Centre for Astrophysics and Supercomputing

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Seeing the Big PictureExtending the
field-of-view of VLBI observations
By Emil Lenc elenc_at_astro.swin.edu.au http//astron
omy.swin.edu.au/elenc/
Supervisor Dr. Steven Tingay Assoc.
Supervisor Prof. Matthew Bailes ATNF
Co-supervisor Dr. Tasso Tzioumis Proposed ATNF
Co-supervisor Dr. Tim Cornwell

• Centre for Astrophysics and Supercomputing
• Swinburne University of Technology

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Seeing the Big PictureExtending the
field-of-view of VLBI observations

• Why is VLBI required?
• Factors limiting the VLBI field-of-view and how
  to overcome them.
• Applications of wide-field VLBI and my current
  findings.
• Future work.
• Summary.

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Why do we need VLBI?The Quest for Resolution
70 metres 1.5 arc-minute resolution (at 3cm /
10 GHz)
0.05 metres 3 arc-second resolution (at 600nm)
Just checking.
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Why do we need VLBI?The Quest for Resolution
At 50 MHz, resolution is 1.5 degrees
At 600 MHz, resolution is 7 arc minutes
305 metres
At 10 GHz, resolution is 25 arc seconds
At visible wavelengths the human eye has
a resolution of about 20 arc seconds!
Arecibo Observatory, Puerto Rico.
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The Bigger the Better?
gt 8 km !
Too Costly!
Green Bank November 15, 1988 Image courtesy of
NRAO/AUI
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The Bigger the Better? Perhaps Not!
Technically "Challenging"!
Green Bank November 16, 1988 Image courtesy of
NRAO/AUI
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The Long Baseline Array (LBA)
Baseline 1,700 km Resolution 10 mas
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The Long Baseline Array (LBA)
Baseline 3,200 km Resolution 5 mas
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Global VLBI
Baseline 12,000 km Resolution 1 mas
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Space VLBI
Baseline 35,000 km Resolution 100 µas
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VLBI Tunnel Vision

• VLBI field-of-view limited by
• Primary beam
• Bandwidth
• Time-averaging
• Non-coplanar effects
• Averaging of data leads to tunnel vision.

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Limiting FactorsThe Primary Beam
The primary beam of a radio telescope defines the
ultimate limit of its field-of-view
Primary Beam FWHM (radians) 1.22 ? / D
Example 64m Parkes Telescope at 2.3GHz
has a FWHM of 8.5.
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Limiting FactorsBandwidth Smearing
The extent of smearing depends on the parameter
v
A form of chromatic aberration. An effect which
in a similar guise affects optical telescopes.
(1 ?f / 2fo) (u2v2)1/2
(1 - ?f / 2fo) (u2v2)1/2
Synthesized beam size
u
Distance from phase centre
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Limiting FactorsBandwidth Smearing
v
Smearing effects reduced with smaller bandwidth
channels.
u
Utilizing numerous smaller channels can increase
uv-coverage
given that the source does not vary greatly
across the band.
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Limiting FactorsBandwidth Smearing
Example 566km LBA Observation at 2.3GHz, 16 MHz
bandwidth.
6.6 Field-of-view
0.5 MHz channels gt 210 Field-of-view
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Limiting FactorsTime-Averaging Smearing
Time-Averaging smearing occurs because the
baseline vector rotates with the Earth during an
observation.
v
ta
Results in complex distortions and amplitude
reductions
u
Averaging time
Distance from phase centre
Synthesized beam size
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Limiting FactorsTime-Averaging Smearing
Example 566km LBA Observation at 2.3GHz, 10s
averaging time.
1.3 field-of-view
2 second averaging gt 7.3 field-of-view
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Limiting FactorsNon-coplanar Effects
ATCA is coplanar - 2D Fourier Transform can
recover sky brightness from uv data.
For non-coplanar arrays the 2D FT is only an
approximation for small fields-of-view.
Results in complex distortions and amplitude
reductions
Distance from phase centre
Image Credit Cornwell 2003
Synthesized beam size
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Seeing the Bigger PictureOvercoming non-coplanar
effects

• Faceting
• Break the image into smaller fields each with a
  different phase centre.
• 3-D Fourier Transforms
• The non-coplanar problem is really a 3-D problem.
• Slow and very inefficient.
• W-Projection
• Projects w out of the problem and then use 2-D
  FT.
• An order of magnitude faster than faceting.

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Limiting FactorsNon-coplanar Effects
Example 566km LBA Observation at 2.3GHz.
2 field-of-view.
With w-projection, limited only by the primary
beam size and level of bandwidth and
time-averaging smearing!
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Applying Wide-field VLBI
Starbursts
Jets
Lenses
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Applying Wide-field VLBIStarburst galaxies

• Supernovae
• Star formation history
• Free-free absorption

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Applying Wide-field VLBIAGN jet interaction

• Resolve points of interaction.
• Constrain models for X-ray emission.
• Jet speed and kinetic power.

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Applying Wide-field VLBIGravitational Lenses

• Study their structure and evolution.
• Constrain mass distribution of foreground object.
• Estimate expansion rate of the Universe.

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Starburst Galaxies NGC 253
LBA (Parkes, ATCA, Mopra, Tidbinbilla)
Observation of NGC 253.
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NGC 253Comparison with other radio observations
Xs mark sources detected by Ulvestad et al
(1997) at higher frequencies.
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NGC 253 Free-Free Modelling Observed Sources
Xs mark sources detected by Ulvestad et al
(1997) at higher frequencies.
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NGC 253A Null Detection Still Tells Us Something!
Xs mark sources detected by Ulvestad et al
(1997) at higher frequencies.
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NGC 253Source 5.48-43.3
Radius of supernova 45 mas
Assuming free expansion of about 10,000 km/s this
suggests 5.48-43.3 went supernova approximately
53 years before this observation.
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More Starbursts
Have applied for LBA time to observe more
starbursts at 1.4 GHz and 2.3 GHz
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Jet Interactions Pictor A
Mystery
What's causing the residual side lobes?
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Pictor A UV Coverage
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Jet InteractionsAn AGN survey
Imaged 30 AGN at 1.4 GHZ and 2.5 GHz (selected
from Jones and McAdam, 1992).
Found 4 interesting AGN PKS B0344-345
PKS B0703-595 PKS B1308-441 PKS
B1637-771
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Future Work

• On my way to observe NGC 4945, NGC 1313 and M83
  at 2.3 GHz with the LBA AS I SPEAK!
• Will observe PKS B0344-345 and two other AGN in
  June with the ATCA at high resolution (22 GHz).
• Have approval to observe NW hotspot of Pictor A
  with the VLBA the highest ever spatial
  resolution image of an AGN hotspot.
• Stay tuned!

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Summary

• VLBI can provide high resolution images but
  unless careful only over small fields of view.
• Wide-field VLBI can increase the field-of-view
  from arc-seconds to arc-minutes.
• Wide-field VLBI can be used to tell us more about
  starburst galaxies, AGN jets and gravitational
  lenses.