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

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

Carlos De Breuck (ESO)
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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.

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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 Richers 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.

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Detecting normal galaxies at z3

• CO emission now detected in 25 zgt2 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.

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Detecting normal galaxies at z3

• ALMA sensitivity depends on
• Atmospheric transparency
  Chajnantor plateau at
  5000m
  altitude is superior to all
  existing mm
  observatories.
• Noise performance of receivers can be reduced by
  factor 2 (approaching quantum limit). Also gain
  v2 because ALMA will simultaneously measure both
  states of polarization.
• Collecting area remaining factor of 7 to 10 can
  only be gained by increasing collecting area to
  gt7000 m2.

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Detecting normal galaxies at z3

• At z3, 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 0.45ltealt0.75
• can be achieved (20 µm antenna
  surface
  accuracy).
• Tsys depends on band, atmosphere,
  for 115 GHz, Tsys 67 K
  obtainable.

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Detecting normal galaxies at z3

• Total CO luminosity of Milky Way Lco(1-0)
  3.7x108 K km s-1pc2 (Solomon Rivolo 1989).
• COBE found slightly higher luminosities in
  higher transitions (Bennett et al 1994) ? adopt
  Lco 5x108 K km s-1pc2.
• At z3 ? 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 Lco(3-2) / Lco(1-0) 1.

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Detecting normal galaxies at z3

• For ?CDM cosmology, ?v300 km/s, the expected
  peak CO(3-2) flux density is 36 µJy.
• Require 5s detection in 12h on source (16h total
  time).
• ? ND27300 m2.
• Achievable with N64 antennas of D12m diameter.

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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 gt50 of the
  uv-plane.
• Short tracking (lt2 hours) to reduce atmospheric
  variations
• ? requires ND gt 560 for a maximum baseline of 3
  km.
• Achievable with 64 12m antennas.

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Precise 0.1 resolution images

• Array cannot measure smallest spatial
  frequencies (ltD).
• 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.

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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 lt0.01 (18.5 km baseline at 650 GHz)
Dynamic range 100001 (spectral) 500001 (imaging)
Flux sensitivity Sub-mJy in lt10 min (median conditions)
Antenna complement 64 antennas of 12m diameter
Polarization All cross products simultaneously
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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.
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Design Reference Science Plan

• 128 projects full list available from
  http//www.eso.org/projects/alma
• Use ALMA sensitivity calculator
• http//www.eso.org/projects/alma/science/bin/sensi
  tivity.html
• Total time 3-4 years of ALMA observing.

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Design Reference Science Plan
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Molecular line studies of submm galaxies

• gt50 of the FIR/submm background are submm
  galaxies.
• Trace heavily obscured star-forming galaxies.
• Optical/near-IR identification very difficult.
• Optical spectroscopy ltzgt2.4.
• Confirmation needed
  with CO spectroscopy.

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Molecular line studies of submm galaxies

• 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)

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Molecular line studies of submm galaxies

• 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.

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Construction has begun!
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ALMA FE key specifications
ALMA Band Frequency Range Receiver noise temperature Receiver noise temperature Mixing scheme Receiver technology
ALMA Band Frequency Range TRx over 80 of the RF band TRx at any RF frequency Mixing scheme Receiver technology
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
- between 370 373 GHz Trx is less then 300 K

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

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