Ever tried to comprehend the magnitude of the cosmos and some of its most energetic processes? If so, the X-ray halo around the merging galaxies known as NGC 6240 may just blow your mind.
You’ve probably already seen the pretty picture by now, but take a minute to try and imagine the scale. This gas cloud is 350,000 light-years across, and the particles that make it up are a scalding 7.5 million degrees Celsius. This incredibly diffuse yet energetic gas has a mass of 10 billion suns, but a cubic foot in this space has an average density of just two or three particles in it. Yup, it’s HUGE.
The mess of a galaxy that is NGC 6240 as seen in the optical image is the result of a recent merger between two spiral galaxies. When these two disks collided to form the beautiful wreckage that we see, each galaxy brought its own diffuse, X-ray emitting halo of hot gas to the mix. Though in a merger no stars ever collide due to their great distances between each other, the gas in the galactic disk and halo most certainly do mix and collide with spectacular results.
The X-ray emission shows abundance of certain elements in the recent observations taken with NASA’s Chandra X-Ray Observatory that indicate that there was a prolonged period of star formation, likely caused by the merger of gas in the galactic disks. Instead of being a short, intense burst, this period of star formation proceeded at a steady rate over a period of some 200 million years.
Star formation creates stars of all sizes, but the largest ones are the first to die, exploding in energetic supernovae that create heavy elements and spread them far and wide. Some particles will seed the gas clouds that create new stars, but some have escaped out into this X-ray halo where the gas is simply too diffuse and moving too quickly to ever form stars.
NGC 6240 provides a unique opportunity to study galaxies at a certain stage of evolution. It is a ULIRG, or ultra-luminous infrared galaxy, and a nearby one at that. Such galaxies produce a prodigious amount of infrared light, and thus can be seen out to great distances in the universe. However, it is not always clear how much of that light comes from star formation and how much from the actively feeding supermassive black holes at the center of galaxies. Since NGC 6240 is relatively nearby, at “just” 400 million light years away, we can study the processes in detail and extrapolate out to galaxies at further distances.
NGC 6240 has a bonus if you love black holes. Each galaxy brought its own supermassive central black hole to the merger, so this mess of a collision still has the two original black holes actively feeding on the gas surrounding them. Eventually, these will spiral in towards the center of the merged galaxy and collide to form an even larger black hole. Scientists looking for gravity waves eagerly anticipate such events, but this particular merger won’t happen for a long time to come?
What is the ultimate fate of this system? Eventually, the galaxy will settle down into a elliptical galaxy with its stellar orbits randomly mixed due to the merger. It is not clear how much of the hot x-ray gas will remain in the system and how much will escape, but whatever is left will settle into a new gas halo around the young elliptical galaxy. But knowing all that doesn’t make it any less pretty, does it?
Image: The new X-ray image from Chandra (purple) overlaid on an optical image of NGC 6240. Credits: NASA/CXC/SAO/E.Nardini et al, NASA/STSci
This work was published in Astrophysical Journal, and a preprint is available on arXiv.org.