Hubble traces “string of pearls” star clusters in galaxy collisions

Galaxy collisions do not destroy stars, though this might seem to contradict conventional wisdom, not to mention storylines in countless Hollywood blockbusters. In fact, according to researchers, the rough-and-tumble dynamics trigger new generations of stars, and presumably accompanying planets.

Since its launch into low Earth orbit in 1990, the Hubble Space Telescope has gathered dozens of terabytes of data, including a multitude of interstellar images, providing invaluable information for astronomers like Western University's Sarah Gallagher.

Hubble, one of the most productive and versatile space telescopes ever deployed, zeroed in on 12 interacting galaxies that have long, tadpole-like 'tidal tails' of gas, dust and a plethora of stars. The telescope's exquisite sharpness and sensitivity to ultraviolet light uncovered 425 clusters of newborn stars along these tails, which look like strings of pearls. Each cluster contains as many as 1 million newborn stars. The most massive of these outshine all the others, and glow with blue light.

Clusters in tidal tails have been known about for decades, but scientifically dating them is a relatively new pursuit for the space science community.

"When you have two galaxies, they start off as these nice disk galaxies with well-organized spirals but as they start moving closer together, the gravity on the near side is much stronger than the gravity on the far side, so they pull out these very long streamers, called tidal tails," said Gallagher, director of Western's Institute for Earth and Space Exploration. "It is like unravelling a spiral galaxy, where you are just pulling out gas and stars into these very, very long streamers that can be much larger than even the galaxy before it started the interaction."

A team of astronomers, including Gallagher, used Hubble archival data to get ages and masses of these tidal tail star clusters. They found that these clusters are very young - only 10 million years old. And they seem to be forming at the same rate along tails stretching for thousands of light-years. The findings were published on Feb. 8 in the journal, Monthly Notices of the Royal Astronomical Society.

"When looking at star clusters captured by Hubble, you get to see a large gathering of stars, all born at exactly the same time," said Gallagher, also an astronomy professor in Western's Faculty of Science. "And then you can look at how bright they are and what the colour of their light is. This can tell you just how massive and how old they are."

These imaging datasets (that show brightness and colour) not only act as important tracers for determining when a star cluster formed within the tidal tail and its size, but also where they form.

"It's surprising to see lots of young objects forming in the tails. It tells us a lot about cluster formation efficiency," said Michael Rodruck, director of Keeble Observatory at Randolph-Macon College and lead author of the study. "With tidal tails, you see the buildup of a new generation of stars that otherwise might not have existed."

The tails look like they are taking a galaxy's spiral arm and stretching it out into space a bit. The exterior part of the arm gets pulled like taffy from the gravitational tug-of-war between a pair of interacting galaxies.

"Before the collision, the galaxies were rich in dusty clouds of molecular hydrogen that simply may have remained inert, but they got jostled and bumped into each other during the collision. This compressed the hydrogen to the point where it precipitated a firestorm of star birth," said Rodruck.

The fate of these strung-out clusters is uncertain. They may stay gravitationally intact and evolve into globular star clusters - like those that orbit outside the plane of our Milky Way Galaxy. Or they may disperse to form a halo of stars around a spiral galaxy or get cast off into an individual galaxy to become wandering intergalactic stars.

"This string-of-pearls star formation may have been more common in the early universe when galaxies collided into each other more frequently. These nearby galaxies observed by Hubble are a proxy of what happened long ago, and therefore are laboratories for looking into the distant past," said Rodruck.

The team involved in the new study was able to successfully date the clusters due to Hubble's sensitivity to ultraviolet plus optical light, specifically the U-band, which can capture the light from the hottest, youngest stars.

"In order to get the masses right, you must get the ages right," said Gallagher. "As these clusters age, they become redder, and they also get fainter. To know how massive they are, you need to know the age, you need an accurate estimate of the age, and the addition of the U-band, which is the colour of the new images, is essential for getting the ages right."

From Hubble to CASTOR

With Hubble set to retire around 2030, it has taken a bit of a back seat to the James Webb Space Telescope, which launched in 2021. And while Webb is considered the most powerful space telescope in human history, there are some things even it can't do.

"Hubble does some pretty cool stuff and it can do some things that the Webb Telescope cannot," said Gallagher. "One of the things about this project, again, is that it used the U-band, which is an ultraviolet band, and Webb detects infrared light. Webb couldn't even see the blue light from these clusters because it's not designed to do that. If you are looking at these young clusters, and you want to figure out how old they are, and how massive they are, then it is important that you get the blue light, and right now we can only do that with Hubble."

Gallagher, who currently serves as the president of the Canadian Astronomical Society, is one of the science leads working on the Cosmological Advanced Survey Telescope for Optical and ultraviolet Research (CASTOR), which is a proposed Canadian Space Agency mission that would image the skies at UV and blue-optical wavelengths simultaneously. CASTOR is on track for a launch in 2030 pending continued government and private funding - timed perfectly for when Hubble is expected to retire.

"CASTOR is a true successor to Hubble. CASTOR would be able to study these types of clusters, as well. It will be sensitive to the same light and colours, something the Webb Telescope is not able to duplicate," said Gallagher.

Regardless of what comes next, Gallagher never forgets to take a minute to look up and imagine.

"One thing that I always like to think about is that this environment we're studying is actually quite different than our Milky Way environment," said Gallagher. "I wonder what it would be like to be on a planet around one of the stars in one of those clusters. You would be able to see two big galaxies in the sky that are so close - that is always cool to think about."

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