Ordinary Meeting, 2005 April 23

 

Quasars, Black Holes and Galaxy Formation

Dr Almaini opened by outlining the talk to come. First, he would take a historical approach to introduce active galaxies and quasars, before coming to one of the biggest questions raised by them: what power source could fuel such luminous objects? He would then be arguing that super-massive black holes were the only plausible candidate, before asking whether such an object existed in our own galaxy, the Milky Way. Finally, he would air some of his own, more controversial, opinions on the effect of such black holes on the evolution of galaxies.

Setting the scene, he explained that all of the individual stars in the night sky were within the Milky Way, a flat spiral-shaped grouping, 30,000 light-years across, of one hundred billion such stars. Also visible were many other distant galaxies of stars, the nearest example of comparable size being the famous Andromeda Galaxy (M31) – its distance of three million light-years typical of the spacing between galaxies of such size. At the opposite extreme of distance, the speaker showed the famous Hubble Deep Field North (HDF-N), in which more than 1,500 distant galaxies could be seen in an area of sky only one arcminute square.

It was possible to measure the distances to these galaxies, Dr Almaini explained, using Hubble's Law combined with the Doppler Effect. The latter made possible the measurement of galaxies' recessional velocities; it dictated that the observed light from an object moving away from us would be redshifted – light it emitted at one wavelength would be observed at a redder wavelength. The magnitude of this redshift was usually measurable: many chemical elements emitted or absorbed light at sharply defined wavelengths, giving rise to readily identifiable features in the spectra of galaxies, and the shifts of the wavelengths of these relative to laboratory measurements yielded the galaxy's redshift.

The usefulness of this arose from the curious phenomenon, first noted by Slipher in the 1920s, that all distant galaxies appeared to be receding from us. More specifically, in 1929, Edwin Hubble had famously discovered a proportionality between their recessional velocities and their distances. Showing Hubble's original data, the speaker commented that the scatter was so great that his confidence in the discovery seemed surprising. Superior modern data had, however, vindicated him, conforming remarkably tightly to his proportionality, now known as Hubble's Law. As a result, if the recessional velocity of a galaxy was known, its distance could be estimated by assuming it to follow this pattern.

The observation of distant galaxies provided a wealth of opportunities to study how they had evolved through cosmic history. Many of those in the HDF, the speaker explained, were 8-10 billion light-years distant. On account of the time taken for their light to reach us, we saw them as they had been 8-10 billion years ago, when the Universe had been less than half its present age. By comparing them with nearby counterparts, we could identify changes in the typical properties of galaxies over the intervening time.

Moving on specifically to active galaxies, Dr Almaini recalled that such systems had first been identified by Carl Seyfert, who had observed in the 1920s that some galaxies, such as NGC 4051, had unusually bright point-like nuclei which could not be resolved, and appeared like stars. With the advent of gamma-ray astronomy in the 1940s, it had become apparent that they also emitted curious spectra, featuring unusually strong fluxes of high-energy photons. It had been speculated that populations of very hot massive young stars (O or B type) in their cores might explain these spectra, but no such model quite matched their shape.

Further mysteries had been unearthed in the 1950s with the birth of radio astronomy – booming at this time largely as a result of the technology and expertise amassed during the Second World War. Many of these "Seyfert galaxies" were also found to be radio sources, and many of the more powerful examples were termed "radio galaxies". The structuring of this radio emission, as well as its sheer power, was often surprising. For example, Cygnus A, the brightest radio source in the sky, showed two distinct vast bulges of emission. Modern high-resolution radio telescopes showed two narrow jets of material emanating from some central source, feeding each of these lobes.

However, not all radio galaxies could be identified as Seyfert galaxies: others appeared to be associated with point-like objects which looked like stars. All attempts to determine the nature of these stars from their spectra had failed, and they became known as quasi-stellar objects, or "quasars" for short. Their spectra exhibited broad emission lines, suggestive of intensely hot gas, but the lines could not be matched to any known elements. It was not until 1963 that Walter Schmitt had realised that the spectral features of quasar 3C 273 would match the redshifted spectrum of a galaxy receding at the tremendous velocity of 48,000 km/s. By Hubble's Law, this suggested that it was not a star at all, rather a very distant and hugely powerful galaxy, with a source at its centre whose luminosity was 1,000 times that of most galaxies. It seemed likely that it masqueraded as a star only because the glare of its bright nucleus completely concealed the nebulosity of the faint surrounding galaxy.

The speaker remarked that this had remained a matter of faith until remarkably recently: nobody had successfully observed a galaxy around a quasar until the advent of the exquisite resolution of the Hubble Space Telescope (HST). This, which was sufficient to allow the intensely bright emission of quasars to be confined to only a small number of pixels, at last allowed the detection of faint nebulosity around them. Many quasars were now known; the most distant as of 2004 had been discovered by the Sloan Digital Sky Survey (SDSS) at a redshift of 6.4, or a distance of 12.6 billion light-years. One curious and not yet well understood phenomenon, which became apparent upon the examination of catalogues of observed quasars, was that they appeared to have been much more common when the Universe had been one quarter its present age as compared to the present day: the number of quasars in the Local Universe was much lower than would be expected from the number seen looking back at the distant universe.

Around 10-20% of them were observed to emit highly collimated jets of material, often terminated with the vast lobes of radio-bright material already mentioned. Whilst this also was not well understood, it demonstrated that whatever powered quasars had to be of very substantial mass: to remain stable whilst producing such energetic and focussed beams of material over sufficient time – millions of years – to accumulate the observed radio lobes required a substantial inertia.

However, there was also evidence that their power sources were physically very compact. For example, quasar 3C 279 had been noticed to vary in its B-band (radio) brightness between magnitudes 17 and 12 over a period of years, implying its power source to be no more than a few light-years across – otherwise these variations would be happening on timescales where no information could traverse across it to cause its entirety to vary in synchrony. More recently, X-ray emission from a number of sources, including NGC 4051, had been observed to vary on timescales of hours, implying a power source no larger than the solar system, yet outshining the whole Milky Way, several hundredfold. By the 1960s, many astrophysicists had been asking what the nature of such power sources could be.

No chemical reaction could come close to the energy output required. Even filling the whole solar system with dynamite would not account for the energy emitted by a quasar. Nuclear fusion processes, of the kind which power the Sun, would be more efficient, but these still could not produce enough energy. It was now realised that the release of gravitational energy alone could account for the power achieved by quasars. Dr Almaini explained that the process could be visualised, without reference to relativity, as being rather like dropping an object on Earth. Intuitively, we would expect the object to land on the floor with a bang, emitting energy as heat and sound. This might not be terribly spectacular, but an object dropped from the Earth's orbit onto the Sun would land with a velocity of 620 km/s, and so release rather more energy. If the Sun were compressed down to a diameter of only 10 km – roughly that of a dense neutron star – then the object would strike at one third of the velocity of light. Thus, it could be seen that very dense objects had the potential to release very large amounts of gravitational energy from infalling matter.

The question remained as to whether there existed such compact objects in the Universe. Neutron stars were too faint to observe directly, but the speaker argued that there was compelling evidence for their existence. Historically, the earliest had come in 1968, when Cambridge astronomers Antony Hewish and Jocelyn Bell had stumbled upon a rapidly pulsating radio source in the midst of a supernova remnant in the Vela/Puppis cloud, and soon after, several other similar sources, some pulsating as rapidly as ten times per second. Christened pulsars, they were concluded to be rapidly rotating stars, emitting intense radio waves along the directions of their magnetic poles. As stars' magnetic poles were typically not aligned with their rotation axes, this emission would flash in and out of our line of sight as they rotated, rather like that of a lighthouse. But the rotation speeds were phenomenal; any ordinary star would be ripped apart by centrifugal forces if it spun so fast. Only a neutron star could spin ten times per second whilst remaining intact. In subsequent years, some pulsars – termed millisecond pulsars – had been found to rotate many hundreds of times per second, only serving to strengthen this conclusion.

The speaker went on, however, to outline evidence that the dense objects at the centres of active galaxies were, in fact, too dense even to be neutron stars, leading to the conclusion that they were black holes. Such objects were thought to form in supernova explosions, at the ends of the lives of massive stars – those so massive that they could not form neutron stars without gravity collapsing them still further, down to a single point in space. No force in nature could halt their relentless collapse. Prior to the 1980s, their formation had been conjectured by various gravitational theorists, but observational evidence for their existence had been rather circumstantial. The strongest such evidence had derived from stars which appeared to be orbiting rapidly around partners which were too faint to see, yet which needed to be very massive to account for their partners' orbital speeds. Often vast winds were seen from such stars, apparently being drawn towards their dark companions, often accompanied by weak X-ray emission from the vicinity of the black hole's proposed position, believed to arise from the heating of material as it spiralled inward towards its destiny.

However, Dr Almaini explained that at around this time, observations of active galaxies had also begun to show evidence for bodies too dense to be neutron stars. In M87, for example, spectral observations of the emission from gas within the central nuclear region had revealed emission lines which could be identified as being from known elements, but each had an unexpected profile. It appeared that they were being Doppler shifted towards redder wavelengths on one side of the nucleus, and towards bluer wavelengths on the other. This might be expected if the gas was orbiting around some unseen body at the galaxy's centre, but the orbital velocities, in excess of 100 km/s, required it to be nearly a billion times more massive than the Sun.

The same effect had since been observed in many other active galaxies; perhaps the strongest evidence for a black hole came from NGC 4258, where gas orbiting within half a light-year of the galaxy's centre appeared to be circling some unseen body of 39 million times the Sun's mass. A very tightly bound cluster of neutron stars might just be able to account for so much mass in such a small volume, but the formation of such a cluster was difficult to explain. A black hole seemed a considerably more natural explanation.

Having established that there was a weight of evidence for black holes at the cores of other galaxies, considerable attention had turned to investigating whether one might reside at the centre of the Milky Way, Dr Almaini explained. As far back as John Herschel's work, an over-density of globular clusters around Sagittarius had hinted the centre of the Galaxy to lie in this direction, though it was not until Shapley's measurement of the distances of these in the 1920s, illuminating their spherical distribution about a point in that direction, that this could confidently be propounded. In more recent times, a compact bright radio source by the name of Sgr A*, discovered in 1974, was believed to mark the exact spot of the Galactic centre. However, observing this central region was notoriously difficult, owing to the thick clouds of dust which enshrouded its innermost nucleus, concealing it from view in visible light. It was possible to observe it only in the infrared, radio, and X-rays, which passed relatively unhindered through the dust.

Over the past decade, the speaker explained, strong evidence had emerged for the presence of a black hole in this system. It derived from the motions of a cluster of stars known as the Central Cluster, localised within a few light-days of Sgr A*. The earliest indications of this cluster's existence dated from 1968, and, with improving instrumentation over subsequent decades, it had now become possible to directly observe its individual stars. In the early 1990s, a dedicated camera, SHARP, had been commissioned on the 4-metre New Technology Telescope (NTT) at the European Southern Observatory (ESO) in La Silla, Chile, with the purpose of monitoring their positions and measuring their velocities. Over the period 1994-2000, a time-lapse movie of their movements had been created, showing them to be orbiting at vast speed around some unseen body close to the position of Sgr A*. From these orbits, the mass distribution in the region could be estimated, but the result proved quite incompatible with any suggestion that the stars themselves might be the dominant masses in the region. Instead, some unseen body, of mass 3.7 million times that of the Sun, contained within a volume comparable to that of the solar system, seemed to dominate.

Describing this as the most remarkable and convincing result that he had seen in his career, Dr Almaini added that it meant that perhaps the most unarguable evidence for the existence of black holes anywhere in the Universe related to one within our own Galaxy.

The speaker closed his talk on a speculative note, outlining his personal, and he admitted slightly controversial, view, that black holes might play a critical role in regulating star formation in galaxies. He presented two pieces of evidence to support this idea. Firstly, he explained that the masses of the central black holes in a number of spiral galaxies had been measured, along with the total mass contained within their central bulge regions – an approximate proxy for the mass of the entire galaxy. The correlation between these masses was found to be remarkably tight. Whilst one might intuitively expect more massive galaxies to host more massive black holes, such tight correlation seemed indicative of some intimate physical connection between the two quantities.

Secondly, he explained that among distant galaxies were observed some very unexpected specimens. Galaxies typically fell into two categories: spiral galaxies, which were actively forming new stars, and elliptical galaxies, which contained no gas from which new stars could form. It was thought that the ellipticals formed from collisions and mergers between spiral systems, and so, among the most distant galaxies – those observed soonest after the Big Bang – few would be expected to be of this type. Yet ellipticals were observed, at distances of up to eight-billion light-years. That spiral galaxies could have formed, converted their gas into stars, merged, and then have been flushed of their gas to form an elliptical galaxy, all before this time, was hard to reconcile with pre-existing ideas: the time-scale was simply too short. Specifically, it was thought that the flushing of gas from elliptical systems was driven by blast waves from stars ending their lives in supernova explosions, but the time that this process would take would greatly exceed the time available to form the most distant elliptical systems observed.

The speaker put forward the opinion that these two puzzles had a common solution. The outflows of material from quasars might be responsible for blowing all of the gas out of galaxies at a certain point in their evolution, preventing any further star formation. The first puzzle would be explained because the time available for forming stars might depend upon the mass of the galaxy's central black hole. The second would be explained because the quasar might be able to strip the galaxy of its gas far more rapidly than supernova winds.

Following the applause for Dr Almaini's lively presentation, the President invited questions. Mr Nick James asked whether the stars shown orbiting the Milky Way's central black hole would eventually fall into it, and whether this would end the Galaxy's present period of quiescence. The speaker replied that this was likely: as the stars orbited, viscous drag from surrounding gas would cause them to gradually spiral into the black hole. However, it was difficult to estimate the time-scale over which this would take place, because our knowledge of the density of gas in this region was somewhat limited. In fact, the speaker added that the present quiescence of the black hole was something of a mystery: massive stars invariably produced substantial outflows of gas, and consequently, one might estimate the gas density to be quite high. But in that case one would expect to see it falling into the black hole: it seemed paradoxical that the black hole was quiescent, given the presence of massive stars nearby.

A member asked whether there was any evidence for jets around Sgr A*. The speaker replied that whilst there was radio emission from the region, it did not appear to be collimated into jets. Another member asked what the consequences would be for life on Earth if Sgr A* were to become active. Dr Almaini reassured him that, though spectacular, they would be largely benign: the region might glow as brightly as the Full Moon, but otherwise the activity would be harmless to humanity. Finally, Dr Nick Hewitt said that he recalled that dynamical studies of the Andromeda Galaxy, M31, had suggested it to have a binary black hole at its centre; he asked what this might tell us. Dr Almaini confirmed this recollection, and replied that it might be evidence that M31 had formed from two smaller galaxies. However, the time-scale for the coalescence of black holes after galaxy mergers was uncertain, and so it was difficult to comment further.

Following further applause, the President welcomed Mr Martin Morgan-Taylor, from the Campaign for Dark Skies (CfDS) to give a brief update on a recent development in legislation concerning light pollution.

Ashburn

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39.04°N
77.49°W
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