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Basic Concepts. An interferometer measures coherence in the electric field between pairs of points (baselines). Direction to source. wavefront. c t. Bsin . . B. T2. T1. Correlator. - PowerPoint PPT Presentation
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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)
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
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
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
clean imageclean image
dirty imagedirty imagevisibilitiesvisibilities
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
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++)
Dirty Mosaic Clean Mosaic
ALMA Imaging Simulations
Dusty Disks in our Galaxy: Physics of Planet Formation
Vega debris disk simulation: PdBI & ALMA
Simulated ALMA imageSimulated PdBI image
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
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)
• 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
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”
Orion at 650 GHz (band 9) :A Spectral Line Forest
Schilke et al. (2000)LSB USB
ALMA: A Unique probe of Distant Galaxies
Galaxies z < 1.5 Galaxies z > 1.5
submillimeter optical
Gravitational lensing by a cluster of galaxies
(simulations by A. Blain)
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
HCNHCO+
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 BW93.296.1
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
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
50 antenna + Single Dish ALMA Clean resultsClean 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.