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. Mullard Radio Astronomy Observatory, Cambridge, UK. Telescopes built in ‘50s and ‘60s • Largely limited to frequencies of a few GHz – set by antennas, receivers and properties of sites. 9th July 2014 Royal Society New Fellows Seminar 2

Higher Frequencies • 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. 9th July 2014 Royal Society New Fellows Seminar 3

Interstellar Molecules • Densities in space are so low that no polyatomic molecules were expected. • Townes thought it worth a try, so initiated search for NH 3 at 23.7GHz with 20-foot dish in 1967. • Succeeded after quite a Cheung et al. (1968), struggle. Noise ~4000K. Phys. Rev. Lett., 21 , 1701 • Detections of H 2 O and H 2 CO followed. • CO detected at 115GHz 9th July 2014 Royal Society New Fellows Seminar 4

More Interstellar Molecules • Became clear there is a rich chemistry in Spectrum of the Orion Nebula at interstellar clouds – around 1.3mm both in the gas phase wavelength. and on the surfaces of 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. 9th July 2014 Royal Society New Fellows Seminar 5

Sutton et al. (1985), ApJ Suppl, 58 , 341 9th July 2014 Royal Society New Fellows Seminar 6

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 observatory – the James Clerk Maxwell Telescope. • Site changed to Mauna Kea in Hawaii. 14,000ft but good access and infra-structure. 9th July 2014 Royal Society New Fellows Seminar 7

Mauna Kea, Big Island, Hawaii 9th July 2014 Royal Society New Fellows Seminar 8

With very dry conditions on high sites we can work at sub-millimeter wavelengths (300 to 1000GHz) 1 0.9 0.8 Atmospheric Transmission 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 200 400 600 800 1000 Frequency GHz • Need even more accurate dishes, specialized receivers… 9th July 2014 Royal Society New Fellows Seminar 9

James Clerk Maxwell Telescope – 15m dish, ~25 μ m surface accuracy 9th July 2014 Royal Society New Fellows Seminar 10

Assembly - 1985 9th July 2014 Royal Society New Fellows Seminar 11

“Homologous” Steel Backing Structure 9th July 2014 Royal Society New Fellows Seminar 12

Goretex membrane keeps out Sun and Wind 9th July 2014 Royal Society New Fellows Seminar 13

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 9th July 2014 Royal Society New Fellows Seminar 14

SCUBA-2 • The main camera now on the JCMT • ~10,000 detectors • “Walk-in” cryostat • Operates at ~0.3K • Cost more than the original telescope and took longer to build, test and commission it. SCUBA-2 on the JCMT Image: Joint Astronomy Center Hilo 9th July 2014 Royal Society New Fellows Seminar 15

Serendipity • The case for building the JCMT was based on studies of interstellar molecules and the formation of stars in our own 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. Isaak et al. (1994), MNRAS, 269 , L28 9th July 2014 Royal Society New Fellows Seminar 16

Red-shifting a dusty galaxy 1mm wavelength z = 1 z = 7 9th July 2014 Royal Society New Fellows Seminar 17

A lot of distant galaxies Geach et al. (2012), • SCUBA-2 survey of MNRAS, 432 , 53 an area about half the apparent diameter of the moon. • These galaxies are at best very faint at optical wavelengths because they are full of dust. • Their total luminosity is very large because they are forming huge numbers of stars. 9th July 2014 Royal Society New Fellows Seminar 18

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 = 1 μ m) 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. 9th July 2014 Royal Society New Fellows Seminar 19

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! 9th July 2014 Royal Society New Fellows Seminar 20

Chajnantor Plateau, Northern Chile, ~16,500ft 9th July 2014 Royal Society New Fellows Seminar 21

12m diameter ALMA antenna 9th July 2014 Royal Society New Fellows Seminar 22

Reference Correlator • 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. 9th July 2014 Royal Society New Fellows Seminar 23

Correlator: ~2 x 10 17 operations per second 9th July 2014 Royal Society New Fellows Seminar 24

Google ‐ Earth view of site with antennas in the most extended configuration – baselines to 16km 9th July 2014 Royal Society New Fellows Seminar 25

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 9th July 2014 Royal Society New Fellows Seminar 26

ALMA transporter in action 9th July 2014 Royal Society New Fellows Seminar 27

ALMA Operations Support Facility, ~9500ft Image Credit: ALMA (ESO, NAOJ, NRAO) W. Garnier 9th July 2014 Royal Society New Fellows Seminar 28

C + Emission Line in BR 1202 z = 4.69 Wagg et al. (2012), ApJ, 752 , L30 9th July 2014 Royal Society New Fellows Seminar 29

Protostar accreting mass ALMA Image Artists impression Casassus et al. (2013), Nature 493 , 191 9th July 2014 Royal Society New Fellows Seminar 30

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! Maercker et al. (2012), Nature 490 , 232 9th July 2014 Royal Society New Fellows Seminar 31

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 9th July 2014 Royal Society New Fellows Seminar 32

Gravitationally lensed high-z galaxies Vieira et al. (2013), Nature 495 , 344 9th July 2014 Royal Society New Fellows Seminar 33

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. 9th July 2014 Royal Society New Fellows Seminar 34