The Hubble Space Telescope imaged this galaxy after it emerged from behind a fast-moving star. The galaxy – the fuzzy blue patch to the right of the star and now dubbed the Peekaboo Galaxy – has a curious lack of heavy metals. Image via NASA/ ESA/ and Igor Karachentsev (SAO RAS)/. Image Processing: Alyssa Pagan (STScI).
Peekaboo Galaxy emerges from behind star
Astronomers said in December, 2022 that they’ve discovered a little galaxy peeking out from behind the glare of a bright foreground star. At 20 million light-years from Earth, the Peekaboo Galaxy is relatively nearby. But it has characteristics more like galaxies in the distant, early universe. Only 1,200 light-years across – in contrast to 100,000 light-years for our home galaxy, the Milky Way – the tiny galaxy HIPASS J1131–31 got the nickname Peekaboo because of its emergence in the past 50-100 years from behind a fast-moving star that previously had obscured it from view.
The discovery is a combined effort of telescopes on the ground and in space. That includes confirmation by NASA’s Hubble Space Telescope. Together, the research shows tantalizing evidence that the Peekaboo Galaxy is the nearest example of the galaxy formation processes that commonly took place not long after the Big Bang, 13.8 billion years ago.
Gagandeep Anand of the Space Telescope Science Institute in Baltimore, Maryland, was co-author of the new study on Peekaboo’s intriguing properties. He said:
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.
The Monthly Notices of the Royal Astronomical Society have accepted the results for publication.
An extremely metal-poor galaxy
Astronomers describe galaxies like Peekaboo as extremely metal-poor (XMP). In astronomy, metals refers to all elements heavier than hydrogen and helium. The very early universe was almost entirely made up of primordial hydrogen and helium, elements forged in the Big Bang. Stars forged heavier elements over the course of cosmic history, building up to the generally metal-rich universe we find ourselves in today. Life as we know it consists of heavier element building blocks like carbon, oxygen, iron and calcium.
While the universe’s earliest galaxies were XMP by default, astronomers have also found similarly metal-poor galaxies in the local universe. Peekaboo caught astronomers’ attention because not only is it an XMP galaxy without a substantial older stellar population, but at only 20 million light-years from Earth, it’s at least half the distance of the previously known young XMP galaxies.
Discovery of the Peekaboo Galaxy
Bärbel Koribalski first detected Peekaboo as a region of cold hydrogen. That was more than 20 years ago with the Australian Parkes radio telescope Murriyang, in the HI Parkes All Sky Survey. Koribalski is an astronomer at Australia’s national science agency CSIRO and a co-author of the latest research study on Peekaboo’s metallicity. NASA’s space-based Galaxy Evolution Explorer mission’s far-ultraviolet observations showed it to be a compact blue dwarf galaxy. Koribalski said:
At first we did not realize how special this little galaxy is. 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.
A closer look at the galaxy
NASA’s Hubble Space Telescope was able to resolve about 60 stars in the tiny galaxy. Almost all the stars appear to be a few billion years old or younger. To complete the picture, SALT took measurements of Peekaboo’s metallicity. Together, these findings underline the major difference between Peekaboo and other galaxies in the local universe. Galaxies in the local universe typically have ancient stars that are many billions of years old. Peekaboo’s stars indicate that it is one of the youngest and least chemically enriched galaxies ever detected in the local universe. This is very unusual, as the local universe has had about 13 billion years of cosmic history to develop.
However, the picture is still a shallow one. Anand explained the Hubble observations were made as part of a “snapshot” survey program called The Every Known Nearby Galaxy Survey. This survey is an effort to get Hubble data of as many neighboring galaxies as possible. The research team plans to use Hubble and the James Webb Space Telescope to do further research on Peekaboo. They hope to learn more about its stellar populations and their metal-makeup. Anand said:
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.
A close-up on the Peekaboo Galaxy from the Hubble Space Telescope. It’s official name is HIPASS J1131–31. Image via NASA/ ESA/ and Igor Karachentsev (SAO RAS)/. Image Processing: Alyssa Pagan (STScI).
Bottom line: The Peekaboo Galaxy, discovered after emerging from behind a fast-moving star, has an extremely low amount of metal, yet its distance isn’t as far in the universe as astronomers would expect.