Back in the Fall of 2002 NASA scientist spotted two black holes in the galaxy NGC 6240, yielding for the first time proof of two super-massive black holes together in the same galaxy. These scientist predicted that the black holes will eventually merge in a few hundred million years. According to their reports, the daunting event will most certainly discharge fierce radiation and gravitational waves, consequently leaving in the aftermath a fearfully massive black hole.
The discovery drove researchers to reconstruct events that produced the images NASA’s Chandra X-ray Observatory revealed. Using Chandra they were able to”see” that the black holes were surrounded by hot swirling vortices (plural of vortex) of matter, which was interpreted as a pair of colliding galaxies called NGC 6240. It is believed that these two galaxies merging into one gave researchers a glimpse into perhaps how larger galaxies are formed. Because the process is a prolonged violent collision, scientist were able and are still able to glean much useful information for forming a hypothesis on the evolutionary sequence of galaxy formation. Thus, many in the scientific community believe NGC 6240 provides an important “missing link” in this evolutionary process.
Scientists have used Chandra (one of the Great Observatories, along with the Hubble Space Telescope, Compton Gamma Ray Observatory, and the Spitzer Space Telescope) to make a detailed study of the enormous cloud of hot gas enveloping two large, colliding galaxies. This unusually large reservoir of gas contains as much mass as 10 billion Suns, spans about 300,000 light-years, and radiates at a temperature of more than 7 million degrees.
This giant gas cloud, which scientists call a “halo,” is located in the system called NGC 6240. Astronomers have long known that NGC 6240 is the site of the merger of two large spiral galaxies similar in size to our own Milky Way. Each galaxy contains a supermassive black hole at its center. The black holes are spiraling toward one another, and may eventually merge to form a larger black hole.
Another consequence of the collision between the galaxies is that the gas contained in each individual galaxy has been violently stirred up. This caused a baby boom of new stars that has lasted for at least 200 million years. During this burst of stellar birth, some of the most massive stars raced through their evolution and exploded relatively quickly as supernovas.
The scientists involved with this study argue that this rush of supernova explosions dispersed relatively high amounts of important elements such as oxygen, neon, magnesium, and silicon into the hot gas of the newly combined galaxies. According to the researchers, the data suggest that this enriched gas has slowly expanded into and mixed with cooler gas that was already there.
During the extended baby boom, shorter bursts of star formation have occurred. For example, the most recent burst of star formation lasted for about five million years and occurred about 20 million years ago in Earth’s timeframe. However, the authors do not think that the hot gas was produced just by this shorter burst.
What does the future hold for observations of NGC 6240? Most likely the two spiral galaxies will form one young elliptical galaxy over the course of millions of years. It is unclear, however, how much of the hot gas can be retained by this newly formed galaxy, rather than lost to surrounding space. Regardless, the collision offers the opportunity to witness a relatively nearby version of an event that was common in the early universe when galaxies were much closer together and merged more often.
In this new composite image of NGC 6240, the X-rays from Chandra that reveal the hot gas cloud are colored purple. These data have been combined with optical data from the Hubble Space Telescope, which shows long tidal tails from the merging galaxies, extending to the right and bottom of the image.
A paper describing these new results on NGC 6240 is available online and appeared in the March 10, 2013 issue of The Astrophysical Journal.
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA’s Science Mission Directorate in Washington.
The understanding taking hold in our present times is that the formation of multiple systems of supermassive black holes is common in the Universe. Many galaxies undergo collisions and mergers with other galaxies, most of which contain supermassive black holes. It is thought that pairs of massive black holes can explain some of the unusual behavior seen by rapidly growing supermassive black holes, such as the distortion and bending seen in the powerful jets they produce. Pairs of massive black holes in the process of merging are expected to be the most powerful sources of gravitational waves in the Universe and arguably reveal our evolutionary process in their collision.