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Posts Tagged ‘Quasars’

There is now overwhelming evidence that our Galaxy harbours a supermassive black hole at its centre. Not only that, but the Hubble Space Telescope has discovered that all spiral galaxies harbour supermassive black holes at their centres, and the mass of that black hole is directly proportional to the mass of the galaxy in which it resides. The reasons for this are still unclear.

The idea of supermassive black holes driving the prodigious energy output at the centre of some galaxies was first proposed by Edwin Salpeter in a 1964 paper. Salpeter is probably better known for his work on the initial mass function of star formation, but in this paper (follow this link to read it), Salpeter proposed that supermassive black holes may be the energy source behind the then newly discovered quasars.

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In a 1964 paper, Edwin Salpeter (possibly more famous for his work on the initial mass function of star-formation) was the first to propose supermassive black holes as the energy source of the newly-discovered quasars (or QSOs).

In 1971, Donald Lynden-Bell and Martin Rees wrote an important paper entitled “On quasars, dust and the galactic centre”, (follow this link to the paper). It was the first paper to suggest that our own Galaxy, the Milky Way, may harbour a supermassive black hole at its centre.

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The possibility that our Milky Way harboured a supermassive blackhole at its centre was first proposed by Donald Lynden-Bell and Martin Rees in 1971.

Another important paper entitled “Accretion onto Massive Black Holes” was written in 1973 by Pringle, Rees and Pacholczyk (follow this link), who considered the observable effects that matter accreting onto a (super)massive black hole would have.

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In a 1973 paper, Pringle, Rees and Packolczyk considered the observable effects of the accretion of matter onto a supermassive black hole.

Pringle etal. draw two main conclusions, the second of which is possibly the more important; that material falling onto a (super)massive black hole will emit a huge amount of radiation.

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The two main conclusions of the Pringle etal. (1973) paper.

In a 1974 review article in The Observatory entitled “Black Holes”, Martin Rees further stated the arguments for supermassive black holes at the centres of galaxies.

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In a 1974 review article in The Observatory, Martin Rees wrote that “a black hole might lurk in the centres of most galaxies.”. 35-40 years later, he was shown to be correct.

He stated (my highlight)

If we regard quasars as hyperactive galactic nuclei, then a black hole might lurk in the centres of most normal galaxies.

How prescient were these words!

Later in the same year, radio astronomers Bruce Balick and Robert Brown discovered a compact radio source in the constellation Sagittarius. They announced their result in a paper entitled “Intense Sub-Arcsecond Structure in the Galactic Center” (here is a link to the paper).

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In 1974, Bruce Balick and Robert Brown used the Very Large Array of radio telescopes in New Mexico to discover a compact radio source at the centre of the Milky Way. We now call this source Sagittarius A*

Using the Very Large Array of radio telescopes in New Mexico, Balick and Brown found a sub-arcsecond radio source at both 2695 MHz (11cm) and 8085 MHz (3.7cm). We now call this source Sagittarius A*, and it is believed to be where the Galaxy’s supermassive black hole resides. Here is their image obtained at 2695 MHz (at right, the image at left is the base-line coverage in the u-v plane, the interferometry plane of the array).

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Bruce Balick and Robert Brown discovered a sub-arcsecond radio source in the constellation Sagittarius. We now call this source Sagittarius A*. Their discovery was made at 2695 MHz (which corresponds to a wavelength of 11 centimetres)

Since these discovery observations, Sagittarius A* (or Sgr A* as it is often known) has been observed at many other wavelengths (but not in visible light, the dust extinction is too great). For example, here is a combined infrared and X-ray image.

The supermassive black hole located 26,000 light years from Earth in the center of the Milky Way.

A composite infrared and X-ray image of Sagittarius A*

And, here are some images taken by the SPIRE camera on the Herschel Space Observatory at 250, 350 and 500 microns.

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Images of Sagittarius A* taken by the SPIRE camera on the Herschel Space Telescope. The observations are at (from left to right) 250, 350 and 500 microns.

Later this week I will blog about the observational evidence for this compact object being a supermassive black hole.

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The discovery of quasars in the 1960s played a crucial role in helping show that the Universe was different in the past. This had important implications for the testing of the two competing cosmological theories of the time – the “big bang theory” and the “steady state theory”. The steady state theory, whose main proponent was Fred Hoyle, argued that the Universe had always existed and did not change in time. The big bang theory, on the other hand, argued that the Universe had a finite beginning in time, and that since this beginning had expanded, cooled and evolved.

The discovery of quasars was made using radio astronomy, and in fact the word “quasar” stands for “quasi-stellar radio source”. Later similar objects would be found that did not emit strongly in the radio, and so the term “QSO” (quasi-stellar object) was suggested. Both acronyms are used today, pretty much interchangeably. The term “quasar” is often used incorrectly for QSOs which do not have strong radio emission.

The beginnings of radio astronomy

There was a little radio astronomy done before the 2nd World war. Karl Jansky accidentally discovered radio waves coming from space in 1931. After a little research he realised that the signal he was detecting with his radio receiver was coming from the centre of our Milky Way galaxy. However, Jansky was an engineer working for Bell Labs, and his request for funding to follow up this discovery and do a more complete survey of radio emissions from space was rejected, and Jansky re-assigned to another project and not given the freedom of following up on this discovery. The unit of flux in radio astronomy, the Jansky, is named after him.

In 1944 Dutch PhD student Hendrik van de Hulst predicted the existence of an emission line from neutral hydrogen, due to a hyperfine splitting in the ground state. I will explain in a separate blog the theory behind this, but basically the emission line comes about when an electron jumps between two very closely spaced energy levels in the ground state of neutral hydrogen. Because the energy difference is so small, the wavelength of the ensuing photon is extremely long – 21cm to be precise. This is in the radio part of the electromagnetic spectrum.

With a combination of this prediction, and the developments made in radar during the 2nd World war, the post-war years saw a boom in radio astronomy. One of the first groups to be established was at Cambridge University. The group was initially led by Martin Ryle, who had worked during the war with the Telecommunications Research Establishment on the design of antennae for airborne radar equipment. After the war, Ryle got a fellowship at the Cavendish Laboratories, and it was there that he established what became known as the Cambridge Radio Astronomy group.

The first Cambridge Radio catalogue (1C)

In 1950 he and his group published the first ever catalogue of radio sources. The paper, published in Monthly Notices of the Royal Astronomical Society (Ryle etal. 1950, MNRAS, 110, 508-523) was entitled “A preliminary survey of radio stars in the Northern Hemisphere”. In this paper they listed the positions of 50 discrete “radio stars”, along with the intensity of each source. The positions were not known very accurately, because the long wavelength used (3.7 metres) meant the resolution of their antenna array could not locate the sources’ exact positions to better than about 1 degree of arc. This was a major problem in identifying which astronomical objects the radio sources were.

The second Cambridge radio catalogue (2C)

The second catalogue by the Cambridge group was published in 1955. Entitled “A survey of radio sources between declinations -38 degrees and +83 degrees”, the lead author was John Shakeshaft of the group, with Ryle as second author. It was published in the Memoirs of the Royal Astronomical Society, (MmRAS 1955, 67, 106-154).

Just like the first catalogue, the second one was done at a wavelength of 3.7 metres. In this second catalogue, 1936 radio sources were found. Of these, 500 of the most intense could have their positions determined to an accuracy of about +/- 2 arc minutes in Right Ascension, and about +/- 12 arc minutes in declination. The team found most of the sources were of small angular diameter, and were distributed isotropically over the sky (that is to say not in any particular direction).

About 30 of the sources were of larger angular diameter, between 20 and 180 arc minutes, but the majority of these larger sources were close to the plane of the Mily Way galaxy and so the authors suggested that they represented a “rare class of galactic object”. They then went on to say that about 100 of the sources appeared to be related to objects which were in the New General Catalogue or the Index Catalogue; both optical catalogues of nebular objects which had been put together in the 1800s and the first decade of the 1900s.

The Third Cambridge radio survey (3C catalogue)

In 1959 the Radio Astronomy group at Cambridge produced their 3rd catalogue, using an upgraded antenna array. This time the observations were done at a frequency of 159 MHz (which corresponds to a wavelength of 1.9 metres). The paper, entitled “A survey of radio sources at a frequency of 159 Mc/s” was published in the Memoirs of the Royal Astronomical Society with D.O. Edge as lead author, and listed 471 sources (MmRAS 1959, 68, 37-60). It was revised in 1962 by Bennett, so the revised 3C catalogue had 470 sources (Bennet, 1962, MNRAS, 125, 75-86). Initially astronomers used the 3C catalogue to try to find optical counterparts to these radio sources. After 1962, with Bennet’s improved catalogue, the revised catalogue (3CR) was used.

The discovery of quasars

The first object in the 3C catalogue to which an optical counterpart was found was the object 3C 48, in 1960 by Thomas Matthews and Allan Sandage (both of Caltech). Using radio interferometry to narrow down its position, and then subsequent direct optical photographs, they found that 3C 48 corresponded to a faint blue star-like object. When its spectrum was taken, it looked unlike the spectrum of any star. First of all it contained emission lines (the spectra of stars usually show absorption lines), and these lines were broad not narrow as is usually the case with blue stars. But, most puzzlingly, the pattern of lines did not seem to fit any pattern that astronomers had seen before.

By 1963 Matthews and Sandage had found three starlike counterparts to three sources in the 3C catalogue, and published this in the Astrophysical Journal (“Optical Identification of 3c 48, 3c 196, and 3c 286 with Stellar Objects”, 1963, ApJ, 138, 30-56). The nature of the three sources was not known, but at least it seemed that they had been identified.

A breakthrough happened in 1962. One of the other 3C sources, 3C 273, was predicted to pass behind the Moon on several occasions. Using the Parkes Radio Telescope in Australia, Cyril Hazard and John Bolton were able to make measurements which allowed Caltech astronomer Maarten Schmidt to find its optical counterpart. Using the Mount Palomar 200-inch telescope, Schmidt obtained a spectrum of the star-like object. The spectrum was as confusing as that of 3C 48, he could see emission lines but was not able to identify them, the pattern just didn’t seem to make any sense.

After much head scratching, Schmidt realised that the lines corresponded to hydrogen emission lines, but they were redshifted to such an extent that he had failed to recognise them. The redshift he measured for 3C 273 was nearly 16% of the speed of light, an unheard of redshift at that time. Assuming Hubble’s law which relates redshift to distance, this put 3C 273 at a huge distance from Earth, much further away than any galaxy ever seen. This work was published in a one page letter in Nature in 1963 – “3C 273: a star-like object with large red-shift”. Nature 197, 1040-1040.



Caltech astronomer Marteen Schmidt, who in 1963 discovered quasars.

Caltech astronomer Maarten Schmidt, who in 1963 discovered quasars.



Very soon afterwards, Jesse Greenstein and Matthews at Caltech identified the redshift of 3C 48, and found it to be 37% of the speed of light, over twice as far away as 3C 273! (“Red-Shift of the Unusual Radio Source 3C48”, 1963, Nature 197, 1041–1042).

The acronym “quasar” was coined by astrophysicist Hong-Yee Chiu. In 1964, he said in an article in Physics Today magazine

So far, the clumsily long name ‘quasi-stellar radio sources’ is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form ‘quasar’ will be used throughout this paper.

As more research was conducted it was found that not all these quasi-stellar objects with extremely high redshifts had strong radio emission. some were “radio quiet”, so the name “QSO” was coined, but because the term “quasar” had been in use for quite a while by this time, many or most astronomers refer to these objects as quasars whether they are radio-loud or radio-quiet.

An evolving Universe

I will go into more detail in a future blog about what we think quasars are. It was in the early 1980s, using the Hubble Space Telescope, that their host galaxies were observed for the first time. They are the extremely active core of galaxies, but are only found in the distant universe. The lowest redshift quasar is at a redshift of 0.056. To put this into context, the redshift of the Norma Cluster is 0.01570, so the nearest quasar lies some 3 times further away. The further away one looks, the more common quasars are. As they are not found in the local Universe, and as looking far away means looking back in time, quasars are very clear evidence of the evolution of the Universe. They are one of the strongest pieces of observational evidence for an evolving Universe, and thus one of the pieces of evidence which helped show that the steady state theory of the Universe was wrong.

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