So here is the paper. On a quick read, this is just another paper on what happens to gas when and star formation when an AGN gets involved. If you’ve listened to many episodes of Astronomy Cast, you might have heard me explain this before. AGN (short for Active Galactic Nuclei) are actively feeding black holes in the centers of galaxies. These giant monsters are capable of calmly devouring vast quantities of gas and dust with accretion rates (the rate they take stuff in) of anywhere from 1 to 100 Solar Masses a year. AGN can occur in all sorts of different galaxies, and once upon a time, our own Milky Way galaxy might have had one in its core. Spiral galaxies with lots of gas and dust can recover from their AGN phase to go on to billions of years of happy go lucky star formation. Elliptical galaxies aren’t so lucky.

If you look out at the sky with a reasonably large telescope and you explore the distribution of colors and shapes of galaxies, you’ll find that spirals are generally (but not always) blue, and elliptical galaxies are generally (but not always) red. Any of you who have played with Galaxy Zoo have probably seen this too, and cursed your fair share of boring red blobs. The only way to get a blue galaxy is to look at a system that contains lots of hot stars, and hot stars don’t live that long, so either you are looking at something with ongoing star formation or something that produced stars until very recently. This tells us that in general, spiral galaxies are actively forming stars, and ellipticals are not. As we look at these systems across other wavelengths, looking for gas and dust using Infrared and radio and all the colors of light in between, we also find that elliptical galaxies are poor in gas and dust and spirals are rich in those exact same things. This means ellipticals generally just don’t even have the stuff in them necessary to form stars.

The question is, how did they get that way?

The standard answer has always been that there is a burst of star formation (often triggered during the formation of the elliptical, or the merger of a spiral galaxy into an elliptical), and the gas and dust not involved in this sudden burst is driven into the super massive black hole in the center of the system (and when I say super, I mean something 10,000,000,000 solar masses or so in size!) This process causes the galaxy to first light up with star formation, then the AGN turns on, and in the end the AGN is all that is left signaling the recent event. The timescale for this process was originally thought to be determined largely by the rate of star formation, but star formation is a slow thing, with many billion years being required to eat away the gas and dust. But… with timescales that long we should see more blue(r) ellipticals. The longer something lives, the more likely we are to see it. (Think of being in a room with a variety of lights that randomly turn on and off that are on all sides of you. Those that turn on for the longest period of time are the ones that you are most likely to see, while those that flick on the smallest fraction of a second are very unlikely to be something you see well enough to learn anything about them.)

So here we were with a model and a universe that weren’t in perfect alignment.

But then came this paper, to tell us we had underestimated the effects of an AGN. In this paper, they look at a series of 24 galaxies (10 Star Forming, 10 AGN+Star Forming, and 4 AGN). What they found was about 200 Million years after the Star Forming turns on, the AGN kicks into full swing, and the Star Formation dies quickly – it dies in a fraction of the time predicted from models that use star formation to devour the dust. This indicates that the AGN is having more of an effect on the dust than previously thought.

They put forward in this paper that AGN feedback is responsible for destroying the gas that would have otherwise gone into star formation. This process was already thought to exist, but the magnitude of the role it plays hadn’t been assumed to be this big! It appears that the actively feeding AGN is able to heat and expel the gas very quickly. We knew this happened in GIANT galaxies, with GIANT AGN (think M87), the type of systems that often sport massive jets and other rather radical high energy phenomenas. What we didn’t know was that low luminosity AGN do the exact same thing. And that is just cool. (Or actually “That’s Hot”).

This paper gives us a cleaner insight into how effectively black holes can kill things. That is a very flippant remark, but to put it more scientifically – we now have new insights into how actively feeding black holes can disrupt the surrounding gas so effectively that they shut down star formation essentially instantly (on cosmic timescales at least). From alive to AGN to dead and red in a billion years flat, that may be the life of even the most mundane elliptical galaxy.

While this paper does use a lot of technical language, it explains itself well and is well referenced. If you feel like chewing through something that will hopefully change how we look at ellipticals, give it a read.

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On a completely unrelated note: There is a remarkably ugly huricane on its way to Texas. One of the worst nights of my life involved a tropical storm parked over Houston that caused flooding and tornadoes as far north as Austin (the barn I used to keep horses at flooded and several hundred horses had to be evacuated in the middle of the night with water rising faster then a human could run). Just looking at the satillite imagery is making quesy with anxiety. My heart goes out to everyone on the Gulf coast. Stay safe everyone.

The truth is, the cluster and our Sun had a falling out.

Once upon a time, somewhere in our galaxy, our Sun’s atoms were part of a giant molecular cloud. Approximately 7 billion years ago, that molecular cloud was bumped. Exactly what did the bumping no one knows. That anonymous bump so shocked the dark molecular cloud that in recoiled and collapsed in on itself. At first this inward spiral wasn’t at all dramatic, and an imaginary space traveler looking at this shocked cloud with her imaginary eyes might not have perceived the motion. Over time, however, momentum built up, and the collapse gained speed, with the densest parts of the cloud pulling themselves into fragments, as more ethereal parts were left behind to collapse more slowly. In one of these collapsing regions a womb of gas and dust that was neither too big nor too small began to glow as a single star exhaled its first breath of heat. As it grew and began to illuminate its surroundings, a disk formed; a disk containing just enough stardust to someday form 8 planets and a lot of harder to categorize smaller bits.

While this star, which would come to be called “The Sun,” was busy forming, its nursery mates were similarly busy growing, glowing, and in some cases even going an extra step and exploding. This stellar nursery was filled with screaming stars that wept radio waves and threw off high energy jets as they tried to find their way onto the main sequence. While these stars wailed and grabbed at matter, they also traveled as a pack around the galaxy. While we can’t do more than guess at the Sun’s original orbital position, we know that today it takes about 135 million years for the Sun to orbit the galaxy. Let’s assume for a minute that the Sun emerged from the center of of that cluster. This would put it in a position to watch some of its nursery mates race ahead around the galaxy, take less time to orbit, while other of its nursery mates slowly fell behind, taking longer to orbit (and a few just explode themselves into oblivion as supernovae). After a few orbits and a few hundreds of millions of years, these differences in speed caused the fastest (and slowest) stars to fall out of the cluster, as their positions no longer made it possible for the casual observer to match them up with their cluster of origin. Over time, differences in orbital velocities drew more and more of the stars away from their siblings. Eventually, it became impossible to tell exactly which stars made up those sibling stars to the Sun.

The Sun, like its sisters and brothers, simply fell out of the cluster as it raced around the galaxy, just as a runner might fall away from the pack.

We are an orphan system, alone in the galaxy. Unlike the majority of stars, our Sun has no companion. Having escaped the chaos of our home, we are now simply alone.

Using the INTEGRAL satellite, astronomers discovered 20 new binary systems that consist on neutron stars orbiting supergiant stars. These findings were presented by Dr. Sylvain Chaty (University Paris 7 / Service d’Astrophysique) at the first GLAST Symposium in Palo Alto, California. Using multi-wavelength observations, he and his team identified 20 sources of X-Ray light, and then did followup observations in optical and infrared light using ESO facilities (image above, credit: Paris 7). The IR and optical observations showed that the X-Rays originated from neutron stars passing through clouds of material surrounding super giant stars. The super giant stars 30 times the mass and 20 times the radius of the sun, and are at a stage in their life when they are puffing off roughly one Earth mass of material per year. This material forms a circumstellar cloud around the supergiant that blocks the majority of the giant stars light from reaching us here on Earth. Instead of a giant star, what we see is a cloud of hot dust and gas radiating in the infrared. (all warm things – including human readers of this blog – emit thermal energy in the form of infrared light). When a neutron star enters this gas cloud, its extreme gravity compresses and heats the gas around it until the material emits X-Ray light. In some of the observed systems, the neutron star’s entire orbit keeps it within the gas and dust cloud. In other systems, the neutron star’s orbit is shaped more like the orbit of a comet, and it spends some of its time in the cloud emitting X-Rays, and other time outside of the cloud. There is a really neat movie of this available on Chaty’s website.

In a second announcement at the same GLAST Symposium, a team of astronomers who operate the High Energy Stereoscopic System (HESS) in Namibia described how gamma-ray light can now be convincingly associated with star forming regions that contain massive young stars. These stars, Wolf-Rayet stars, are some of the highest mass stars known, and they live very short lives that are punctuated with a supernova burst and the formation of a black hole or neutron star. The HESS team found diffuse X-Ray emission surrounding a binary system of two Wolf-Rayet Stars. This system is the highest mass binary system known. The diffuse gamma rays come from an area roughly 28 pc in diameter – these is several 1000 times greater than the separation between the two stars! They suggest that the gamma-ray emission may be created by when accelerated particles from the high-energy region around the binary interacting with slower moving materials from the star forming region surrounding the binary. In this scenario, the binary Wolf-Rayet stars blow open a blister in the star forming region in which they reside. Particles within these region are shocked and accelerated. At the skin of the blister, surrounding material is able to leak in, and when it collides with the higher energy particles it is shocked into emitting gamma rays. This scenario has theorized for for a long time and was detailed in 1997 by Whiteoak and Uchida, but this is the first time all the pieces – gamma rays, Wolf-Rayet stars, and a star forming region – have all been found together.

So, what about the lobster? Currently we live during a period biologists refer to as the Holocene extinction. This is an extinction event that is largely driven by man. As we eat things, build things, and chemically treat thing we are killing off vast numbers of animals. It (may have) started with the wooly mammoth, a favorite cuisine of early man, and it continues today with sharks, sword fish, and modern elephants. I could continue my “We’re killing our planet” tirade, but others do it more effectively (see here, here, and here). So, back to the lobster. In a world where so many species are dieing off, it is really cool to find a new type of lobster, and indications that in the Philippines there may be 1000s of new critters just waiting to be classified. And, on a morbid note, each critter we find is one more critter whose genetic structure we can collect and save. Several groups have suggested DNA should be collected from as many life-forms on earth as possible so our planet can someday be re-populated with lost species. Admittedly, this would require cloning, and science still doesn’t know how to clone things consistently, but . . . You can’t clone species you don’t have dna for, so, I’m all for freezing genetic samples.

So, another day, another addition of species not covered in a text book that increase the diversity of our universe. Go science, go.

As I understand it, there are a couple different models. In one, a molecular cloud will very slowly, over lots and lots of time collapse due to gravity (some clouds formed with our galaxy still haven’t collapsed all the way into stars!). Higher density regions will collapse faster, and lower density regions will either get sucked into higher density regions, or just collapse very very slowly. Pretty much everything in the galaxy has some angular momentum due to inherent rotation. As the densities within a giant molecular cloud collapse, they begin to spin and flatten. There is a period of time during which gravity is pulling material into the center of the density while the radiation pressure from the warm gas is ejecting the material in jets. Luckily, the system is able to not blow itself apart in the process, and gravity wins. When a star turns on – when nuclear reactions start up in the center, the light from the star creates so much pressure on surrounding material that the inflow of mass stops and the star clears out the area around it.

Now, if all the stars in the universe where formed simply through the very very slow gravitational collapse and fragmentation of molecular clouds, we would live in a very boring universe. Shocks (such as those from supernovae, spiral density waves, and collisions) can speed up the collapse of gas by pushing stuff together (condensing it). In this scenario, only the highest density regions survive to form stars, and the lower density gas dispersed. Here is a way to picture it: Imagine you have a rake with very flexible light wieght tines. Thanks to the help of a squirrel, you have one small patch of lawn with an over density of leaves. When you rack that one section of leaves, leaves that aren’t part of the original clump get pushed into it, and the force from the rake condences the pile. If the clump gets big enough, with a lot of large friction with the ground, the tines may bend and leave that clump behind. In a similar way, the shock wave can push together a large density of material, and if the material is dense enough gravity will hold it together and it will grow into a star.

In colliding galaxies, massive amounts of star formation will be triggered by shockwaves from the collapse, but the material that doesn’t get turned into stars may get strewn through space or pushed into the central black holes. No matter its fate, after the collision, the two galaxies will be dead, and star formation will have ceased.

image credit goes to HST.