Hubble Finds Truth Is Stranger Than Fiction: Mysterious “Ghost” Stars Wandering Around for Billions of Years

Orphaned Stars Were Lost into Intergalactic Space Long Ago

In the 1960s sci-fi television show “Lost in Space” a small family of would-be planetary colonists get off course and lost in our galaxy. But truth is stranger than fiction when it comes to Hubble Space Telescope discoveries. Thanks to Hubble, astronomers now know about entire families of stars – and presumably their planetary systems – that don’t even have a galaxy to call home. We are nestled inside the sprawling Milky Way galaxy, an empire of stars. But there are many stars wandering about inside giant clusters of hundreds or thousands of galaxies. These stars are not gravitationally tied to any one galaxy in a cluster. The nighttime sky would appear inky black and starless to any inhabitants orbiting their parent sun, save for the feeble soft glow of neighboring galaxies peppering the sky.

Collectively, the dim dispersed glow from these wayward stars forms a background called intracluster light that is evidence they are lurking around. Although the first clues came in 1951, Hubble can easily detect this light even though it’s 1/10,000th the glow of the night sky as seen from the ground-based telescopes. Billions of years ago galaxies would have been smaller than seen today, and they probably shed stars pretty easily because of a 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 assembly history of entire galaxy clusters.

Hubble Space Telescope Finds that Ghost Light Among Galaxies Stretches Far Back in Time

In massive clusters of hundreds or thousands of galaxies, countless stars wander between the galaxies like lost souls, emitting a dim light. These stars are not gravitationally bound to any of the galaxies in a cluster.

The nagging question for astronomers is:
how come the stars are so spread out in the cluster in the first place? Several competing theories include the possibility that the stars were stripped of galaxies from a cluster, or that they were shaken up after a galaxy merger, or that they were present during the early years of a cluster’s formation. several billion years ago. 

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

The survey includes 10 clusters of galaxies nearly 10 billion light-years apart. These measurements must be made from space because the dim light inside the cloud is 10,000 times dimmer than the night sky seen from the ground.

The survey showed that the ratio of light inside the cluster to the total light in the cluster remained constant, looking back billions of years ago. “This means that these stars were homeless at the beginning of cluster formation,” said James Jee of Yonsei University in Seoul, South Korea. His results were published in the January 5 issue of the journal Nature. 

^{_{Image of galaxy clusters MOO J1014+0038 (left panel) and SPT-CL J2106-5844 (right panel) captured by 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 when 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 object’s orientation on the celestial sphere. North points to the north celestial pole which is not a fixed point in the sky, but it currently lies near the star Polaris, in the circumpolar constellation Ursa Minor. Celestial coordinates are analogous to a terrestrial map, though east and west are transposed 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 100,000 years for light to travel a distance equal to the length of the bar.) One light-year is equal to about 5.88 trillion miles or 9.46 trillion kilometers.
A parsec is also a measure of length or distance. One parsec is approximately 3.26 light-years across.
Note that the distance in light-years and parsecs shown on this scale bar applies to the galaxy cluster, not to foreground or background objects.
Credit: Science: NASA, ESA, STScI, James Jee (Yonsei University), Image Processing: Joseph DePasquale (STScI)}}

Stars may be scattered outside their galactic birthplace as a galaxy moves through gaseous matter in intergalactic space as it orbits the center of the cluster. In the process, the smoke trail 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 star production in the cluster. Indeed, the internal lighting section will increase over time so far if stripping is the main player. But that’s not the case in new data from Hubble, which shows a constant fraction over billions of years. “We don’t know exactly what made them homeless. Current theories can’t explain our results, but they were somehow produced in large numbers in the early universe,” Jee said. “In the early years of their formation, galaxies were probably quite small, and they easily made stars because of their weaker gravity.”

Hyungjin Joo of Yonsei University said: “If we detect the origin of the stars inside the cluster, it will help us understand the assembly history of the entire galaxy cluster, and they could play a role. The role is the visible trace of the dark matter surrounding this cluster.” first author. paper. Dark matter is the invisible scaffolding of the universe, holding galaxies and galaxy clusters together.

If the wandering stars were produced by a relatively recent intergalactic ball game, they would not have had enough time to disperse throughout the cluster’s gravitational field and would therefore not follow the distribution. cluster dark matter. But if the stars were born in the first years of the cluster, they would be completely dispersed throughout the cluster. This would allow astronomers to use wayward stars to map the distribution of dark matter across the cluster. 

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

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

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

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