Radio Astronomy - Leiden Observatory - Leiden University Radio astronomy covers 6 decades in...

Radio Astronomy !MSc. course !

Lecture 1 Course overview, introduction to Radio

Astronomy (historical perspective)

Prof. Mike Garrett !(ASTRON/Leiden/Swinburne)

Acknowledgements

I’ve tried to steal the best ideas and bring them together into a coherent picture that broadly covers radio astronomy - the technique and the science. !!Books that can be useful in the course of this lecture series: !Tools of Radio Astronomy (Fourth Revised and Enlarged Edition) Kirsten Rohlfs, Thomas L. Wilson, Publisher: A&A Library/ Springer!!Interferometry and Synthesis in Radio Astronomy (Second Edition), A.R. Thompson, J.M. Moran, G.W. Swenson Jr., Publisher: Wiley-VCH.!!!Very Long Baseline Interferometry Techniques and Applications, Marcello Felli, Ralph E. Spencer, Publisher: Kluwer!!Astronomical Society of the Pacific Conference series Volume 180, Synthesis Imaging in Radio Astronomy II, G.B. Taylor, R.A. Perley, Publisher:Astronomical Society of the Pacific Conference !!Very Long Baseline Interferometry and the VLBA: Napier, Diamond & Zensus., ASP Conf series, Vol 82, 1995. !!An introduction to Radio Astronomy second edition, Bernard F. Burke and Francis Graham-Smith, Publisher: Cambridge University Press!!Radio Astronomy (2nd edition), John D. Kraus Publisher: Solutions Manual !!!Radio telescopes second edition, W.N. Christiansen, J.A. Högbom, Publisher: Cambridge University Press!!!

I also acknowledge the following sources of information for this lecture:!!Publications: !"Early years of Radio Astronomy", W.T.Sullivan, ed., Cambridge Univ.Press (1984)!“Radio Astronomy Transformed - Aperture Arrays: Past, Present & Future” - M.A. Garrett (2013) - http://arxiv.org/pdf/1307.0386v1.pdf!

!Presentations (some can be googled): !M.A. Garrett, 100 years of Radio Astronomy, Public lecture to RSE. !I. Morrison, 50 years of Jodrell Bank. !R. Strom, History of Radio Astronomy!History of Radio Astronomy - NRAO web pages: www.nrao.edu/index.php/learn/radioastronomy/radioastronomyhistory

http://arxiv.org/pdf/1307.0386v1.pdf

http://www.nrao.edu/index.php/learn/radioastronomy/radioastronomyhistory

1. Radio Astronomy - Basic TermsRadio astronomy covers 6 decades in frequency across the e-m spectrum! !

- Microwaves (1cm - 30m)!

- Millimetre (1mm to 10 mm) !

- Sub-millimetre (< 1mm, up to 0.4 mm)!

e.g. FM radio and TV stations (~ 3 metres); !

wireless internet (wifi), microwave ovens/mobile phones (10-15cm).

Earth’s atmosphere is transparent to radio waves from mm to decametre wavelengths:

!Special: Radio waves largely unaffected by dust... !!

==> Can observe day and night!!

==> Studies of the early obscured Universe are possible.

e.g. Milkyway Galaxy across the e-m spectrum

from radio !frequencies

to gamma-rays!

N.B. absorption in the galactic centre in optical window due to dust obscuration.

James Clerk Maxwell (right) develops the laws of electro-magnetism (Maxwell’s laws). !

Maxwell’s equations describe light as a wave-like phenomenon in which the electric field (E) and the magnetic field B are at right angles to each other, as well as to the direction of the propogation of the wave. !

Energy alternates periodically between the electric and magnetic fields. The disturbance propogates at a constant speed (c) in a vacuum. !

Historical Background (1870-1930)

Maxwell’s laws also show that electro-magnetic radiation can span a wide-range of wavelengths and that light is just a subset of the electro-magnetic spectrum.

Heinrich Hertz became the first person to artificially generate waves of a different wavelength from those of NIR/visible light.

Sir Oliver Lodge attempted (but failed) to detect radio waves from the Sun (1894).

Mike Garrett / NAC 9

Centrally fed ground antennas - G. Marconi (1901)

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Early Antenna Arrays (1900)


Impressive scale (1900):


“My previous tests had convinced me that when endeavouring to extend the distance of communication, it was not merely sufficient to augment the power of the electrical energy of the sender, but that it was also necessary to increase the area or height of the transmitting and receiving elevated conductors” - Marconi.

Poldhu, Cornwall, UK 1901:


Some problems too...:

Mike Garrett / NAC

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St. John’s Poldhu


Marconi’s success led to the discovery of the ionosphere (right) & its reflective and refractive properties w.r.t. (low-frequency) radio wave propogation:

Solar radiation is the main ionising source - ionosphere height changes between day & night.

Marconi’s transatlantic signals required multiple reflections off ionosphere...

400 km


Professional Science community very sceptical of Marconi’s attempts at “over the horizon” transatlantic communications...

λ/4

Marconi room on Titanic

RMS Titanic

Early radio communications used v. long wavelengths (see image above)


Y-Stations WWI

Radio WWI trenches

Radio Broadcasting (post WWI)


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Cosmic Radio Emission - first claims...

Published: September 2, 1921Copyright © The New York Times

N. Tesla

1930’s Karl Jansky - communication engineer at Bell Laboratory begins to investigate sources of radio noise that adversely affected communications - cross-country, transatlantic etc. !

Identified thunderstorms (near and far) and something else....

Alexander Graham Bell invents the telephone - Bell telephone Company created in 1877 - it will play a significant role in radio astronomy over the next 100 years:

1932 Discovery of cosmic radio waves (Karl Jansky):!

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Galactic centre

v=25MHz; dv=26kHz

Early History of Radio Astronomy!

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New York Times:

Jansky detected emission from the centre of the galaxy:

"There is no indication of any kind, that these results are the form of some intelligence striving for intra-galactic communication."


Grote Reber (1911-2002)- the first radio astronomer

The first radio astronomer (Grote Reber)!

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Built the first parabolic radio telescope:!

- "Good" angular resolution!

- Good visibility of the sky!

- Detected Milky Way, Sun, Cas-A, Cyg-A, Cyg-X @ 160 & 480 MHz (ca. 1939-1947). !

- Published his results in ApJ!

- Multi-frequency observations !!

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• Ground-breaking work (1937-1947): !

• discovered that most radio emission was "non-thermal"

Reber's multi-frequency observations revealed the non-thermal nature of radio emission:!

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Reber's multi-frequency observations revealed the non-thermal nature of radio emission (UNEXPECTED!)!

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The things that make radio astronomy "special":

==> Special: !Dawn of high-energy !

astrophysics


Reber - the maverick!

Solar powered Home in !Tasmania

Electric car - Pixie!Built first

high altitude observatory

in Hawaii

Reber’s ashes have been distributed around all the major radio astronomy observatories in the world.

1939-1945: World War II

Major military powers were busy with other things...

Scientific papers (like Reber’s) were difficult to get hold of...

In USA, UK, Russia, Germany and Japan scientists were conscripted to help with the war effort - as we shall see, the technology developed during the war would have a major influence on the emergence of radio astronomy as a major new field over the next few decades...

But in some countries (like the Netherlands) the pursuit of fundamental research continued to some extent...


Oort had shown that the Milky Way rotated around its nucleus in Sagittarius. !!He was greatly interested in studying the central part of the Milky Way but was unable to make progress due to dust obscuration...

Jan Oort (Leiden) first heard of Reber’s work in 1944.

Henk van de Hulst

In the spring of 1944, Oort has an idea: !!Oort to student van der Hulst: !“We should have a colloquium on the paper by Reber; would you like to study it? And, by the way, radio astronomy can really become very important if there were at least one spectral-line in the radio spectrum.”!!

van de Hulst & Oort

Based on Oorts suggestion, Henk van de Hulst (then a student) investigated various possibilities - his main result was that a spectral line associated with neutral hydrogen (just the most abundant element in the Universe!) at 21 cm might be observed! !

Oort was keen to try and detect the HI line but the technical expertise was not available in NL, and funding was scarce. !!Eventually Oort managed to arrange observations with a disused German radar antenna.

Meanwhile on the other side of the atlantic, there was interest, resources and the right people. Purcell (left) and Ewen (centre) built a radio antenna to search for HI.

Ewen and Purcell detected HI in March1951.

6 weeks later Oort & Muller detected HI using the German radar telescope at Kootwijk:

Some examples of what galactic hydrogen line profiles look like (right). Peaks along the same line of sight and arising at different frequencies were used to map out the spiral structure of the Milky Way.

In 1949, Oort, v.d. Hulst et al. formed the “Stichting Radiostraling Zon en Melkweg” (SRZM), later renamed “NFRA” and more recently “ASTRON” (the Netherlands Institute for Radio Astronomy).!!With the detection of neutral hydrogen, Oort built the first, large steerable radio telescope (1956):

Building the Dwingeloo 25-metre telescope (1956)

Raising the dish

Grand opening with Queen Juliana: 17 April 1956.

• Previous images taken from a movie made by H. Kleibrink: !

http://www.youtube.com/user/magarrett1964?feature=mhum#p/u/7/iAPAkWSQ8RM

http://www.youtube.com/user/magarrett1964?feature=mhum#

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1944: van der Hulst predicts discrete 1420 MHz (21 cm) emission from neutral Hydrogen (HI).

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1951: HI detected by Ewen & Purcell and Oort & Muller.

Radio Spectral Lines - neutral hydrogen

Special: Radio astronomy traces the FIRST and most ABUNDANT element in the Universe.

Significance of HI today...

The scale & structure of the Milky Way... free of dust obscuration

The scale, and dynamics of external galaxies

Tracing dark matter in !galaxies well beyonf the optical extent of a galaxy

Significance of HI... (continued)

Cold HI gas also detected in absorption - inflow and outflow around Active Galactic Nuclei

HI observations also reveal galaxy interactions:

The future: probing the dark ages - the Epoch of Reionisation when the first stars and blackholes formed and neutral hydrogen became ionised (z =30-6). !!Probing the equation-of-state of dark energy via huge HI galaxy surveys,


But what was happening elsewhere in the world? !In 1939, Britain was fighting a war against nazi Germany on its own. Radar had been invented by Robert Watson-Watt and was to play a decisive role in the defense of the British coastline, especially during the battle of Britain. !Occasionally it appeared that the British radar was being “jammed” by the Germans. J.S. Hey, a British army research officer was sent to investigate. !

He discovered that the source of “jamming” was the Sun!!!This discovery remained classified until after the end of the war (1947). !

TRE (Telecommunications Research Establishment) developed radar during WWII - young physicists were conscripted - after the war they would return to civilian research and establish the field of radio astronomy in the UK and Australia: !!Ryle, Lovell, Hey, Hewish, Bolton, Bowen, Ratcliffe, Hanbury Brown, Graham-Smith…

Left: Taffy Bowen - not only a pioneer of radar but a member of the Tizard delegation that sold the UKs secrets to the US (e.g. cavity magnetron and jet engine) in order to get cash for fighter planes and arms. Bowen went on to become CEO of CSIRO in Australia, also establishing the Radio Division (now CASS).


Sir Bernard Lovell & Jodrell Bank

Lovell - air to surface radar (H2S)

"blind bombing"!(1939-1945)


Building the MkI Jodrell Bank Telescope (1950-57)

15 Dec 1945


Sputnik (1957), Cuban crisis, MI5, Moon...


Mk1 1957

Jodrell tracks Luna 9 soft-landing on the moon


Mk1 1957July 1969, 64-m Parkes Telescope “The Dish”, returns live TV signals to Earth


The “Lovell” telescope upgraded in 2003

Single dishes versus Interferometers

The quest for resolution drove radio astronomers towards interferometric techniques.

But for an interferometer array the resolution is where B is the maximum baseline.

For a single dish the resolution is where D is the diameter of the telescope.

D

B

Radio interferometers

In 1954 interferometer measurements showed that the radio emission from one of the brightest sources in the sky, Cyg-A came from bright regions on either side of a host galaxy.

In the 1950’s the first radio arrays (interferometers) were constructed. People like Martin Ryle (pictured) pioneered the the interferometry technique, receiving the Noble Prize in 1974 for developing the concept of aperture synthesis (see lecture 4).

Cyg-A:

As interferometers began to survey hundreds of bright radio sources, many of them exhibited this “classical double-structure”.

Cyg-A:

“Modern” (1990!) VLA image of the radio galaxy Cyg-A

Radio core - identified with optical galaxy. !

Collimated jet

lobe

“hotspot”

In those early days, Cygnus-A was also puzzling because the strength of the signal was sometimes observed to vary wildly - with timescales of a few seconds. !!Intrinsic variations of order dt seconds, implied implausibly small physical size (r) for the variable region of r < ~ cdt. !!Rather than intrinsic variability, the researchers of the day rightly identified the source of the fluctuations as ionospheric scintillation:

Scintillation effects only occur if the source angular size < size of irregularities in the ionosphere. If the source is large, the effect gets washed out across all the irregularities.

So... Irrespective of whether the fluctuations were intrinsic or due to scintillation, the radio source associated with the core of Cyg-A must be small!

The need for sub-arcsecond resolution led to the technique of VLBI (Very Long Baseline Interferometry). First over distances of ~ 50 km via remote transportable antennas (Jodrell Bank, UK).

In 1967 the first transatlantic baselines (USA-Sweden) detected very compact radio sources. B ~ 8000km ==> milliarcsecond resolution ==> very small sources. !!At cosmological distances 1mas ~ 10 pc!

In 1974 the first super-luminal, expanding radio sources were detected with VLBI ==> relativistic motions in radio sources.

From galaxies to planets!

In 1955, Bernie Burke & Kenneth Franklin detected strong periodic burst of radio emission from Jupiter.

Bernard Burke - still active in radio astronomy.

Discovery of Quasars -1963

Maarten Schmidt (1963, Nature, 197, 1040) examined a visible spectrum of this "star" and identified the characteristic spectrum of emission lines in the Balmer series of hydrogen Doppler-shifted to longer wavelengths by 16 percent!

In the 1950s/60s the great challenge was to identify radio sources with optical counterparts. !!In 1963 one of the brightest radio sources then known, 3C273, was identified with a faint blue 13th mag star-like source (Hazard et al. Nature)...

Almost immediately Greenstein & Matthews (1963, Nature, 197, 1041) identified a similar starlike object at the position of the radio source 3C 48, but with an even higher redshift, z=0.37. These objects became known as QUASARs (Quasi-Stellar Radio Sources). The of study of objects we now refer to as active galactic nuclei (AGN) had begun.

3C 273 had to be not only the most distant known object in the universe but also intrinsically more luminous than the brightest galaxies known at the time. !!

By the early 1960’s it was becoming clear that many types of cosmic objects were unexpectedly detected as strong sources of radio emission: !!- in the solar system the Sun and Jupiter, !!- in the Milky Way stellar explosions (supernovae) and interstellar gas clouds!!- but most common of all were the ubiquitous double sources associated with active galaxies at cosmological distances, with the radio emission appearing far beyond the optical extent of the galaxy (see Cen-A - extreme right). The luminosity of these sources could not be explained by an conventional sources of energy e.g. nuclear fusion.....

Above: Bright ring-like radio source was identified at the position of Tycho’s 1572 nova.

What else makes radio astronomy “special” ?

The Radio sky (left) is generally quite different from the optical sky: !=> most of the radio sources are located at cosmological distances!

Radio sky surveys also show a large excess in the faint radio source counts - the first evidence of rapid cosmic evolution (1960).

The NRAO 5GHz sky survey projected on the sky around Greenbank, USA.

Cosmic Microwave Background detected -1965

The Cosmic Microwave Background: Nobel prize Penzias & Wilson (1978) and Mather & Smoot (1990). !!Like Jansky before them, Penzias & Wilson (also Bell labs) were trying to characterise sources of noise at cm wavelengths (for satellite coms).

They measured a non-negligible source of noise that was present in all directions. At first they thought it might be due to pigeons that frequented the antenna - set traps and cleaned the antenna:

But still the excess temperature that Penzias & Wilson measured would not go away. Penzias happened to have a conversation with Bernie Burke (MIT) who knew that a physicist Robert Dickie at Princeton was predicting a background signal associated with the cooling of radiation from the big bang.

When Dickie heard of Penzias & Wilson’s detection he told his students (top right): “Boys we’ve been scooped”!

Left: measurements made by various groups of the CMB at different radio frequencies:

Future satellite missions were to measure the blackbody nature of the CMB in exquisite detail. !Deviations from a BB were measured to be ~ dT/T < 10E-6 Kelvin!!!

Penzias & Wilson and Dicke et al. published their results side-by-side in Nature. P&W did not say much about the CMB - they were still a little sceptical that this was really the source of the background radiation!!!In 1978 they alone received the Noble Prize for the CMB detection.

Dicke

Discovery of Pulsars -1967

Pulsars discovered by Jocelyn Belll-Burnell (PhD student) and Anthony Hewish, as a by-product of Interplanetary Scintilation Studies (ISS) in Cambridge.

The signal was recorded on chart paper - telescope produced about 30 metres per day! !!Jocelyn carefully waded through the paper (and interference), and started to notice an unusual source that appear to be periodic (T ~ 2 secs) she called this “scruff”. !!By 1968, four periodic sources had been discovered.

After instrumental causes were ruled out, other physical explanations were considered - including that the signal were being beamed to us by other intelligent life-forms in the galaxy. !!See youtube “2009: Pulsars (Bell Burnell)”

Hewish reveived the Nobel prize for this discovery. Jocelyn Bell did not...!!Pulsars Radio observations of Pulsars (rotating neutron stars) produce the most stringent tests on Einstein’s GR and provide an extreme laboratory in which to investigate the equation of state of degenerate matter.!!They are also nature’s best clocks - a property that astronomer aim to use in the future to detect low-frequency gravitational waves.

Tommy Gold (see also the CMB slides) on the basis of the short period and pulse width, the extreme stability of the period and the fact that the period was only ever observed to decrease, suggested that these periodic sources “Pulsars” must be associated with radio emission from extremely dense, fast-rotating neutron stars. !!Gold published his idea, and Hewish & Bell their discovery in Nature in 1968.

Binary pulsar

Pulsars are nature’s very best clocks. !!Discovery of binary pulsar by Hulst and Taylor led to another two Nobel Prizes. !!Measurements of change in PSR orbit first evidence of gravitational waves.

History of Radio Astronomy (untill 1980)

Prehistory:!!1870’s: James Clerk Maxwell predicts existence of electromagnetic radiation with any wavelength!!1888: Heinrich Hertz demonstrates transmission and reception of radio waves !1901: Marconi demonstrates the potential of wireless radio systems as a means of global communication !!Early pioneers: !!1932: Karl Jansky discovers cosmic radio waves from the galactic centre (USA)!1937-1944: Grote Reber’s First Surveys of the Radio Sky (USA)!1942: Sun discovered to be a radio source by J.S. Hey (UK)!1936-1945: Development of radar before and during world war II e.g. Sir Bernard Lovell!1944: Prediction by Henk van der Hulst and Oort (NL) of neutral Hydrogen spectral line emission at 1.4GHz. !1951: Detection of neutral Hydrogen Ewen & Purcell (USA) and van der Hulst & Oort (NL) !!!Major discoveries: !!1960-63: Radio sources identified with blue quasi-stellar objects with huge recession velocities!1960s: First radio interferometers constructed; Aperture Synthesis developed (Ryle, UK) - Noble Prize!1965: Cosmic Microwave Background (CMB) detected (Penzias & Wilson, USA) - Noble Prize!1967: Pulsars (rotating neutron stars) discovered by Jocelyn Bell & Tony Hewish (UK) - Noble Prize!!

History of Radio Astronomy (until 1980)

Large scale radio telescope facilities: !!1956/7: Construction of the first large steerable telescopes (Dwingeloo-NL, Jodrell Bank-UK...)!1967: First successful transatlantic Very Long Baseline Interferometery observations.!1970: Westerbork Synthesis Array telescope starts operations (NL)!1976: Very Large Array (VLA) becomes operational (USA) etc. etc.

300 000 lj

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Parabola dish domination: 1970-2010...?


21cmWhy dishes?: ! - discovery of the 21cm HI line !- interest in discrete radio sources and their identification !- the need for resolution and development of long baseline interferometry interferometry !- Low frequencies “hard”: RFI, ionosphere etc. !- reliability and flexibility of early phased arrays was poor !- many radar scientists had developed cm-wave technology during the war.


1937:

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One exception - Reber... visionary

1970s:

300 000 lj

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Meanwhile outside of Radio Astronomy an explosion of civilian and military applications using Antenna Arrays occurred (also called Aperture Arrays)

300 000 lj

LOFAR

OSMA,1999

THEA, 2004

Ardenne et al.!

In the early 1990s, ASTRON (NL) began to rediscover the potential of Aperture Arrays.!

R&D focused on:!

- concept demonstration, ! - integration, ! - cost reduction.

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Mid-frequency Aperture Array R&D at ASTRON (1995-2010)

AAD,1997

EMBRACE, 2010

300 000 lj

LOFAR

OSMA,1999

THEA, 2004

Ardenne et al.!

Aperture Arrays R&D focused on:!

- concept demonstration, ! - integration, ! - cost reduction.

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Mid-frequency Aperture Array R&D at ASTRON (1995-2010)

AAD,1997

EMBRACE, 2010

LOFAR - Low Frequency Array

LOFAR (2010) - Low Frequency Array

History of Dutch radio astronomy

1944: Significance of radio astronomy (for galactic studies - Oort) realised and the prediction of the 21cm spectral line of neutral hydrogen predicted by v.d. Hulst. !!1949: Foundation of “Stichting Radiostraling Zon en Melkweg” (SRZM), later “NFRA” and more recently “ASTRON” (the Netherlands Institute for Radio Astronomy).!

1951: Detection (simultaneously with US researchers) of the 21cm line with 7.5 metre dish at Kootwijk. !!1956: Opening of the first fully steerable (25-metre) paraboloid in Dwingeloo!!1970: Opening of the WSRT (Westerbork Synthesis Radio Telescope!

Kootwijk

Dwingeloo

1976-79: WSRT extended from 12 to 14 telescopes, extending the baseline length from 1.4 to 2.8 km.

Westerbork built - 1970

Good sensitivity and tens of arcsecond resolution

WiFi IEEE 802.11 standard - John O’Sullivan (1970-90)

Very Large Array (VLA) operational - 1980

Good sensitivity and arcsecond resolution

1989: Participation in the sub-mm James Clarke Maxwell Telescope (JCMT) in Hawaii (together with UK and Canada).

1992: Foundation of JIVE (Joint Institute for VLBI in Europe) - hosted by ASTRON in Dwingeloo.

1996-2002: WSRT upgrade - new receivers, correlator. !!2000-2012: Participation in design/construction of Atacama Large Millimeter Array (ALMA)!!!2010: LOFAR (Low Frequency Array) opened by Queen Beatrix!!

Netherlands: recent history

!2015: Upgrade of WSRT to APERTIF (Phased Array Feeds).!!

12 June 2010

ALMA first science 2014

Sub-mm astronomy: 0.3 - 3.5 mm

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