Hubble shows ‘baby’ galaxy is not so young after all

Astronomers using Hubble have made observations of the galaxy I Zwicky 18 which seem to indicate that it is in fact much older and much farther away than previously thought.

Observations of I Zwicky 18 at the Palomar Observatory about 40 years ago seemed to show that it was one of the youngest galaxies in the nearby Universe. The studies suggested that the galaxy had erupted with star formation thousands of millions of years after its galactic neighbours, like our galaxy the Milky Way.

Back then it was an important finding for astronomers, since this young galaxy was also nearby and could be studied in great detail - something not easy with observations made across great distances when the universe was much younger.

But the new Hubble data has quashed that possibility. The telescope found fainter, older, red stars contained within the galaxy, suggesting its star formation processes started at least one thousand million years ago and possibly as much as 10 thousand million 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 European Space Agency/Space Telescope Science Institute, who led the new study. Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is mostly composed of hydrogen and helium, the main ingredients created in the Big Bang. In other words, the stars within it have not created the same amounts of heavier elements as seen in other galaxies nearby.

Thus, the galaxy’s primordial makeup suggests that its rate of star formation was much lower than that of other galaxies of similar ages. 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 an outstanding mystery as to why I Zwicky 18 formed few stars in the past, and why it is forming so many new stars right now.

The new Hubble data also suggests that I Zwicky 18 is 59 million light-years from Earth, almost 10 million light-years more distant than previously believed. By extragalactic standards, this is still in our own backyard yet the galaxy’s larger-than-expected distance may now 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 sensitivity and resolution.

Aloisi and her team discerned the new distance by observing blinking stellar distance-markers within I Zwicky 18. Some massive stars, called Cepheid variables, pulse with a regular rhythm. The timing of their pulsations is directly related to their brightness. By comparing their actual brightness with their observed brightness, astronomers can precisely measure their distance.

The team determined the observed brightness of three Cepheids and compared it to the actual brightness predicted by theoretical models specifically tailored for the low metal content of I Zwicky 18 to determine the distance to the galaxy. The Cepheid distance was also validated with another distance indicator, specifically the observed brightness of the brightest red stars in a characteristic stellar evolutionary phase (the so-called ‘giant’ phase).

Cepheid variables have been studied for decades, especially by Hubble, and have been instrumental in the determination of the scale of our universe. This is the first time, however, that variable stars with such 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.

Notes for editors:

The Hubble Space Telescope is a project of international cooperation between NASA and ESA.

Aloisi and her team’s results appear in the 1 October issue of the Astrophysical Journal Letters in ‘I Zw 18 Revisited with HST ACS and Cepheids: New Distance and Age’.

Aloisi’s team consists of Francesca Annibali, Jennifer Mack, and Roeland van der Marel of the Space Telescope Science Institute, Marco Sirianni of ESA and Space Telescope Science Institute, 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.

For more information:

Alessandra Aloisi, Space Telescope Science Institute, USA
E-mail: aloisi @ stsci.edu

Lars Lindberg Christensen, Hubble/ESA, Garching, Germany
E-mail: lars @ eso.org

Donna Weaver, Space Telescope Science Institute, USA
E-mail: dweaver @ stsci.edu

Ray Villard, Space Telescope Science Institute, USA
E-mail: Villard @ stsci.edu