Shades of SHADES - The SCUBA HAlf Degree Extra-galactic Survey

Towards the end of 2002, the first SCUBA data were taken in what is designed to be the most ambitious extragalactic survey undertaken to date at the JCMT.

This major, collaborative, UK-led survey, now named SHADES (The SCUBA HAlf Degree Extra-galactic Survey) will ultimately cover 0.5 sq. degrees to a 4sigma detection limit of S₈₅₀ = 8mJy and will in effect require approximately one third of the usable UK time on the JCMT over the next 3 years.

This survey is designed to maximise the impact of the JCMT on cosmology over this period by answering the three fundamental questions:

i) What is the cosmic history of massive dust-enshrouded star-formation activity?

ii) Are SCUBA sources the progenitors of present-day massive ellipticals?

iii) What fraction of SCUBA sources harbour a dust-obscured AGN?

To address these issues requires a wide-area survey yielding a substantial (~200) sample of bright unconfused sub-mm sources with meaningful redshift estimates (Δz ~ ± 0.5). Given this information we can answer the first question with the addition of sufficiently deep and unconfused far-infrared photometry to constrain the bolometric luminosities. Crude but near-complete redshift information is also sufficient for us to answer the second question provided our survey covers sufficient area and contains enough sources to measure the clustering of bright SCUBA sources on scales up to ~10Mpc. To answer the third question requires additional measurements of the rest-frame mid-infrared SEDs of the sources with SIRTF.

To meet these requirements our team is undertaking a combined SCUBA + BLAST survey of two well-studied fields (accessible to both facilities). Prior to the launch of Herschel, the balloon-borne far-infrared telescope BLAST offers the only means by which to measure unconfused 500µm, 350µm and 250µm flux densities for a substantial number of SCUBA-selected sub-mm sources. For those SCUBA sources detected by BLAST we will be able to determine their sub-mm-far-infrared SEDs and hence estimate their redshifts to an accuracy delta-z ~ +/- 5 (as we demonstrate from extensive simulations). The SCUBA sources not detected by BLAST are expected to lie at z > 4, and will thus form a key sample for direct spectroscopic CO redshift measurement, feasible for z > 4 sources due to appearance of two CO lines within the 35-GHz bandwidth expected to be available on the LMT and/or GBT by 2004.

The resulting dataset will provide sufficient information (even prior to completion of follow-up observations at other wavelengths) to delineate the evolution of far-infrared luminosity density as a function of redshift, and to measure the spatial clustering of SCUBA sources. Moreover, given these redshift constraints we can exploit the power of mid-infrared observations with SIRTF to constrain the fraction of SCUBA sources which house obscured AGN.

Figure: The large image shows a complete simulation of the sort of 850µm map, covering 0.5 sq. degrees, which will result from this programme. This map has been produced from an N-body simulation and the image shown here incorporates real bolometer noise from the 8-mJy survey, and the real SCUBA beam (including negative sidelobes) assuming a 30-arcsec EW chop). The strong clustering anticipated if SCUBA sources are the projenitors of massive ellipticals is clear in this synthetic image. Due to the limitations of greyscale representation, we have made this 300 Megabyte image available for detailed inspection by anonymous ftp (ftp://ftp.roe.ac.uk/pub/jsd/scuba_blast/scuba_blast.fits.gz). For comparison, the SCUBA surveys of the HDF (Hughes et al. 1998) and the ELAIS N2 field of the 8-mJy survey (Scott et al. 2002) are shown, roughly to scale, in the center and to the right respectively.

The legacy value of this survey is potentially enormous as these important basic results can be refined and enhanced through the wealth of supporting deep multi-frequency data in our chosen fields (e.g. deep XMM and VLA images of both fields are already available, and the UKIRT WFCAM Ultra Deep Survey, with supporting optical ESO surveys will be centred on the Subaru Deep Field). We also plan further follow-up from Hawaii (UKIRT, Gemini North, Subaru, Keck, CSO) and Chile (Gemini South, VLT, VISTA, and ultimately ALMA) as well as mm imaging/spectroscopy with the Mexican LMT.

We aim to complete the survey over 3 years. To achieve this the survey requires (and has now been provisionally awarded) 30 good-weather shifts on the JCMT for 6 semesters.

A prediction (i.e. a synthesized image) of the sort of results we can expect from SHADES is shown in Figure 1, which also indicates the scale of this new survey as compared with some extragalactic surveys which have already been completed at the JCMT since the advent of SCUBA.

Further details on this project can be found by visiting the SHADES website at Edinburgh (http://www.roe.ac.uk/ifa/shades). This website also provides links to the websites of the key supporting facilities involved in maximising the science output of the SCUBA data (such as BLAST and WFCAM). It also contains a full listing of the members of the international SHADES team, and links to the home pages of all collaborating institutions.

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