Why do galaxies align?
Michael West, Astronomy | Published: Wednesday, October 3, 2018
The matter in our universe forms filaments and threads like a grand cosmic web, tugging galaxies and clusters into place along the way
Michael West, Astronomy | Published: Wednesday, October 3, 2018
The matter in our universe forms filaments and threads like a grand cosmic web, tugging galaxies and clusters into place along the way
One of the most striking features of the distribution of matter in the universe is its filamentary appearance, with long, luminous strands of galaxies woven together into a vast cosmic web.
Nowhere is this more evident than the Perseus-Pisces Supercluster. This colossal chain of galaxies snakes across more than 50° of the northern sky, fed by a network of smaller filaments that resemble tributaries flowing into a river. Embedded within these filaments are densely populated groups and clusters of galaxies. Between them lie immense voids.
Our own Milky Way Galaxy resides in the outskirts of a similar structure known as the Laniakea Supercluster. (Laniakea means “immeasurable heaven” in Hawaiian.) Home to an estimated 100,000 galaxies, it’s a tangled knot of filaments stretching half a billion light-years from end to end. University of Hawaii astronomer Brent Tully, whose team discovered Laniakea in 2014, likened it to “finding out for the first time that your hometown is actually part of a much larger country that borders other nations.”
Everywhere we look, galaxies trace out the paths of these filaments. But it turns out galaxies don’t just illuminate the cosmic web — they’re also shaped by it.
When the stars align
In 1874, less than a decade after the Civil War ended and long before anybody knew for certain what galaxies were, astronomer Cleveland Abbe wondered how “nebulae,” as galaxies were known in those days, are oriented in space.
To answer this question, Abbe chose 59 of the most extended nebulae in Sir John Herschel’s famous Catalogue of Nebulae and Clusters of Stars and measured their direction of elongation. His surprising conclusion was that the nebulae appeared to favor certain orientations with respect to the Milky Way. But his study drew little attention and was soon forgotten; Abbe moved on to a more successful career in meteorology.
Forty years later, American astronomer Edward Fath revisited Abbe’s question. After measuring the orientations of hundreds of galaxies on photographic plates taken at Mount Wilson Observatory, he reported in 1914 that they “appear to be oriented at random.”
Decades of lively debate followed. English amateur astronomer Francis Brown spent more than 30 years investigating galaxy alignments in his spare time. In a series of papers published between 1938 and 1968, he presented evidence that galaxy orientations in certain regions of the sky were far from random. But many astronomers remained skeptical, suggesting that the results might be a consequence of measurement errors, selection effects, or even psychological biases.