Using the NASA/ESA Hubble Space Telescope, astronomers have observed seven interacting galaxy systems that have long, tadpole-like tidal tails of gas, dust and a large number of stars. Hubble’s exquisite sharpness and sensitivity to ultraviolet light have discovered 425 newborn star clusters along these tidal tails; Each cluster contains up to 1 million newborn blue stars.
Star clusters in tidal tails have been known for decades. When galaxies interact, gravitational tidal forces carry long streams of gas and dust.
Two popular examples are antennas and galaxy mice with its long, narrow finger-like projections.
In the new research, astronomer Michael Rodruck of Randolph-Macon College and his colleagues used a combination of new observations and archival data to obtain ages and masses of tidal tail star clusters.
The researchers discovered that these groups are very young: only 10 million years old.
And they appear to be forming at the same rate along tails that stretch across thousands of light years.
“It’s a surprise to see a lot of young objects in the tails,” said Dr. Rodruck, lead author of a paper published in the Monthly Notices of the Royal Astronomical Society.
“This tells us a lot about the efficiency of cumulus formation.”
“With tidal tails, new generations of stars will be built that otherwise would not have existed.”
Tidal tails appear to be taking the spiral arm of a galaxy and stretching it out into space.
The outer part of the arm is dragged like candy by the gravitational tug-of-war between a pair of interacting galaxies.
Before the mergers, galaxies were rich in clouds of molecular hydrogen dust that may have simply remained inert.
But the clouds pushed and collided with each other during the encounters.
This compressed the hydrogen to the point where it precipitated a storm of star birth.
“The fate of these extended star clusters is uncertain,” the astronomers said.
“They can remain gravitationally intact and evolve into globular star clusters, such as those orbiting outside the plane of our Milky Way.”
“Either they can disperse to form a halo of stars around their host galaxy, or be ejected and become wandering intergalactic stars.”
“This pearl-string star formation may have been more common in the early Universe, when galaxies collided with each other more frequently.”
“These nearby galaxies observed by Hubble are a representation of what happened long ago and are therefore laboratories for looking into the distant past.”
Michael Rodruck et al. 2023. Star clusters in tidal debris. MNRAS 526 (2): 2341-2364; doi:10.1093/mnras/stad2886