what is millimetre-wave astronomy and why is it different?

What is Millimetre-Wave Astronomyand why is it different?

Michael BurtonUniversity of New South Wales

Some Millimetre Basics• MM: 1–~12mm, Sub-MM: 0.3–1mm• CMBR (T = 2.7K = 1mm)• Molecular rotational lines

– Polar molecules (have dipole moment)eg CO (E1 = 5K), HCN, CS, HCO+

• Cold thermal continuum (dust)– Thermal processes: F ~ B ~ 2kT2/c2 . x

• Problem: Atmosphere (O2, H2O)……

The Millimetre Advantage

• Thermal Processes B 0.5-2 2

• Decay Rates (linear molecules) 3 • Doppler Widths 0.5 [?]• Level Population (T>>TJ; gJJ) • Number of Photons -1

• Energy • Spatial Resolution -1

Transparancies

• Electromagnetic Spectrum• MM transmission for 4mm H2O

• MM transmission for 11mm H2O• Some bright MM-lines

Brightness Temperature

Atmospheric Transmission

The 3mm Millimetre Spectrum

Physical Parameters you can derive!

• Temperature: Tex, TBrightness

• Density: nH2 (~ncrit range of densities present!)

• Column Density: N (when optically thin)• Optical Depth: (use isotope ratios)• Mass (with scale length)• Abundances: different species• Velocities: line widths, centres, shapes

Infall, outflow, mass transfer rates

Constrain the properties of your source!!

16272-4837SEST molecular line survey

–Gradient: Trot = 27 ± 4 K

–Intercept: N(H2) = 1 x 1024 cm-2

( comes in as well)

– Size + Column: n(H2) = 6 x 105 cm-3

– With Volume: Mass = 6 x 103 M

Garay et al, 2002

16272-4837: SEST kinematical studies

– Evidence for infall(profile of optically thick lines)

- Modelling: Vinfall ~ 0.5 km s-1

- Speed + Density + Size:

dMinfall/dt ~10-2 M yr-1

– Evidence for outflow from wings- Extent: Voutflow = 15 km s-1

Brooks et al, 2002

OpticallyThick

OpticallyThin

WideWings

Mopra: Current Capabilities

• 22-m Telescope for > ~3mm• 85–115 GHz SIS receiver (2.6 – 3.5 mm)• 35” beam @ 100 GHz• Tsys ~ 150K(@85GHz) – 300K (@115GHz)• Beam Efficiency:

mb (86 GHz) = 0.49, mb (115 GHz) = 0.42 xb (86 GHz) = 0.65, xb (115 GHz) = 0.55

• Bandwidth 64, 128 or 256 MHz (200 - 800 km/s)• 1024 Channels (0.2 - 0.8 km/s per channel)• 2 Polarizations

– 1 frequency or 1 polarization + SiO 86 GHz• Must Nod – No chopping• OTF Mapping

Methanol Maser-selected Hot Molecular Core SurveyCH3CN CH3OH HCO+ H13CO+ N2H+ HCN HNC

7 lines; 86 Sources

Purcell

‘On the Fly’ Mapping with Mopra:The Horsehead Nebula

Optical 12CO 13CO

6 arcminTony Wong

Tony Wong 0.17 km/s channel spacing

OTF Mapping Specifications

• For a 300” x 300” map:– ~1400 spectra (31 x 46)– ~35” resolution– 0.17 km/s resolution – 120 km/s bandwidth– Dual polarization ~ 0.3K per channel, per beam– ~70 minutes / grid– Upto 7 grids / transit– Processed with LIVEDATA +

GRIDZILLA packages

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The DQS in 13CO: Mopra OTF Mapping

How many photons have we lost (or gained)?

00sec(z)

z

Signal on-source:

( ){ })sec()sec( 001 zsou

zatmrecSig eTeTTCT ττ −− +−+=

Trec

Tsou

Tatm

Sky (Reference, Off)

Source (On)

Difference

Some Radiative Transfer

•Radiative Transfer dI/ds = - I +

•Kirchoff (LTE) / = B(T)

Radiative Transfer dI/d = I + B(T)

•Solution I(s)= I(0)e- (s) + B(T)(1 - e- (s))

Source Atmosphere

Obtaining Data:Signal from Source and Reference

• TSig = C{TR+TA(1-e-0secz)+TS e-0secz}• TRef = C{TR+TA(1-e-0secz)}• [TSig-TRef]/[TRef] = TS e-0secz

/ {TR+TA(1-e-0secz)}

Show Plots of Opacity + Brightness Temperature

• TBB = C{TR+TA}• [TSig-TRef]/[TBB - TRef] = TS/TA

Calibrating Data:Gated Total Power

• GTPRef = C’ TRef

• GTPPaddle = C’{TA + TR}

• [GTPPaddle - GTPRef] / GTPRef

= TAe- 0secz / {TR+TA(1-e-0secz)}

• GTPHot - GTPCold = C’{THot - TCold} Atmosphere Liquid Nitrogen

Calibrating Data:

• {[TSig-TRef]/[TRef]} / {[GTPPaddle - GTPRef] / GTPRef }

= TSource / TAtmosphere

• Actually TSource = T’Source / Efficiency– Usually written as TMB = TA

* / (note the different notation)

Mopra Upgrades

• 8 GHz Digital Filter Bank– Zoom modes– 4(?) lines simultaneously

• MMIC receiver– Easier tuning– Higher Tsys

– May loose 115 GHz end?• 7 mm receiver

– New ATNF project?• Focal Plane Array???• Ultra-wide band correlator???

– Needs source of funds……

Australia’s MM–Wave Radio Telescopes

3 mm

12 mm

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Australia Telescope Compact Array• National Facility

– Built for 1–10 GHz operation• MM-upgrades

– 3 mm (85-~105 (115) GHz)• 5 x 22m antennas• EW-array + NS-spur

– Currently 84.9-87.3+88.5-91.3 GHz– 12 mm (22-25 GHz)

• 6 x 22m antennas• 2 GHz bandwidth upgrade • 7 mm (45 GHz) upgrade planned

– 6 antennas• FPAs???

– With ultra-wide-band correlators??

Water Vapour and Phase Fluctuations

MillimetreInterferometry

Poses special challenges:• Significant atmospheric opacity, mostly due to H2O

• Fluctuations in H2O produce phase shifts• These increase with both baseline and frequency• Instrumental requirements (e.g. surface, pointing, baseline

accuracy) are more severe• Need more bandwidth to cover same velocity range (1

MHz (mm) km/s)

R SaultDesai 1998

Brightness Temperature H2O Turbulence Seeing

ALMAAtacama Large Millimetre Array

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Antarctica??