oct 16, 2008, sfig, zhiyu zhang, seminar 2008 introduction of radio interferometry and the evla...

Oct 16, 2008, SFIG, Zhiyu Zhang, Seminar 2008

Introduction of Radio Interferometry

and the EVLA

Zhiyu Zhang

Oct 16, 2008, SFIG, Zhiyu Zhang

Introduction of interferometers Fundamental of interferometry

Next generation radio interferometers

Introduction of E-VLA

Science cases

Summary

OutlineOutline


Introduction of interferometers

Real time

m–cm: WSRT, GMRT, VLA, ATCA, etc.

Mm: CARMA, PdBI, etc.

Sub-mm: SMA etc.

VLBI

E-MERLIN, EVN, VLBA, LBA, Space-VLBI(VSOP) etc.

Optical: VLT-I, and?


Advantage and shortcoming

Best resolution – VLBI ~10-6 arcsec, Much better than single dish telescopes at the same band!!!

Stable -- even part of it can work.

Flat baseline – much better than single dish

3-D data cube -- 4-D information. position, intensity, and spectral line.

Tracing position – Spacecrafts, asteroid, etc.

Rapid response – Pulsar

Very long time integration – more than one day, can be thousands of hours____________________________________

Missing flux -- can be partly adjusted

Expensive -- cost for correlators

Deconvolution algorithm -- fast Fourier transition, not only one solution

Complicated calibration – phase, flux, bandpass etc.

Data reduction – hard to study


FundamentalFundamental

Angular resolution ~ λ/B & frequency

Resolution -- depends on shortest & longest baseline

Field of view (FOV) -- equal to single dish main beam

UV-coverage -- depends on earth rotation, and configuration of antennas. interpolating & imaging quality

Antenna configuration – beam response and better UV-coverage

Phase Calibration -- calibration for position

Bandpass calibration -- calibration for spectra

Flux calibration -- calibration for flux

De-convolve algorithm -- CLEAN, MEM, Hybrid, etc.


DSingle dish: = /D

B

Interferometer: = /B

Scheme


combine signals from two antennas separated by baseline vector b in a correlator; each sample is one “visibility”

• each visibility is a value of the spatial coherence function V (b) at coordinates u and v obtain sky brightness distribution by Fourier inversion:

s

b

Visibility


UV-Coverage

ALMA snapshot

Central hole


Clean Imaging

Weighting

Self-calibration

Resampling

3D-2D

Algorithm:

CLEAN

MEM

Hybrid


Introduction of a few of next generation interferometers

ATA

EVLA

ALMA

LOFAR

SKA

350x 6m64 x 12m

25000 elements

one square kilometer


Introduction of the VLA

Built 1970’s, dedicated 1980,27 x 25m diameter antennasTwo-dimensional 3-armed array designFour scaled configurations, maximum baselines 35, 10, 3.5, 1.0 Km. Eight bands centered at 0.074, 0.327, 1.4, 4.6, 8.4, 15, 23, 45 GHz100 MHz total IF bandwidth per polarizationFull polarization in continuum modes.Digital correlator provides up to 512 total channels – but only 16 at maximum

bandwidth.

VLA in D-configuration(1 km maximum baseline)


Introduction of the EVLA

Sensitivity: Continuum sensitivity improvement over the VLA by factors of 5 to 20, to give point-source sensitivity better than 1 microJy between 2 and 40 GHz.

Frequency Accessibility: Operation at any frequency between 1.0 and 50 GHz, with up to 8 GHz bandwidth per polarization.

Spectral Capability: Full polarization (8 GHz bandwidth per polarization), with a minimum of 16,384 channels, frequency resolution to 1 Hz, and 128 independently tunable sub-bands.

Resolution: Angular resolution up to 200 / (frequency in GHz) milliarcseconds with tens of Kelvin brightness temperature sensitivity at full resolution.

Low-Brightness Capability: Fast, high fidelity imaging of extended low-brightness emission with tens of arcseond resolution and microKelvin brightness sensitivity.

Imaging Capability: Spatial dynamic range greater than 106, frequency dynamic range greater than 105, image field of view greater than 109 with full spatial frequency samplng.

Operations: Dynamic scheduling, based on weather, array configuration, and science requirements. "Default" images automatically produced, with all data products archived.


Ultrasensitive Array

New Mexico Array

VLA

by 2012

Two Phases

Ten new antennas

Range up to 250 Km from EVLA

+WIDAR

Wideband Interferometric Digital ARchitecture

Receivers

PHASE I

PHASE IIProposed


EVLA Phase I - Key Science Examples Measuring the three-dimensional structure of the Sun's magnetic field Mapping the changing structure of the dynamic heliosphere Measuring the rotation speed of asteroids Observing ambipolar diffusion and thermal jet motions in young stellar objects Measuring three-dimensional motions of ionized gas and stars in the centre of the Galaxy Mapping the magnetic fields in individual galaxy clusters Conducting unbiased searches for redshifted atomic and molecular absorption Looking through the enshrouding dust to image the formation of high-redshift galaxies Disentangling starburst from black hole activity in the early universe Providing direct size and expansion estimates for up to 100 gamma-ray bursts every year

Main Science Projects

EVLA Phase II - Key Science Examples AU-scale imaging of local star forming regions and proto-planetary disks Resolving the dusty cores of galaxies to distinguish star formation from black hole accretion Imaging at the highest resolution at any wavelength of the earliest galaxies (z~30) Imaging of galaxy clusters with 50 kpc or better resolutions at arbitrary redshifts Imaging of thermal sources at milliarcsecond scales Resolving individual compact HII regions and supernova remnants in external galaxies as distant as M82 Tying together the optical and radio reference frames with sub-milliarcsecond precision Measuring accurate parallax distances and proper motions for hundreds of pulsars as distant as the Galactic Center Providing 50 pc or better resolution for galaxies at any redshift Monitoring and imaging the full evolution of the radio emission associated with X-ray and other transients


Resolution vs. Frequency VLA vs. EVLAA key EVLA requirement is continuous frequency coverage from 1 to 50 GHz.This will be met with 8 frequency bands:

Two existing (K, Q)Four replaced (L, C, X, U)Two new (S, A)

Existing meter-wavelength bands (P, 4) retained with no changes. Blue areas show existing coverage. Green areas show new coverage.


Resolution vs. Frequency VLA vs. EVLA


NOISE regimes


Parameter VLA EVLA FactorPoint Source Sensitivity (1-, 12 hours) 10 Jy 1 Jy 10

Maximum BW in each polarization 0.1 GHz 8 GHz 80

Frequency channels at max. bandwidth 16 16,384 1024

Maximum number of frequency channels 512 4,194,304 8192

Coarsest frequency resolution 50 MHz 2 MHz 25

Finest frequency resolution 381 Hz 0.12 Hz 3180

(Log) Frequency Coverage (1 – 50 GHz) 22% 100% 5

Sensitivity, Bandwidth & Frequency resolution


1δ, 12 h integrationSensitivity


Continuum Sensitivity vs. Frequency

VLA

EVLA

ALMA


SKA will have

A sensitivity

of hundred times

of the VLA

Arp 220 as a template of High Z galaxies


4 P L C X U K Q

F (GHz) 0.073-0.0745 0.3-0.34 1.34-1.73 4.5-5.0 8.0-8.8 14.4-15.4 22-24 40-50

(cm) 400 90 20 6 3.6 2 1.3 0.7

(′) 600 150 30 9 5.4 3 2 1

(″) 24 6 1.4 0.4 0.24 0.14 0.08 0.05

Flx(mJy, 10 min)

150 1.4 0.056 0.054 0.045 0.019 0.10 0.25

T(K) 103-104 150-180 37-75 44 34 110 50-190 90-140

Full Band Coverage 1 ‘continuum’ (maximum sensitivity) observations

2 spectral line surveys


FIRSTA. Faint Images of the Radio Sky at

Twenty-centimetersB. Flux limit = 1mJyC. Resolution limit = 5″D. ~90/sq degreeE. Coincide with SDSS

NVSS (1993.9-1996.10)A. NRAO VLA Sky Survey B. Configuration D and DnCC. F = 1.4 GHz

.D >-40°E. Completeness limit ~ 2.5 mJyF. Resolution ~ 45″G. 1.8 106 sources

Survey Speed

EVLAVLA


Arp 220, Z=8?High Z CO survey


K-band spectra, in Massive SFR, One tuning pair (4 pairs totally)


64 Spectral Lines, with Full Polarization, and different spectral resolution

with 4-bit Re-Quantization

1. 18.6 - 20.6 GHz which covers 3 RRL + 1 Mol, 12 sub-band pairs (SBP) free

2. 20.6 - 22.6 GHz which covers 2 RRL + 3 Mol, 11 SBP free

3. 22.6 - 24.6 GHz which covers 2 RRL + 14 Mol, all SBP used

4. 24.6 - 26.6 GHz which covers 1 RRL + 14 Mol, 1 SBP free

24 SBP

40 SBPContinuum

Spectra


Ideas

1. super-fine spectral resolution observation towards molecular clouds – get the information of each clump. ( High performance in spectral observation)

2. RRL observation – Broad line region in AGNs? (High sensitivity and very broad bandwidth, and later high resolution to resolve)

3. H2CO maser survey in the Galaxy? (Hi sensitivity, fast survey speed, without confusion from surrounding absorption)

4. Weak molecular absorption lines towards continuum sources like SNR or quasar, as dense gas tracer.

5. I am still thinking about some interesting attempts.


Thank You!


Backup Slides


Now w points to the source, u to the east, and v toward the North.The direction cosines l and m ( on the celestial sphere plane) increase to the east and the north respectively.

UV-plane


Assume small frequency width (Δν) and no motion of the source.Now consider radiation from a small solid angle dΩ from direction S

Stationary, Monochromatic, Two element Interferometer

It is multiply but not plus because of

lower noise.


Making a SIN Correlator


Adding a time delay Change the spatial resolution to the resolution of time (much easier to handle)


Visibility

We now define a complex function, V, from the two independent correlator outputs:

This gives us a beautiful and useful relationship between the source brightness, and the response of an interferometer:


A Schematic Illustration

1 The correlator can be thought of ‘casting’ a sinusoidal coherencepattern, of angular scaleλ/b radians, onto the sky.

2 The correlator multiplies the source brightness by this coherence pattern, and integrates (sums) the result over the sky.

3 Fringe separation set by baseline length and wavelength

Long baseline gives close-packed fringes.

Short baseline gives widely-separated fringes.


Can’t be sampled, missing flux


UV Coverage of the VLA/EVLA

Missing flux


Dirty beam


Software

Newstar :WSRT, Nobeyama, etc.

GILDAS(MIRA) :IRAM PdBI

MIRIAD :WSRT, ATCA, CARMA, SMA etc.

AIPS :VLA, VLBA, etc.

MIR :SMA

AIPS++ => CASA :ALMA, EVLA, PdBI, etc.

Uniform, convenience, good at imaging

Powerful, complicated, good at calibration and

specific usage

Powerful, Uniform, Convenience,

Still in programming


H7

1

H7

0

H6

9

H6

8

H6

7

H6

6

H6

5

H6

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H6

3

H6

2

K-band spectra line excitation temperature?

oct 16, 2008, sfig, zhiyu zhang, seminar 2008 introduction of radio interferometry and the evla...

Documents

frequency resolution

spectraflux calibration

positionbandpass calibration

maximum bandwidth

lowbrightness capability

polarizationfull polarization

frequency accessibility

sky brightness distribution