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Long baseline interferometry - part 1 (eMERLIN/VLBI science: (Galaxy…
Long baseline interferometry - part 1
Connected v non connected arrays:
Connected
ntennas connected by optical fibre and regulated by a common reference clock
Approach dictates when antennas can be placed - in practice, means connected
Common timestamp for all data - data can be correlated in real-time
Non-connected
Antennas not physically connected to common reference clock ; electronics locked
Data recorded on disk packs at ech antenna and shipped to correlator
Usually the only approach when antennas separated by >100s of km
Differ in baseline length
eMERLIN and european VLBI network (EVN)
eMERLIN
7 telescopes in uk
Observing modes: Lovell only, eMerlin - lovell, eMerlin + lovell
Connected array - data correlated in real time at Jodrell bank and available immediately
Open to proposals from scientific community
EVN
22 telescopes, mostly in northern europe
Extremely long baselines (High angular resolution esp. E-W) by low surface brightness sensitivity
Non-connected array - data correlated after observations at JIVE (NL)
Most telescopes independently operated - EVN as a whole only available for a few scheduled blocks
eMerlin available as part of EVN for limite periods during the year
eMerlin enhances EVN sensitivity/EVN enhances eMerlin resolution
Probes ~ mas scales; 100x sharper imaging than hubble in theory
Data recorded and shipped (Digitally and/or physically) to JIVE (NL) for correlation
eMERLIN/VLBI science:
Galaxy formation/evolution
at 1.4GHx radio surveys are dominated by two populations
Star forming galaxies (dust-unbiased view f radio emission from SNe, tracing recent star formation)
AGN tracing effects of accretion onto SMBH and the mea
Can see through dust
Decomposing nearby galaxies
VLA
Major workhorse in radio astronomy since 1980
Short baselines giving high brightness sensitivity
Unable to spatial resolve high-redshift galaxies
Use VLA to add sensitivity to eMERLIN or use eMERLIN to add resolution
eMERGE
eMERGE; an ultra deep survey to spatially resolve ~2000 tar-forming galaxies and AGN
Create the first comparison between optical (stellar light) and radio (synchrotron) sizes
On average, sycrotron emission for most galaxies traces similar extent to stellar light
Cosmology
Gravitational lensing
Exquisite control of systematics required:
PSF (fully deterministic, for interferometer as function of wavelength
Intrinsic galaxy alignments (can constrain in via polarisation)
Precise redshift (every interferometer observation is a spectroscopic observation)
No approach free from issues, but radio has "different" issues to optical/IR
Innermost regions of AGN
Accretion of gas on to SMBHs in galaxies produces intense emission
AGN radio emission comes from compact (<1kpc) core regions and/or from expanded envelopes
Cygnus A - jet axs misaligned with axis connecting hotspots
Discovery of second black hole undergoing recent flare-up
Event horizon telescope
mm-wave VLBI, delivering 10micro arcsec resolution imaging of central black hold of M87
Explosive phonomena/transients
Every interferometer observation contains frequency information
~1mas resolution of VLBI sufficient to detect radio emission from alien civilisations
Stars and stellar masers
Quadruple star-system with colliding winds
Previously unknown transient sources
Lowest upper-limits of mass-loss rates for majority of these sources
Planet Earth building blocks:
eMERLIN survey, cm-wave observations of protoplanetary disks around 20 stars
Overcoming the mm-gap - difficult to explain how mm-sized dust grains (ALMA) form cm-sized "pebbles" without falling apart
Also mm-emission likey optically-thick in the inner-region of the disk where rocky planets form
eMERLIN resolution gives 1AU sampling of distribution of dust pebbles in the regions where planets (may) form
Earth and space
All astronomica observatoin defined reltive to standardised set of coordinates
VBI - highest resolution images in all of astronomy
ICRF3 define grid of 4500 distant quasars which serve as basic reference points
Interferometry only wors if precise locations of antennas known;with reference coordinates
Future prospects
Ongoing: improvements in data transport between telescopes;better retention/distribution
Longer-term: African VBI Network and SKA VLBI - integration of excommunicated
Operational developmets: new recievers to improve frequency coverae, EVN "subarray"