NGC 1277, a lenticular galaxy in the constellation Perseus,
NGC 1277 is composed exclusively of aging stars that were born 10 billion years ago. But unlike other galaxies in the local Universe, it has not undergone any further star formation.
Astronomers nickname such objects as ‘red and dead’ galaxies, because the stars are aging and there aren’t any successive generations of younger stars.
“We can explore such original galaxies in full detail and probe the conditions of the early Universe,” said lead author Dr. Ignacio Trujillo, from the Instituto de Astrofisica de Canarias, Spain.
Dr. Trujillo and colleagues found that NGC 1277 has twice as many stars as our Milky Way Galaxy, but physically it is as small as one quarter the size of the Milky Way. Essentially, this galaxy is in a state of ‘arrested development.’
Perhaps like all galaxies NGC 1277 started out as a compact object but failed to accrete more material to grow in size to form a magnificent pinwheel-shaped galaxy.
“Approximately one in 1,000 massive galaxies is expected to be a relic galaxy, like NGC 1277,” the researchers said.
“We were not surprised to find it, but simply consider that it was in the right place at the right time to evolve the way it did.”
The telltale sign of the galaxy’s state lies in the ancient globular clusters of stars that swarm around it.
Massive galaxies tend to have both metal-poor (appearing blue) and metal-rich (appearing red) globular clusters.
The red clusters are believed to form as the galaxy forms, while the blue clusters are later brought in as smaller satellites are swallowed by the central galaxy. However, NGC 1277 is almost entirely lacking in blue globular clusters.
“I’ve been studying globular clusters in galaxies for a long time, and this is the first time I’ve ever seen this,” said Dr. Michael Beasley, also from the Instituto de Astrofisica de Canarias.
The red clusters are the strongest evidence that the galaxy went out of the star-making business long ago.
However, the lack of blue clusters suggests that NGC 1277 never grew further by gobbling up surrounding galaxies.
By contrast, our Milky Way contains approximately 180 blue and red globular clusters. This is due partly to the fact that our Milky Way continues cannibalizing galaxies that swing too close by in our Local Group of a few dozen small galaxies.
It’s a markedly different environment for NGC 1277. The galaxy lives near the center of the Perseus cluster of over 1,000 galaxies.
But NGC 1277 is moving so fast through the cluster, at 2 million mph, that it cannot merge with other galaxies to collect stars or pull in gas to fuel star formation. In addition, near the galaxy cluster center, intergalactic gas is so hot it cannot cool to condense and form stars.
The astronomers started looking for ‘arrested development’ galaxies in the Sloan Digital Sky Survey and found 50 candidate massive compact galaxies.
Using a similar technique, but out of a different sample, NGC 1277 was identified as unique in that it has a central black hole that is much more massive (17 billion solar masses) than it should be for a galaxy of that size.
This reinforces the scenario that the supermassive black hole and dense hub of the galaxy grew simultaneously, but the galaxy’s stellar population stopped growing and expanding because it was starved of outside material.
“I didn’t believe the ancient galaxy hypothesis initially, but finally I was surprised because it’s not that common to find what you predict in astronomy,” Dr. Beasley said.
“Typically, the Universe always comes up with more surprises that you can think about.”
Michael A. Beasley et al. A single population of red globular clusters around the massive compact galaxy NGC 1277. Nature, published online March 12, 2018; doi: 10.1038/nature25756