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The SCUBA-2 "All-Sky" Survey


Of all the wavebands in the electromagnetic spectrum the most poorly surveyed to date remains the sub-millimetre, despite the being one of the most high-impact areas in observational astronomy (as evidenced by the high citation counts for JCMT papers and the development of the ALMA project). Only a tiny fraction of the sky has been surveyed at high angular resolution in the sub-mm; while COBE produced a full-sky map in the sub-mm it did so with only 7 angular resolution. The SCUBA-2 "All-Sky" Survey, or SASSy, is a JCMT Legacy Survey project designed to redress this balance and exploit the rapid mapping capability of SCUBA-2 to ultimately map the entire sky visible from the JCMT to an angular resolution of 14" at 850 m. The benefits of such a wide-field survey are many, ranging from a complete census of infrared dark clouds (IRDCs) to the potential discovery of some of the most luminous high-redshift galaxies in the Universe.

SASSy comes in two phases: a pilot phase in which approximately a quarter of the sky visible from the JCMT will be mapped in the form of two 10 wide strips; and an extended phase where the mapping strategy to complete the remaining sky is defined by the discoveries made in the pilot phase. The two strips that will be mapped in the pilot phase are shown in the figure below. The first is centred upon the galactic plane and is known as GP-Wide. The second, known as Pole-to-Pole or P2P, is oriented perpendicular to the galactic plane and passes through the North & South Galactic Poles and the North Ecliptic Pole. Each of these strips will be mapped down to a sensitivity of 30 mJy/beam. Following the completion of this phase our aim is to survey the remaining sky to the same depth, although the precise details will be determined from the results of the pilot phase.

SASSy has been awarded 500 hours of time on the JCMT to undertake the pilot phase and will commence in the 2007B semester.

Figure 1: The regions that will be mapped during the pilot phase of SASSy overlaid on an all-sky
projection of the IRAS 100 m sky. GP-Wide is shown by a red outline and P2P is outlined in

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Survey Design

SASSy is optimised to fully exploit poorer weather conditions in order to minimise the impact of such a large survey on the rest of the JCMT Legacy Survey programme. The central role of SASSy is to act as a detection experiment and identify infrared dark clouds, star-forming cores & dusty high-redshift galaxies in as unbiased a manner as possible. By only working at 850 m we will use the new capabilities of SCUBA-2 (improved sky noise cancellation, sensitivity and water vapour calibration) to work in less favourable weather bands than traditionally used for SCUBA operations. SASSy will operate in the grade 4 weather band, i.e. 0.12 < τ(225GHz) < 0.2. By running SASSy in this weather band we also provide a fallback project at essentially all Right Ascensions for those surveys that require more demanding weather conditions.

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Galactic Science Goals

SASSy aims to answer the following specific Galactic science questions:

How many IRDCs are there in our Galaxy and how are they distributed?
The distribution of IRDCs mirrors that of the galactic mid-IR background, thus it is not possible to determine the latitude dependence of IRDCs, their numbers in the outer Galaxy where the mid-IR background is low, nor their numbers in the inner Galaxy where more distant clouds are lost against the bright emission of the Galactic Plane. As the emission level of the clouds is at or below the MSX noise floor, with MSX data alone it is only possible to determine a lower limit to the opacity and hence column density of the clouds. The GP-wide strip of SASSy is perfectly matched to the MSX and UKIDSS Galactic Plane Survey regions (|
b| ≤ 5). We will detect over 3000 IRDCs by their sub-mm emission in this region, as well as those in the outer Galaxy missed by MSX, and will determine whether the fall-off in galactic latitude is a real effect or background induced.

What is the relation of IRDCs to star formation and Galactic structure?
With 850 m fluxes from SASSy and MSX constraints we will initially determine an upper limit to the temperature of these clouds that will be refined by later ASTRO-F far-IR detections and/or Herschel follow-up from either pointed observations or the proposed HiGAL survey ( Molinari et al 2005). In combination with distances determined either from ancillary UKIDSS or 2MASS photometry (Maheswar et al. 2004) or via follow-up millimetre-wave spectroscopy we will measure the mass, column density and galactic position of each IRDC. The GP-wide portion of SASSy will comprise a catalogue of over 3000 IRDCs with well determined basic properties, allowing us to investigate whether these objects are as a whole indeed associated with the early stages of high-mass star formation. As IRDCs occupy a much smaller region of phase space (i.e. possess much simpler kinematics) then either stars or GMCs, it is possible to use the IRDCs as ``test particles'' to infer the Galactic potential well. Such studies will be a valuable tool to reveal the underlying structure of our Galaxy, especially in the outer galaxy where our knowledge of the Galactic potential is poor.

Is there an underlying unknown population of star formation?
The P2P section of SASSy will allow a first attempt at answering this question. By searching for dense cores over a range of galactic latitudes that cut through a number of known cirrus clouds we will be able to determine the number of star-forming cores in this region that are similar to those identified by Heithausen et al. (2002) in high-latitude cirrus clouds. We will cross-correlate the positions of the detected cores with those of known clouds from the Dobashi et al. (2005), Dame et al. (2001) and Magnani et al. (1996) catalogues to infer whether these cores indicate the presence of as yet unknown molecular clouds. We will also establish the temperature, mass, column density and distance to each detected core (using photometric distances where possible for high-latitude objects).

What is the fraction of clustered vs isolated star formation?
The contrast between the local star forming cores identified in the GP-wide survey region with those isolated cores associated with cirrus clouds in the P2P region will allow us to begin to investigate their differences with respect to clustered star formation in the well-known molecular clouds. For example, do the physical properties of isolated (possibly high-latitude) protostellar cores vary from those found in more clustered regions like Orion? However, only by extending the GP-wide and P2P regions to encompass a larger fraction of the sky will we be able to quantify the fraction of star formation found in each environment.

What is the answer to the distributed T-Tauri problem?
An unbiased volume-limited sample of protostellar cores will enable us to locate the early stages of star formation at their birthplaces, rather than some 3 Myr after they have formed (Feigelson 1996). Only by identifying the birthplaces of the field T-Tauris can we solve the distributed T-Tauri problem and set tight constraints upon molecular cloud lifetimes and sizes.

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Extragalactic Science Goals

Is there an undiscovered population of extreme luminosity objects?
The favourable K-corrections in the sub-mm mean that SASSy is sensitive to galaxies across the Hubble volume and the 2000 sq. degrees of the P2P strip at z > 0.5 comprises a co-moving volume some 6x that of the entire z < 0.5 Universe. With SASSy we therefore have the potential to find galaxies that are too rare to have any local counterparts.The depth of SASSy is carefully optimised to detect extragalactic populations that are inaccessible to deeper and narrower cosmological surveys and the bright (~150 mJy) galaxies that SASSy is sensitive to are expected to be the most luminous and briefest phases in the star formation history of galaxies (e.g. Blain et al 2004).

What are the number counts of bright sub-mm galaxies?
Historically the existence and number densities of the most luminous galaxies have provided some of the most demanding challenges to semi-analytic models of galaxy evolution. Accounting for the mJy-level SCUBA starburst galaxy population has been a major theme of semi-analytic descriptions of galaxy evolution in the past few years. The number counts of brighter galaxies (in the > 100 mJy regime) are completely unknown; current models (e.g. Pearson 2005, Granato 2001, Rowan-Robinson et al 2001) predict counts that differ by up to several orders of magnitude. The depth and area of SASSy are selected to resolve this issue.

What is the fraction of lensed sub-mm sources?
Gravitational lensing statistics are a potentially powerful probe of the geometry of the Universe. The extremely steep source counts of sub-millimetre galaxies may make them strongly susceptible to gravitational magnification bias and some models imply that up to around ~ 40% of galaxies at the ~ 100 mJy level may be strongly lensed (e.g. Perrotta et al. 2003). If so, bright sub-millimetre galaxy
surveys will be an extremely efficient and unprecedented selection method for strong lenses, as well as having the advantages of immunity to dust extinction effects and the benefit of well-understood SEDs in this wavelength range. SASSy is ideally placed to find populations of bright, lensed sub-mm sources and to use them to place new constraints on cosmological parameters such as Ω

The Planck Connection?
The Planck satellite will provide an all-sky survey (although deeper at the Ecliptic Poles), with channels at 857, 545, 353, 217, 143 and 100 GHz in the mm/sub-mm, as well as 70, 44 and 30 GHz at longer wavelengths. The eventual end-of-mission sensitivity that Planck is capable of reaching in the 850 m band (353 GHz) is similar to that planned by SASSy. The production of the final Planck point source catalog as well as the Planck data on the CMB requires that each separate emission component, be it galactic dust, cirrus emission, foreground galaxies, S-Z clusters or the microwave background itself, must be separated from all the others. By comparing the Planck component-separated maps for the P2P region to the 850 m map of SASSy we will be able to test the separation process and provide an informed measurement of the contribution of the galactic foreground at small scales to the Planck CMB team. SASSy is uniquely powerful in this regard since it will be able to sample the sky over the entire range of galactic latitudes at resolutions $20$ times higher than the Planck maps.

Is there an undiscovered population of cold local galaxies?
Sub-mm surveys of the local Universe have so far mainly been based upon the IRAS point source catalogue (e.g.~SLUGS; Dunne et al.~2000) or on HI catalogues (e.g.~SINGS). Recently, searches for Type Ia supernovae hosts have been uncovering galaxies dominated by cold ~ 20 K dust, have been found with high L850 at z=0.5 (Farrah et al. 2004). Similar galaxies at z=0.1 would have F60m ~ 70 mJy, and so would not appear in any of the IRAS galaxy catalogues, but they would have 850 m fluxes detectable by SASSy. Are there populations of cold, ultraluminous galaxies in the local (i.e. z ~ 0.1) Universe? The only way to determine this is with a large area, shallow survey such as SASSy.

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The SASSy Consortium

Membership of the SASSy Consortium is still open. Until survey operations have begun researchers from JCMT partner countries (UK, Canada, the Netherlands) can apply for membership by contacting the survey coordinators. Memberships is also open to potential members from non-partner countries on a case-by-case basis. Non-partner country members will normally need to demonstrate an “added-value” to the consortium for membership to be granted. The decision to admit new members will be made by the coordinating team.

Coordinating Team:
Mark Thompson (University of Hertfordshire), Steve Serjeant (Open University), Tim Jenness (Joint Astronomy Centre), Douglas Scott (University of British Columbia)

Consortium Members:
George J. Bendo (Imperial College London), Chris Brunt (University of Exeter), Harold Butner (Joint Astronomy Centre), Antonio Chrysostomou (University of Hertfordshire), Dave Clements (Imperial College London), Jim Collett (University of Hertfordshire), Kristen Coppin (University of British Columbia), Iain Coulson (Joint Astronomy Centre), Bill Dent (UKATC), Frossie Economou (Joint Astronomy Centre), Nye Evans (Keele University), Per Friberg (Joint Astronomy Centre), Andy Gibb (University of British Columbia), Jane Greaves (University of St Andrews), Jennifer Hatchell (University of Exeter), Wayne Holland (UKATC), Mike Hudson (University of Waterloo), Andrew Jaffe (Imperial College London), Hugh Jones (University of Hertfordshire), Johan Knapen (University of Hertfordshire), Jamie Leech (Joint Astronomy Centrre), Bob Mann (University of Edinburgh), Henry Matthews (HIA/NRC Canada), Toby Moore (Liverpool John Moores University), Ange Mortier (University of Kent), Dave Nutter (Cardiff University), Chris Pearson (ESA/JAXA), Michele Pestalozzi (University of Hertfordshire), Alexandra Pope (University of British Columbia), John Richer (University of Cambridge), Russell Shipman (SRON/Kapteyn Institute), Mattia Vaccari (Imperial College London), Ludovic Van Waerbeke (University of British Columbia), Serena Viti (University College London), Bernd Weferling (Joint Astronomy Centre), Glenn White (University of Kent/Open University/RAL), Jan Wouterloot (Joint Astronomy Centre), Ming Zhu (Joint Astronomy Centre)

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Contact: Iain Coulson. Updated: Wed Feb 14 12:27:54 HST 2007

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