SKA Working
Group 1: Milky Way and Nearby Galaxies
Compliance Matrix for Level 0 Science
Goals
Numerical scale calibration :
5 = complete compliance
3 = compliance could be reached with significant (but not fundamental)
changes to the proposed design
1= no possible compliance
Links to earlier versions: The SKA Concept
SKA designs-requirements
matrix: HI Preliminary
all-WG compliance matrix
HI in the Universe
HI (neutral atomic hydrogen) is the fundamental tracer of neutral gas in the universe at all times and on all scales. The SKA will provide an
unprecedented view of the structure and motion of gas inside and outside
of galaxy disks. The SKA
will be able to survey HI 21-cm emission from large numbers of spiral and
dwarf irregular galaxies
(out to z~1), and to map
the distribution of the HI in the Milky Way at unprecedented resolution
and sensitivity. This encompasses
level-1 science for working groups 1, 3 and 4.
General comments
on design compliance for HI science :
A hypothetical mapping project
would need brightness temperature sensitivity 1 K over spectral channel
width 5 kHz with beam size 10 arc seconds. We should be able to survey
at a rate of 1 square degree per hour or faster with this combination of
sensitivity and resolution. At 21-cm wavelength this requires a large
fraction of the collecting area (~50%) at baselines shorter than 5 km.
* Frequency range: 1.4 GHz only
* Multibeaming: not needed provided the field of view requirement is
met.
* Array configuration: continuous u-v coverage needed giving angular
resolution over the full range from 1 arcsec to 30 arcmin at 1.4 GHz (20m
to 40 km baselines).
* Field-of-view: minimum of 1 square degree for core of array at 1.4
GHz with full brightness sensitivity. A significant fraction of 1 square degree must
be able to be imaged for baselines out to 50 km
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HI specific notes:
Chinese design for HI
COMPLIANCE RATING: 2
COMMENTS:
* The field of view is insufficient for imaging local galaxies,
or for surveying regions of the Milky Way.
* This science goal requires continuous u-v coverage giving resolution
from 1 arc second to ~30 arcmin. The gap in spacings between 200-1000 metres,
is a major problem for mapping HI, which contains power on all spatial scales.
* It would be extremely beneficial to be able to do 21cm spectral line
and continuum simultaneously
* Spacings shorter than 200m seem only to be available by making single-dish
measurements and then combining them with the interferometer data at the
reduction stage. This will limit dynamic range when imaging large faint structure.
* The main specification of HI surveys is to reach a brightness sensitivity
of 1K in survey mode (approx 1 deg^2/hr) at a resolution substantially better
than 1 arcmin. It will be challenging to achieve these goals with this design
to achieve a brightness sensitivity of 1K with any subset of the array because
of the lack of close packing.
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European design for HI
COMPLIANCE RATING: 4
COMMENTS:
* What is the u-v coverage within the core? If only individual stations
correlated, sensitivity to large spatial scales will be limited.
* While the brightness sensitivity in this design is limited by small
collecting area of core, multibeaming makes up for this, letting one
survey several areas at once.
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Indian design for HI
COMPLIANCE RATING: 5
COMMENTS:
* While available specifications for this design meet our requirements,
more information is needed on array configurations etc.
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USA design for HI
COMPLIANCE RATING: 4
COMMENTS:
* While the brightness sensitivity of this design is limited at intermediate
resolutions, the very dense core ensures that efficient HI
surveying can still be done.
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Canadian design for HI
COMPLIANCE RATING: 3
COMMENTS:
* This science goal requires continuous sensitivity at resolutions from
1arc second to ~30 arcmin. The gap in spacings between 200-1000 metres is
a major problem for mapping HI, which contains power on all spatial scales.
* It would be extremely beneficial to be able to do 21cm spectral line
and continuum simultaneously
* Spacings shorter than 200m seem only to be available by making single-dish
measurements and then combining them with the interferometer data at the
reduction stage. This will limit dynamic range when imaging large faint structure.
* The main specification of HI surveys is to reach a brightness sensitivity
of 1K in survey mode (approx 1 deg^2/hr) at a resolution
substantially better than 1 arcmin. It will be challenging to achieve
these goals with this design to achieve a brightness sensitivity of 1K
with any subset of the array because of the lack of close packing.
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Australian Cylinder design
for HI
No problems noted.
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Australian Luneberg Lens
design for HI
No problems noted.
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The Magnetic Universe
Magnetic fields are
fundamental to most physical processes in the Universe, but are usually overlooked.
The techniques and processes with which one can easily map the strength
and structure of magnetic
fields are unique to radio astronomy. The SKA will be able to map the strength and direction
of the magnetic fields in the Milky Way and nearby galaxies in several ways.
These include Zeeman splitting of the 21-cm and OH maser lines, measurement
of the linear polarization of synchrotron emission (both in discrete sources
and the diffuse Galactic background), and measurement of the Faraday rotation
of linear polarization toward pulsars, extragalactic sources, and the diffuse
synchrotron emission. Some of these applications require very high brightness
sensitivity, including good sensitivity to very short uv
spacings. This topic encompasses level-1
science goals for working groups 1, 2, 6 and 8.
General comments on design compliance:
A hypothetical polarization
mapping project would need brightness temperature sensitivity 0.01 K over
bandwidth 5 MHz with beam size 10 arc seconds. We should be able to
survey Stokes Q, U, and V at a rate of 1 square degree per hour or
faster with this combination of sensitivity and resolution. This requires
about half the total collecting area be in a central core with diameter ~
5 km.
* Frequency range: 0.4-8 GHz (0.4-12 GHz desirable)
* Multibeaming: not needed
* Array configuration: continuous u-v coverage needed over scales giving angular resolution over the
full range of angular resolution ~0.5 arcsec to 30 arcmin at 1.4 GHz
(20m to 40 km baselines).
* Field-of-view: 1 square degree at 1.4 GHz; need to be able to image
full FOV out to baselines of 100 km.
* Other: Polarization purity of -40 dB needs to be attainable over
the entire field of view. -30 dB should be attainable in hardware, with
a further 10 dB after calibration.
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Polarization specific notes:
Chinese design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 2
COMMENTS:
* The field of view is insufficient for imaging local galaxies, or for
surveying regions of the Milky Way.
* This science goal requires continuous sensitivity with resolution from
0.5" to ~30' for mapping diffuse emission in other galaxies, as well as for
studying individual objects in our own Galaxy. Also, the relatively small
number of elements (30) might limit the continuity of the u-v coverage at
longer spacings.
* Spacings shorter than 200m seem to only be available by making single-dish
measurements and then combining them with the interferometer data at the
reduction stage. Calibration will need to be extremely accurate for polarimetry
studies; also, this will limit dynamic range when imaging large faint structure.
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Indian design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 4
COMMENTS:
* Can a 1 deg^2 FOV be achieved at 0.5 arcsec resolution (at 1.4 GHz)?
Or will the antenna elements be grouped into stations?
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European design for Galactic magnetic fields
& polarization
COMPLIANCE RATING: 1
COMMENTS:
* The limited frequency coverage of this design will make it very difficult
to study polarization, because of heavy depolarization at
lower frequencies.
* This science goal requires continuous sensitivity with angular resolution
from 0.5" to ~30'. It needs to be determined whether the 70 non-VLBI stations
in this design can provide sufficient u-v coverage and dynamic range to meet
this goal.
* Can a 1 deg^2 FOV be achieved at 0.5 arcsec resolution (at 1.4 GHz)?
Or does the station concept limit the FOV?
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Australian lens design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 3
COMMENTS:
* The limited frequency coverage of this design will make it difficult
to study polarization, because of depolarization at frequencies below
~8-10 GHz.
* The station size of 250 metres limits the FOV at subarcsec resolution
to << 1 deg^2
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Australian cylinder
design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 4
COMMENTS:
* The offset line feed design (preferred) may have difficulty providing
the required polarization performance. Asymetrical beam patterns in
Stokes Q and U may be unavoidable.
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USA design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 3
COMMENTS:
* The station size of 84 metres limits the FOV at subarcsec resolution
to << 1 deg^2
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Canadian design for Galactic magnetic fields & polarization
COMPLIANCE RATING: 3
COMMENTS:
* This science goal requires continuous sensitivity with angular resolution
from 0.5" to ~30' for mapping diffuse emission in other galaxies, as well
as for studying individual objects in our own Galaxy. Also, the relatively
small number of elements (60) might limit the continuity of the u-v coverage
at longer spacings.
* Spacings shorter than 200m seem to only be available by making single-dish
measurements and then combining them with the interferometer data at the
reduction stage. Calibration will need to be extremely accurate for polarimetry
studies; also, this will limit dynamic range when imaging large faint structure.
* Polarimetry studies can greatly benefit from simultaneous observations
in two separate frequency bands, for Faraday rotation
studies. With only one independent IF, one can do these observations
in serial fashion, but at lower efficiency.
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