2.3
Deep fields
The depth (and therefore the lookback time) of cosmology has been much ex- tended by the use of deep fields. The observation of deep fields (long integra- tion times to discover distant sources) became central to extragalactic astronomy with the 10-day Director’s Discretionary program on the Hubble Space Telescope (HST) in December 1995 (Williams et al. [1996]). Focussing on an apparently blank field in its continuous viewing zone, Hubble took images for ten consec- utive days. Instead of reaching its confusion limit18 as many expected, many
hundreds of very distant, previously unobserved galaxies were discovered. Thou- sands of galaxies are now known in the Hubble Deep Field (HDF). The brightest submillimetre source (HDF850.1) was discussed in Section 1.6 (see Figure 1.5). This initial Hubble Deep Field in the northern hemisphere (HDF-N) was later expanded to 0.124 deg2 and when follow-up multi-wavelength observations were
made with other NASA space telescopes was re-titled The Great Observatories Origins Deep Survey North (GOODS-N).
Hubble followed this with a deep field in the southern hemisphere (HDF-S). However, the premier deep field in the southern hemisphere has become the Chan- dra Deep Field South (CDF-S; an extended area is called ECDF-S), which was re-christened GOODS-S, and comprises 0.25 deg2. Another Hubble field which
extended further in depth (to magnitude mAB ∼ 29 for point sources) was the
Hubble Ultra Deep Field (HUDF; Beckwith et al. [2006]) and many other deep fields have since been explored. The main deep fields used in extragalactic cos- mology, with their central coordinates and sizes, and some of the main surveys carried out to date in each field, are listed in Table 2.1.
There is of course a trade off between the depth and area of a field when using
18The confusion limit is the flux below which sources cannot be detected individually because
of random fluctuations of the background sky brightness (from sources or other structures below the detection flux limit). It is determined by the angular resolution of the telescope (its diameter in relation to the observed wavelength) and it also varies by position on the sky (e.g. it will be affected by foreground cirrus). It is the point beyond which the sensitivity of an instrument cannot be improved by increasing the observation time because the fluctuations in the image become dominated by the the fixed pattern noise from astronomical sources instead of by random noise from the instrument and the sky. A discussion of confusion is given in Condon [1974].
2.3 Deep fields
Table 2.1: Well-studied extragalactic deep fields (in order of declination). Chapters 3 to 5 deal with the ADF-S; Chapter 6 covers sources selected from several of these deep fields.
Deep Field Centre (J2000) Size Selection of Key Deep Surveys RA (h m s) Dec (d m s) deg2
AKARI-NEP 17 55 24 +66 37 32 0.5 AKARI, WSRT, CFHT, Herschel-NEP, Subaru HDF-N (GOODS-N, CDF-N) 12 36 49 +62 12 58 0.04-0.124 HDF, SCUBA, Chandra, XMM-Newton, FIDEL,
HerMES , PEP, CANDELS, GOODS-Herschel Spitzer FLS 17 18 00 +59 30 00 4.0 xFLS, HerMES
Lockman Hole: ISO Deep Field 10 52 43 +57 28 48 1.2 SWIRE, SHADES, ISOPHOT, PEP, HST/WFPC2, XMM-Newton, VLA, DXS, HerMES, ROSAT ELAIS N1 16 08 44 +56 26 30 2.74 ELAIS, SWIRE, DXS, HerMES
Extended Groth Strip 14 17 52 +52 29 46 1.0 FIDEL, CUDSS, AEGIS, HerMES, CFHTLS-D3, CANDELS, SCUBA2 CFRS-14 14 17 49 +52 30 23 (bisects the Groth Strip)
SSA 13 13 12 17 +42 38 05 0. 3 VLA, Subaru/Subprime-Cam, ISOCAM ELAIS N2 16 39 44 +41 15 43 2.98 ELAIS, SWIRE, HerMES
Bo¨otes / ELAIS N3 14 32 06 +34 16 48 9.3 XBo¨otes, ELAIS, NDWFS, Spitzer 24 HerMES, SCUBA2
NGP 13 18 00 +29 00 00 150 H-ATLAS, GAMA Subaru Deep Field 13 24 39 +27 29 26 0 26 Subaru, UKIRT, GALEX
COSMOS 10 00 29 +02 12 21 2.0 COSMOS, COSBO, VLA, VLT, Spitzer, CFHTLS-D2, Subaru, XMM-Newton, HerMES,
UltraVISTA, PEP, CANDELS GAMA A 09 00 00 00 00 00 36 GAMA, H-ATLAS, GALEX GAMA B 12 00 00 00 00 00 36 GAMA, H-ATLAS, GALEX GAMA C 14 30 00 00 00 00 36 GAMA, H-ATLAS, GALEX WHDF 00 22 30 +00 21 00 0.014 WHDF
VIMOS 4 / SSA 22 22 17 00 +00 20 00 8.75 VVDS, DXS
VVDS 02 26 00 -04 30 00 2.0 VVDS, VVDS Radio, SWIRE, DXS, CFHTLS-D1, HerMES
Cetus 02 10 00 -04 30 00 9.2 NDWFS
XMM-LSS-SXDF / UDS 02 18 00 -07 00 00 1.3 SWIRE, SXDS, VLA, SHADES, DXS, UDS, HerMES, CANDELS
NTT 12 05 20 -07 44 20 0.0015 NTT-SUSI
CFHTLS-D4 22 15 31 -17 43 56 0.74 CFHTLS, XMM-Newton
HUDF 03 32 39 -27 47 00 0.003 HUDF, UDF09, UDF12, CANDELS, XDF CDF-S (GOODS-S, ECDFS) 03 32 28 -27 48 30 0.11 - 0.25 CDFS, SWIRE, BLAST, LABOCA, FIDEL,
GEMS, LABOCA, HerMES, PEP, SCUBA2, CANDELS, GOODS-Herschel
FORS Deep Field (FDF) 01 06 04 -25 45 46 0.01 VLT-FORS SGP B 23 15 36 -32 54 00 66 H-ATLAS, GAMA SGP A 02 26 48 -33 00 00 66 H-ATLAS, GAMA ELAIS S1 00 38 24 -43 32 02 4.15 ELAIS, SWIRE, HerMES ADF-S 04 44 00 -53 20 00 12 AKARI, BLAST, Spitzer, AzTEC,
ATCA, HerMES, Herschel-PACS SSDF 23 30 00 -55 00 00 94 Spitzer, SPT, Herschel-SPIRE, XMM,
VISTA, ATCA
Marano 03 15 09 -55 13 57 1 Studies with ISOCAM, VLT, ATCA etc HDF-S 22 32 56 -60 33 03 0.001 HDF-S
2.3 Deep fields
a telescope’s time. Whilst wide fields are essential for understanding large scale structure and anisotropies in the universe, deep fields are essential for studies extending back to high redshifts. There is also a trade-off between fields with low foreground contamination (e.g. away from the galactic plane), and easy acces- sibility by ground-based instruments (available to a wider range of instruments nearer the equator). Foreground contamination may include emission from the Solar System (e.g. zodiacal thermal emission from dust and asteroids), from the Galaxy (from Galactic cirrus and from local stars, although at least a few fore- ground stars are needed for accurate calibration) and from known extragalactic sources. Absence of very bright optical sources (e.g. local bright stars or clusters of galaxies) is a factor in selecting which part of the sky is observed. Some deep surveys have selected several non-contiguous regions of the sky (e.g. ELAIS which comprised six regions across both hemispheres, and HerMES which included al- most all of the well-known deep fields).
The Hubble Space Telescope Frontier Fields project (another Director’s Dis- cretionary observing time project) has recently started a 3-year program to make deep observations in six fields centred on strongly lensing galaxy clusters at 0.3 < z < 0.55. The aim is to make use of the magnification caused by gravi- tational lensing19 to detect the earliest galaxies back to z ∼ 10 (age of Universe
< 0.5 Gyr)(Richard et al. [2014]). The first data was released in July 2014 (Coe et al. [2015]; Ishigaki et al. [2015]); observations are scheduled to run to July 2015.
The work in Chapters 3, 4 and 5 is based on ADF-S, one of two deep fields chosen by AKARI (Matsuhara et al. [2006]), the other being near the north ecliptic pole, AKARI-NEP. The two AKARI fields were chosen as particularly low cirrus regions. The work in Chapters 6 and 7 is based on a selection of sources in various well-known deep fields.