Wide Field Surveys and Astronomical Discovery Space
Wide Field Surveys and Astronomical Discovery Space
A.Lawrence
Institute for Astronomy, SUPA∗, University of Edinburgh,
Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ
A review for publication in Astronomy and Geophysics
Feb 27th 2007
Abstract
I review the status of science with wide field surveys. For many decades surveys have been the
backbone of astronomy, and the main engine of discovery, as we have mapped the sky at every
possible wavelength. Surveys are an efficient use of resources. They are important as a fundamental
resource; to map intrinsically large structures; to gain the necessary statistics to address some
problems; and to find very rare objects. I summarise major recent wide field surveys - 2MASS,
SDSS, 2dfGRS, and UKIDSS - and look at examples of the exciting science they have produced,
covering the structure of the Milky Way, the measurement of cosmological parameters, the creation
of a new field studying substellar objects, and the ionisation history of the Universe. I then look
briefly at upcoming projects in the optical-IR survey arena - VISTA, PanSTARRS,WISE, and LSST.
Finally I ask, now we have opened up essentially all wavelength windows, whether the exploration
of survey discovery space is ended. I examine other possible axes of discovery space, and find them
mostly to be too expensive to explore or otherwise unfruitful, with two exceptions : the first is the
time axis, which we have only just begun to explore properly; and the second is the possibility of
neutrino astrophysics.
1 Why are wide field surveys important ?
Some astronomical experiments are direct, in that measurements are made of some piece of sky, and
these measurements are then used for a specific scientific analysis. The essence of a survey however is
that extracting science is a two step process. First we summarise the sky, usually by taking an image
and then running pattern recognition software to produce a catalogue of objects each with a set of
measured parameters. When this summary is made, we can then do the science with the catalogue;
the archive becomes the sky. There are many such archives, distributed around the world in online
structured databases; querying such databases is a growing mode of scientific analysis. This of course is
why survey databases have played such a central role in the worldwide Virtual Observatory initiatives.
Why is this two-step process a good thing to do ? Firstly, it is cost effective, because we can performmany
experiments using the same data. Secondly, surveys are a resource that can support other experiments.
This can mean for example creating samples of objects which are ‘followed up’, i.e. observed in detail,
on other facilities (eg getting spectra of galaxy samples). Conversely, interesting objects discovered
by other experiments can be matched against objects in the standard survey catalogues, so that one
quickly has the optical flux of a new gamma-ray source. (Should this be called follow-down ?). Finally,
surveying the sky can produce surprises. First looks in new corners of parameter space have often
∗Scottish Universities Physics Alliance
http://arxiv.org/abs/0704.0809v1
Wide Field Surveys : A.Lawrence 2
Table 1: Examples of major astronomical surveys from recent decades
Type Survey Examples
Radio 3C, PKS, 4C, FIRST
IR IRAS-PSC, ELAIS, 2MASS, UKIDSS
Optical APM, SuperCOSMOS, SDSS, CFHTLS
X-ray 3U, 2A, HEAO-A, 1-XMM
z-surveys CfA-z, QDOT, 2dFGRS, SDSS-z
discovered completely new populations of objects. Historically, surveys have been the main engine of
discovery for astronomy.
Why are wide angle surveys important, as opposed to the deepest possible pencil beams ? The key point
here is that in Euclidean space, time spent surveying more area increases volume much faster than time
spent going deeper. (The argument that wide angles produce large samples faster breaks down when the
differential source count slope is flatter than 1, which for example occurs for galaxies fainter than about
B∼ 23. Also of course, sometimes, one simply has to go deep, for example to survey at some given large
redshift.) Many astronomical problems need large samples of objects to address them. Sometimes this
is because one wants accurate function estimation – for example to test theories of structure formation,
one wants to estimate the galaxy clustering power spectrum to an accuracy of around 5% in many bins
over a wide range of scale. Sometimes large samples are needed to recover a very weak signal from
noise – for example the net alignment of many random galaxy ellipticities produced by weak lensing
by intervening dark matter. The second reason for maximising volume as quickly as possible is to find
rare objects, such as the hoped for Y dwarfs and z = 7 quasars; to a given depth there might be only a
handful over the whole sky. Finally, some objects of astronomical study simply have intrinsically large
angular scale - for example the Milky Way, the galaxy clustering dipole, or open clusters of stars, which
can be tens of degrees across.
2 Major surveys
Surveys are the core of astronomy. This has always been true of course, from Ptolemy through the New
General Catalogue, to the Carte du Ciel, but it has been certainly been the case in the last few decades.
Table 1 lists some of the best known imaging surveys in each wavelength regime. (I have also included a
few redshift surveys as a distinct set). This is only a selection, and is biased towards my own favourites,
so apologies to those whose own surveys aren’t listed. The point to note is that these names are as
immediately recogniseable to every astronomer as are the names of famous telescopes and satellites -
Palomar, AAT, Ariel-V, etc. The data in these catalogues are of everyday use and have been the source
of many discoveries. Many of the older surveys were classic examples of opening a completely new
window on the Universe - 3C, IRAS, and 3U in particular, though I think it is also fair to include the
CfA redshift survey in this category, as it gave us the first real feel for the three dimensional structure of
the Universe, with bubbles, filaments, and walls. The 1-XMM catalogue is slightly different, in that it
wasn’t planned as a coherent single survey, but is the uniformly processed summation of XMM pointings
over the sky.
Over the last 5-10 years the most important major new surveys have been in the optical-IR - 2MASS,
SDSS, 2dFGRS, and now UKIDSS, which started in 2005. I will summarise each of these briefly in turn.
Some highlight science results are in the next section.
The Two Micron All Sky Survey : 2MASS.
Wide Field Surveys : A.Lawrence 3
Figure 1: All sky distribution of 2MASS catalogues. Point sources are shown as white dots. Extended
sources are coloured according to estimated redshift, based either on known values, or estimated from K
magnitude. Blue are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04)
and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). Taken from Jarrett (2004).
2MASS broke new ground, as it was the first real sky survey at near infa-red wavelengths. At near-
IR wavelengths we see roughly the same Universe as in the visible light regime, but with some key
improvements. Extinction is much less; we can see pretty much clean through the Milky Way, and can
find reddened versions of objects such as quasars. Cooler objects such as brown dwarfs can be found,
with the most extreme objects essentially invisible in standard optical bands. Cleaner galaxy samples
can be constructed, with high redshift objects easier to find. Colour combinations with optical bands
have proved especially good at finding rare objects, such as the new T-dwarf class of brown dwarfs.
2MASS used two dedicated 1.3m telescopes, in Mt Hopkins, Arizona, and CTIO, Chile. Each telescope
was equipped with a three-channel camera, each channel consisting of a 256×256 HgCdTe array, so
that observations could be made simultaneously at J (1.25 microns), H (1.65 microns), and Ks (2.17
microns). One interesting innovation was the use of large pixels, maximising survey speed, requiring
micro-stepping to improve sampling. The survey started in June 1997 June and completed in February
2001. The full data release occurred in March 2003, including both an Atlas of images and a catalogue of
almost half a billion sources. To a point source limit of 10σ, the catalogue depth is J=16 H=15 Ks=14.7,
almost five orders of magnitude deeper than any comparable IR survey. However, for the colours of
many astronomical objects, this is still two orders of magnitude shallower than modern optical surveys.
The core reference for 2MASS is Skrutskie et al. 2006). Further information can be found at the IPAC
(http://www.ipac.caltech.edu/2mass/) and UMASS(http://pegasus.phast.umass.edu/) sites. Data ac-
cess is through the IRSA system at http://irsa.ipac.caltech.edu/ .
The Sloan Digital Sky Survey : SDSS.
The SDSS project has produced a survey of 8,000 square degrees of sky at visible light wavelengths,
approximately two magnitudes deeper than the historic Schmidt surveys, and in addition has carried out
a spectroscopic survey of objects selected from the imaging survey. The project used a dedicated 2.5m
telescope at Apache Point, New Mexico, and a camera covering 1.5 square degrees. Survey operations
used a novel drift scan approach; the 30 CCDs on the camera are arranged in five rows each sensitive
to a separate filter band (u,g,r,i,z); the telescope is parked in a given position and the sky allowed to
drift past. The spectroscopic survey is carried out using a 600-fibre system, on separate nights spliced
into the imaging programme. This then required the data processing pipeline to keep up in almost real
time. Public data access has been announced in a series of staged releases, culminating in June 2006
with DR5, which contains a catalogue of around 200 million objects, and spectra for around a million
http://www.ipac.caltech.edu/2mass/
Wide Field Surveys : A.Lawrence 4
galaxies, quasars, and stars. An extended programme, cunningly called SDSS-II, has now commenced,
and is expected to continue through 2008.
SDSS has been arguably the most successful survey project of recent times, with many hundreds of
scientific papers based directly on its data, and having an impact on a very large range of astronomical
topics - large scale structure, the highest redshift quasars, the structure of the Milky Way, and many
other things besides. This may seem surprising, as visible light sky surveys covering the whole sky have
been available for decades, and available as digitised queryable online databases for some years (eg the
Digitised Sky Survey (DSS : see http://archive.stsci.edu/dss/) or the SuperCOSMOS Science Archive
(SSA : see http://surveys.roe.ac.uk/ssa). There are several reasons for the success of SDSS. The first
reason is of course the spectroscopic database, matched only by 2dF (see below). The second reason
is the wider wavelength range, with filters carefully chosen and calibrated to optimise various kinds of
search. The third reason is the improvement in quality - not only is SDSS a magnitude or two deeper
than the Schmidt surveys, but the seeing is markedly better. The fourth reason, shared by 2MASS,
is the quality of the online interface - well calibrated, reliable, and documented data were available
promptly, and with the ability to do online analysis rather than just downloading data. This has made
it easy for astronomers all over the world to jump in and benefit from SDSS.
The core reference for SDSS is York et al. 2000). Further information can be found at http://www.sdss.org,
which also contains links to data access via SkyServer.
The UKIRT Infrared Deep Sky Survey : UKIDSS.
UKIDSS is the near-infrared equivalent of the SDSS, covering only part of the sky, but many times
deeper than 2MASS. The project has been designed and implemented by a private consortium, but on
behalf of the whole ESO member community, and after a short delay, the world. It uses the Wide Field
Camera (WFCAM) on the UK Infrared Telescope (UKIRT) in Hawaii, and is taking roughly half the
UKIRT time over 2005-2012. WFCAM has an instantaneous field of view of 0.21 sq.deg, much larger
than any previous large facility IR camera. Put together with a 4m telescope, this makes possible an
ambitiuous survey. It is estimated that the effective volume of UKIDSS will be 12 times that of 2MASS,
and the effective amount of information collected 70 times larger. UKIDSS is not a single survey, but a
portfolio of five survey components. Three of these are wide shallow surveys, to K∼ 18−19, and covering
a total of ∼ 7000 sq.deg - the Galactic Plane Survey (GPS); the Galactic Clusters Survey (GCS); and
the high latitude Large Area Survey (LAS). Then there is a Deep Extragalactic Survey (DXS), covering
35 sq.deg to K ∼ 21, and an Ultra Deep Survey (UDS), covering 0.77 sq.deg. to K ∼ 23. In all cases,
there is the maximum possible overlap with other multiwavelength surveys and key areas, such as SDSS,
the Lockman Hole, and the Subaru Deep Field.
The aim of UKIDSS is to provide a public legacy database, but the design was targeted at some specific
goals - for example, to measure the substellar mass function, and its dependence on metallicity; to find
quasars at z = 7; to discover Population II brown dwarfs if they exist; to measure galaxy clustering at
z = 1 and z = 3 with the same accuracy as at z = 0; and to determine the epoch of spheroid formation.
Like SDSS, data are being released in a series of stages. At each stage the data are public to astronomers
in all ESO member states, and world-public eighteen months later. Data are made available through a
queryable interface at the WFCAM Science Archive (WSA : http://surveys.roe.ac.uk/wsa). Three data
releases have occured so far – the “Early Data Release”, and Data Releases One and Two (DR1 and
DR2) which contain approximately 10% of the likely full dataset.
UKIDSS is summarised in Lawrence et al. (2007) , and technical details of the releases are described in
Dye et al. (2006) and Warren et al. (2007).
Redshift surveys : 2MRS/6dFGRS; 2dFGRS and SDSS-z.
Systematic redshift surveys based on galaxy catalogues from imaging surveys were one of the big success
stories of the 1970s–90s, culminating in the all-sky z-survey based on the IRAS galaxies, the PSC-z
(Saunders et al. 2000). The most ambitious surveys to date have however been carried out over the last
five years. The first example is the construction of a complete all-sky redshift survey based on galaxies in
http://archive.stsci.edu/dss/
http://www.sdss.org
http://surveys.roe.ac.uk/wsa
Wide Field Surveys : A.Lawrence 5
the 2MASS Extended Source Catalog (XCS) to a depth of KS=12.2, containing roughly 100,000 galaxies.
In the south, observations are carried out at the UK Schmidt, as part of the 6dfGRS project (Jones et
al. 2004, http://www.aao.gov.au/local/www/6df/); in the North observations are being carried out by
a CfA team at Mt Hopkins, Arizona (see http://cfa-www.harvard.edu/∼huchra/2mass/). The survey
is part way through, but has already been used to measure the dipole anisotropy of the local universe
(Erdodgu et al. 2005).
Two very successful projects have completed redshift surveys of smaller area, but reaching considerably
deeper, containing hundreds of thousands of galaxies. The first, in the Northern sky, is SDSS-z, the
spectroscopic component of SDSS, as described above. The second, in the southern sky, is the 2dF
Galaxy Redshift Survey (2dFGRS; Colless et al. 2001). This was based on galaxies selected from the
APM digitisation of UK Schmidt plates (Maddox et al. 199x), and observed using the Two Degree
Field (2dF) facility at the Anglo-Australian Telescope, which has 400 independent fibres. The 2dFGRS
obtained spectra for 245591 objects, mainly galaxies, brighter than a nominal extinction-corrected mag-
nitude limit of bJ=19.45, covering 1500 square degrees in three regions. The final data release was in
June 2003. More information, and data access, is available at http://www.mso.anu.edu.au/2dFGRS/.
These two surveys have produced a range of science, but have concentrated on making the best possible
measurement of the power spectrum of galaxy clustering, and together with WMAP and supernova
programme results, have produced the definitive estimates of the cosmological parameters, leading to
the current ‘concordance cosmology’.
3 Recent survey science highlights
I have picked out a handful of results from the optical-IR surveys of the last few years, including the
first results from UKIDSS, to illustrate the power of the survey approach.
Panoramic mapping : the structure of the Milky Way.
Two topics which clearly benefit from a map covering 4π sr, and with low extinction, are the structure
of the Milky Way, and the structure of the local extragalactic universe. Figure 1, taken from Jarrett
(2004), illustrates the impact 2MASS has on both these topics, showing both the Point Source Catalog
(mostly stars) and the Extended Source Catalog (mostly galaxies at z<0.1). For the first time, we can
see the Milky Way looking like other external galaxies, with disc, bulge, and dust lane. Some of the
most important scientific results however have come from looking at subsets of the stellar population.
Figure 2, from Majewski et al. (2003), shows the sky distribution of M giants selected from the 2MASS
PSC, a selection which traces very large scale structures while removing the dilution of local objects,
using a few thousand stars out of the catalogue of half a billion. From APM star counts we already knew
of the existence of the Sagittarius dwarf, swallowed by the Milky Way (Ibata, Irwin and Gilmore 1994),
but now we can see its complete structure including an extraordinary 150 degree tidal tail. Its orbital
plane shows no precession, indicating that the Galactic potential within which it moves is spherical. The
Earth is currently close to the debris, which means that some very nearby stars are actually members of
the Sagittarius dwarf system. Interestingly, Sagittarius seems to contribute over 75% of of high latitude
halo M giants, with no evidence for M giant tidal debris from the Magellanic clouds.
SDSS, although not a panoramic survey, has also been very important for Galactic structure and stel-
lar populations, with the five widely spread bands making it possible to derive stellar types and so
photometric parallaxes. Juric et al. (2005) derive such parallaxes for 48 million stars. They fit a com-
bination of oblate halo, thin disk, thick disk, but also find significant ‘localised overdensities’, including
the known Monoceros stream, but also a new enhancement towards Virgo that covers 1000 sq.deg. This
then maybe another dwarf galaxy swallowed by the Milky Way.
Large sample statistics : galaxy clustering and the cosmological parameters.
http://www.aao.gov.au/local/www/6df/
http://www.mso.anu.edu.au/2dFGRS/
Wide Field Surveys : A.Lawrence 6
Figure 2: Smoothed maps of the sky in equatorial coordinates showing the 2MASS point source catalogue
optimally filtered to show the Sagittarius dwarf; southern arc (top), and the Sagittarius dwarf northern
arm (bottom). Two cycles around the sky are plotted to demonstrate the continuity of features. The
top panel uses 11 < Ks < 12 and 1.00 < J − Ks < 1.05. The bottom panel uses 12 < Ks < 13 and
1.05 < J −Ks < 1.15. Taken from Majewski et al. (2003).
Figure 3: Cone diagram showing projected distribution of galaxies in 2dFGRS. Taken from Peacock
(2002).
Wide Field Surveys : A.Lawrence 7
Figure 4: (a) Power spectrum from 2dFGRS, compared to various model predictions. Taken from Per-
cival et al. (2001). (b) Correlation function of Luminous red Galaxies in the SDSS-z sample, showing
the first baryon acoustic oscillation peak. Taken from Eisenstein et al. (2005)
The SDSS-z and 2dFGRS surveys illustrate the power of the survey approach in two ways. First,
significant volume is needed to map out large scale structures and overcome shot noise on the largest
scales. Figure 3 is a cone diagram for all the 2dFGRS galaxies, showing the richness of structure that
is only possible to map out with both a large volume and density. Second, large numbers are needed to
make a good enough estimate of the power spectrum of galaxy clustering. This is illustrated in Fig 4a,
which shows the power spectrum derived from 2dFGRS compared to various model predictions (data
from Percival et al. (2001), figure from Peacock (2002)). To distinguish models with differing matter
density in the interesting range requires accuracy of a few percent over a very wide range of scales;
to have a chance of measuring small scale features predicted by models including a significant baryon
fraction requires many samples across this wide range, with of the order 103 galaxies per bin to achieve
the required accuracy. These wiggles are due to acoustic oscillations in the baryon component of the
universe at early times. In the Percival et al. paper, only a limit could be placed on these oscillations,
but they were statistically detected in the fimal 2dFGRS data (Cole et al. 2005). However, in another
good example of filtering out a tracer sub-sample from a very large sample, the first baryon peak was
much more clearly seen in the correlation function of Luminous Red Galaxies (LRGs) selected from
SDSS-z (Eisenstein et al. 2005; Huetsi 2005; see Fig 4).
2dFGRS and SDSS-z were the first redshift surveys to have large enough scale and depth to overlap
the fluctuation measurements from the CMB, enabling degeneracies in the estimation of cosmological
parameters to be broken, and accuracy to be increased by a factor of several. Several key papers made
joint analyses of the galaxy and CMB datasets (Percival et al. 2002; Efstathiou et al. 2002; Tegmark et
al. 2003; Pope et al. 2004) arriving at broadly consistent answers. We now know what kind of universe
we live in : a geometrically flat universe dominated by vacuum energy (75%), with some kind of cold
dark matter at about 21% and ordinary baryons 4%. The equation of state parameter for the dark
energy has been limited to w < −0.52 (Percival et al. 2002), and the total mass of the neutrinos to
m <1 eV (Tegmark et al. 2003; Elgaroy et al. 2002)
The Deep eXtragalactic Survey (DXS) of UKIDSS will produce a galaxy survey over a volume as large
as that of 2dFGRS or SDSS, but at z = 1. A redshift survey of this sample is a prime target for future
work.
Rare objects : Brown Dwarfs.
Infrared surveys have transformed the study of the substellar regime, blurring our idea of what it means
Wide Field Surveys : A.Lawrence 8
7000 7500 8000 8500 9000 9500 10000 10500
Wavelength
Dashed: LBQS composite
Dotted: Telfer continuum
SDSS J1030+0524
z=6.28
Figure 5: (a) High resolution spectrum of high redshift quasar found in SDSS. Gunn-Peterson troughs
due to Lyα and Lyβ are the black sections from 8500Å to 9000Å and from 7000Å to 7500Å. Taken
from White et al. (2003); original discovery spectrum in Becker et al. (2001). (b) Spectral energy
distributions for a z = 7 quasar and a T-dwarf, compared to filter passbands from SDSS and UKIDSS.
Taken from Lawrence et al. (2007).
to be a star. For many years, until the first discovery of the very faint IR companion of GL 229 (i.e.
GL229B) by Nakajima et al. (1995), the possibility of star-like objects which never ignite nuclear burning
was only a speculation. Within a year of the start of 2MASS, Kirkpatrick et al. (1999) had found 20
brown dwarfs in the field, increasing the number of known brown dwarfs by a factor of four, and had
defined two new stellar spectral types - L and T. (These strange designations were determined by the
fact that various odd stellar types had already used up nearly all the other letters of the alphabet.)
The transition from M to L was defined by the change of key atmospheric spectral features from those
of metal oxides to metal hydrides and neutral metals; the transition from L to T by the appearance of
molecular features such as methane - as seen in solar system planets. The effective temperature for L
dwarfs is in the range T∼1500 – 2000 K, and for T-dwarfs T∼1000 –1500 K. As of the time of writing,
almost 600 brown dwarfs are known. Most of these are L-dwarfs, but almost 60 T-dwarfs have now been
found in a series of 2MASS papers (see Ellis et al. 2005 and references therein).
The much deeper UKIDSS search is expected to make significant further advances in two ways. The
first is by pushing to ever cooler and fainter objects, hopefully finding examples of a putative new stellar
class labelled ‘Y dwarfs’ (the last useable letter left ... see Hewett et al. 2006), finding T-dwarfs further
than 10pc, and plausibly finding Population II brown dwarfs if they exist. The second advance expected
from UKIDSS is the determination of the substellar mass function, through the Galactic Clusters Survey
(GCS), and testing whether it is universal or not. These hopes are already being borne out by early
UKIDSS results; Warren et al. (2007b) report the discovery of the coolest known star, classified as T8.5;
and in early results from the GCS programme, Lodieu et al. (2007) have found 129 new brown dwarfs in
Upper Sco, a significant fraction of all known brown dwarfs, including a dozen below 20 Jupiter masses,
finding the mass function in the range 0.3 – 0.01 solar masses to have a slope of index α = 0.6 ± 0.1.
Rare objects : the ionisation history of the Universe.
An excellent example of the ‘needle in a haystack’ search is looking for very high redshift quasars.
Only the most extremely luminous quasars are detectable at these distances, but the space density of
such objects is very low; even in a survey with thousands of square degrees there may be only a few
present. Luminous and high redshift quasars are interesting for a variety of reasons, but a key target
for four decades has been their use as beacons to detect the re-ionisation of the inter-galactic medium.
The baryon content of the early universe must have become neutral as it cooled down, but something
subsequently re-ionised it, as attempts to find the expected ‘Gunn-Peterson trough’ (Gunn and Peterson
1965) in the spectra of high redshift quasars had failed for many years. This finally changed in 2001 as
SDSS broke the z = 6 quasar redshift barrier (Fan et al. 2001) and Becker et al. (2001) made the first
Wide Field Surveys : A.Lawrence 9
detection of a Gunn-Peterson trough at z = 6.28. Figure 5 shows the improved spectrum from White
et al. (2003).
Unfortunately this exciting result seemed to conflict with the CMB measurements by the WMAP year-1
data. The degree of scattering required implied that ionisation had already taken place by z=11 – 30
(Kogut et al. 2003). Rather than being seen as a contradiction, it seems likely that that re-ionisation was
not a single sharp-edged event, but an extended and very likely complex affair, perhaps with multiple
stages and even spatial inhomogeneity (see White et al. 2003). This opens an entire new field of
investigation for understanding the history of the early universe. Rather than a single object locating
the transition edge, it is now important to find as many beacons as possible at z>6, and to find some
beacons in the range z = 7–8. This is one of the key aims of UKIDSS, in combination with SDSS data,
looking for z-dropouts. A problem however is that JHK colours of high-z quasars and T-dwarfs become
very similar. For this reason, UKIDSS is using a Y-band filter centred at 1.0 µm. Figure 6 illustrates
the point, comparing the spectrum of a quasar redshifted to z = 7 with that of a T-dwarf brown dwarf.
4 Next steps in optical-IR surveys
Three key optical-IR survey projects are to begin soon (VISTA, PanSTARRS, and WISE), with the
ultimate in wide-field surveys (LSST) now in the planning stage. Here I briefly summarise each of these.
VISTA.
The Visible and Infrared Survey Telescope for Astronomy (VISTA) is a 4m aperture dedicated survey
telescope on Paranal in Chile. It was originally a UK project, aimed ta bothe optical and IR surveys,
but became an IR-only ESO telescope during the accession of the UK to ESO. The infrared camera
operates at Z, Y , J , H , and KS , and contains 16 arrays each of which has 2048×2048 0.33” pixels,
covering 0.6 sq.deg. in each shot. VISTA therefore operates in the same parameter space as UKIDSS,
but will survey three times faster, and furthermore, 100% of the time is dedicated to IR surveys. The
majority (75%) of the telescope time is reserved for large public surveys. At the time of writing, these
surveys are in the process of final approval, but are likely to include a complete hemisphere survey to
K=18.5, surveys of the Galactic Bulge and the Milky Way, a thousand sq.deg. survey to K=19.5, a 30
sq.deg. survey to K=21.5, and a 1 sq.deg. survey to K=23. VISTA is expected to begin operations in
late 2007. The VISTA web page is at http://www.vista.ac.uk/, and a recent reference is McPherson et
al. (2006).
PanSTARRS.
The power of a survey facility is measured by its étendue, the product of collecting area times field
of view. The cost of a telescope, and the difficulty of producing very wide fields, scales steeply with
telescope aperture. The idea behind the ‘Panoramic Survey Telescope and Rapid Response System’
(PanSTARRS), a University of Hawaii project, is to produce the maximum étendue per unit cost by
building several co-operating wide field telescopes of moderate size. The design has four 1.8m telescopes
each with a mosaic array of 64×64 CCD chips covering 7 sq.deg, which will produce an étendue an order
of magnitude larger than the SDSS facility. As well as enabling one to produce deep surveys faster,
this makes it plausible to cover very large areas of sky repeatedly - thousands of square degrees per
night. The prime aim of PanSTARRS is to detect potentially hazardous NEOs, but it will also be used
for stellar transits, microlensing studies, and locating distant supernovae to constrain the dark energy
problem. The accumulated sky survey will be many times deeper than SDSS, and the expected image
quality and stability from Hawaii should allow the best ever mapping of dark matter via weak lensing
distortions.
A prototype single PanSTARRS system (‘PS1’) has recently been built and is being commissioned at
the time of writing. The operation and science analysis for PS1 involves an extended ‘PS1 Science
Consortium’ with additional partners, from the US, Uk and Germany. Over three years, it is expected
http://www.vista.ac.uk/
Wide Field Surveys : A.Lawrence 10
 13  13.5  14  14.5  15
Log(ν) Hz
Sky survey comparisons : Extragalactic
PS1UHSWISE
LRG z=2
quasar z=2
quasar z=7
 13  13.5  14  14.5  15
Log(ν) Hz
Sky survey comparisons : Brown dwarfs
PS1UHSWISE
T=1000K
D=30pc
M=10MJ, T=5Gyr
D=1pc
Figure 6: Spectral energy distributions of various objects compared to 5σ sensitivities of key sky surveys.
Green triangles are JHK sensitivities for a proposed extension to the UKIDSS Large Area Survey, the
UKIRT Hemisphere SUrvey (UHS). Blue circles are for the WISE mission, from Mainzer et al 2005.
Red circles (PS1) are for the PanSTARRS-1 3π survey, taken from project documentation. The left hand
frame compares extragalactic objects - a giant elliptical at z = 2; the mean quasar continuum SED from
Elvis et al 1994 redshifted to z = 2; and a high redshift quasar spectrum redshifted to z = 7. The right
hand frame compares two model brown dwarf spectra, from Burrows et al (2003, 2006). The red line
(lower curve at low frequency) is for an object with effective temperature of 1000K and surface gravity
of 4.5, placed at a distance of 50pc. The black line (upper curve at low frequency) is for an object with
mass of 10 Jupiter masses and age 5 Gyr, placed at a distance of 1pc.
to produce a 3π steradian survey at grizy to z=23 with 12 visits, a Medium Deep Survey visiting 12 7
sq.deg. fields with a 4 day cadence, building up a survey to z=26, and special stellar transit campaigns
and microlensing monitoring of M31. The data will become public at the end of this science programme.
Information about PanSTARRS can be found at http://pan-starrs.ifa.hawaii.edu/public/
WISE.
The Wide Field Infrared Survey Explorer (WISE) is a NASA MIDEX mission scheduled for launch in
2009 that will fill the gap between UKIDSS/VISTA in the near-IR and IRAS and Akari in the far-IR,
surveying the sky in four bands simultaneously (3.3, 4.7, 12, and 23µm). The sky survey at 3 and 5µm
is completely new territory; as 12 and 23µm WISE covers the same territory as IRAS but will be a
thousand times deeper. WISE carries a 40cm cooled telescope with a 47 arcmin field of view. It is
designed to have a relatively short lifetime - 7 months - but in this time will make a mid-infrared survey
of the entire sky in all four bands. WISE will produce significant advances in a number of areas, but
especially for objects expected to have temperatures in the hundreds of degrees - the very coolest brown
dwarfs, protoplanetary discs, solar system bodies, and obscured quasars. Information about WISE can
be found at http://wise.ssl.berkeley.edu/ and in Mainzer et al. (2006).
The depths of PS1, UKIDSS-VISTA, and WISE surveys are well matched and will produce a stunning
sky survey dataset over a factor of a hundred in wavelength. This is illustrated in Fig. 6, taken from a
recent proposal to extend UKIDSS to a complete hemisphere survey.
LSST.
The Large Synoptic Survey Telescope (LSST) aims at the maximum possible étendue, aiming at the
same kind of science as PanSTARRS - hazardous NEOs, GRBs, supernovae, dark matter mapping via
weak lensing - but a factor of several faster; it should be able to produce a survey equivalent to SDSS
every few days. The design has an 8.4m telescope with a 10 sq.deg. field of view. The planned standard
mode of use is to take 15 second exposures, and keep moving, covering the whole sky visible from LSST
http://pan-starrs.ifa.hawaii.edu/public/
http://wise.ssl.berkeley.edu/
Wide Field Surveys : A.Lawrence 11
in bands ugrizy once every three days. This produces 15TB of imaging data every night. The aim is
to keep up with this flow in quasi-real time, producing alerts for transient objects within minutes. This
requires approximately 60 TFlops of processing power - a huge amount today, but following Moore’s
Law, very likely to be equivalent to merely the 500th most powerful computer in the world by 2012..
The LSST data management plan has a hierarchy of archive and data centres, reminiscent of the LHC
Grid, with the primary mission facility acting like a ‘beamline’, where a variety of research groups can
rent space for their own experiments on the data flowing past. The LSST site has now been chosen
(Cerro Pachon in Chile), but the project is not yet fully funded. More information can be found at
http://www.lsst.org
5 The end of survey discoveries ?
In 1950, the universe seemed to consist of stars, and a sprinkling of dust. Over the last fifty years, the
actual diverse and bizarre contents of the universe have been successively revealed as we surveyed the sky
at a series of new wavelengths. Radio astronomy has shown us radio galaxies and pulsars; microwave
observations have given us molecular clouds and the Big Bang fossil background; IR astronomy has
shown us ultraluminous starburst galaxies and brown dwarfs; X-ray astronomy has given us collapsed
object binaries and the intra-cluster medium; and submm astronomy has shown us debris disks and
the epoch of galaxy formation. As well as revealing strange new objects, these surveys revealed new
states of matter (relativistic plasma, degenerate matter, black holes) and new physical processes (bipolar
ejection, matter-antimatter annihilation). Having opened up gamma-rays and the submm with GRO
and SCUBA, there are no new wavelength windows left. Has this amazing journey of discovery now
finished ?
Wavelength is not the only possible axis of survey discovery space. Let us step through some other axes
and examine their possibilities. In doing this, we will to some extent go over ground already trodden by
Harwit (2003), but with a particular emphasis on surveys rather than discovery space in general, and
with an eye to what is economically plausible.
Photon Flux. Historically, going ever deeper has been as productive as opening new wavelength
windows, the classic example of course being the existence of the entire extragalactic universe, which
did not become apparent until reaching ten thousand times fainter than naked eye observations, requiring
both large telescopes and the ability to integrate. We can now see things ten billion times fainter than the
naked eye stars. However, we have reached the era of diminishing returns. The flux reached by a telescope
is inversely proportional to diameter D but its cost is proportional to D3. Significant improvements can
now only be achieved with world-scale facilities, and orders of magnitude improvements are unthinkable.
The easy wins have been covered already - our detectors now achieve close to 100% quantum efficiency;
we have gone into space and reduced sky background to a minimum; and multi-night integrations have
been used many times. We will keep building bigger telescopes, but it no longer seems the fast track to
discovery.
Spectral resolution. Detailed spectroscopy of individual objects is of course the key technique of
modern astrophysics. Spectroscopic surveys of samples drawn from imaging surveys have been carried
out at many wavelengths, and have been particularly important for measuring redshift and so mapping
the Universe in 3D; we were not expecting the voids, bubbles and walls that we found in the galaxy
distribution in the 1980s. This industry will continue, but there is no obvious new barrier to break.
Narrow band imaging surveys centred on specific atomic or molecular features (21cm HI, CO, Hα) have
been fruitful, but again its not obvious there is anywhere new to go.
Polarization. Polarisation measurements of individual objects are a very important physical diagnostic,
but are polarisation surveys plausible ? Surveys of samples of known objects to the 0.1% level have
been done, with interesting results but no big surprises. Perhaps blank field imaging surveys in four
Stokes parameters would turn up unexpected highly polarised objects ? This has essentially been done
http://www.lsst.org
Wide Field Surveys : A.Lawrence 12
in radio astronomy but not at other wavelengths.
Spatial resolution. This is the dominant big-project target of the next few decades, and of course is
the real point of Extremely Large Telescopes. Put together with multi-conjugate Adaptive Optics, we
hope to achieve both depth and milli-arcsec resolution at the same time. However, the royal road to
high spatial resolution is through interferometry. Surveys with radio interferometers in the twentieth
century showed the existence of masers in space, and bulk relativistic outflow. In the twenty first
century we will be doing microwave interferometry on the ground (ALMA) and IR interferometry in
space (TPF/DARWIN), hoping to directly detect Earth-like planets around nearby stars. So there is
excitement for at least some time to come; however, as with photon flux, we are hitting an economic
brick wall. Significantly bigger and better experiments will be a very long time coming.
Time. The observation of temporal changes has repeatedly brought about revolutionary changes in
astronomy, the classic examples being Tycho’s supernova, and the measurement of parallax. The last
two decades has seen a renaissance in this area, with an impressive number of important discoveries
from relatively cheap monitoring experiments - the discovery of extrasolar planets from velocity wobbles
and transits; the discovery of the accelerating universe and dark energy from supernova campaigns; the
location of substellar objects from survey proper motions; the existence of Trans-Neptunian Objects,
and Near Earth Objects; the final pinning down of gamma-ray burst counterparts; and the limits on
dark matter candidates from micro-lensing events. The next decade or two will see more ambitious pho-
tometric monitoring experiments, such as PanSTARRS and LSST, and a series of astrometric missions,
culminating in GAIA, which will see external galaxies rotating. Overall, the ‘time window’ is well and
truly opened up. However, the temporal frequency axis is far from fully explored. My instinct is that
this technique will continue to produce surprises for some time.
Non-light channels : particles. Cosmic ray studies have been important for many decades, but
you can’t really do surveys - indeed the central mystery has alway been where they come from. Dark
matter experiments are confronting what is arguably the most important problem in physics, let alone
astrophysics, but again no survey is plausible. The big hope is neutrino astrophysics. Neutrinos should
emerge from deep in the most fascinating places that we could otherwise never see - supernova cores, the
centres of stars, the interior of quasar accretion discs. Measurement of solar neutrinos has solved a long
standing problem, and set a challenge for particle physics - but what about the rest of the Universe ?
New experiments such as ANTARES (under the sea) and AMANDA (under the ice) seem to be clearly
detecting cosmic neutrinos, but no distinct sources have yet emerged. Possibly the next generation
(ICECUBE) will get there. This looks like the best bet for genuinely unexpected discoveries in the
twenty first century.
Non-light channels : gravitational waves. Like neutrinos, we know that gravitational waves have
to be there somewhere, and their existence has been indirectly proved by the famous binary pulsar
timing experiment. However after many years of exquisite technical development, we still have no
direct detection of a gravitational wave. The space interferometer mission LISA should finally detect
gravitational waves, unless current predictions are badly wrong. However even LISA will not produce a
genuine survey. We will detect many events and understand more astrophysics, but will have essentially
no idea where they came from, except that hopefully some will correlate with Gamma-ray bursts. If we
see totally unexpected signals, it will be very hard to know what to do next.
Hyper-space planes : the Virtual Observatory. As we explore the various possible axes one by
one, many if not most of them are running out of steam, or too expensive to pursue. But we are a
long way short of exploring the whole space - for example narrow line imaging in all Stokes parameters
versus time. This exploration does not necessarily need complex new experiments. More survey-quality
datasets come on line every year. As formats, access and query protocols, and analysis tool interaction
protocols all get standardised, the virtual universe becomes easier for the e-astronomer to explore, and
unexpected results will emerge. This, of course, is the agenda of the worldwide Virtual Observatory
initiative.
Wide Field Surveys : A.Lawrence 13
6 Conclusions
Surveys are perhaps the most cost effective and productive way of doing astronomy. In recent years
optical, infra-red, and redshift surveys have produced spectacular results in determining cosmological
parameters, finding the smallest stellar objects, decoding the history of the Milky Way, and much else
besides. Surveys underway now and over the next few years should also produce impressive science.
Having been the main engine of discovery for decades, there is a worry now that we have already explored
every axis of discovery space. The best hopes for unexpected discoveries may be in massive time domain
surveys, in neutrino astrophysics, and in exploring the full multi-dimensional space through the Virtual
Observatory.
Wide Field Surveys : A.Lawrence 14
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	Why are wide field surveys important ?
	Major surveys
	Recent survey science highlights
	Next steps in optical-IR surveys
	The end of survey discoveries ?
	Conclusions
	REFERENCES
