Radio Astronomy: from Cottage Industry to Mega-Projects Richard - - PowerPoint PPT Presentation

Radio Astronomy: from Cottage Industry to Mega-Projects

Richard Hills Astrophysics Group, Cavendish Laboratory University of Cambridge

9th July 2014 Royal Society New Fellows Seminar 1

Radio Astronomy in the ‘60’s

• Already well-established with substantial projects being

carried out by a university groups in UK, NL, Aus, etc.

• Largely limited to frequencies of a few GHz – set by

antennas, receivers and properties of sites.

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Mullard Radio Astronomy Observatory, Cambridge, UK. Telescopes built in ‘50s and ‘60s

Higher Frequencies

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• More accurate dishes: rms

surface errors ~0.5mm for 30GHz (wavelength 10mm).

• New receiver technology –

Schottky diode mixers.

• Water, both vapour and

droplets, absorbs high frequency signals.

• So use high, dry sites with

few clouds:

• Jack Welch’s 20-foot (6 m)
• dish. University of California

Hat Creek Observatory.

Interstellar Molecules

• Densities in space are so

low that no polyatomic molecules were expected.

• Townes thought it worth

a try, so initiated search for NH3 at 23.7GHz with 20-foot dish in 1967.

• Succeeded after quite a
• struggle. Noise ~4000K.
• Detections of H2O and

H2CO followed.

• CO detected at 115GHz

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Cheung et al. (1968),

• Phys. Rev. Lett., 21, 1701

More Interstellar Molecules

• Became clear there is

a rich chemistry in interstellar clouds – both in the gas phase and on the surfaces

• f dust grains.
• Provides a tool for

studying conditions in these clouds.

• Can observe new

stars and planetary systems being formed.

• Great intrinsic interest in the chemistry. Also implies the

earth was formed from material rich in organic compounds.

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Spectrum of the Orion Nebula at around 1.3mm wavelength.

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Sutton et al. (1985), ApJ Suppl, 58, 341

UK Millimetre-Wave Telescope

• After a number of false starts UK decided to get involved

and in 1975 started design studies for a 15m dish with < 0.05mm accuracy on a high dry site. A national facility.

• Negotiations over first choice of site – La Palma – were

very slow and research council budgets got squeezed.

• Dutch joined in 1981 and

the Canadians in 1987 so it became an international

• bservatory – the James

Clerk Maxwell Telescope.

• Site changed to Mauna

Kea in Hawaii. 14,000ft but good access and infra-structure.

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Mauna Kea, Big Island, Hawaii

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With very dry conditions on high sites we can work at sub-millimeter wavelengths (300 to 1000GHz)

• Need even more accurate dishes, specialized receivers…

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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 200 400 600 800 1000

Atmospheric Transmission Frequency GHz

James Clerk Maxwell Telescope – 15m dish, ~25 μm surface accuracy

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Assembly - 1985

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“Homologous” Steel Backing Structure

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Goretex membrane keeps out Sun and Wind

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Sensitive Receivers

• No technology available for

frequencies of 100 – 1000GHz

• Need both spectral-line and

broad-band (“continuum”) detectors

• Cooled to reduce noise
• Later moved to super-conducting

devices and on from individual detectors to “cameras”

• Several orders of magnitude

increase in complexity, sensitivity and cost between ~1985 and today

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SCUBA-2

• The main camera

now on the JCMT

• ~10,000 detectors
• “Walk-in” cryostat
• Operates at ~0.3K
• Cost more than the
• riginal telescope

and took longer to build, test and commission it.

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SCUBA-2 on the JCMT Image: Joint Astronomy Center Hilo

Serendipity

• The case for building the JCMT was based on studies of

interstellar molecules and the formation of stars in our

• wn galaxy, but it ended up making its name elsewhere.
• It turned out that we could detect the thermal emission

from cold dust in external galaxies and even more surprisingly that this was possible out to high redshifts.

• In 1993 we detected such

emission from an object at a redshift of z = 4.69.

• That means that the signals

had been travelling for more than 12 billion years.

• JCMT is best known for this

type of observation.

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Isaak et al. (1994), MNRAS, 269, L28

Red-shifting a dusty galaxy

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1mm wavelength z = 1 z = 7

A lot of distant galaxies

• SCUBA-2 survey of

an area about half the apparent diameter of the moon.

• These galaxies are

at best very faint at

• ptical wavelengths

because they are full

• f dust.
• Their total luminosity

is very large because they are forming huge numbers of stars.

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Geach et al. (2012), MNRAS, 432, 53

Angular Resolution

• Is set by ratio of wavelength to diameter of telescope.
• Once one has detected an interesting object, the key to

understanding its nature is to make detailed images.

• In the near IR (λ = 1μm) a 10m telescope has a diffraction

limit of ~0.02 arc sec.

• To match this at λ = 1mm we need a telescope 10km in

diameter! Only reasonable option is “aperture synthesis”.

• Strong case for this in order to investigate both nearby

proto-stars and high redshift galaxies: in 1990s projects were proposed in Japan, US and Europe.

• Became clear that merging them would have great

advantages and that this was probably the only way to get funding. Agreement reached in ~2000.

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ALMA: Atacama Large Mm/sub-mm Array

• 66 antennas with accuracy < 25 microns.
• At 5000m altitude in northern Chile.
• Partnership between:

Europe: 14 countries, North America: USA, Canada and Taiwan, East Asia (NAOJ): Japan, Taiwan and South Korea, in cooperation with the Republic of Chile

• Construction cost ~£1billion.
• Started “Early Science” in Sept 2011.
• Opening ceremony March 2013. Well over a thousand

people there!

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Chajnantor Plateau, Northern Chile, ~16,500ft

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12m diameter ALMA antenna

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• ALMA is an aperture synthesis telescope. Telescopes are limited in

the amount of detail they can see (their angular resolution) by the ratio of the aperture diameter to the wavelength. We can’t make a single aperture 15km across so we build it up a little at a time. Correlator Reference

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Correlator: ~2 x 1017

• perations

per second

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Google‐Earth view of site with antennas in the most extended configuration – baselines to 16km

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Configuration scheme going from the largest to the smallest: 20 by 20km, 4 by 4, 0.5 by 0.5 and the Compact Array 0.1 by 0.1km

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ALMA transporter in action

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ALMA Operations Support Facility, ~9500ft

Image Credit: ALMA (ESO, NAOJ, NRAO) W. Garnier

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C+ Emission Line in BR 1202 z = 4.69

Wagg et al. (2012), ApJ, 752, L30

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Protostar accreting mass

ALMA Image Artists impression

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Casassus et al. (2013), Nature 493, 191

First Nature paper

Gas around an evolved star. Star was known to be loosing mass. Expected to see a

• shell. Instead find

that the outflow is a spiral!

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Maercker et al. (2012), Nature 490, 232

Central region of a galaxy (NGC 1433) with a massive black hole in the centre

Image Credit: LMA (ESO/NAOJ/NRAO)/NASA/ESA/F. Combes

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Gravitationally lensed high-z galaxies

Vieira et al. (2013), Nature 495, 344

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Note on Big Projects - positives

 International collaborations are the only way of obtaining sufficient resources to do really ambitious things.  Once established the funding is relatively well-protected.  They bring in talented people from many different backgrounds – can provide more diverse ideas and solutions to problems.

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Note on Big Projects - negatives

X Increased complexity of organization, meeting demands

• f many different authorities, etc., may mean that there

are actually inefficiencies of scale. X Very long times between conception and full operation, together with need to optimize system for specific goals, means there is a risk that they become out-dated and reduces the possibilities for serendipitous discoveries. X Difficult to find projects for students which combine technical and astronomical aspects. Danger that they don’t really understand their data and won’t be able to create the next generation of instruments. X Operations costs may be under-estimated. If they are 10% of capital and facilities run for 30 years total for

• perations is ~3 times the capital.

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Conclusions

• I believe that the evolution of high frequency radio

astronomy is an excellent example of the development of a field driven by scientific curiosity.

• It is important to note that the way in which this has

happened could not have been predicted.

• As such fields develop we must be careful to achieve a

balance of resources between “mega-projects” and smaller- scale activities which allow more innovation.

Acknowledgement

• Working in this field has been a great pleasure. All the

progress described has only been possible as a result of the dedicated work of a huge number of talented people. I am delighted to pay tribute to all of them for their efforts.

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