Some 130 million light years away, within the constellation Canis Major, two twinkling spiral galaxies are in the process of colliding, treating our eyes to a dazzling show.
The pair—NGC 2207 and IC 2163—has hosted the explosive deaths of three stars as supernovas in the last 15 years, and is also spawning stars at an intense rate. But scientists have become particularly interested in these merging galaxies for another reason: They are home to one of the largest known collections of super bright X-ray objects. These so-called “ultraluminous X-ray sources” have been spotted using NASA’s Chandra X-ray Observatory, which is partly responsible for the lustrous composite image shown above.
Just like the Milky Way, these galaxies are teeming with bright sources of X-rays called X-ray binaries. These are systems in which a normal star is closely orbiting a collapsed star, such as a neutron star or black hole. Strong gravitational forces from the compact star draw material from the normal star, a process known as accretion. And when the material hits the companion star, it is heated to millions of degrees, a process that generates a huge amount of X-rays. While intense, the emission from these systems pales in comparison to that from ultraluminous X-ray sources (ULXs).
As the name suggests, ULXs are exceedingly bright; they emit more radiation in the X-rays than a million suns would at all wavelengths. Although they are not well understood, many believe they could be black holes of approximately 10 solar masses that are collecting, or accreting, material onto a disk, emitting X-rays in an intense beam. ULXs are also extremely rare; our galaxy doesn’t have one, most galaxies don’t, but those that do usually only have one. Between NGC 2207 and IC 2163, however, there are 28.
Scientists have been studying the X-ray emissions from the colliding pair for some time now using data collected by Chandra (the pinkish purple in the composite above), GALEX, which observes in UV, and the Spitzer Space Telescope, which gathers infrared data. The Hubble Space Telescope also collected optical light data on the merging galaxies, which was included in the striking composite.
X-ray. Chandra X-ray Observatory
According to the study, the combined observations highlight a strong correlation between X-ray sources and the rate at which stars are forming. As can be seen in the composite, X-ray sources seem to be most concentrated in the spiral arms of the galaxies, where lots of stars are in the process of formation. This also suggests that the companion stars are probably very young.
It’s well established that galactic collisions trigger intense rates of star formation. The encounter causes shock waves to ripple through clouds of dust and gas, which ultimately leads to their collapse and thus the formation of stars. Indeed, the researchers estimate in The Astrophysical Journal that the stars associated with the observed ULXs are very young, perhaps only 10 million years old.
Infrared. Chandra X-ray Observatory.
Optical. Chandra X-ray Observatory