Bright clusters of galaxies --- seen as far back as ten billion light years in cosmic time --- appear to be aligned to their local surroundings on scales of millions of light years, an international team of astronomers now reports.
In a paper appearing in the journal Nature Astronomy, lead author Michael West and colleagues note that Hubble Space Telescope (HST) observations of 65 galaxy clusters suggest that the brightest galaxies in these clusters have a unique formation history; one “influenced by development of the cosmic web over billions of years.” The team was just as surprised to find that these alignments date back to an epoch when our 13.8 billion year-old cosmos was only a third of its current age.
Credit: ESA/Hubble, NASA, HST Frontier Fields.
A new study based on observations with the Hubble Space Telescope has shown that the most massive galaxies in the universe have been aligned with their surroundings for at least ten billion years.
As the authors note, astronomers once assumed that galaxies were randomly oriented across the cosmos, but now it’s clear that at least some have what they term preferred orientations with respect to their surroundings.
The most likely scenario to explain such orientations is that the largest galaxies in such large clusters grew by cannibalizing smaller ones. West, an astronomer at Lowell Observatory in Flagstaff, Ariz., likens the largest such galaxies to spiders waiting for prey at the center of a web. “Only it’s galaxies rather than bugs that are being eaten, and the web isn't circular, it's elongated in certain directions,” he said.
West says the orientations of such galaxies apparently come about because large galaxies capture smaller ones along local strands of the cosmic web. But he says only the brightest member of each of these galaxy clusters seems to be elongated in the same direction as its host cluster. However, the brightest galaxies are almost always found at the very center of their host cluster, he says.
Galaxies in other locations, West says, have led somewhat different lives, and consequently haven't ended up aligned with their surroundings. But he was most surprised by the fact that the team observed these alignments when the universe was only one third its current age. That is, during an epoch when galaxies, galaxy clusters, and the vast cosmic web were still in their infancy.
What's next?
The aim is to look far enough back in cosmic time to observe an epoch before these clusters became aligned. That would help researchers rule out one or more theories for their origin.
West says these alignment revelations are important for cosmology, because they give us new insights into the birth and evolution of the cosmos’ most massive galaxies.
Perhaps the biggest conundrum however is why less bright galaxies show little if any such alignment.
“ Clearly, there's something special about the brightest galaxies compared to others , something that sets them apart from the usual galactic riffraff,” said West.