The largest “depleted galaxy” ever observed apparently did not form according to the most popular model.
The galaxy, designated 2MASX J17222717+3207571, is remarkable for its size (a total stellar mass of 4.44 trillion solar masses) and for the stars missing from its core (175 billion solar masses). By comparison, the Milky Way tips the cosmic scale at a meager 60 billion solar masses.
Astronomers call a galactic core “depleted” when the mass density of stars in its core does not follow the progression set by the outer part of the galaxy. The density of stars in most galaxies increases smoothly from the outer edge to the center. Astronomers can plot the actual progression in a galaxy and compare it with the expected, smooth progression. In some galaxies, the actual number of stars in the central bulge falls short of what is expected.
Depleted cores show up in only a small percentage of galaxies, but they are fairly common in very large galaxies, according to Alister Graham, a professor of astronomy at Swinburne University of Technology’s Centre for Astrophysics and Supercomputing. “It’s only galaxies with a spheroidal distribution of stars more massive than about 200 billion solar masses. As a general rule, this excludes all spiral galaxies.”
After a close inspection of what was thought to be the largest depleted galaxy core last year and finding that the initial analysis was mistaken, Graham and Paolo Bonfini, from Centro de Radioastronomía y Astrofísica in Mexico, decided to take a closer look at the next two largest depleted galaxies.
Graham and Bonfini, who was the lead author, published their results in the Sept. 23, 2016, Astrophysical Journal (Vol. 829, No. 81, “The Quest for the Largest Depleted Galaxy Core: Supermassive Black Hole Binaries And Stalled Infalling Satellites”; http://iopscience.iop.org/article/10.3847/0004-637X/829/2/81/pdf).
The smaller of the two galaxies, designated 2MASX J09194427+5622012, appears to have formed by the merger of two similar-size smaller galaxies. Each of the two merging galaxies had a supermassive black hole at its center. As the two cores merged into each other, some of the stars were flung out, the way an Earth spacecraft gains speed as it slingshots around a gas giant planet such as Jupiter.
“Some stars may be kicked right out of the galaxy,” says Graham, “while others now orbit in the middle and outer regions of the galaxy.”
Galaxy ’012 apparently formed through the “binary black hole scouring scenario,” a widely accepted model for how galaxies merge and form depleted cores.
The revelation came from the bigger depleted galaxy, which shows no evidence of containing a supermassive black hole. Galaxy ’571 has a core that is evenly distributed with stars — except for a handful of dense knots of stars near the core’s edge.
“When this model was first introduced, the cores it produced were so large, and the central mass deficits so great, that it was considered excessive,” says Graham. “Nothing so big had ever been observed. But that all changed with 2MASX J17222717+3207571 (‘571), which perfectly matches the unusually large, constant-density core seen in the simulations.”