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(Based on a talk given at the Annual Meeting of TA, 2010 at Wakefield) PART 1: Black holes and how we observe them Introduction As we peer deep into the night sky with the most advanced modern telescopes, we see that the Universe is literally teeming with galaxies (Figure 1). These magnificent objects, thousands of light-years across and comprising many billions of stars, are among the most fascinating and beautiful objects in the sky. However, galaxies also have a dark side: spectacular observations in recent years have revealed that essentially all large galaxies, including our own Milky Way, are home to remarkable and exotic objects called supermassive black holes. In this two-part article we begin with a brief overview of the physics behind black holes and how astronomers observe them. In the next part we move on to the nature and populations of galaxies and describe the detailed picture that has emerged for how galaxies form and change over cosmic time. Finally, we discuss how recent observational and theoretical work has shown that galaxies and their supermassive black holes are intimately linked as they grow and evolve over the age of the Universe. Figure 1 This composite Hubble Space Telescope image shows a variety of galaxies observed in a 3.5 arcmin square region for a total of 39.6 hours in the constellation Fornax. The nearby galaxies display a variety of structures and sizes, while more distant sources (some of which appear simply as small, barely resolved objects) are galaxies at earlier stages in the evolution of the Universe. The bright central object is a foreground star in our own Milky Way. This image is a typical view of the distant Universe and shows that the cosmos is literally teeming with galaxies. Credit: NASA, ESA, and the Hubble Heritage Team (STSci/AURA). Black holes Almost everyone has heard of black holes. They occupy a special place in the public imagination and rightfully so, for they are among the most exotic and interesting objects in nature. However, what exactly are black holes, and how do we know they exist? Perhaps the simplest and most intuitive definition of a black hole is an object whose gravity is so strong that nothing can escape, even at the speed of light. To understand this better, we can consider the idea of escape velocity. Imagine we are standing on the Earth and throwing a ball up in the air. The faster the initial speed of the ball, the higher the ball will go before it comes back down. Based on our understanding of gravity, as two objects move apart the forces between them decreases proportional to the square of the distance -- this is given by the famous equation which many may remember from their physics education: F = GMm/R2 (where F is the force, G is the gravitational constant, M and m are masses of the objects, and R is the distance between them). So as we throw the ball higher and higher, the pull of gravity on the ball becomes weaker and weaker, and we can imagine throwing the ball with such high speed that it leaves the gravitational field of the Earth completely and flies far into space. The speed at which this happens is called the escape

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velocity; for the Earth it is approximately 11 kilometres per second, which is why we need such powerful rockets to launch vehicles into deep space. The equations of gravity tell us that the escape velocity for a spherical object (such as a star or planet) is given by vesc = ¥GM/R. Thus, if we make an object more massive (larger M) or compress it (smaller R), then we increase the escape velocity. Taking this to its extreme, we can imagine taking an object as massive as the Sun and compressing it down to a radius of only 3 km, which implies an enormous density (a teaspoonful of this material on Earth would weigh many billions of tonnes!). This object would then be so massive and yet so small that the escape velocity at its surface would be equal to the speed of light. Since nothing in the Universe can travel faster than light, we infer that nothing can escape this object’s strong gravitational field. Such a remarkable entity is what we call a black hole. The concept of escape velocity gives us a clear and intuitive way of thinking about black holes, but unfortunately does not provide a full description of the physics behind these objects. For this we require a more complete description of gravity, which was provided by Einstein in his theory of general relativity in the early 20th century. Einstein's remarkable insight was that gravity is not simply a force between two objects that acts at a distance, but instead represents a fundamental curvature in the fabric of space and time in the Universe. Massive objects curve the spacetime around them, and the motion of objects follows straight lines in this curved space; thus the Moon orbits the Earth because it is following the curvature of space induced by the Earth's gravity. This is an extraordinary idea, but it makes some robust predictions, chief among which is that light rays (which have no mass, and so traditionally were not thought to experience gravity) are deflected as they pass close to the Sun. The observation of this effect provided among the first experimental proof of Einstein's theory. In light of general relativity, to fully understand black holes we must think of them as objects for which the strong gravity bends space so much that even light cannot escape. In this sense, black holes truly are black -- if we could see a black hole directly, we would see a black sphere, surrounded by images of background objects that had been deflected or

lensed by the strong gravity of the hole (Figure 2).

Figure 2

A simulation of an

observer’s view of a black

hole placed in front of the

Milky Way. The black hole

itself appears as a dark

sphere, while the

gravitational bending of

light by the black hole’s

gravity produces

“lensing” effects around

the outside of the event

horizon. Credit: Ute Kraus,

University of Tübingen.

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How do we observe black holes? Since black holes by definition cannot emit light, an obvious question is how do we observe them? While we cannot see black holes directly, we can observe how their strong gravitational fields influence the matter and gas around them. At present observations have revealed two separate types of black holes. The first have relatively small masses, roughly tens of times that of the Sun, which we can observe in binary orbits around normal stars. In many such systems, the normal star has run out of hydrogen fuel in its core and begins to expand, causing some of its mass to be captured by the gravity of the black hole companion. As gas spirals onto the hole, viscous interactions cause the material to heat up to temperatures of millions of degrees and glow extremely brightly, particularly at X-ray wavelengths (Figure 3). These objects, called X-ray binaries, are among the brightest X-ray sources in the sky. By studying the Doppler shifts of the companion star's spectrum as it orbits, we can measure the mass of its black hole companion. More than twenty of these black hole binaries are now known and their study is an active and extraordinarily fruitful area of research. Figure 3

An artist’s impression of a black hole X-ray binary. The system consists of a black hole (left) in a binary orbit with a normal star (right). Matter streams from the normal star onto the black hole and forms an accretion disc, which produces an enormous luminosity in X-rays. In some cases this accretion also launches relativistic jets of material from the region around the black hole. Such X-ray binaries provide very compelling evidence for the existence of black holes with masses of roughly ten times that of the Sun. Credit: ESO/L. Calçada.

For this discussion, however, the black holes we're interested in are much, much larger, with masses of millions to billions times that of the Sun, and they are found in the centres of galaxies. The best-studied example of such a supermassive black hole lies at the heart of our own Milky Way. This object, known as Sgr A*, is identified as a weak and variable radio source right at the dynamic centre of our Galactic bulge. In the last twenty years infrared observations, which can peer through the veil of dust in the disc of our Galaxy, have revealed a population of stars orbiting the location of the radio source. By measuring the orbits of the stars, we can deduce that the central object has a mass of roughly four million times that of the Sun, despite being remarkably faint (Figure 4). Decades of study of this object have led to the overwhelming consensus that it is indeed a supermassive black hole. Amazingly, this black hole is by no means unusual. We now know that essentially all large galaxies have supermassive black holes at their centres. In the second part of this article, we will describe the nature of galaxies and show how central supermassive black holes may play a fundamental role in how galaxies grow and evolve over the age of the Universe.

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Figure 4: Measured positions of stars around the centre of the Milky Way. The circles show annually measurements of the position for a number of stars, taken using infrared observations with the W.M. Keck Telescopes. The positions are overlaid on an infrared adaptive optics image from 2010. The sizes of the stellar orbits are on the order of tens of light-days, and from computing their Keplerian motions, the mass of the central object (marked by a star) can be estimated to be roughly 4 million times the mass of the Sun.Credit: This image was created by Prof. Andrea Ghez and her research team at UCLA.

References and further reading Gillessen, Stefan, et al. 2009, “Monitoring Stellar Orbits Around the Black Hole in the Galactic Center”, The Astrophysical Journal, 692, 1075-1109 Ghez, Andrea M. et al. 2008, “Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits”, The Astrophysical Journal, 689, 1044-1062 Mella, Fulvio, 2003, The Black Hole at the Center of Our Galaxy, Princeton University Press Remillard, Ronald A. & McClintock, Jeffrey E. 2006, “X-Ray Properties of Black-Hole Binaries”, Annual Reviews of Astronomy and Astrophysics, 44, 49-92 Shapiro, Stuart L. & Teukolsky, Saul A. 1994, Black Holes, White Dwarfs, and Neutron Stars: The Physics of Compact Objects, Wiley

Thorne, Kip 1994. Black Holes and Time Warps: Einstein's Outrageous Legacy. W W Norton & Company.

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SUPERMASSIVE BLACK HOLES AND THE GROWTH OF GALAXIES Ryan Hickox (continued from TA Vol 47 No 563 p294-297) PART 2: Galaxies and the beasts within Introduction In last month's issue of The Astronomer, we explored the physics behind black holes and the observational techniques astronomers use to study these fascinating and exotic objects. Observations have revealed that supermassive black holes, with masses up to a billion times that of the Sun, reside at the centres of essentially all large galaxies in the Universe. The second part of this article will examine the nature and origin of the galaxies themselves, and explore the fundamental role of black holes in the growth and evolution of galaxies over cosmic time. The nature of galaxies Galaxies are possibly the most majestic objects in the night sky, thousands of light-years across and containing hundreds of billions of stars. The grand spiral of our own Milky Way is just one of billions of such systems in the observable Universe, and galaxies are found in a fascinating range of shapes and sizes. Despite this variety, it has long been recognised that galaxies can be classified into two main types: disk-dominated and bulge-dominated systems (Figure 1). Figure 1 This Hubble Space Telescope image of the galaxy cluster Abell S0740 shows clear examples of the two main types of galaxies: a disk-dominated spiral galaxy (bottom left) with blue colours and significant ongoing star formation, and a bulge-dominated elliptical galaxy (top right) with red colours, and little ongoing star formation. Understanding the processes that gave rise to these two types of galaxies is one of the major challenges in cosmology. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Disk galaxies have flattened, rotating distributions of stars and often spiral structure like the Milky Way, and they tend to be undergoing significant star formation, as dense clouds of gas collapse under gravity inside the disk. This star formation activity produces a characteristic blue colour, as the light from the galaxy is dominated by a small population of massive, luminous, and hot stars that shine brightly before they run out of fuel and explode as supernovae. By contrast, bulge-dominated or "elliptical" galaxies tend to have higher masses than disk galaxies, and have roughly spheroidal morphologies, little large-scale rotation, and red colours indicating no ongoing star formation. A long-standing puzzle for astronomers has been why these massive bulge-dominated galaxies are "dead" - no longer forming stars - while their disky counterparts still have significant star formation. Another puzzle regards the origin of the relationship between

galaxies and their central black holes. A remarkable recent discovery is that the mass of the black hole is strongly correlated with the properties of the stellar bulge of a galaxy, rather than with the galaxy as a whole (Figure 2). This correlation suggests a fundamental link between the growth of the black hole and the structure and evolution of its host galaxy, yet the precise nature of this relationship is still poorly understood.

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Figure 2 Schematic diagram of the correlation between black hole mass and the stellar bulge of galaxies. Galaxies that are more massive and have larger bulges tend to host more massive galaxies. The remarkable tightness of this correlation suggests a link between the evolution of galaxies and their central black holes. Credit: K. Cordes, S. Brown (STScI). Galaxy formation and evolution To understand the answers to these and other questions about the nature of galaxies, we must first explore our general picture of how galaxies form and grow over the lifetime of the Universe. Recent years have seen the emergence of a remarkably successful “standard model” of cosmology, in which most of the mass in the Universe is made up of dark matter for which the only strong interaction is via the gravitational force. This cosmological model has been studied using large computer simulations that show how structure in the Universe forms: at early times, small over-dense regions collapse under gravity to become bound halos of dark matter. These halos grow and become increasingly massive with time, and are distributed in a filamentary structure known as the cosmic web (Figure 3). Figure 3: The large-scale spatial distribution of galaxies, from observational surveys of the real Universe (top and left), and from theoretical models based on the growth of dark matter structures and the physics of galaxy and star formation (bottom and right). The models are remarkably successful in reproducing the cosmic web traced out by galaxies as they form and evolve inside dark matter halos. Credit: VIRGO Consortium (Springel et al. 2006)

It is in dark matter halos that galaxies form and grow, as normal matter (mainly hydrogen and helium gas) cools and falls to the centre of the halo, attracted by the gravitational pull of the dark matter. Eventually, the gas becomes cold and dense enough that it begins to fragment and collapse into the first stars. Initially, a young galaxy settles into a disk-like structure that preserves the angular momentum of the cooling gas, and continues to grow as more and more material falls onto the disk along the cosmic web. Additional growth occurs through collisions, as two more galaxies merge together to form a single larger system. In the merger of two massive disk galaxies, the orbits of the stars can become disrupted and randomized, and this is believed to be the process by which the bulges of the most massive galaxies are formed. In recent years, theoretical models have been developed that include the processes of galaxy growth, star formation, and merging inside of dark matter halos, and can successfully reproduce many aspects of the overall population of galaxies. These models can also match how galaxies are distributed in space, residing in groups and clusters that trace out the cosmic web of dark matter (Figure 3). It is therefore clear that we are beginning to close in on an accurate physical picture to explain the formation and growth of galaxies.

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The remarkable role of black holes Despite this exciting progress, theoretical models have still faced some fundamental problems. Of these, perhaps the most troubling is the near-universal prediction that massive galaxies should be able to continually accrete new gas that cools and falls from the cosmic web into their large dark matter halos. This cooling gas would naturally form stars, thus producing massive, bulge-dominated galaxies with huge star formation rates and strikingly blue colours, in stark contrast to the observations showing that elliptical galaxies are usually red and dead. Among the normal processes of gas dynamics and star formation, there is nothing that can stop this "cooling catastrophe", which has thus proved a serious dilemma for galaxy formation theory. Figure 4: Examples of the huge amounts of energy released by rapidly growing black holes. The first panel shows Hubble Space Telescope images of quasars, for which the radiation released by the growing black hole outshines the the light from all the galaxy’s stars. The second panel shows a radio image of the powerful radio galaxy Cygnus A, which displays enormous jets of material moving close to the speed of light that are ejected from the black hole, extending hundreds of thousands of light-years into intergalactic space. Credit: J. Bahcall, M. Disney, and NASA (first panel), NRAO/AUI (second panel). In the last five years or so, a solution this problem has emerged from a rather unexpected source. We know that every massive, bulge dominated galaxy has a large central black hole, and that the black hole acquires its mass through the accretion of interstellar material over cosmic time. We also know that this accretion process can liberate huge amounts of energy (Figure 4), either in the form of radiation (as we observe in the most radiatively powerful objects in the Universe, the quasars) or as energetic outflows moving close to the speed of light (as in the spectacular

galaxy-scale jets of radio galaxies). Hydrodynamic simulations have shown that this enormous energy input from growing black holes can either blow gas out of a galaxy completely and thus quench ongoing star formation, or can re-heat the surrounding gas and thus stop it from cooling and forming new stars. In a number of cases, we can actually observe these processes in action, as energetic outflows from growing black holes evacuate spectacular bubbles in the gas atmospheres of galaxies (Figure 5). A host of new simulations have included energy input from black holes in tracing the growth of galaxies, and

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find, in general, that the black hole can indeed produce enough energy to shut off star formation in massive, bulge-dominated systems. Some prescriptions for black hole feedback can also naturally explain the remarkable correlations between black hole mass and bulge properties. It is therefore increasingly well established that to fully understand the evolution of galaxies, we must account for the energy released by their central black holes. Figure 5: X-ray and radio images of the massive galaxy cluster Hydra A, showing how energy released from black hole jets can profoundly affect the gaseous atmospheres around galaxies. The outer diffuse emission (in blue in the colour version) is Chandra X-Ray Observatory data showing X-rays from the hot atmosphere in the cluster, thus tracing the distribution of the gas. Clearly, the radio-emitting black hole jets (show in pink in the colour version) are evacuating huge bubbles in the gas atmosphere. The strong interaction between the jet and the gas transfers energy from the black hole to the much larger atmosphere, keeping the gas hot and stopping gas from cooling to form new stars in the central galaxy Credits: X-ray: NASA/CXC/U.Waterloo/C.Kirkpatrick et al.; Radio: NSF/NRAO/VLA; Optical: Canada-France-Hawaii-Telescope/DSS. Open questions and future horizons These discoveries about the importance of black holes in galaxy evolution have generated huge excitement and interest in the astronomical community. However, some key questions remain: What types of growing black holes are most important for shutting off star formation? Do galaxy-scale winds from quasars even exist? What physically causes the relationship between black holes and bulges? And finally, what comes first, the galaxy or the black hole? To answer these questions, new generations of observational tools will allow an increasingly detailed picture of galaxies, their central black holes, and their evolution over cosmic time. For example, the KMOS infrared spectrograph on the Very Large Telescope will enable much more sensitive searches for evidence of galaxy-wide outflows in distant quasars. The Atacama Large Millimetre Array, which comes on line in 2011, will be able to detect molecular clouds in galaxies from which stars form, and so will test directly whether energy input from growing black holes is disrupting these clouds. In the more distant future, the planned Wide Field X-ray Telescope satellite will identify the X-ray emission from millions of growing black holes, and so enable precise statistical studies of how growing black holes are linked to their host galaxies and large-scale structures. Along with ever more sophisticated theoretical simulations, these new observations promise to yield a much deeper physical understanding of how the galaxies around us, and their central black holes, came to exist. The fascinating story of black holes and galaxies has only just begun. References and further reading Bower, Richard G. et al. 2006, “Breaking the Hierarchy of Galaxy Formation”, Monthly Notices of the Royal Astronomical Society, 370, 645-655 Gultekin, Kayhan et al. 2009, “The M-ı and M-L Relations in Galactic Bulges, and Determinations of Their Intrinsic Scatter”, The Astrophysical Journal, 698, 198-221 Hickox, Ryan C. et al. 2009, “Host Galaxies, Clustering, Eddington Ratios, and Evolution of Radio, X-Ray, and Infrared-

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Selected AGNs”, The Astrophysical Journal, 696, 891-919 Hopkins, Philip F. et al. 2008, “A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies. I. Galaxy Mergers and Quasar Activity”, The Astrophysical Journal Supplement, 175, 356-389 Kitchin, Chris 2007, Galaxies in Turmoil: The Active and Starburst Galaxies and the Black Holes that Drive Them, Springel McNamara, Brian R. & Nulsen, Paul E. J. 2007, “Heating Hot Atmospheres with Active Galactic Nuclei”, Annual Reviews of Astronomy and Astrophysics, 45, 117-175 Robertson, Brant E. et al. 2010, “Early Star-Forming Galaxies and the Reionization of the Universe”, Nature, 468, 49 Springel, Volker, Frenk, Carlos S. & White, Simon D. M. 2006, “The Large-Scale Structure of the Universe”, Nature, 440, 1137-1144 Internet: ryan.hickox@durham.ac.uk (Editor: Although the title was shown on p294 of last month’s paper issue, it was omitted from some colour PDF files in error for which apologies) ______________________________________________________________________________ AURORAL NOTES Edited by Tom McEwan

2011 March. All times UT 01-02 Ian Brantingham (Banff) - 01:00, quiet light, green, through cloud. Tom McEwan (Dalry) - 22:50-

23:02, quiet arc through cloud, faint, h 4°, 7°. 06-07 Ian Brantingham (Banff) - 00:30-00:50, faint horizon light.

10-11 Ian Brantingham (Banff) – 21:00-02:00, faint horizon light, 10°, with weak rays at 01:00.

11-12 Ian Brantingham (Banff) – 21:00-02:00, quiet homogeneous arc, faint, 10-15°.

METEOR NOTES Edited by Tony Markham Eta Aquarids 2011 With New Moon occurring on May 3rd, the Eta Aquarid meteor shower is favourably timed in 2011. Activity starts in late April and continues through to mid May with peak rates occurring during May 4-5. The radiant is located at RA 22h20m, Dec –01. The shower arises from the Earth’s post-perihelion encounter with the meteoroid stream of comet 1P/Halley ; the Orionids arising from the pre-perihelion encounter. However, whereas there have been several occasions during the past 20 years on which enhanced Orionid rates have been reported, there have been no corresponding reports of enhanced Eta Aquarid activity. This could be due to less Eta Aquarid observations having been made. With the shower radiant being located near the celestial equator and the Sun in early May being at a more northerly declination, observers at northern latitudes can only see Eta Aquarid activity late in the night – and most meteor observers are located at such latitudes.