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 H 2 O, CO 2, O 2 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:

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 NSF ATNF 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, Australia 6-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

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