What is Millimetre-Wave Astronomyand why is it different?

Michael Burton

University 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 arcmin

Tony 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??