Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Hubble Science Briefing: The Real World: Black Hole Edition Chandra/CXC Dr. Eileen Meyer Space Telescope Science Institute 4 September 2014 Outline Part I: What is a Black Hole, Really? Part II: Black Holes in the Universe Part III: Not-so-Black Holes (Outbursts, Outflows, and Jets from super-massive Black Holes) Part IV: The Big Picture: Why do we care about Black Holes? 2 Part I: What is a Black Hole, really? There are many black holes in this picture, even if they don’t appear like the artist’s conception above. The ones we’re most sure about lurk in the centers of massive galaxies. (Image: Hubble Deep Field, NASA/ESA) 3 A black hole is a region of spacetime which is so dense that nothing, not even light, can escape (once inside the point of no return, called the event horizon). Many people associate black holes with Einstein and the Theory of General Relativity (“GR”, published in 1915). But, the idea actually existed as far back as the 18th century, using only a Newtonian understanding of Gravity: For a large enough mass (M) and small enough radius (R), you can get an escape velocity greater than the speed of light, the universal speed limit. In other words, a very compact object could be impossible to escape. 4 It is not an exaggeration to say that GR changed our entire view of the Universe. Under GR, space and time form a 4-dimensional construct called spacetime, which is curved in the presence of mass. We usually depict it as two-dimensional because it is easier to visualize. 5 GR is a theory that accurately predicts the behavior of the large-scale Universe. However, it says nothing at all about electromagnetism, or atoms, or anything on the very small scale (the scales on which we use quantum field theory (QFT) to understand what is going on). To put it most simply: we have a theory for large, heavy things (where QFT is not important). We have another theory for small, light things (where gravity is not important). We don’t have a theory for small, heavy things. The major problem in physics is to unify GR and QFT into a “grand unified theory” (GUT). 6 GR is incomplete precisely because it predicts black holes. Or more accurately, because it predicts singularities. Under GR, it is possible for a massive star to run out of the fuel needed to balance out gravity, and collapse. In fact, GR predicts that the star collapses to an infinitely small point with infinite density. Many people think this is true in real life, but it is not. The truth is, singularities (infinite physical quantities) tend to show up in physical regimes where the theory doesn’t apply anymore. 7 All physical theories have limits. Example: An opera singer can sing at a frequency that causes a glass to vibrate. The simplest theory, which describes this very well up to a point, says that these oscillations of the glass will grow to an infinite size, as the acoustic waves reinforce them. Of course, in real life, the glass will break. 8 Do black holes really exist? Yes. (see Part II) We expect black holes to exist, though we are in ignorance of precisely the description of matter inside them. Image credit: Ute Kraus 9 How to produce a black hole Normal stars exist in a state of equilibrium. Every point inside a star is balanced perfectly so that the gas and radiation pressure balance the selfgravity of the star. Any imbalances will cause the star to re-adjust (either shrink or puff up) until equilibrium is reached. Stars live for millions to billions of years (depending on their mass) in this way, slowly using up their Hydrogen and Helium through Nuclear Fusion. 10 How to produce a black hole But, eventually, all stars run out of “fuel”. When that happens, suddenly the inward force of gravity is unopposed, which causes the star to shrink in size and become denser and denser (“stellar collapse”). “Stellar Mass” Black Holes are those which are formed from stars a little more massive than our sun (say 5 to 15 times the mass of our sun). 11 A Common Misconception Black holes do not “suck”. Their gravitational force on any object is simply proportional to their mass, like every other object in the Universe. Thought experiment: if you replaced the sun with a black hole of the same mass, we would not be sucked in! Our orbit would not change at all. (Though we would freeze to death.) 12 (End of Part I) Next… Part II: Black Holes in the Universe Part III: Not-so-Black Holes (Outbursts, Outflows, and Jets from super-massive Black Holes) Part IV: The Big Picture: Why do we care about Black Holes? 13 Part II: Black Holes in the Universe Massive stars become black holes at the end of their lives (usually in quite spectacular fashion: gamma-ray bursts and supernovae). These are about 5 – 15 times the mass of our sun. Supernova N49 in the Large Magellanic Cloud Supernova N63A 14 Evidence for Stellar-Mass Black Holes Our theories predict black holes will form at the end of the life of a massive star, and we have observed the explosions that we believe accompany them. Supernova 1987A But, direct observation is difficult (after all, light cannot escape a black hole)! However, we can observe indirect signatures that point to a black hole. One case of this are systems called X-ray binaries. 15 X-ray Binaries Stellar-mass black hole X-ray binaries are extremely bright: just one can outshine all the other stars in a galaxy in X-rays. Where does all that energy come from? Normal companion star in close binary Powered by Accretion: The infalling matter releases gravitational potential energy, which ultimately heats up the accreting material to extremely high temperatures, so hot that they produce huge amounts of X-rays. 16 On a different scale entirely: Super-Massive Black Holes Most astronomers had no reason to suspect black holes more massive than several times our sun existed. Until we started noticing something odd about certain galaxies. At left, the optical spectrum for a normal galaxy. It is fairly smooth with no big spikes and only a few dips. Amount of Light (Brightness) These spikes and dips tell us about the chemical compositions of the stars in the galaxy, among other things. Increasing wavelength 17 The “strange” galaxies, first observed in the early 1900s, had very “spikey” spectra! We call these spikes “emission lines” – they indicate the presence of very hot gas in large quantities. Amount of Light (Brightness) Increasing wavelength These strange cases showed emission lines like you see in nearby Nebulae became known as “Active Galaxies” or AGN 18 Active Galaxies (AGN) AGN also have extremely luminous ‘Nuclei’. In some cases, the nuclei are so bright, we can’t even see the rest of the galaxy! Even more strange: they can “flicker”, changing brightness by a factor of 2 – 10 in less than a year. 19 Super-Massive Black Holes (SMBH) AGN must be powered by something extremely compact. After a lot of effort, we now know that AGN can only be powered by a super-massive black hole, with a mass of 1 million to 10 billion times the mass of the sun. The power source for AGN is the same as in X-ray binaries: accretion onto a black hole. But because the black hole is a million to a billion times bigger, the brightness is much more extreme. 20 How to detect a super-massive Black Hole How do you go about “weighing a black hole”? The most direct evidence is actually from the center of our own galaxy. Our SMBH is “only” about 4 million times the mass of the sun. 21 How to detect a super-massive Black Hole We also use Gas Dynamics to look for the signature of extremely fast-moving gas near the black hole. The range of speeds observed (dispersion) is directly proportional to the central mass. If you can show that the mass must be contained in a small enough region, then it must be a supermassive black hole. Image credit: NASA/ESA 22 (End of Part II) Next… Part III: Not-so-Black Holes (Outbursts, Outflows, and Jets from super-massive Black Holes) Part IV: The Big Picture: Why do we care about Black Holes? 23 Part III: Not-so Black Holes As should be clear by now, the main way we detect black holes is from the radiation coming from matter very close to them. We rely on the fact that the “extreme environment” near black holes are unique enough that we can thus identify them. 24 Accretion in AGN The main reason AGN are so bright is because of their accretion disks. A common misconception is that these black holes are swallowing enormous amounts of gas all the time. In fact it’s probably only a solar mass per year, or even a lot less. 25 AGN are still observed from far away Here is the typical artist’s picture of an AGN However, MOST of the components here cannot be directly imaged. They are inferred based on looking at different types of radiation. 26 Jets from AGN However, in about 10% of all AGN, there are some very, very big associated structures that we can see quite clearly. It should also be mentioned that the artist’s conception is quite out of scale! 27 In fact, we have even made movies of them. This is a Hubble image of the 1700-parsec long jet from an AGN known as “M87”. It is one of the nearest jets to Earth. Using over 400 images taken of this jet over nearly 13 years, we were able to actually measure the motions of the plasma as it extends out into space. 28 In fact, we have even made movies of them. (please see accompanying video file) Or visit http://hubblesite.org/newscenter/archive/releases/exotic/2013/32/ 29 The most extreme recyclers of energy in the Universe? In addition to the jets, it is now understood that many (probably all) AGN produce outflows of gas from the central part of the galaxy. These can be huge (3000 solar masses per year, which can clear a galaxy of the cold gas needed to support forming new stars) and also very, very fast (e.g., 3030 40% of the speed of light). A common misconception (partly due to scientific shorthand): AGN jets and winds are often described as coming “from the black hole”. This is technically incorrect! It is still true that nothing escapes a black hole once it has fallen in. The extreme gravity of the black hole is essential to the whole system, but the outflows are not coming out of the black hole event horizon. It is more accurate to say these jets and winds come from the accretion disk around the black hole. 31 (End of Part III) Next… Part IV: The Big Picture: Why do we care about Black Holes? 32 Part IV: The Bigger Picture (and some Questions) Currently, Astronomers are trying to put together the big picture: how the Universe evolved from the time of the Big Bang to the galaxies and clusters of galaxies we see today. 33 (Credit: NASA/WMAP Science Team) 34 Did AGN reionize the Universe? The redshift of an AGN tells us how far away it is. In Astronomy, light from distant objects was released earlier and earlier in the Universe. At a redshift of 10-20, the Universe was reionized by something very energetic, that existed everywhere. One possibility is AGN, another is the very first massive stars. We have only observed AGN (aka quasars) as far back as redshift 7, so we have to extrapolate to the epoch of reionization. 35 ESO/M. Kornmesser How did the SMBH form? We have evidence of extremely super-massive black holes (10 billion solar masses) at very high redshifts – when the Universe was only a few hundred million years old! You can’t grow a 10 billion solar mass BH in that time using accretion. 1. 2. Collapse of super-massive early stars (100-1000 times mass of the sun), followed by accretion? Direct Collapse of gas in the very early Universe (200-500 Million years old) into a “quasistar” This is an area of active research! 36 Do all galaxies host an AGN? Image Credit: Galaxy Zoo 37 How do jets impact their environment? NASA/ESA/STScI 38 Summary Black holes exist! They can be stellar-mass, up to billions of times the mass of the sun (and we’re looking for intermediate ones now). Black holes that we know about (except in our own galaxy) are usually accreting matter which radiates very brightly. Black holes are associated with some of the most energetic phenomena around, producing huge luminosities, particularly in the centers of galaxies. The SMBH at the centers of galaxies probably played a key role in how the Universe evolved into what we see today. 39