How to make a supermassive black hole

Apparently, the recipe is:
Step 1 - Crash two galaxies together
Step 2 - Wait for the two smaller black holes at the center of these galaxies to merge together, during which time lots of gas falls into them.
Step 3 - Go to Step 1.

Figuring this out was not as simple as it looks. Go here,
here, and here, and here to learn more.

How is a black hole like a Volcano?

Go here to find out.

That Flipping Black Holes

No, I'm not using a euphemism for swearing. Black holes can change the direction in which they spin (i.e, flip), and this can have some impressive results. Go here to learn more!

The Heart of Darkness

Well, of black holes at least. Go here to learn more.

Puff the Small Black Hole...

.. it can make such a big hole. Remember how a couple of days ago I linked to a post stating that astronomer now think that most of the X-ray emission from a black hole is generated by fast-moving material ejected (called "jets") which are from the disk of infalling material? Well, there is now evidence that these jets can clear out large cavities around the black hole, pushing the surrounding medium far away - as you can read here and here or in its full scientific glory here (library access required, sorry). This isn't new necessarily, evidence for such behavior from the supermassive black holes located in the center of galaxies has been around a for a while, but seeing it in a low-mass black hole that is nearby is new and suggests we may be able to study this process a lot better than before. Enjoy!

X-rays from Black Holes

Huh? Well, yes, black holes are "black" because their gravity is so strong that any light emitted from inside them can't escape. However, light can be emitted from outside black holes, and in fact it often is - it believed that many of the most luminous objects in the universe (quasars) are powered by material falling into a black hole. A recent study by RXTE suggests that the X-ray emitted by these objects is not from material falling into the black hole, but material flowing away from the black hole very rapidly in a narrow cone, called jets (go here for details). Very interesting indeed. To learn more about RXTE, listen to this interview I broadcast a while back.

Spinning Black Holes

One of the many mysteries surrounding astrophysical black holes is why some are powering powerful jets - narrow beams of relativistic particles, and others don't. An intriguing possibility is that is related to the spin of the black hole and the surrounding disk of material falling into the black hole - if the two are aligned, you don't get jets, and if they are misaligned, you do. Read here for details. This would be very hard to test, and it isn't clear how you might get such a misalignment, but it is in an intriguing possibility.

The "Quiet" Black Hole

at the center of M31, has its own bursts, as revealed by Chandra. Go here to read more.

Young Supermassive Black Holes?

Astronomers believed that at the center of essentially every galaxy is a supermassive black hole (supermassive = more material than a million Suns. Some are believed to have a mass more than a billion times that of the Sun). Where these black holes come from, no one really knows, but they gain mass by accreting (think swallowing) the surrounding gas. This gas forms a disk as it falls into the black hole, in which gas particles can stick together to form molecules - which astronomers call "dust." The older the disk, the more dust that should be there - or so goes the current thinking. By this logic, using Spitzer astronomers just found a couple supermassive black holes with very young disks. Go here to read more. Not necessarily the most convincing argument, but interesting nevertheless.

Speaking of stellar-mass black holes...

... it turns out that their formation might help their parent explode after all. Black holes as black widows seems fairly fitting, don't you think? Go here to read about it. This is one of the very few times where we actually observed the supernova and measured the properties of the produced compact object - and the first time for the limited set of explosions that it appears a black hole and not a neutron star was produced. Very exciting indeed.

The furthest black hole...

.. so far. And stellar mass black hole (the ones produced during the death of the most massive stars), not the supermassive black holes believed to be at the center of practically every massive galaxy (in which case, the most distant one is just the most distant galaxy). Go here to learn how it was discovered...

Interview with Dr. Katie Mack (Princeton University)

Broadcast way too long ago, and recorded well before that, finally available here is my interview with Dr. Katie Mack of Princeton University on her research into microscopic black holes and other bizarre particles which might have been created during the Big Bang. For more information, check out her webpage here and a comic strip that features her work here. Hope you enjoy, and as always, please email or leave below and comments, questions, or concerns you might have. Thanks for listening!

Description of January 21st Radio Show: Galaxy Formation

Previously available here, below is a description of the January 21st episode of this radio show, where I try to describe the current thinking (and many questions) on how galaxies formed:

• Galaxy Formation: Pretty much every galaxy is believed to have a super-massive black hole (SMBH) at its center, but is unclear if the galaxy formed before the SMBH or vice versa. Recent radio observations of distant galaxies suggest that the SMBH formed first, and then the galaxy around it (link), though recent optical observations of other distant galaxies suggest the opposite. As matter falls into the SMBH, it heats up and radiates a lot of energy, which is absorbed by the surrounding gas, causing it to heat up and possible decreasing the amount of future gas will fall into the SMBH. This process was originally believed to be very sporadic and violent, but recent evidence suggests this process may be gentler in some galaxies (link). Most galaxies are either spiral galaxies currently forming stars (like the Milky Way, referred to as "blue spirals" because young, massive stars cause the galaxy to appear bluish) or elliptical galaxies consisting almost entirely of old stars (referred to as "red ellipticals" or "red, dead galaxies" which old, low mass stars give the galaxy a red color). A population of red spirals do exist, and are either cases where the spiral galaxy ran out of gas and dust to form new stars, or have so much gas and dust that the blue light produced by young, massive stars is absorbed and the galaxy appears red (link). Galaxies where a lot of material appears to be falling into (a process astronomer call "accretion") the SMBH are often referred to as AGN, or "Active Galactic Nuclei." Discoveries of AGN with the Swift telescope have found differences in the types of galaxies that host nearby and distant AGN (link). It is currently believed that the massive (Milky Way-sized) galaxies observed today or the result of smaller, proto-galaxies merging together in earlier times. This process predicts that the relationship between different current properties of a galaxy (e.g., its size, mass, total light output, gas content) is complicated because they are effected differently by the merging process. However, a recent study found that all of these parameters just depend on the mass of the galaxy - which is very puzzling indeed. Even if this is true, there is a wide diversity in the properties of nearby galaxies, and to study this the Hubble Space Telescope has been measuring the properties of individual stars in these galaxies to determine the history of how quickly they formed stars, etc. (link). One feature of spiral galaxies is that some of these appear to have "bars" in the center (like the Milky Way does, actually), while others don't. A recent survey of galaxies done as part of the COSMOS project has found that the fraction of spirals with bars has tripled over the last 7 billion years (link) . It is currently believed that the merger of two galaxies together will lead to a short-lived but intense increase in the rate at which the resultant galaxy forms stars - called a starburst galaxy. If this correct, then astronomers expect that all startburst galaxies should either show morphological evidence for a recent merger, or be in a crowded regions of galaxies where mergers are likely to occur. An exception to this was NGC 1569, but recent Hubble observations allowed a new measurement of the distance to the galaxy, placing it in the midst of 10 other galaxies (link). Starbursts galaxies are a very active area of research since they play an important role in understanding the history of star formation in the universe, and observations of distant galaxies suggests that star formation in the universe peaked ~2 billion or so years after the Big Bang (about 12 billions years ago; link). The rate of star formation in these galaxies is huge, star formation rates as high as 4000 new stars a year (the Milky Way current makes about 10) has been estimated in some galaxies (link).
• News: Op-ed article in the November 24, 2008 New York Times on on-going problems at NASA with astronomy programs and others going over budget. Also in the New York Times, Dr. Aaron Hirsh write a guest column for their "The Wild Side" arguing for "citizen science" - stepping up a widespread data taking and analysis network. Astronomy has some similar networks, e.g. amateur astronomers searching for supernovae and other transient events, and projects like Galaxy Zoo.
• Wednesday Morning Astronomer: In this article, Gregg Easterbrook speculates that the excess of radio emission from deep outer space is the sound of interstellar war (No. Radio waves are actually light, not sound waves), and discusses recent observations of the Sun's motion around the center of the Milky Way that suggests the inner part of the Milky Way has 50% more mass than previously thought. One reader writes in that this was discovered using a simple method - which is very true, but very hard to do precisely.
• Calendar of upcoming Astronomy/science events in the greater Poughkeepsie/New York City area.
Galaxy Formation (continued): As I mentioned earlier, the merger of galaxies is believed to be the most important process in producing the galaxies we see today. Not surprisingly, what happens when two galaxies merge is very complicated, and much of our current understanding comes from studying the Antennae Galaxies - the nearest example of a galaxy merger. Recent observations of this pair of interacting galaxies change the distance from 65 million light-years to 45 million light-years, important in measuring the properties of these galaxies (link). A recent survey of interacting galaxies suggest that all galaxies have undergone a "major merger" in the last 6 billion years, and the peak in the merger rate of galaxies corresponds with the peak of the fraction of starburst galaxies in the universe - suggesting a link between the two. Evidence for a recent merger in a very distant galaxy has been uncovered thanks to gravitational lensing - which makes this galaxy appear much brighter than it ordinarily would (link). New observations suggests that SMBH were common inside galaxies 12 billion years ago, based on observations of two colliding galaxies at this time (link). The merging of two galaxies is also believed to drive gas towards the center of the galaxy, where it falls into the SMBH - creating a Seyfert galaxy. If so, the distribution of hydrogen gas inside such galaxies show evidence for being disrupted by such a merger - and they do (link). I said before that the merger/collision of two galaxies can lead to a period of intense star formation. Well, there is evidence that in some cases, this actually can cause star formation to stop (link). Additionally, the energy radiated by gas falling into the SMBH triggered by a galaxy merger/collision can also stop star formation in the outer part of the gas (link). Not just galaxies can merge / collide, this happens to galaxy clusters as well, and one example of a colliding galaxy cluster is surrounding by a diffuse haze of very low frequency radio emission (link). These collisions may also explain the existence of the magnetic field which exists in the void between galaxies inside galaxy clusters. (article)

As always, please leave below or email me any questions, comments, or concerns you might have. Thank you for listening!

Description of January 7 Radio Show: Milky Way Galaxy

Long available here, below is a description of the January 7th episode of the radio show, where I continue the "Tour of the Universe" with a description of the Milky Way:

• Galactic Center and Galactic Plane - At the very center of the Milky Way, there is believed to a black hole which has a mass approximately a million or so times that of the Sun called Sgr A*. Pretty strong evidence for the existence of such an object comes from studying the orbit of stars very close to Sgr A* (link). Measuring the mass of similar black holes believed to be at the centers of other galaxies is much more difficult since there you can't resolve the orbits of individual stars. Other methods which have been proposed are the tightness of their spiral arms (link), the temperature of the hot, X-ray emitting gas surrounding the galaxy (if it is a galaxy without much ongoing star formation; link). Surrounding Sgr A* is a disk of stars including the Sun, referred to as the Galactic Plane. They stars orbits Sgr A*, and a new, precise measurement of the rotation of stars in the Milky Way was recently made using a particular class of pulsating stars called Cepheids - which have also been used to measure the distance to other galaxies (link). The spiral arms in the Milky Way are not believed to concentration of particular stars, but where a significant number of stars are born "at once" - which is why they appear brighter than other parts of the Milky Way and contain a vast majority of the most massive (and therefore, very short lived) stars in the Galaxy. There is evidence that our Sun has traveled a considerable distance from its birth site in the Milky Way. Since young stars are clustered along spiral arms, and young, massive stars are the dominant source of ultraviolet radiation in a galaxy, ultraviolet images like the one the Swift satellite recently made of M33 (link), are good ways of studying the spiral structure in a galaxy. Spiral arms, and other features of the galactic disks make the results of gravitational interactions between galaxies (link). The Milky Way also contains clusters of stars, the densest and oldest of which are called "globular clusters", which are believed to be stars formed at the same time out of the same cloud of gas. A recent study of the a particular globular cluster measured its age using three different methods and got three different answers - a bit of a puzzle (link). As discussed on a previous show, the most massive stars in the Milky Way are believed to end their life in a supernova explosions, which actually plays a very important role in the properties of the gas which fills the Galactic plane of the Milky Way. Recent Chandra and Very Large Array observations have discovered the remnant of the most recent supernova explosion in the Milky Way, believed to have occurred only 140 years ago (link). The material released in these explosion expand with a very high velocity, heating the surrounding gas to very high temperatures and compressing into a thin shell of material which make for lovely Hubble images - and very interesting science (link). The hot gas and cosmic rays produced in the interaction between the material ejected in a supernova and its surrounding might explain the flow of gas out of the Milky Way's Galactic Plane, and carve bubbles out of nearby cold gas as observed in the Tarantula Nebula in the Large Magellanic Cloud (link). This is believed to explain why clusters of young stars are often surrounding by "holes" in gas, though this is not the case for nearby dwarf galaxy IC 2574. In most galaxies like the Milky Way, ionized hydrogen is only found in the center of these galaxies, most likely the result of star formation occurring only in certain regions and not distributed uniformly around a galaxy (link). Spitzer observations of a nearby spiral galaxy M101 shows that organic molecules are only present towards its center and not its edges (link; image. In the Milky Way, most star formation is currently taking place near the center of the galaxy, but not so in M83 where GALEX recently observed star formation at the outer edge (link)
• Wednesday Morning Astronomer: I understand and don't necessarily disagree with his point in this article, but astronomers did not "cavalierly" come up with the idea of Dark Matter - it was first proposed to explain some observations in the 1930s I believe, did not gain acceptance for at least 30 years, and still bothers many people, dark matter has been "located" (though not yet explained), the every controversial "dark flow" observed in distance galaxy clusters might be due to an unknown force, but not one outside our universe but one important on distances greater than the speed of light times the age of the universe which defines the "observable universe." I know that is pretty subtle, but it is an important distinction.
• Calendar of upcoming Astronomy/Science events in the greater Poughkeepsie/New York City area.
• Globular Clusters and Galactic Halo: Outside the Galactic Plane there are dense concentrations of millions of old stars referred to as globular clusters. Recent observations suggest that, even though they are "only" 9-13 billion years old, the structure of stars inside globular clusters is still evolving (link). It is though the some globular clusters are actually the remnants of the centers of dwarf galaxies which have been absorbed by the Milky Way. Since such galaxies are also believed to have black holes at their center, the presence of a massive black hole (much more massive than the Sun) might be proof this occurring. Such a black hole might have been found in globular cluster Omega Centauri (link), which - unlike other globular clusters - also contains dust (link). A similar process might explain the large number of globular clusters in M87, the giant elliptical galaxy in the center of the Virgo cluster - it "stole" them from lower mass galaxies which got too close (link). By measuring the velocity of the stars in the halo of the Milky Way, the diffuse "cloud" of stars which surrounds the Galactic Plane, it is possible to estimate the total mass of our galaxy. A recent such measurement suggests a lower mass than previously estimated (link). How the Milky Way's halo got there is an open, and interesting question. The two possibilities are that the stars formed out of the same cloud of gas that collapsed to form the Milky Way, or that it is the remnants of galaxies which have merged into the Milky Way. Recent studies of the structure of the halo suggest the second possibility, as do the presence of streams of stars in the halos of two nearby galaxies (here). It is expected that there are stars in the vast empty space between galaxies, and astronomers are searching for them (link). Last, but not least, it appears that most of the mass of galaxies is not in stars, gas, or dust, but in "dark matter" - this mysterious stuff that has mass but doesn't seem to produce light. Dark Matter is present in the dwarf galaxies which orbit the Milky Way - in fact, one of these has the highest ratio of dark to "normal" matter of any known object (link), and the galactic plane might be enclosed in a larger disk of dark matter(link). The distribution of dark matter in a galaxy is not expected to be entirely smooth but contain clumps and streams (link) which might be measurable using the motion of nearby stars in the night sky. An alternative to dark matter is the Newton's equation for gravity is wrong on very large distances (called MOdified Newtonian Dynamics, or MOND), which can do a good job reproducing the orbit of dwarf galaxies around the Milky Way (link). There are many fewer known dwarf galaxies around the Milky Way than expected, a problem for our current understanding of galaxy formation, though possible solutions have been suggested (link)

Thank you very much listening, and please email or post below any questions, comments, or concerns you might have.

Description of December 3rd Radio Show: Black Holes

Long available here, below is a description of the 2008 December 3rd episode of this radio show with focused on black holes. On this program, I discussed the following:

• Black Holes: Black Holes are objects believed to be so dense that light can not escape if it gets to close (i.e., past the "event horizon"). Additionally, its gravity is so strong that material that makes up a black hole can not arrange itself into a structure that can withstand its own gravity, so this material is thought to collapse into a point. But if one can't see too close to a black hole, how does one know? Well, it the above is correct, that black holes should not have a surface, unlike a neutron star. If a neutron star or a black hole is close enough to a normal star, its gravity is so strong that it will rip material off the surface of the normal star and cause it to fall on itself. This process is called "accretion", and as discussed previously on this radio show by Dr. Tod Strohmayer, this has process have been observed for many neutron stars and black hole candidates. In the case of a neutron star, this material will pile up on the surface of the neutron star, and this pile will got hotter and denser until it is so hot and dense that the hydrogen in this material fuses into helium, releasing a burst of heat and light which is observable by X-ray telescopes such as the Rossi X-ray Timing Explorer. One would not expect such bursts from material falling onto a black hole - which is more massive than a neutron star - since their is no surface for the material to collect. These X-ray bursts have been detected from neutron stars (for example this link), but not from any black hole candidates. Therefore, even there seems to be evidence that black holes indeed do not have surfaces. Additionally, black holes appears to have a mass either a few times that of the Sun (called "stellar-mass black holes"), or millions to billions times that of the Sun - like the black hole believed to reside in the center of our galaxy, the Milky Way (called "super-massive black holes"). Why there seems to be a lack of black holes with a mass between these two extremes (for example, a thousand solar masses) isn't known. One possibility is that such black holes exist, but reside in the middle of globular clusters where they would be hard to detect. A recent survey of globular cluster RZ2109 did not find such a black hole, suggesting that if this was correct they are extremely rare (link). Studies of the super-massive black holes suggest that their may be an upper-limit to how massive they can be, around 10 billion times that of the Sun (link). Since black holes do not have any structure, they are often thought to be some of the simplest objects in the universe (really!) - according to Einstein's theory of General Relativity, in order to completely describe the properties of a black hole you need to know its mass, its spin, and its electric charge (and real black holes in the universe and expected to have zero electrical charge). If so, stellar-mass black holes and super-massive black holes should accrete matter the same way - as observed for black holes in the nearby galaxy M81 (link). As mentioned before, it a normal star passes too close to a black hole, the black hole's gravity will cause material from the star to fall towards, and eventually fall inside, the black hole. As this material falls towards the black hole, it gets hotter - so hot that it produces a lot of X-rays. By looking at the periodic flickering of this light, it is possible to estimate the mass of the black hole. This was recently done for one black hole in the Milky Way, at they estimate a mass of just 3.8 times that of the Solar Mass, the lowest mass black hole known (link). Periodic flickering has also been observed from the super-massive black holes in the centers of other galaxies, and can be used to estimate their mass as well. This was done for galaxy RE J1034+396, which has a mass a million times that of the Sun (link). What causes the light from gas falling into a black hole to flicker is not known, but a recent study of the visible light flickering and X-ray light flickering from a Galactic black hole suggests that its magnetic field plays an important role (link). As mentioned before, there is a super-massive black hole in the center of our galaxy called Sgr A*, and material falling into this black hole also produces regular flares of light (link). In fact, there is evidence that, around 300 years ago, it produced a flash of light so bright that today we are seeing some of the light reflecting off molecular clouds near the Galactic center (link). A major goal of astronomers today is to make an image of light coming from the event horizon around a black hole. The best chance to do this is using radio telescopes to observe Sgr A*, and radio astronomers have gotten down to only 3 times the expected size of the event horizon (link 1, link 2). It is possible to use to orbits of objects around black holes to test general relativity (GR), just as one does this with neutron stars. This was recently done for the two black holes believed to be orbiting each other in center of galaxy OJ 287, and the time between the closest approach of these two black holes agreed with what GR predicts. The material that flows into a black hole is believed to form a disk (called an "accretion disk") around the black hole before it passes the event horizon. Quasars are believed to be galaxies where the optical light of this disk shines much, much brighter than all the stars in the rest of the galaxy. If so, the spectrum of the quasar - how bright the quasar is at different colors - should resemble that of a disk. This is hard to measure in great detail since the light at some colors is absorbed by material between the Earth and the galaxy, but a recent study suggests that it does (link). Additionally, for some unknown reason, the presence of gas flowing into the black hole is often associated with the presence of gas flowing AWAY from the black hole with very high velocities (>60 million mph). Astronomers at UCSC and University of Florida recently observed this outflow turn on around a quasar (link). Sometimes this very collimated outflow (called a "jet) is pointed directly at the Earth, in which case the galaxy is called a "blazar." Recent radio observation of a blazar suggest that these jets are powered by the magnetic field of the black hole (link). Quasars come in very different varieties, with one class recently observed to produce many more X-ray than previously thought to be possible (link). The presence of very, young stars around super-massive black holes like Sgr A* strongly suggest that stars can form in the accretion disk around them (article, link 1). Optical observations of these accretion disk have also detected weird optical filaments around them, suggesting this gas is confined by a weak magnetic field possibly generated by the black hole (link). While all galaxies are currently thought to have a super-massive black hole in their center, very few galaxies are quasars. Why some galaxies are quasars are most are not is unknown, but it is though that quasars are the result of two galaxies recently merging together.
• Calendar of upcoming Astronomy / science events in the greater Poughkeepsie / New York City area.
• Wednesday Morning Astronomer:
• News:

December 3rd Radio Show: Black Holes

I know this is unconscionably late, and I still owe you detailed descriptions of past shows, but here is the December 3rd episode of the radio show, where I discuss black holes big and small. Thank you very much for listening, hope you enjoy it, and hope you having a great holiday season.

Are you worried...

... that the Large Hadron Collider is going to create a black hole which is going to swallow the Earth? Don't be.

If people are interesting in this, I can talk about it in more depth on a future program. Please let me know.

Interview with Dr. Tod Strohmayer online

Available here is the interview I conducted with Dr. Tod Strohmayer of Goddard Space Flight Center on what you can learn about neutron stars and black holes from precise timing of their X-ray emission using the Rossi X-ray Timing Explorer, currently the oldest working X-ray observatory. Dr. Strohmayer maintains a very nice webpage which discusses his research, which I encourage all of you to check out.

As always, please email or leave below and question, comments, or concerns you might have. Thank you very much for listening!