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

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

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 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.