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Mysterious “ghost” stars that have been wandering for billions of years

Mysterious “ghost” stars that have been wandering for billions of years

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In the 1960’s sci-fi television show Lost in Space, a small family of would-be colonists goes off course and gets lost in our galaxy. But truth is stranger than fiction when it comes to Hubble Space Telescope discoveries. Thanks to Hubble, astronomers now know entire families of stars – and presumably their planetary systems too – that don’t even have a galaxy as their home. We are embedded in the sprawling Milky Way, a realm of stars. But there are many stars wandering about in huge clusters of hundreds or thousands of galaxies. These stars are not gravitationally bound to a galaxy in a cluster. The night sky would appear inky and starless to all inhabitants orbiting its parent sun, save for the faint, gentle glow of neighboring galaxies dotting the sky.

Collectively, the faint, scattered glow from these wayward stars forms a backdrop called intracluster light, evidence they lurk nearby. Although the first hints came in 1951, Hubble can easily see this light, although it is 1/10,000th the glow of the night sky as seen by ground-based telescopes. Billions of years ago, galaxies would have been smaller than what you see today, and likely shed stars fairly easily due to weaker gravitational pull. (The escape velocity from our Milky Way is over 1 million miles per hour). Understanding the origin of intracluster light could give astronomers new insights into the formation history of entire galaxy clusters.

Ghost Light Galaxy Cluster

These are Hubble Space Telescope images of two massive galaxy clusters named MOO J1014+0038 (left) and SPT-CL J2106-5844 (right). The artificially added blue color is translated from Hubble data that captured a phenomenon called intracluster light. This extremely faint glow traces a uniform distribution of light from wandering stars scattered across the cluster. Billions of years ago, stars were ejected from their parent galaxies and are now drifting through intergalactic space. Credit: Science: NASA, ESA, STScI, James Jee (Yonsei University), Image Processing: Joseph DePasquale (STScI)

The Hubble Space Telescope notes that the ghost light between galaxies reaches far into the past

In vast clusters of hundreds or thousands of galaxies, countless stars wander the galaxies like lost souls, emitting an eerie veil of light. These stars are not gravitationally bound to a galaxy in a cluster.

The nagging question for astronomers was: How could the stars be so scattered about the cluster? Several competing theories include the possibility that the stars were removed from a cluster’s galaxies, or that they were tossed about after galaxies merged, or that they were present early in a cluster’s founding years many billions of years ago.

A recent infrared survey by NASA’s Hubble Space Telescope looking for this so-called “intracluster light” sheds new light on the mystery. The new Hubble observations suggest that these stars have been wandering for billions of years and are not a product of recent dynamic activity within a galaxy cluster that would dislodge them from normal galaxies.

The survey included 10 clusters of galaxies located up to nearly 10 billion light-years away. These measurements must be made from space because the faint intracluster light is 10,000 times dimmer than the night sky when viewed from the ground.

The study shows that the fraction of light within the cluster relative to the total light in the cluster remains constant, looking billions of years into the past. “This means that these stars were already homeless in the early stages of star cluster formation,” says James Jee of Yonsei University in Seoul, South Korea. His findings were published in the January 5 journal Nature.

Ghostlight Galaxy Cluster Compass

Image of galaxy clusters MOO J1014+0038 (left) and SPT-CL J2106-5844 (right) taken with Hubble’s Wide Field Camera 3, with color key, compass arrows, and scale bar for reference.
This image shows near-infrared wavelengths of light. The color key shows which filters were used in collecting the light. The color of each filter name is the color used to represent the wavelength that passes through that filter.
The compass graphic points to the orientation of the object on the celestial sphere. North points to the north celestial pole, which is not a fixed point in the sky but is currently near the star Polaris in the circumpolar constellation Ursa Minor. Celestial coordinates are analogous to a terrestrial map, although east and west are swapped because we are looking up rather than down.
The scale bar is labeled in light-years (ly) and parsecs (pc).
A light year is the distance that light travels in one Earth year. (It takes light 100,000 years to travel a distance equal to the length of the bar.) A light-year is about 5.88 trillion miles, or 9.46 trillion kilometers.
A parsec is also a measure of length or distance. A parsec is approximately 3.26 light-years across.
Note that the distance in light-years and parsecs shown on this scale bar is for the galaxy cluster, not for foreground or background objects.
Credit: Science: NASA, ESA, STScI, James Jee (Yonsei University), Image Processing: Joseph DePasquale (STScI)

Stars can be scattered outside of their galactic birthplace as a galaxy moves through gaseous material in the space between galaxies as it orbits the center of the cluster. Air resistance pushes gas and dust out of the galaxy. However, based on the new Hubble survey, Jee rules out this mechanism as the main cause of the production of stars within the cluster. That’s because if stripping were the key player, intracluster light fraction would increase over time to the present. But that’s not the case in the new Hubble data, which shows a constant fraction over billions of years.

“We don’t know exactly what made her homeless. Current theories can’t explain our results, but somehow they were produced in large quantities in the early universe,” Jee said. “In their early formative years, galaxies may have been quite small and they bled out stars fairly easily due to a weaker gravitational pull.”

“Finding the origin of intracluster stars helps us understand the formation history of an entire galaxy cluster, and they can serve as visible traces of dark matter enveloping the cluster,” said Hyungjin Joo of Yonsei University, the first author of the paper. Dark matter is the invisible framework of the universe that holds galaxies and galaxy clusters together.

If the wandering stars were generated by a comparatively recent pinball game between galaxies, they would not have enough time to disperse across the cluster’s entire gravitational field and therefore would not track the cluster’s dark matter distribution. But if the stars were born in the cluster’s early years, they have fully dispersed throughout the cluster. This would allow astronomers to use the wayward stars to map the distribution of dark matter across the cluster.

This technique is new and complements the traditional method of mapping dark matter by measuring how the entire cluster distorts light from background objects due to a phenomenon called gravitational lensing.

Intracluster light was first discovered in the Coma galaxy cluster in 1951 by Fritz Zwicky, who reported that one of his most interesting discoveries was the observation of luminous, faint intergalactic matter in the cluster. Because the Coma Cluster is one of the closest clusters to Earth (330 million light years) with at least 1,000 galaxies, Zwicky was able to spot the ghost light even with a modest 18-inch telescope.

The near-infrared capability and sensitivity of NASA’s James Webb Space Telescope will greatly extend the search for intracluster stars deeper into the Universe and should therefore help solve the mystery.

Reference: “Intracluster light is always abundant at redshift beyond unity” by Hyungjin Joo and M. James Jee, 4 January 2023, Nature.
DOI: 10.1038/s41586-022-05396-4

The Hubble Space Telescope is an international collaborative project between NASA and ESA. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, DC

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