The density of stars increases smoothly as you move towards the center of normal sized galaxies. It has however been known for decades that there is a star shortage in the center of many big galaxies.

Astronomers at Swinburne University of Technology have recently investigated the millions of missing stars from the cores, or centers of two big galaxies. They believe they may have solved the riddle and the conclusion they have arrived at is not in line with traditional thinking on the matter. The team did however confirm that one of the depleted cores is the largest ever detected.

Dr Paolo Bonfini from the Universidad Nacional Autónoma de México and lead author of the paper explains that the galaxy with the smaller depleted core (the smaller of the two galaxies studied) probably formed from the collision of two similar galaxies. He suspects each of these were seeded with a black hole several billion times the mass of our Sun.

When black holes migrate towards the center of the newly forged galaxy, the stars already there are ousted and hurled outward in a gravitational slingshot maneuver. Two massive black holes work together and gang up on individual stars in a galaxy’s core, pushing them away. Swinburne’s Professor Alister Graham, co-author of the study, notes that less fortunate stars venturing too close to either black hole is torn apart and swallowed. This process has been studied numerous times and is well documented.

Graham also explains that high-energy UV and X-ray flares are produced when a star is shredded by the massive gravitational field around each black hole. Finally, a series of gravitational waves is emitted when the black holes themselves merge.

Things are however different if a galaxy collision involves a smaller satellite galaxy being consumed by a larger galaxy. Simulations of this scenario show that if the captured galaxy has a center that is densely crowded, only the outer stars are stripped off this tightly bound region and the rest survives largely intact.

Professor Ben Moore, galaxy modeler and Director at the University of Zurich‘s Center for Theoretical Astrophysics and Cosmology in Switzerland, notes that the stars are pumped out of the core of the large galaxy by the braking process that occurs when the captured galaxy moves towards the center of the large galaxy. The simulations reveal that they do this in such a way that it creates a core of constant stellar density. The semi-digested satellites then float around this core.

This galaxy contains several dense knots of stars near the edge of its center and the stars in the center of the bigger galaxy are uniform in their distribution. This is different from the smaller of the two galaxies the scientists studied. Each of these knots is capable of causing a lot of damage, as its mass is equal to nearly the entire stellar mass of our own Milky Way galaxy, or 45 billion times the mass of our Sun.

Marc Postman at the Space Telescope Science Institute in the USA discovered the larger galaxy’s giant core in 2012. It is approximately ten times larger than cores typically observed in other big galaxies, which is unusual. Graham suspects that the giant core didn’t form from massive black holes, but from captured satellite galaxies.

The smaller galaxy with the smaller core is near the Ursa Major constellation and is about 30 times the mass of the Milky Way. The bigger galaxy lies amid a large cluster of galaxies in the direction of the Hercules Constellation and is 75 times more massive than the Milky Way. It is also the brightest of the galaxies in that area. Both galaxies are approximately 4 billion light years away from us.

A new window on the study of galaxy cores and the processes that shape the evolution of massive galaxies has been opened by this research. The James Webb Space Telescope, which should be launched in 2018, will probably reveal how often satellite galaxies are consumed by enabling astronomers to image more galaxy cores better.

The full study was published in the Astrophysical Journal.