NASA's Hubble Space Telescope has found a galaxy that is the equivalent of the painting of Dorian Gray, a portrait in an Oscar Wilde novel that appears mysteriously to age.
Like the fictional painting, the galaxy I Zwicky 18 appears to look older the more astronomers study it. What astronomers once thought was a toddler galaxy by galactic standards may now be considered an adult.
The galaxy's youthful appearance was identified some 40 years ago through observations at the Palomar Observatory. Those studies showed that the galaxy erupted with star formation billions of years after its galactic neighbors. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Astronomers were thrilled that a newly forming galaxy like I Zwicky 18 could be studied nearby to learn about galactic evolution, which is normally only observable at great distances.
New Hubble data have quashed that possibility. The telescope found faint, older stars contained within the galaxy, suggesting its star formation started at least 1 billion years ago and possibly as much as 10 billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.
"Although the galaxy is not as youthful as was once believed, it is certainly developmentally challenged and unique in the nearby universe," said astronomer Alessandra Aloisi from the Space Telescope Science Institute and the European Space Agency in Baltimore, Md., who led the new study.
Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. Heavier elements are forged within the cores of stars and blasted into space when the stars die. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the Spitzer Space Telescope, the Chandra X-ray Observatory, and the Far Ultraviolet Spectroscopic Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now.
The Hubble data also suggest that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed. While this is still in our own backyard, as measured by extragalactic standards, the galaxy's larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy until now. In fact, the faint, old stars in I Zwicky 18 are almost at the limit of Hubble's resolution and sensitivity.
Aloisi and her team discerned the new distance by observing flashing stellar mile-markers within I Zwicky 18. These massive stars, called Cepheid variable stars, pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. By comparing the stars' actual brightness with their observed brightness, astronomers can precisely measure their distance. The team determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18's deficiency in heavy elements, indicating the galaxy's stars formed before these elements were abundant in the universe. This analysis allowed the astronomers to determine the galaxy's distance. The Cepheid distance also was validated through another distance indicator, specifically the observed brightness of the brightest red stars older than 1 billion years.
Cepheid variable stars have been studied for decades and have been instrumental in the determination of the scale of our universe. This is the first time, however, that variable stars with so few heavy elements were found. This may provide unique new insights into the properties of variable stars, which is now a topic of ongoing study.
Aloisi and her team published their results in the Oct. 1 issue of the Astrophysical Journal Letters.
Aloisi's team consists of Francesca Annibali, Jennifer Mack, and Roeland van der Marel of the Space Telescope Science Institute; Marco Sirianni of the Space Telescope Science Institute and the European Space Agency; Abhijit Saha of the National Optical Astronomy Observatories; and Gisella Clementini, Rodrigo Contreras, Giuliana Fiorentino, Marcella Marconi, Ilaria Musella, and Monica Tosi of the Italian National Astrophysics Institutes in Bologna and Naples.
Lars Lindberg Christensen
Hubble/ESA, Garching, Germany
Space Telescope Science Institute/European Space Agency, Baltimore, Md.