Introduction
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 µm. 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 µm sky. GP-Wide is shown by a
red outline and P2P is outlined in black
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 µm 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.
Back to
top
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 µm 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.
Back to top
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
ΩΛ.
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 µm 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 µm
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
F60µm ~ 70
mJy, and so would not appear in any of the IRAS galaxy
catalogues, but they would have 850 µm 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.
Back to top
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)
Back to top