A new study based on observations with the NASA/ESA Hubble Space Telescope has shown that so-called brightest cluster galaxies (BCGs) — the very bright galaxies that inhabit the cores of rich galaxy clusters — have been aligned with their surroundings for at least 10 billion years.
The giant elliptical galaxy in the center of this Hubble image is the most massive and brightest member of the galaxy cluster Abell 2261. Spanning a little more than one million light-years, the galaxy is about 10 times the diameter of the Milky Way Galaxy. Abell 2261 is located approximately 3 billion light-years away. Image credit: NASA / ESA / M. Postman, STScI / T. Lauer, NOAO / CLASH Team.
“The most massive galaxies in the Universe appear to know about their surroundings,” said lead author Dr. Michael West of Lowell Observatory and co-authors.
“It’s well established that the major axes of BCGs are often elongated in the same direction as the galaxy cluster in which they reside and, furthermore, that clusters themselves are aligned with their neighbors, a remarkable coherence of structures over many millions of light years.”
“With few exceptions, however, most studies of BCG alignments have focused on relatively nearby systems. Examining more distant clusters can provide a glimpse into how these alignments have evolved over time, yielding insights into the processes that have shaped galaxies over the history of the Universe.”
With this motivation, Dr. West and his colleagues from Finland and the United Kingdom used Hubble to observe 65 BCGs whose light has taken billions of years to reach our planet.
The team found that the BCGs were already aligned with their surroundings when the Universe was only 1/3 of its current age.
“It’s an important new piece of the puzzle, because it says that whatever caused these alignments happened early,” Dr. West said.
“Our results show that galaxy alignments were established very early in the Universe’s history,” said co-author Dr. Roberto De Propris, from the University of Turku.
There are different theories for why such alignments occur.
One is that giant galaxies grow by accreting smaller neighbors along preferred directions that reflect the cosmic web, a vast network of filaments connecting galaxies on large scales.
Another theory suggests that, given enough time, gravity’s relentless tug will slowly reorient the largest galaxies until they are aligned with the surrounding distribution of galaxies.
While the discovery of galaxy alignments at early epochs does not rule out either scenario, it does place increasingly tight time constraints.
The astronomers are eager to look further back in time by observing more remote clusters. But studying galaxies at the dawn of the time is not easy, even with Hubble.
The study was published this week in the journal Nature Astronomy.
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Michael J. West et al. 2017. Ten billion years of brightest cluster galaxy alignments. Nature Astronomy 1, Article number: 0157; doi: 10.1038/s41550-017-0157
