1 / 23

Basic Concepts

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.

nikkos
Download Presentation

Basic Concepts

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Basic Concepts • An interferometer measures coherence in the electric field between pairs of points (baselines). Direction to source wavefront ct Bsin  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)

  2. 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 (kl) Bsin  T2  u (kl) B T1 T2 T1 • Actually we obtain data at both (u,v) and (-u,-v) simultaneously, since the two antennas are interchangeable. Ellipse completed in 12h, not 24!

  3. 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

  4. 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

  5. visibilities dirty image  clean image

  6. Data flow “Every astronomer, including novices to aperture synthesis techniques, should be able to use ALMA” Data flow: • Data taking • Quality Assurance(QA)programme • Data reduction pipeline • Archive • User

  7. 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++)

  8. ALMA Imaging Simulations Dirty Mosaic Clean Mosaic

  9. Dusty Disks in our Galaxy: Physics of Planet Formation Vega debris disk simulation: PdBI & ALMA Simulated PdBI image Simulated ALMA image

  10. 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

  11. 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)

  12. 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

  13. SMA 850 mm of Massive Star Formation in Cepheus A-East SMA 850 mm dust continuumVLA 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.

  14. Orion at 650 GHz (band 9) : A Spectral Line Forest LSB USB Schilke et al. (2000)

  15. ALMA: A Unique probe of Distant Galaxies Galaxies z < 1.5 Galaxies z > 1.5

  16. Gravitational lensing by a cluster of galaxies submillimeter optical (simulations by A. Blain)

  17. 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 • 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 CO HCO+ HCN 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

  18. Why do we need all those telescopes? Mosaicing and Precision Imaging • SMA ~1.3 mm observations • Primary beam ~1’ • Resolution ~3” 3.0’ ALMA 1.3mm PB ALMA 0.85mm PB CFHT 1.5’ Petitpas et al. 2006, in prep.

  19. 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

  20. Similar effect to adding both total power from 12m and ACA  need to fill in 15m gap in ALMA compact config. + 24m SD 50 antenna + Single Dish ALMA Clean results Model Clean Mosaic + 12m SD

More Related