Supermassive Black Holes: The ultimate Galaxy Killers?
Dr Rebecca Smethurst, Christ Church, University of Oxford
Dr Smethurst is a research fellow at Christ Church working to try and understand how galaxies and their central supermassive black holes evolve together. In order to do this she gets the public to help out classifying galaxy shapes online with galaxyzoo.org. She is currently using the SDSS-IV: MaNGA survey data to figure out if supermassive black holes can stop a galaxy from forming stars. She is trying to prove whether this is actually happening across the observable galaxy population.
She is also a very keen science communicator and does a lot of media work. She has her own YouTube channel, Dr. Becky, where she posts weekly videos on unsolved mysteries, weird objects in space and a monthly round up of space news with her usual level of enthusiasm for all things science. She was short-listed for the Institute of Physics Early Career Physics Communicator Award and was named Audience Winner of the UK National Final of the FameLab 2014 Competition.
There are over 1 billion galaxies in the Universe, each home to over a billion stars and one central supermassive black hole weighing in at up to a billion times the mass of the Sun. This lecture focused on the research of astrophysicists trying to understand the current conflict between observations of galaxies and their supermassive black holes and our current best model of the Universe.
My notes from the lecture (if they don’t make sense then it is entirely my fault)
Astrophysicists have been trying to get rid of the ? in the title of this lecture for five years.
We don’t know all the answers – the work is interesting because of this.
We understand where to find the supermassive black holes. They are at the centres of galaxies. They have masses of about several billion times that of the Sun.
The term ‘galaxy’ as we know it is relatively new. Galaxies were initially discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.
In the 1920s Edwin Hubble started to measure distances to these ‘spiral nebulae’. In 1936 he produced a classification of galactic morphology that is used to this day.
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology and is regarded as one of the most important astronomers of all time.
Tuning-fork-style diagram of the Hubble sequence
Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.
Hubble’s scheme divided galaxies into three broad classes based on their visual appearance (originally on photographic plates):
Elliptical (also affectionately known as blobs) galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter “E”, followed by an integer n representing their degree of ellipticity on the sky. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters. They are dominated by old stellar populations, giving them red colours.
Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol “S”. Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol “SB”. Spiral galaxies are named by their spiral structures that extend from the centre into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the darker background stellar distribution behind the waves, hence making the waves visible.
Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity. They have used up or lost most of their interstellar matter and therefore have very little ongoing star formation.
Hubble wanted to understand where these different shapes came from. How do these tell us how the Universe evolved?
William Parsons (above right), 3rd Earl of Rosse KP PRS HFRSE (17 June 1800 – 31 October 1867), was an Anglo-Irish astronomer who had several telescopes built.
Above left is a drawing of the Whirlpool Galaxy by Rosse in 1845
In the 1840s Lord Rosse used his 72-inch (6 feet/1.83 m) telescope at Birr Castle, Parsonstown, County Offaly to perform astronomical studies and he discovered the spiral nature of some nebulas, today known to be spiral galaxies. In 1845 this telescope Leviathan was the first to reveal the spiral structure of M51, a galaxy nicknamed later as the “Whirlpool Galaxy”, and his drawings of it closely resemble modern photographs.
Whirlpool Galaxy (M51A or NGC 5194), the smaller object in the upper right is M51B or NGC 5195. This modern version gives more detail with side whirlpool galaxies.
The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an interacting grand-design spiral galaxy with a Seyfert 2 active galactic nucleus. It lies in the constellation Canes Venatici, and was the first galaxy to be classified as a spiral galaxy. Its distance is estimated to be 23 million light-years away from Earth.
The galaxy and its companion, NGC 5195, are easily observed by amateur astronomers, and the two galaxies may be seen with binoculars. The Whirlpool Galaxy has been extensively observed by professional astronomers, who study it to understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions.
Many more galaxies have been found since Hubble’s time.
One of the key aims of the astronomers who designed the Hubble Space Telescope was to use its high optical resolution to study distant galaxies to a level of detail that was not possible from the ground.
The Hubble Space Telescope in orbit as seen from the departing Space Shuttle Atlantis, flying Servicing Mission 4 (STS-125), the fifth and final Hubble mission.
It was decided to focus the Hubble telescope on a dark area of the sky to see if anything was there.
The Hubble Deep Field (HDF) is an image of a small region in the constellation Ursa Major, constructed from a series of observations by the Hubble Space Telescope. It covers an area about 2.6 arcminutes on a side, about one 24-millionth of the whole sky, which is equivalent in angular size to a tennis ball at a distance of 100 metres. The image was assembled from 342 separate exposures taken with the Space Telescope’s Wide Field and Planetary Camera 2 over ten consecutive days between December 18 and 28, 1995.
The Hubble Deep Field
The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe.
The Hubble Ultra Deep Field is located within Fornax, and the Fornax Cluster, a small cluster of galaxies, lies primarily within Fornax.
The patch of sky is less than 2 percent of the area of the full Moon as seen from Earth.
The Hubble Ultra Deep Field from 2004 represents the deepest portrait of the visible universe ever achieved by humankind. Using the improved capabilities of the Advanced Camera for Surveys, the camera installed during the 2002 servicing mission, a new Deep Field was observed, in the constellation of Fornax (the Furnace). Only about 5 of the bright spots in the photograph are stars. The rest are galaxies (about 5000 galaxies).
There are believed to be 100 trillion galaxies in the Universe. The Hubble deep field image is the closest we will get to a time machine. It is allowing us to see 13 billion years ago. It is a slice through the history of the Universe.
Timeline of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. On the left, the dramatic expansion occurs in the inflationary epoch; and at the centre, the expansion accelerates (artist’s concept; not to scale).
The Big Bang theory is the prevailing cosmological model for the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a very high-density and high-temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB), large scale structure and Hubble’s law (the farther away galaxies are, the faster they are moving away from Earth). If the observed conditions are extrapolated backwards in time using the known laws of physics, the prediction is that just before a period of very high density there was a singularity which is typically associated with the Big Bang.
In order to understand how the Universe was formed computer simulations have been developed.
Scientists have created the first realistic model of the universe, capable of recreating 13 billion years of cosmic evolution. The simulation is called “Illustris,” and it renders the universe as a cube (350 million light-years on each side) with, its creators say, unprecedented resolution: The virtual universe uses 12 billion 3-D “pixels,” or resolution elements, to create its rendering. And that rendering includes both normal matter and dark matter.
The rendering, importantly, also includes elliptical and spiral galaxies—bodies that, because of numerical inaccuracies and incomplete physical models, we’d been unable to see with such detail in previous simulations of the universe. It also does a better job than previous renderings of modelling the feedback from star formation, supernova explosions, and supermassive black holes.
The model, reported in the journal Nature, also takes us back to almost the origin of the universe—just 12 million years after the Big Bang. And that’s where the time machine component comes into play. Since light travels at a fixed speed, Illustris gives astronomers the ability to correlate light with time (so, say, a galaxy that’s a billion light-years away will look to us like it did a billion years ago). That’s an important new capability. While the Hubble and similar space telescopes allow us to gaze on the early universe, Illustris lets us follow a single galaxy as it evolves over time.
Illustris has 41,000 galaxies in its simulation—a mix of spiral
galaxies like our Milky Way along with elliptical galaxies. It represents five years of work on the part of the scientists from, among others, the MIT/Harvard-Smithsonian Center for Astrophysics and the Heidelberg Institute for Theoretical Studies in Germany. And it exists now in large part because computing technology has finally caught up with our aspirations for understanding the workings of the universe.
How many of the different size galaxies are there?
The Current Status of Galaxy Formation
Numerical simulations of large-scale structure have met with great success. However these same simulations fail to account for several of the observed properties of galaxies. On large scales, ∼0.01 − 100 Mpc, the assumption of cold, weakly interacting dark matter has led to realistic maps of the galaxy distribution, under the assumptions that light traces mass and that the initial conditions are provided by the observed temperature fluctuations in the cosmic microwave background. On smaller scales, light no longer traces mass because of the complexity of galaxy and star formation. Baryon physics must be added to the simulations in order to produce realistic galaxies. It is here that the modelling is still inadequate.
Role of feedback in modifying the galaxy luminosity function (SN = supernova and AGN = active galactic nuclei)
Could cold dark matter be the reason why the simulations don’t quite work?
The simulations are missing things out as the theory gives different results to observations (except when you consider out own Milky Way). Can including Super Massive Black Holes affect the simulations? They do.
An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an “active galaxy”. The radiation from an AGN is believed to result from the accretion of matter by a supermassive black hole at the centre of its host galaxy.
There are a lot of forces occurring. Radiation pressure outwards is occurring. There are lots of magnetic fields. These funnel lots of plasma upwards. This allows the heating up of cold gas to make stars etc.
As matter enters the accretion disc, it follows a trajectory called a tendex line, which describes an inward spiral. This is because particles rub and bounce against each other in a turbulent flow, causing frictional heating which radiates energy away, reducing the particles’ angular momentum, allowing the particle to drift inwards, driving the inward spiral. The loss of angular momentum manifests as a reduction in velocity; at a slower velocity, the particle wants to adopt a lower orbit. As the particle falls to this lower orbit, a portion of its gravitational potential energy is converted to increased velocity and the particle gains speed. Thus, the particle has lost energy even though it is now travelling faster than before; however, it has lost angular momentum. As a particle orbits closer and closer, its velocity increases, as velocity increases frictional heating increases as more and more of the particle’s potential energy (relative to the black hole) is radiated away; the accretion disk of a black hole is hot enough to emit X-rays just outside the event horizon.
Accretion disks are usually assumed to be threaded by the external magnetic fields present in the interstellar medium. These fields are typically weak (about few micro-Gauss), but they can get anchored to the matter in the disk, because of its high electrical conductivity, and carried inward toward the central star. This process can concentrate the magnetic flux around the centre of the disk giving rise to very strong magnetic fields. Formation of powerful astrophysical jets along the rotation axis of accretion disks requires a large scale poloidal magnetic field in the inner regions of the disk.
An astrophysical jet is an astronomical phenomenon where outflows of ionised matter are emitted as an extended beam along the axis of rotation. When this greatly accelerated matter in the beam approaches the speed of light, astrophysical jets become relativistic jets as they show effects from special relativity.
Most of the largest and most active jets are created by supermassive black holes (SMBH) in the centre of active galaxies such as quasars and radio galaxies or within galaxy clusters. Such jets can exceed millions of parsecs in length. Other astronomical objects that contain jets include cataclysmic variable stars, X-ray binaries and gamma-ray bursts (GRB). Others are associated with star forming regions including T Tauri stars and Herbig–Haro objects, which are caused by the interaction of jets with the interstellar medium. Bipolar outflows or jets may also be associated with protostars, or with evolved post-AGB stars, planetary nebulae and bipolar nebulae.
Artist’s impression of the dusty gaseous torus around an active supermassive black hole. ALMA revealed the rotation of the torus very clearly for the first time. Credit: ALMA (ESO/NAOJ/NRAO)
During the 1970s, scientists confirmed that radio emissions coming from the centre of our galaxy were due to the presence of a Supermassive Black Hole (SMBH). Located about 26,000 light-years from Earth between the Sagittarius and Scorpius constellation, this feature came to be known as Sagittarius A*. Since that time, astronomers have come to understand that most massive galaxies have an SMBH at their centre.
Astronomers have come to learn that black holes in these galaxies are surrounded by massive rotating toruses of dust and gas, which is what accounts for the energy they put out. However, it was only recently that a team of astronomers, using the the Atacama Large Millimeter/submillimeter Array (ALMA), were able to capture an image of the rotating dusty gas torus around the supermassive black hole of M77.
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.
These galaxies have supermassive black holes at their centres which are surrounded by accretion discs of in-falling material. The accretion discs are believed to be the source of the observed ultraviolet radiation. Ultraviolet emission and absorption lines provide the best diagnostics for the composition of the surrounding material.
In a typical Seyfert galaxy, the nuclear source emits at visible wavelengths an amount of radiation comparable to that of the whole galaxy’s constituent stars, while in a quasar, the nuclear source is brighter than the constituent stars by at least a factor of 100. Seyfert galaxies have extremely bright nuclei, with luminosities ranging between 108 and 1011 solar luminosities. Only about 5% of them are radio bright; their emissions are moderate in gamma rays and bright in X-rays. Their visible and infrared spectra shows very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4,000 km/s (310 to 2,490 mi/s), and are believed to originate near an accretion disc surrounding the central black hole.
Seyfert II galaxies are surrounded by obscuring toruses which prevent telescopes from seeing all regions of the em spectrum strongly. A lot of light is blocked out.
There is no single observational signature of an AGN. The list below covers some of the features that have allowed systems to be identified as AGN:
Nuclear optical continuum emission. This is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength;
Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted (‘reprocessing’). As it is thermal emission, it can be distinguished from any jet or disc-related emission;
Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons;
Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines;
Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation;
X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present;
X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.
Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the observable universe, it has a large population of globular clusters—about 12,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.
The galactic core of Messier 87 as seen by the Hubble Space Telescope with its blue plasma jet clearly visible (composite image of observations in visible and infrared light)
The Event Horizon Telescope image of the core of M87 using 1.3 mm radio waves. The central dark spot is the shadow of the black hole and is larger than the black hole’s event horizon.
The core of M87 contains a supermassive black hole, designated M87*, whose mass is billions of times that of the Earth’s Sun; estimates have ranged from (3.5 ± 0.8) x 109 M☉ to (6.6 ± 0.4) x 109 M☉, with a measurement of 7.22+0.34−0.40 x 109 M☉ in 2016. In April 2019, the Event Horizon Telescope released measurements of the black hole’s mass as (6.5 ± 0.2stat ± 0.7sys ) x 109 M☉. This is one of the highest known masses for such an object. A rotating disk of ionized gas surrounds the black hole, and is roughly perpendicular to the relativistic jet. The disk rotates at velocities of up to roughly 1,000 km/s, and spans a maximum diameter of 0.12 parsecs (25,000 AU; 0.39 ly; 3.7 x 1012 km). By comparison, Pluto averages 39 astronomical units (0.00019 pc; 5.8 x 109 km) from the sun. Gas accretes onto the black hole at an estimated rate of one solar mass every ten years (about 90 Earth masses per day). The Schwarzschild radius of the black hole is 5.9 x 10−4 parsecs (1.9 x 10−3 light-years), which is around 120 times the Earth–Sun distance.
M87 has a diameter of around 100,000 light years. Looking at an image of it the black hole would be a tiny spec and the jet would be about the size of the palm of your hand
M87 formed from a big galaxy merging with other galaxies. This is what will happen to the Milky Way.
The tuning fork diagram shows an evolution from simple to complex with no temporal connotations intended. Astronomers now believe that disk galaxies likely formed first, then evolved into elliptical galaxies through galaxy mergers.
When a galaxy forms, it has a disk shape and is called a spiral galaxy due to spiral-like “arm” structures located on the disk. There are different theories on how these disk-like distributions of stars develop from a cloud of matter: however, at present, none of them exactly predicts the results of observation.
It has been shown that the growth of Black Holes must be tightly connected to the processes governing the evolution of galaxies.
Haring & Rix (2004)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.
Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, which is traditionally assumed to be a supermassive black hole. Such black holes by definition cannot be observed directly (light cannot escape them), but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centres of ellipticals. The masses of the black holes correlate tightly with bulge properties. The M–sigma relation relates black hole mass to the velocity dispersion of bulge stars, while other correlations involve the total stellar mass or luminosity of the bulge, the central concentration of stars in the bulge, the richness of the globular cluster system orbiting in the galaxy’s far outskirts, and the winding angle of the spiral arms.
Theoretical studies have included the growth of SMBHs in the framework of cosmological galaxy formation models. Most of them assumed mergers and/or disc instabilities as triggers for BH accretion, and related the effectiveness of each accretion event to the galaxy properties.
Things we don’t know
Can black holes grow in galaxies without mergers? Mergers form bulges. We need to find galaxies without bulges.
Are galaxies affected by supermassive blackholes?
The Sloan Digital Sky Survey has created the most detailed three-dimensional maps of the Universe ever made, with deep multi-colour images of one third of the sky, and spectra for more than three million astronomical objects.
The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P. Sloan Foundation, which contributed significant funding.
A key factor leading to the creation of the project was the problem of what has been referred to as data deluge, where research produces vast sets of information to the extent that research teams are not able to analyse and process much of it. Kevin Schawinski, previously an astrophysicist at Oxford University and co-founder of Galaxy Zoo, described the problem that led to Galaxy Zoo’s creation when he was set the task of classifying the morphology of more than 900,000 galaxies by eye that had been imaged by the Sloan Digital Sky Survey.
When Galaxy Zoo first started, the science team hoped that 20–30,000 people would take part in classifying the 900,000 galaxies that made up the sample. It had been estimated that a perfect graduate student working 24 hours a day 7 days a week would take 3–5 years to classify all the galaxies in the sample once. However, in the first Galaxy Zoo, more than 40 million classifications were made in approximately 175 days by more than 100,000 volunteers, providing an average of 38 classifications per galaxy.
New pictures are still being added
Christopher John Lintott FRAS is a Professor of Astrophysics in the Department of Physics at the University of Oxford. Lintott is involved in a number of popular science projects aimed at bringing astronomy to a wider audience. He is the primary presenter of the BBC series The Sky at Night.
Supermassive black holes in disk-dominated galaxies outgrow their bulges and co-evolve with their host galaxies
The deep connection between galaxies and their supermassive black holes is central to modern astrophysics and cosmology. The observed correlation between galaxy and black hole mass is usually attributed to the contribution of major mergers to both.
The Isaac Newton Telescope or INT is a 2.54 m (100 in.) optical telescope run by the Isaac Newton Group of Telescopes at Roque de los Muchachos Observatory on La Palma in the Canary Islands since 1984.
It was used to obtain spectra of the Ha emission region for 5 additional sources using the Intermediate Dispersion Spectrograph from 21st-23rd May 2014.
The left column shows 5 randomly-selected spectra zoomed to show the Ha region of the spectrum. The right column shows the INT spectra for the 5 galaxies observed using the intermediate dispersion spectrograph. Each panel shows the same rest-frame wavelength range; observed wavelengths are shown on the top axis of each panel, with redshifts in the top right of each panel. All spectra show broadened Ha emission, confirming that the multi-wavelength AGN selection employed here efficiently selects unobscured AGN.
There is not a big sharp peak. Hydrogen gas is moving in the accretion disc. This allows you to calculate the size of the super massive black hole.
(Above left) Black hole-bulge relations: Black hole mass versus bulge stellar mass. Black or blue points show disk-dominated galaxies with bulge masses or upper limits. The best fit line to these data is shown as a solid line, with regions of ±3σ uncertainty shown as a shaded region. The fit properly incorporates the bulge mass upper limits as censored data. The dotted line and 3σ shading shows the fit if all upper limits are treated as secure measurements. Red open circles show early-type galaxies used to compute a canonical bulge-black hole relation (Haring & Rix 2004); we apply the same fitting method to these data and show the best fit line (red-dashed line) and 3σ uncertainties (red shaded region). (Above right) Black hole-galaxy relations: Black hole mass versus total stellar mass. The best-fit line to the diskdom sample is again shown in black (solid line), with the relation for early-types (Haring & Rix 2004) shown in red (dashed line; unchanged from left panel). The relations are consistent with one another. While the diskdom sample is consistent (in the left panel) with having no correlation between black hole mass and bulge mass and inconsistent with the relation between black hole mass and the bulges of early-type galaxies, the correlations between black hole mass and total stellar mass (right panel) are consistent for populations of disk-dominated and early-type galaxies, despite the very different dynamical and morphological configurations in these samples.
Observations and simulations
1) Can black holes grow in galaxies without mergers? Mergers form bulges. We need to find galaxies without bulges.
Answer – Yes, black holes can grow in galaxies without mergers and most of them do!
How is stuff driven to the centre?
2) Are galaxies affected by supermassive blackholes? Probably
Look at a sample from Galaxyzoo.
How many stars are forming?
Single observation per galaxy
Star Formation Rate Since the Big Bang
The accretion events that power AGN can be extremely energetic and this can have profound effects on a galaxy that harbours a growing SMBH. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that can effectively suppress the surrounding galaxy’s star formation activity. In this way, SMBHs can regulate the growth of their host galaxies by limiting the amount of stars they form. This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. However, observational evidence that this suppression actually occurs in AGN host galaxies is still tenuous at best. One of the goals identified by the CANDELS AGN working group is a better understanding of the connection between star formation activity and AGN activity in galaxies. This may soon be possible as infrared observations from the Herschel Space Observatory are now allowing us to measure the star formation rates of active galaxies far better than previously possible. This will provide the first clues as to whether star formation activity is indeed suppressed in galaxies harbouring highly energetic AGN.
CANDELS is a powerful imaging survey of the distant Universe being carried out with two cameras on board the Hubble Space Telescope.
Just as a thermostat controls the temperature in a house, it’s been discovered that black holes can help to control the thermal activity of the galaxy in which they live. Their regulating properties explain a perplexing mystery around a number of elliptical galaxies. These galaxies are awash with gas and dust, which is perfect for star formation, but the rate of creation of new stars is lower than expected. The reason why had puzzled scientists.
Two new studies, though, may have an answer. One, published in The Astrophysical Journal, examined elliptical galaxies in the early universe. The other, published in the Monthly Notices of the Royal Astronomical Society, examined elliptical galaxies in the nearby universe. They found that supermassive black holes can regulate the temperature of gas in a galaxy, preventing stars from forming, despite an abundance of fuel being available.
A layer of cool gas, perfect for star formation, surrounds a central black hole of an elliptical galaxy. As stars are created, they also serve as fuel for the black hole, which sucks up dust and gas from the stars. Eventually, the greedy black hole has over-gorged on star dust and it spits out a hot, energetic jet of particles. This jet heats up the surrounding, star-forming puddle so that it is too hot to form new stars. The star birth halts while the gas cools down. When it’s cool enough, star formation can restart and the process starts all over again.
The temperature fluctuations of the star-forming gas dictates how much energy the black hole releases, much like a thermostat. This give-and-take relationship the black hole has with the surrounding, cool gas is what regulates star formation and gives these galaxies a lower rate of star birth than expected.
These theories were used to compare a computer simulation of the gas flow in a galaxy to actual photographs. The models produced galaxies that look remarkably similar to the real images. This gives scientists a fantastic resource to predict how the galactic structures were formed.
The process of star formation being stunted begins when an active black hole jet blasts through the galaxy and propels out some gas from the atmosphere. This gas, once separated from the main galaxy, starts to cool down and form cold clumps of gas. These clumps then begin to rain back down into the galaxy; a downpour of star fuel.
These falling droplets of gas eventually cool down enough to form stars. The clouds of gas require extremely low temperatures of only a few degrees above absolute zero to form stars. If they were hotter, then the vibrations of the atoms would overcome the attractive force of gravity, and the atoms could never start to fuse and power a star. The star formation within these clumps of gas was visible using the Hubble telescope’s ultraviolet eye.
This puddle of star-forming gas around the black hole can now serve as fuel for the black hole. This is where the temperature regulation comes in. If lots of stars are born then there’s an increase in the amount of fuel for the black hole to guzzle into its accretion disk: a swirling disk of accelerating matter surrounding a black hole before it is sucked in. An increase in fuel leads to an increase in energy, which is released as a jet, which heats up the surrounding star-forming dust. This delays star birth until the gas can cool down and the whole process restarts.
Lead author of the second study Grant Tremblay, from Yale University, concluded in a statement: “We know that these showers are linked to the jets because they’re found in filaments and tendrils that wrap around the jets or hug the edges of giant bubbles that the jets have inflated.”
Star formation rates are believed by some astronomers to be declining over time.
By looking back at the younger galaxies in the Universe — the star formation rate back then was much higher than it is now! A typical galaxy from long ago is forming more stars on average than a galaxy now.
It’s interesting that the star formation rate has declined, and it’s interesting that it’s declined at the rate we’ve observed. But it’s not going to drop to zero any time soon, and if you sum up the total number of stars in our Universe’s future, it’s actually far greater than the number of stars that have already existed up until this point in time, a far cry from the “only 5% more than we have now” figure you may have read.
Although we might be approaching the peak of star density within our galaxy, we can very strongly say that the vast majority of stars that will ever call our galaxy home haven’t been born yet.
As predicted, there is a clear correlation between the mass of the central black holes and stellar mass in these galaxies. However, it was also noted that in cases where stellar mass was slightly smaller than expected (relative to the mass of their central black holes), star formation rates were lower. In some other cases, galaxies had larger-than-expected stellar masses (again, relative to their black holes) and their star formation rates were higher.
This correlation was not only more consistent than that observed between black hole mass and stellar mass, it occurred independently of other factors (such as shape or density). As Martín-Navaro explained:
“For galaxies with the same mass of stars but different black hole mass in the centre, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes.”
It was also noted that this correlation extends into the deep past, where the galaxies with supermassive central black holes have been consistently producing a comparatively low rate of stars for the past 12.5 billion years. This constitutes the first strong evidence for a direct, long-term connection between star formation and the existence of a central black hole in a galaxy.
Another point was the way it addressed possible correlations between AGN luminosity and star formation. In the past, other researchers have sought to find evidence of a link between the two, but without success. According to Martín-Navarro and his team, this may be because the time scales are incredibly different. Whereas star formation occurs over the course of eons, outbursts from AGNs occur over shorter intervals.
What’s more, AGNs are highly variable and their properties are dependent on a number of factors relating to their black holes – i.e. size, mass, rate of accretion, etc. “We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” said Martin-Navarro.
Further research will be conducted to determine exactly how central black holes arrest star formation. At present the possibility that it could be due to radiation or jets of gas heating up surrounding matter are not definitive. As Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, indicated:
“There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”
Part of determining how the Universe came to be is knowing what mechanisms were at play and the extent of their roles. With this latest study, astrophysicists and cosmologists can take comfort in the knowledge that they’ve been getting it right – at least in this case!
Most galaxies host a supermassive black hole (SMBH) at their nucleus (a supermassive black hole is one whose mass exceeds a million solar-masses.) A key unresolved issue in galaxy formation and evolution is the role these SMBHs play in shaping their galaxies. Most astronomers agree that there must be a strong connection because of the observed correlations between a SMBH’s mass and its galaxy’s luminosity, stellar mass, and the stellar motions in the galaxy. These correlations apply both in local galaxies and those at earlier cosmic epochs. But despite progress in studying SMBHs, how they affect their hosts is still not understood. In some suggested scenarios the SMBH suppresses star formation in the galaxy by expelling material. In others, like the merger scenario, the effect is the opposite: the SMBH boosts star formation by helping stir up the interstellar medium. Computer simulations have been undertaken to try to settle these differences, and they tend to show that cold gas flowing in from the intergalactic medium can feed both SMBH and galaxy growth.
Star formation is one of the principle markers of galaxy growth. Observations of galaxies have tried to measure the star formation by correlating the formation rate with the intrinsic luminosity (star formation heats the dust whose infrared emission can dominate the luminosity). However, the emission from the region around a supermassive black hole that is actively accreting, an active galactic nucleus (AGN), can easily be confused with the emission from star formation. X-rays or the emission of highly excited ions can be used to determine the AGN contributions independently, but these measures may be complicated by intervening dust extinction or other effects. Furthermore there is evidence that in small or less luminous galaxies, or in those at earlier cosmic epochs, other factors like element abundances strongly influenced the galaxy’s development.
CfA astronomer Belinda Wilkes and Joanna Kuraszkiewicz and five colleagues examined 323 galaxies known to host AGN from their strong X-ray emission (as measured by the XMM-Newton telescope) and also to have active star formation underway as determined by their far infrared emission (as measured with the Herschel Space Telescope). The galaxies are all at distances such that their light has been traveling from between about two to eleven billion years. Their statistical analysis of the sample finds that on average the AGN contributes about 20% to the infrared luminosity although it can sometimes be as much as >90%. They reach the important conclusions that there is no evidence (at least in this set of objects) for a strong correlation between the two or that AGN quench the star formation. In fact, it appears that both grow together.
Simulations have shown that gas accreting onto supermassive black holes in galactic centres produces high-energy jets; the released energy can expel enough cold gas to quench star formation.
Our own Milky Way and the nearby Andromeda Galaxy currently appear to be undergoing the quenching transition from star-forming blue galaxies to passive red galaxies.
Astronomers and cosmologists have pondered what role SMBHs have on galactic evolution, with some venturing that they have a profound impact on star formation. And thanks to a recent study by an international team of astronomers, there is now direct evidence for a correlation between and SMBH and a galaxy’s star formation. In fact, the team demonstrated that a black hole’s mass could determine when star formation in a galaxy will end.
The study, titled “Black-Hole-Regulated Star Formation in Massive Galaxies“, recently appeared in the scientific journal Nature. Led by Ignacio Martín-Navarro, a Marie Curie Fellow at the University of California Observatories, the study team also consisted of members from the Max-Planck Institute for Astronomy and the Instituto de Astrofísica de Canarias.
Galaxy Zoo: Evidence for rapid, recent quenching within a population of AGN host galaxies
Quenching galaxies and the search for evidence of AGN feedback https://arxiv.org/pdf/1609.00023.pdf
Coincidence or Cause?
Is the AGN the cause or the consequence of quenching, Could the AGN actually be triggered by the quenching mechanism? Or could the AGN actually cause the quenching?
Integral field survey
In an Integral Field Unit the field of view is divided into many cells or segments to obtain a comprehensive overview of the whole.
IFUs are used in astronomy to study extended objects, such as nebulae, galaxies or a crowded cluster of stars or galaxies in one shot, using a technique known as integral field spectroscopy. In this method, the signal from each cell or pixel of the field is fed into a spectrograph, which then generates a spectrum for each individual pixel. All the resulting spectra are arranged into a datacube (see Figure 1) which contains the entire 2D field of view plus the third dimension drawn from the spectrograph, which splits the light into its different colours or wavelengths (see Figure 2). Astronomers can use the wealth of information from integral field spectrographs to measure, for example, the motion of the gas in a distant galaxy or the distances to the different galaxies found in a field of view.
Viewing 127 spectra from each galaxy instead of 1. What are the time and the rate of quenching in each part of this galaxy? Is there a trend? Can you see star formation switching off?
Quenching is messy.
There does seem to be trends in the rate of decline of star formation caused by Super Massive Black Holes
Dr. Becky on youtube:
There are two theories of Black holes
1) Galaxies formed first then supernovae and then black holes
2) Super massive black hole formed first then the stars.
Integral field spectroscopy gets rid of gaps. You can move the aperture around so there is overlap.