ESA title
Panning on I Zwicky 18
Science & Exploration

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

16/10/2007 2362 views 1 likes
ESA / Science & Exploration / Space Science

The NASA/ESA Hubble Space Telescope has found out the true nature of a dwarf galaxy that was reputed to be one of the youngest galaxies in the Universe.

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.

I Zwicky 18
I Zwicky 18

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.

I Zwicky 18
I Zwicky 18

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.

Zoom in I Zwicky 18
Zoom in I Zwicky 18

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.

I Zwicky 18
I Zwicky 18

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.

I Zwicky 18
I Zwicky 18

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

Related Links

Nebula NGC 3603
Science & Exploration

Extreme star cluster in new Hubble images

02/10/2007 5103 views 2 likes
Read
The Hubble Ultra Deep Field
Science & Exploration

Hubble and Spitzer find 'Lego-block' galaxies in early Univ…

06/09/2007 2263 views 1 likes
Read
The Witch's Broom Nebula within the Veil Nebula
Science & Exploration

Hubble uncovers the Veil Nebula

01/08/2007 7751 views 7 likes
Read
Galaxy NGC 4449
Science & Exploration

Stellar fireworks through Hubble’s eyes

03/07/2007 3956 views 0 likes
Read
The ring of dark matter
Science & Exploration

Hubble sees dark matter ring in a galaxy cluster

15/05/2007 6041 views 4 likes
Read
Globular Cluster NGC 2808
Science & Exploration

Hubble sees multiple star generations in a globular cluster

02/05/2007 2918 views 0 likes
Read
Panorama of the Carina Nebula
Science & Exploration

Spectacular star birth pictures on Hubble’s 17th birthday

24/04/2007 4462 views 2 likes
Read
Barred spiral galaxy NGC 1672
Science & Exploration

Hubble’s view of barred spiral galaxy NGC 1672

03/04/2007 4974 views 4 likes
Read
'Comet Galaxy' being ripped apart by galaxy cluster
Science & Exploration

Hubble sees ‘Comet Galaxy’ being ripped apart by galaxy clu…

02/03/2007 3939 views 2 likes
Read
Hubble's view of planetary nebula NGC 2440
Science & Exploration

The colourful demise of a Sun-like star

14/02/2007 2701 views 0 likes
Read
Hubble's view of N90 star-forming region
Science & Exploration

New stars shed light on the past

08/01/2007 5045 views 2 likes
Read
3D distribution of dark matter in the Universe
Science & Exploration

First 3D map of the Universe’s dark matter scaffolding

07/01/2007 13679 views 8 likes
Read
Spectacular view of V838 Monocerotis light echo
Science & Exploration

Latest views of the V838 Monocerotis light echo from Hubble

27/10/2006 7613 views 13 likes
Read

Related Links