One of the greatest puzzles in astronomy, and one of the most difficult to solve, concerns the formation and evolution of galaxies. What did the first ones look like? How did they get so massive?
A tiny galaxy just 20 million light-years away could be one piece of the puzzle.
A 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 entirely composed of the two lightest elements, hydrogen and helium. Thus, the stars of the first galaxies contained hydrogen and helium. It wasn’t until generations of stars lived and died that the Universe was populated with heavier elements – things like carbon, oxygen and iron – which astronomers call metals. Stars forge these metals through nucleosynthesis, then spill them into the Universe when they die, to be taken up in subsequent generations of star formation. The heavier elements are essential for the formation of planets like Earth and life forms like us.
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So a young galaxy with very low metallicity is an oddball, and observing it can be like looking back in time.
The tiny galaxy is called HIPASS J1131–31. But his nickname, Peekaboo, is more descriptive. Indeed, over the past 50 to 100 years, a fast-moving foreground star that blocked Peekaboo’s view has moved out of the way, allowing Hubble and other telescopes to better observe it.
“Discovering the Peekaboo galaxy is like discovering a direct window into the past, allowing us to study its extreme environment and stars in a level of detail unattainable in the distant, early universe,” said astronomer Gagandeep. (Deep) Anand of space. Telescope Science Institute, and co-author of the new study on the intriguing properties of Peekaboo.
Now that the intermediate star has moved away, astronomers have studied the tiny galaxy’s metallicity and other properties. The results of these observations appear in a new article in the journal Monthly Notices of the Royal Astronomy Society titled “Peekaboo: The extremely metal-poor dwarf galaxy HIPASS J1131-31”. The main author is ID Karachentsev of 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, despite Peekaboo being 20 million light-years away, the Hubble Space Telescope was able to resolve about 60 of its individual stars. Observations with Hubble and other facilities like the Large South African Telescope (SALT) have shown that its stellar population is only a few billion years old. Stars this young should have a higher metallicity than they do because so many stars have lived and died before forming.
Prof Bärbel Koribalski, an astronomer at Australia’s national science agency CSIRO, first detected Peekaboo 20 years ago and is co-author of the new paper.
“At first we didn’t realize how special this little galaxy was,” Koribalski said of Peekaboo. “Now, with combined data from the Hubble Space Telescope, the Large South African 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 generally 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 that they left the main sequence, stopped fusing hydrogen into helium, swelled in volume, and cooled. Because they are cooler, they appear red.
But Peekaboo lacks a similar population of older red stars. Instead, Peekaboo is a compact blue dwarf galaxy, and the color blue signifies a high proportion of young, hot stars. It has some RGB stars, but not as many as expected. “In each case, the RGB is very insignificant compared to the obvious young populations,” the authors write, describing Peekaboo and a few similar galaxies we know of. Since these hot blue stars are so young, they should have higher metallicity.
Peekaboo is extraordinarily interesting, but it’s not completely unique. “The compact Peekaboo dwarf galaxy with a predominantly young stellar population and very low metallicity is not unique in the local volume,” they write in their paper.
Peekaboo is an example of an extremely metal-poor (XMP) galaxy. What sets it apart from other XMP galaxies in the local Universe, such as 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 are twice as far away.
Peekaboo’s stellar population makes it one of the youngest and lowest metallic galaxies in the local universe. 13 billion years have passed in the local universe, and Peekaboo should have developed a higher metallicity than it did. (The term “local universe” describes an observer-centered region of space with a radius of R < 300 Mpc (z < 0.1) containing large-scale structures such as groups, voids, clusters, and superclusters. This is a scale large enough for the Universe to appear both homogeneous and isotropic.)
What Peekaboo can tell us about galaxy evolution will have to wait. This study is based on observations from a survey called the Every Known Near Galaxy Survey, and Hubble’s job was to collect as much data as possible from as many nearby galaxies. The next step is to focus on Peekaboo with Hubble and the JWST to study the stars and metallicity of the tiny galaxy in more detail. This is why his closeness to us is so important.
“Due to Peekaboo’s proximity to us, we can make detailed observations, opening up the possibility of seeing an environment resembling the early universe in unprecedented detail,” Anand said.
For now, there is uncertainty surrounding 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 on the formation and evolution of galaxies, Peekaboo could become an important piece. We’ll have to wait for follow-up observations from the team to find out.
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