basic concepts an interferometer measures coherence in the electric field between pairs of points...
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Basic Concepts• An interferometer measures coherence in the electric field
between pairs of points (baselines).
Direction to source
• Because of the geometric path difference c, the incoming wavefront arrives at each antenna at a different phase.
wavefront
Correlator
B
Bsinc
T1 T2
For good image quality: many baselinesn antennas: n(n-1)/2 spacings(ALMA 50 antennas: 1225 baselines)
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Aperture Synthesis• As the source moves across the sky (due to Earth’s rotation),
the baseline vector traces part of an ellipse in the (u,v) plane.
B sin = (u2 + v2)1/2
v (k)
u (k)T1
T2
• Actually we obtain data at both (u,v) and (-u,-v) simultaneously, since the two antennas are interchangeable. Ellipse completed in 12h, not 24!
B
Bsin
T1 T2
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Synthesis observing
• Correlate signals between telescopes: visibilities• Assign the visibilities to correct position on the u-v disc• Fourier Transform the u-v plane : image
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Deconvolution• There are gaps in u-v plane. Need algorithms such as CLEAN and
Maximum Entropy to guess the missing information• This process is called deconvolution
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clean imageclean image
dirty imagedirty imagevisibilitiesvisibilities
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Data flow
“Every astronomer, including novices to aperture synthesis techniques, should be able to use ALMA”
Data flow:1. Data taking2. Quality Assurance (QA) programme3. Data reduction pipeline4. Archive5. User
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ALMA data reduction• After every observation:• Data reduction pipeline starts
– Flagging (data not fulfilling given conditions)
– Calibration (antenna, baseline, atmosphere, …) bandpass, phase and amplitude, flux
– Fourier transform (u-v to map)– Deconvolution– (Mosaicking, combination, ACA and main array,…)
• Output: fully calibrated u-v data sets and images or cubes (x,y,freq) Archive
• Pipeline part of CASA (f.k.a. aips++)
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Dirty Mosaic Clean Mosaic
ALMA Imaging Simulations
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Dusty Disks in our Galaxy: Physics of Planet Formation
Vega debris disk simulation: PdBI & ALMA
Simulated ALMA imageSimulated PdBI image
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ALMA ResolutionSimulation Contains:Simulation Contains:* 140 AU disk* 140 AU disk* inner hole (3 AU)* inner hole (3 AU)* gap 6-8 AU* gap 6-8 AU* forming giant planets at:* forming giant planets at: 9, 22, 46 AU with local9, 22, 46 AU with local over-densitiesover-densities* ALMA with 2x over-density* ALMA with 2x over-density* ALMA with 20%* ALMA with 20% under-densityunder-density* Each letter 4 AU wide,* Each letter 4 AU wide, 35 AU high35 AU highObserved with 10 km arrayObserved with 10 km arrayAt 140 pc, 1.3 mmAt 140 pc, 1.3 mm
Observed Model Observed Model L. G. MundyL. G. Mundy
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ALMA 950 GHz simulations of dust emission from a face-on disk with a planet
Simulation of 1 Jupiter Mass planet around a 0.5 Solar mass star (orbital radius 5 AU)
The disk mass was set to that of the Butterfly star in Taurus
Integration time 8 hours; 10 km baselines; 30 degrees phase noise(Wolf & D’Angelo 2005)
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• Protoplanetary disk at 140pc, with Jupiter mass planet at 5AU
• ALMA simulation– 428GHz, bandwidth 8GHz– total integration time: 4h– max. baseline: 10km
• Contrast reduced at higher frequency as optical depth increases
• Will push ALMA to its limits
Wolf, Gueth, Henning, & Kley 2002, ApJ 566, L97
Imaging Protoplanetary Disks
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SMA 850 m of Massive Star Formation in Cepheus A-East
Brogan et al., in prep.
2 GHz• Massive stars forming regions are at large distances need high resolution
• Clusters of forming protostars and copious hot core line emission
• Chemical differentiation gives insight to physical processes
SMA 850 m dust continuum VLA 3.6 cm free-free
1” = 725 AU
ALMA will routinely achieve resolutions of better than 0.1”
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Orion at 650 GHz (band 9) :A Spectral Line Forest
Schilke et al. (2000)LSB USB
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ALMA: A Unique probe of Distant Galaxies
Galaxies z < 1.5 Galaxies z > 1.5
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submillimeter optical
Gravitational lensing by a cluster of galaxies
(simulations by A. Blain)
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ALMA into the Epoch of Reionization
Spectral simulation of J1148+5251 at z=6.4
Detect dust emission in 1sec (5) at 250 GHz
Detect multiple lines, molecules per band => detailed astrochemistry
Image dust and gas at sub-kpc resolution – gas dynamics! CO map at 0”.15 resolution in 1.5 hours
HCN
HCO+
CO
CCH
Atomic line diagnostics [C II] emission in 60sec (10σ) at 256 GHz[O I] 63 µm at 641 GHz[O I] 145 µm at 277 GHz[O III] 88 µm at 457 GHz[N II] 122 µm at 332 GHz[N II] 205 µm at 197 GHzHD 112 µm at 361 GHz
Band 3 at z=6.4
4 GHz BW
93.296.1
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Why do we need all those telescopes? Mosaicing and Precision Imaging
3.0
’1.5’
SMA ~1.3 mm observations
• Primary beam ~1’
• Resolution ~3”
Petitpas et al. 2006, in prep.
CFHT
ALMA 1.3mm PB
ALMA 0.85mm PB
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ALMA Mosaicing SimulationSpitzer GLIMPSE 5.8 m image
• Aips++/CASA simulation of ALMA with 50 antennas in the compact configuration (< 100 m)
• 100 GHz 7 x 7 pointing mosaic
• +/- 2hrs
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50 antenna + Single Dish ALMA Clean results
Clean MosaicModel
+ 12m SD + 24m SDSimilar effect to adding both total power from 12m and ACA need to fill in 15m gap in ALMA compact config.
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