A Star was Blocking a Galaxy, but Now it’s Moved Enough That Astronomers can Finally Examine What it Was Hiding

One of the biggest puzzles in astronomy, and one of the hardest ones to solve, concerns the formation and evolution of galaxies. What did the first ones look like? How have they grown so massive?

A tiny galaxy only 20 million light-years away might be a piece of the puzzle.

One critical difference between the Universe shortly after the Big Bang and the Universe we find ourselves in today is the different metallicity. The early Universe was almost completely made of the two lightest elements, hydrogen and helium. So the stars in the earliest galaxies contained hydrogen and helium. Only as generations of stars lived and died was the Universe populated with heavier elements—things like carbon, oxygen, and iron—which astronomers call metals. Stars forge these metals via nucleosynthesis and then spread them out into the Universe when they die, to be taken up in the following generations of star formation. The heavier elements are critical in the formation of planets like Earth and lifeforms like us.

So a young galaxy with very low metallicity is an oddball, and observing it can be like looking back in time.

“At first, we did not realize how special this little galaxy is.”

Prof. Bärbel Koribalski, co-author, CSIRO.

The tiny galaxy is called HIPASS J1131–31. But its nickname, Peekaboo, is more descriptive. That’s because, in the last 50 to 100 years, a fast-moving foreground star that was blocking Peekaboo from view has moved aside, letting the Hubble and other telescopes get a better look at it.

“Uncovering the Peekaboo Galaxy is like discovering a direct window into the past, allowing us to study its extreme environment and stars at a level of detail that is inaccessible in the distant, early universe,” said astronomer Gagandeep (Deep) Anand of the Space Telescope Science Institute, and co-author of the new study on Peekaboo’s intriguing properties.

Now that the intervening star has moved out of the way, astronomers have studied the tiny galaxy’s metallicity and other properties. The results of those observations are in a new paper in the journal Monthly Notices of the Royal Astronomy Society titled “Peekaboo: the extremely metal-poor dwarf galaxy HIPASS J1131–31.” The lead author is I D Karachentsev from the Special Astrophysical Observatory of the Russian Academy of Sciences, Nizhnij Arkhyz, Karachay-Cherkessia.

As a dwarf galaxy, it contains far fewer stars than massive galaxies like our Milky Way. Remarkably, though Peekaboo is 20 million light-years away, the Hubble Space Telescope was able to resolve about 60 of its individual stars. Observations with the Hubble and other facilities like the South African Large Telescope (SALT) showed that its stellar population is only a few billion years old. Stars this young should have higher metallicity than they do because so many stars lived and died before they formed.

This image from the study comes from Hubble’s Advanced Camera for Surveys and shows some of the brightest individual stars detected in Peekaboo. The star that used to block our view is in the bottom right. Image Credit: Karachentsev et al. 2022.

Professor Bärbel Koribalski, an astronomer at Australia’s national science agency CSIRO, first detected Peekaboo 20 years ago and is a co-author of the new paper.

“At first, we did not realize how special this little galaxy is,” Koribalski said of Peekaboo. “Now, with combined data from the Hubble Space Telescope, the Southern African Large Telescope (SALT), and others, we know that the Peekaboo Galaxy is one of the most metal-poor galaxies ever detected.”

Galaxies in the local universe typically contain a good proportion of ancient stars that are billions of years old. These are called Red Giant Branch (RGB) stars, and they’re so old they’ve left the main sequence, stopped fusing hydrogen into helium, swelled in volume and cooled. Because they’re cooler, they appear red.

But Peekaboo lacks a similar population of older red stars. Instead, Peekaboo is a compact blue dwarf galaxy, and the colour blue signifies a high proportion of hot young stars. It contains some RGB stars, but not as many as expected. “In each case, the RGB is very insubstantial compared with the evident young populations,” the authors write, describing Peekaboo and a few similar galaxies that we know of. Since these hot blue stars are so young, they should have a higher metallicity.

This figure from the study is an isochrone fit for the resolved stars in Peekaboo. It shows that the galaxy is dominated by blue main sequence stars and blue loop stars. As the researchers point out in their paper, “The red giant branch is remarkably sparse.” Image Credit: Karachentsev et al. 2022

Peekaboo is extraordinarily interesting, but it’s not completely unique. “The compact dwarf galaxy Peekaboo with a predominantly young stellar population and very low metallicity is not unique in the Local Volume,” they write in their paper.

“Due to Peekaboo’s proximity to us, we can conduct detailed observations, opening up possibilities of seeing an environment resembling the early universe in unprecedented detail.”

Deep Anand, paper co-author, Space Telescope Science Institute.
I Zwicky 18 (lower left) is another blue compact dwarf galaxy. Like Peekaboo, it’s similar to galaxies that formed very early in the Universe and is also an XMP galaxy. Astronomers question if these galaxies contain any significant populations of older stars. Image Credit: NASA, ESA, Y. Izotov (Main Astronomical Observatory, Kyiv, UA) and T. Thuan (University of Virginia)

Peekaboo is an example of an extremely metal-poor (XMP) galaxy. What sets it apart from other XMP galaxies in the local Universe, like I Zwicky 18, is its proximity to Earth. It’s only about 20 million light-years away—almost next door in astronomical terms—while the other XMP galaxies we know of are twice that distance away.

Peekaboo’s stellar population makes it one of the youngest and lowest-metallicity galaxies in the local universe. 13 billion years have passed in the local universe, and Peekaboo should have developed a higher metallicity than it has. (The term ‘Local Universe’ describes a region in space centred on an observer with a radius of R < 300 Mpc (z < 0.1) containing large-scale structures like groups, voids, clusters, and superclusters. It’s a large enough scale that the Universe appears both homogeneous and isotropic.)

What Peekaboo can tell us about the evolution of galaxies will have to wait. This study is based on observations from a survey called the “Every Known Nearby Galaxy Survey,” and Hubble’s job was to gather as much data on as many neighbouring galaxies as it could. The next step is to focus on Peekaboo with the Hubble and the JWST to study the tiny galaxy’s stars and metallicity in greater detail. That’s why its proximity to us is so important.

“Due to Peekaboo’s proximity to us, we can conduct detailed observations, opening up possibilities of seeing an environment resembling the early universe in unprecedented detail,” Anand said.

For now, there’s uncertainty around Peekaboo. “The situation with Peekaboo is decidedly ambiguous,” the authors write in their conclusion. How can it have such low metallicity when 13 billion years have passed in the Local Universe?

As for the puzzle about how galaxies form and evolve, Peekaboo could end up being an important piece. We’ll have to wait for the team’s follow-up observations to find out.


Evan Gough

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