the 2dFGRS

Completeness map for 2dFGRS

The map allows correction of the incompleteness by an appropriate weighting of the observed galaxies at each position in the survey region.

Example II - biased correlation

Sample selection effects can also bias physical correlations.

Ingredients:

two correlated parameters…

intrinsic scatter in relation or errors in measurements…

selection/observation limits…

True (x,y) correlation

Biased correlation

Selection limit on x

x

y

Biased (x,y) correlation

Solution requires:

accurate knowledge of measurement errors

accurate knowledge of selection limits

determine correlation allowing for both errors and limits via a full maximum likelihood fit.

Types of Surveys and Some Useful Examples

Imaging surveys (various wavelengths)

Spectroscopic surveys (redshift etc.)

Surveys of specific object classes

General-purpose surveys (‘sky surveys’)

Single-goal surveys (‘experiments’)WARNING - acronym soup!

All-Sky Imaging Surveys

Optical sky surveys:

UKST Southern Sky Surveys (B and R), esp. APM and SuperCosmos digitized versions

DSS - Digitized Sky Survey

DPOSS - Digitized Palomar Obs. Sky Survey

SDSS - Sloan Digital Sky Survey, ugriz over str to r=23 in north + r=25 in southern deep strips

All-Sky Imaging Surveys

IR sky surveys: 2MASS - Two-Micron All-Sky Survey (JHK) DENIS - Deep Near-Infrared Survey of the

Southern Sky (IJK) IRAS - Infra-Red Astronomy Satellite, 60,120m

All-Sky Imaging Surveys X-ray surveys:

RASS - ROSAT All-Sky Survey, whole-sky map and catalogue of 105 sources in 0.1-2.4keV band.

All-Sky Imaging Surveys Radio surveys:

FIRST - Faint Images of the Radio Sky at Twenty-cm, northern sky down to 0.75mJy

NVSS - NRAO VLA Sky Survey, continuum survey at 1.4GHz, northern sky above >-40°

SUMSS - Sydney U. Molonglo Sky Survey, 843MHz, ~matching NVSS but in south

HIPASS - HI Parkes All-Sky Survey; blind HI survey of southern sky

Deep Imaging Surveys Hubble Deep Fields (N+S) - deepest optical images

of the universe (also ISO, Chandra, etc deep fields) EIS - ESO Imaging Survey, O/IR multi-colour imaging

of 6 3°x2° patches ELIAS - ISO deep imaging of 13° at 7, 15, 90,

175m

PSCz - IRAS Point Source Catalog z-survey, 15,000 galaxies over 90% of sky

LCRS - Las Campanas Redshift survey, 25,000 galaxies, 800°

2dFGRS - 2dF Galaxy Redshift Survey, 250,000 galaxies, bJ<19.5, 2000°

2QZ - 2dF QSO Redshift Survey, 25,000 QSOs, 750°

SDSS - Sloan Digital Sky Survey, 106 galaxies + QSOs, steradians at NGP

Redshift Surveys

Other Surveys

Gravitational Lens Surveys:

MACHO - microlensing in LMC, SMC and bulge

AGAPE - microlensing in Andromeda

EROS - microlensing in the LMC

OGLE - microlensing in LMC, SMC and bulge

Galaxy Surveys:

APM Galaxy Survey - 2x106 galaxies with BJ<20.5 in South Galactic Cap.

Other Surveys Astrometric surveys:

Hipparcos - positions, photometry, proper motions etc for 106 stars down to V=11

Cluster surveys: Abell Cluster Catalogue - 4000 galaxy

clusters over the whole sky

ENACS - ESO Nearby Abell Cluster Survey, redshifts etc for Abell clusters

Secular surveys (time-variability): ASAS - All-Sky Automated Survey,

photometric monitoring of 107 stars with V<14