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