Transcript ALMA 50 antennas: 1225 baselines

Basic Concepts
• An interferometer measures coherence in the electric field
between pairs of points (baselines).
Direction to source
ct

B
T2
T1
Correlator
• Because of the geometric path difference ct, the
incoming wavefront arrives at each antenna at a
different phase. For good image quality: many baselines
n 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.
v (kl)


T1
B
T1
B sin  = (u2 + v2)1/2
T2
u (kl)
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!
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

visibilities
dirty image

clean image
Data flow
“Every astronomer, including novices to aperture
synthesis techniques, should be able to use ALMA”
Data flow:
1. Data taking
2. Quality Assurance (QA) programme
3. Data reduction pipeline
4. Archive
5. 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++)
ALMA Imaging Simulations
Dirty Mosaic
Clean Mosaic
Dusty Disks in our Galaxy:
Physics of Planet Formation
Vega debris disk simulation: PdBI & ALMA
Simulated PdBI image
Simulated ALMA image
ALMA Resolution
Simulation Contains:
* 140 AU disk
* inner hole (3 AU)
* gap 6-8 AU
* forming giant planets at:
9, 22, 46 AU with local
over-densities
* ALMA with 2x over-density
* ALMA with 20%
under-density
* Each letter 4 AU wide,
35 AU high
Observed with 10 km array
At 140 pc, 1.3 mm
Observed
Model
L. 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)
Imaging Protoplanetary Disks
• 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
SMA 850 mm of Massive Star Formation in Cepheus
A-East
SMA 850 mm dust continuum
VLA 3.6 cm free-free
1” = 725 AU
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
ALMA will routinely achieve resolutions of better than 0.1”
Brogan et al., in prep.
Orion at 650 GHz (band 9) :
A Spectral Line Forest
LSB
USB
Schilke et al. (2000)
ALMA: A Unique probe of Distant Galaxies
Galaxies z < 1.5
Galaxies z > 1.5
Gravitational lensing by a cluster of galaxies
optical
submillimeter
(simulations by A. Blain)
ALMA into the Epoch of Reionization
Band 3 at z=6.4
Spectral simulation of
J1148+5251 at z=6.4
 Detect dust emission in 1sec (5s)
at 250 GHz
CO
HCO+
HCN
 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
CCH
96.1
93.2
4 GHz BW
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 GHz
HD 112 µm at 361 GHz
Why do we need all those telescopes?
Mosaicing and Precision Imaging
SMA ~1.3 mm observations
• Primary beam ~1’
3.0’
• Resolution ~3”
ALMA 1.3mm PB
ALMA 0.85mm PB
CFHT
1.5’
Petitpas et al. 2006, in prep.
ALMA Mosaicing Simulation
Spitzer GLIMPSE 5.8 mm 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
Model
+ 12m SD
Clean Mosaic
+ 24m SD
Similar
effect to
adding both
total power
from 12m
and ACA 
need to fill
in 15m gap
in ALMA
compact
config.