Wide Field VLBI Imaging I (Background) Indra Bains.
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Transcript of Wide Field VLBI Imaging I (Background) Indra Bains.
Wide Field VLBI Imaging I
(Background)
Indra Bains
What is wide field imaging?
•A technique used in radio astronomy, specifically Very Long Baseline Interferometry (VLBI) :
–Radio astronomy–Interferometry –VLBI
Optical Sky
Radio Sky
408 MHz Jodrell, MPIfR, Parkes
Radio Window
Atmospheric absorption
Ionospheric reflection
H2O, CO2, O2
Image credit: NASA/IPAC
Why Do Radio Astronomy?ground-based;Fainter or invisible at other s;Physical diagnostics
~ 12 arcmin
VLA + HST; courtesy of Hubblesite
Credit: http://www.astro.umd.edu/~white/
Radio Sources
• Emission mechanisms/sources; eg:– Thermal free-free (e.g. ionized regions around SFRs &
PN)– Thermal atomic/molecular transitions (e.g. hydrogen
recombination lines in ionized regions, molecular transitions in molecular clouds)
– Synchrotron (relativistic electrons spiralling around B field lines e.g. extragalactic jets, SNRs)
– Masers (stimulated line emission eg from SFRs & OH/IR stars)
– Pulsars (pulsed radiation from rapidly spinning neutron stars)
Radio Telescopes
courtesy of the NAIC - Arecibo Observatory, a facility of the NSFATNF Mopra telescope; credit ATNF website
•Require: combination of sensitivity (large collecting area) and resolution (large aperture)•Radio dishes are diffraction limited; Rayleigh criterion = 1.22 /D• e.g. for Mopra, D = 22 m, ~ 11 arcmin at 5 GHz or Arecibo, D ~ 300 m, ~ 50 arcsec•c.f. HST ~ 100 mas
Interferometric Arrays
VLA, New Mexico, USA; image courtesy of NRAO/AUI
27-km A-array has ~ 0.6 arcsec at 5 GHz
5 telescopes of the ATCA, NSW, Australia6-km array has ~ 3 arcsec at 5 GHz
The MERLIN array, UK, with 217 km max
baseline giving ~ 60 mas at 5 GHz
Permit higher resolution imaging via earth rotation aperture synthesis; examples of connected element interferometers are:
Interferometry Credit: WSRT website
•Connected element arrays have e.g. microwave or waveguide links & are correlated in real time•Correlation: signals are multiplied and accumulated•VLBI (Very Long Baseline Interferometry) elements are not connected; data are recorded (H maser keeps time) & transported to be correlated off-line (eVLBI is nearly here in some cases!)
VLBI Arrays I
Very Long Baseline Array , USA. Credit: National Radio Astronomy Observatory / Associated Universities, Inc. / NSF
Credit: EVN website
Typically vlbi ~ few mas (c.f. HST ~ 100 mas )
VLBI Arrays II
Australian LBA. Credit: Emil Lenc
VLBI Science•Requires compact, bright structure•Science eg:
–Galactic & megamasers–Gravitational lensing–Microquasars–AGN–Starbursts–Etc etc
•FOV given by primary beam; but VLBI traditionally has a restricted FOV (< few 100 mas) due to:
–Bandwidth smearing (radial)–Time-average smearing (~azimuthal)–Computational cost (many 10s -> 100s Gb)
4’, 20 kpc
Why WFI?
•Computational cost is now ~ manageable•Advantages over ‘normal’ VLBI:
–large FOV hence more sources/observation–More sensitive (in-beam calibration,less integration overhead lost & fewer residual errors)–Better dynamic range & morphological detail
•Surveys•Larger fields of view to see what else is there
WFI Science
–Deep field sub-mJy & Jy sources (star forming vs AGN at z < 1; c.f. other s)–Surveys eg faint sub-mJy population–Mapping the SNRs in starburst galaxies–Mapping jets & hotspots in radio galaxies–Swinburne research in Emil’s talk!
Credit: M. Garrett (JIVE), T. Muxlow and S. Garrington (Jodrell Bank), EVN
EVN images of AGN in the HDF
END