Posts Tagged ‘Black Holes’

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.


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.


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.


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.


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.


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).


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).


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.


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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On Wednesday of last week (15 June 2016), it was announced by the LIGO-Virgo collaboration that they had made their second detection of gravitational waves. This follows the announcement made by the same team in February of the first ever detection of the waves predicted by Einstein 100 years ago (see my blogposts here and here about that).

The fact that LIGO has now detected two gravitational wave events in the space of a few months suggests that there will be many more; and really does highlight how we are opening up a whole new window on the Universe, as I have said before. To me, this is akin to the development of radio astronomy in the 1950s or X-ray astronomy in the 1960s, when new sources were being detected several times a year. Or, one could say, to the development of the telescope in the early 1600s.

I think that we can not only expect to see more and more detections coming from the LIGO-Virgo team, but also an increase in sensitivity of the detections as time goes on. Even with ground-based detectors I expect the sensitivity to increase, but once we start doing this from space (as ESA plans to do with eLISA), the sensitivity will increase hugely.


The LIGO collaboration has detected a second emission of gravitational waves. This detection, announced last week (15 June 2016), was made on 25 December 2015.

As with the first detection, this second detection seems to be of two black holes merging. However, unlike the first event, which lasted about a tenth of a second, this event was about 1-second long. Also, whereas it is calculated that the two black holes in the September 2015 event (announced in February) had masses of 29 and 36 times the mass of the Sun, the black holes in this event had masses of 11 and 8 times the mass of the Sun. It is the lower mass of the two black holes in this second event which leads to the merger taking longer, as their orbits about each other would have been slower.

We really are living at a very exciting time, to be witnessing this whole new window on the Universe opening up.

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I have been asked by the BBC to talk on the radio about a recent press release made by the Department of Physics and Astronomy at Cardiff University . The press release is from the department’s Gravitational Physics group. Three members of this group, Ioannis Kamaretsos, Dr. Mark Hannam and the group’s head Professor B. Sathyaprakash have modelled what would happen if two black holes were to merge (coalesce). The work forms part of Mr. Kamaretsos’ PhD thesis.

It has long been predicted by Einstein’s theory of General Relativity that such an event would produce ripples in space, due to the intensity of the gravitational fields produced. This is akin to electrons producing electromagnetic waves when they make certain transitions. In fact, over the last 10 years, astronomers have built gravitational wave detectors to detect these predicted ripples in space. From studying the precise nature of these ripples, astronomers believed one could calculate the mass of the final black hole, and how rapidly it spins.

What the Cardiff team are proposing in this new theoretical research is not only can studying the details of the ripples tell us about the final, merged black hole, but also about the properties of the two black holes before the merger. This is the first time that research has suggested that something could be learned about the final black hole’s progenitors.

A screen capture of a movie showing a simulation of two black holes merging

The movie of the simulation can be found here

As of yet, astronomers are yet to detect their first gravitational waves, but the hope is that with the recently completed LIGO detectors, and others, that the first detections are not too far away. Let us hope so.

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