Atacama Large Millimeter Array 27-29 October 2004DUSTY041 Scientific requirements of ALMA, and its capabilities for key-projects: extragalactic Carlos

Atacama Large Millimeter Array

27-29 October 2004DUSTY04 1

Scientific requirements of ALMA, and its capabilities for key-projects:

extragalactic

Carlos De Breuck (ESO)



Primary Scientific Requirements

• ALMA will be a flexible observatory supporting a wide range of scientific investigations in extragalactic, galactic and planetary astronomy.

• ALMA should be “easy to use” (i.e. you do not need to be an expert in aperture synthesis to produce images).

• Three scientific requirements drive the science planning. These are the “Primary Scientific Requirements”.



Primary Scientific Requirements

• The ability to detect spectral line emission from CO or CI in a normal galaxy like the Milky Way at a redshift of 3, in less than 24 hours of observation.

• The ability to image the gas kinematics in protostars and protoplanetary disks around young Sun-like stars at a distance of 150 pc, enabling one to study their physical, chemical and magnetic field structures and to detect the gaps created by planets undergoing formation in the disks. (see John Richer’s talk)

• The ability to provide precise images at an angular resolution of 0.1”. Here the term ‘precise images’ means representing to within the noise level the sky brightness at all points where the brightness is greater than 0.1% of the peak image brightness. This requirement applies to all sources visible to ALMA that transit at an elevation greater than 20°.



Detecting normal galaxies at z=3

• CO emission now detected in 25 z>2 objects.

• To date only in luminous AGN and/or gravitationally lensed. Normal galaxies are 20 to 30 times fainter.

• Current millimeter interferometers have collecting areas between 500 and 1000 m2.




ALMA sensitivity depends on:1.Atmospheric transparency:

Chajnantor plateau at 5000m altitude is superior to all existing mm observatories.

2.Noise performance of receivers: can be reduced by factor 2 (approaching quantum limit). Also gain √2 because ALMA will simultaneously measure both states of polarization.

3.Collecting area: remaining factor of 7 to 10 can only be gained by increasing collecting area to >7000 m2.



•At z=3, the 10 kpc molecular disk of the Milky Way will be much smaller than the primary beam → single observation.•Flux density sensitivity in image from an interferometric array with 2 simultaneously sampled polarizations and 95% quantum efficiency is:

•Aperture efficiencies 0.45<εa<0.75

can be achieved (20 µm antenna surface accuracy).

• Tsys depends on band, atmosphere, … for 115 GHz, Tsys =67 K obtainable.





•Total CO luminosity of Milky Way: L’co(1-0) = 3.7x108 K km

s-1pc2 (Solomon & Rivolo 1989).• COBE found slightly higher luminosities in higher transitions (Bennett et al 1994) → adopt L’

co = 5x108 K km s-1pc2.• At z=3 → observe (3-2) or (4-3) transition in the 84-116 GHz atmospheric band → need to correct, but also higher TCMB providing higher background levels for CO excitation.

• Different models predict brighter or fainter higher-order transitions. Few measurements of CO rotational transitions exist for distant quasars and ULIRGs, but these are dominated by central regions.→ Assume L’

co(3-2) / L’co(1-0) = 1.




• For ΛCDM cosmology, Δv=300 km/s, the expected peak CO(3-2) flux density is 36 µJy.• Require 5σ detection in 12h on source (16h total time).

→ ND2=7300 m2. • Achievable with N=64 antennas of D=12m diameter.



Precise 0.1” resolution images

• 0.1” resolution needed to complement contemporary facilities: JWST, eVLA, AO with 8-10m telescopes, …• High angular resolution and sensitivity complementary.• High fidelity images require a sufficiently large number of baselines to fill >50% of the uv-plane.• Short tracking (<2 hours) to reduce atmospheric variations→ requires ND > 560 for a maximum baseline of 3 km.• Achievable with 64 12m antennas.



Precise 0.1” resolution images

• Array cannot measure smallest spatial frequencies (<D).• Solve by having four antennas optimized for total power measurements (nutating secondaries).• Remaining gap in uv-plane filled in by Atacama Compact Array (ACA): 12 antennas 7m diameter.



Summary of detailed requirements

Frequency 30 to 950 GHz (initially only 84-720 GHz)

Bandwidth 8 GHz, fully tunable

Spectral resolution 31.5 kHz (0.01 km/s) at 100 GHz

Spatial resolution <0.01” (18.5 km baseline at 650 GHz)

Dynamic range 10000:1 (spectral); 50000:1 (imaging)

Flux sensitivity Sub-mJy in <10 min (median conditions)

Antenna complement 64 antennas of 12m diameter

Polarization All cross products simultaneously





Design Reference Science Plan

DISCLAIMER:The Design Reference Science Plan has been set up by expert scientists to serve as a quantitative reference for developing the science operations plan, for performing simulations, and for software design. It assumes the full 64-antenna array ready in 2012.The DRSP does not form the basis for any definition of ALMA early science observing, nor for any priority claims on key or similar projects.




• 128 projects; full list available from http://www.eso.org/projects/alma• Use ALMA sensitivity calculator:http://www.eso.org/projects/alma/science/bin/sensitivity.html

• Total time: 3-4 years of ALMA observing.






Molecular line studies of submm galaxies

•>50% of the FIR/submm background are submm galaxies.• Trace heavily obscured star-forming galaxies.• Optical/near-IR identification very difficult.• Optical spectroscopy: <z>~2.4.• Confirmation needed with CO spectroscopy.




• ALMA will provide 0.1” images of submm sources found in bolometer surveys (LABOCA/APEX, SCUBA-2/JCMT) or with ALMA itself.• 3 frequency settings will cover the entire 84-116 GHz band → at least one CO line. (1h per source)• Confirm with observation of high/lower order CO line. (1h per source)




• Follow-up with ALMA:• High resolution CO imaging to determine morphology (mergers?), derive rotation curves → Mdyn, density, temperature, ... (1h per source)

• Observe sources in HCN to trace dense regions of star-formation. (10h per source, 20 sources)Total: 12h per source, 170h for sample of 50 sources.



Construction has begun!



ALMA FE key specifications

ALMABand

Frequency Range

Receiver noise temperature

Mixing schemeReceiver

technologyTRx over 80% of the RF band

TRx at any RF frequency

1 31.3 – 45 GHz 17 K 28 K USB HEMT

2 67 – 90 GHz 30 K 50 K LSB HEMT

3 84 – 116 GHz 37 K 62 K 2SB SIS

4 125 – 169 GHz 51 K 85 K 2SB SIS

5 163 - 211 GHz 65 K 108 K 2SB SIS

6 211 – 275 GHz 83 K 138 K 2SB SIS

7 275 – 373 GHz* 147 K 221 K 2SB SIS

8 385 – 500 GHz 98 K 147 K DSB SIS

9 602 – 720 GHz 175 K 263 K DSB SIS

10 787 – 950 GHz 230 K 345 K DSB SIS

•Dual, linear polarization channels:•Increased sensitivity•Measurement of 4 Stokes parameters

•183 GHz water vapour radiometer:•Used for atmospheric path length correction

* - between 370 – 373 GHz Trx is less then 300 K

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Atacama Large Millimeter Array 27-29 October 2004DUSTY041 Scientific requirements of ALMA, and its capabilities for key-projects: extragalactic Carlos