Hubble’s ANGST reveals diversity of galaxies
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 01, 2008

To soften the blow of any Hubble angst resulting from the current software glitch (see Hubble trouble for details), the science team have released the details of a survey of 14 million stars in 69 galaxies from the ACS Nearby Galaxy Survey Treasury, otherwise known as ANGST, revealing the true diversity of galaxies, and reminding us just what Hubble is capable of.

Close up views of four galaxies from the ANGST survey, revealing the diversity of nearby galaxies. Hubble's sharp view also reveals the colours and brightnesses of individual stars, which can be used to derive the history of star formation in each galaxy. NGC 253 (top) contains thousands of young blue stars, while NGC 300 (middle) hosts young stars in its spiral arms. NGC 4163 (bottom right) shows a swarm of young blue stars and NGC 3077 (bottom left) reveals dark clumps of material resulting from interactions with its larger neighbours. Image: NASA, ESA, J. Dalcanton and B. Williams (University of Washington).

The detailed survey observed galaxies in our cosmic backyard, the ‘Local Volume’, which hosts galaxies from 6.5 million light-years to 13 million light-years from Earth. The survey revealed, that just like snowflakes, from afar all galaxies look fairly similar since the stars tend to blur together, but up close, and through the sharp eyes of Hubble, they all have their individual defining characteristics. By discerning each star in the galaxy, and by measuring the brightnesses and colours of these individual stars, scientists can derive the local history of star formation within each galaxy and can tease out subtle features in a galaxy's shape.

"Past Hubble observations of the local neighbourhood have provided dramatic insights into the star-formation histories of individual galaxies, but the number of galaxies studied in detail has been rather small," says Julianne Dalcanton of the University of Washington in Seattle and leader of the ANGST survey. "Instead of picking and choosing particular galaxies to study, our survey will be complete by virtue of looking at 'all' the galaxies in the region. This gives us a multi-colour picture of when and where all the stars in the local Universe formed."

Hubble's Advanced Camera for Surveys (ACS) shows individual stars, clusters of stars and nebulae in the spiral galaxy NGC 300, located approximately 7 million light-years away from Earth. The image shows a star-forming region a few thousand light-years farther from the galaxy's centre. The yellow nebulosities are the glow from hot gas that has been heated by radiation from the nearest young, blue stars. More diffuse groupings of young, blue stars are seen farther away from the galaxy's centre, along with faint shells of hot gas. Image: NASA, ESA, J. Dalcanton and B. Williams (University of Washington).

Many stars in nearby galaxies are the fossil equivalents of the active star formation seen in galaxies in the distant Universe, with young and distant galaxies showing signs of frenetic star formation. “However, we can only guess as to what those galaxies might eventually turn into," says Dalcanton. "Using the galaxies in the nearby Universe as a fossil record, we can compare them with young galaxies far away. This comparison gives us a history of star formation and provides a better understanding of the masses, structures, and environments of the galaxies."

Preliminary results of the survey reveal the rich diversity of galaxies, with some composed entirely of ancient stars, while others are still pumping out newborn stars in giant stellar nurseries. There are even a few examples of galaxies that have only started forming stars in the recent past.

"With these images, we can see what makes each galaxy unique," says team member Benjamin Williams of the University of Washington. "When we look at the distribution and development of stars in each survey galaxy, we can learn how differences in the galaxies' histories have produced the
diversity of galaxy shapes and colours."

The dark clumps of material scattered around the bright nucleus of NGC 3077 are pieces of wreckage from the galaxy's interactions with its larger neighbours. The galaxy is a member of the M81 group of galaxies. Image: NASA, ESA, J. Dalcanton and B. Williams (University of Washington).

The ANGST survey also includes maps of many large galaxies, such as M81, which helps astronomers to pinpoint when different parts of the galaxy formed. For example, the scientists find that the massive spiral galaxies formed most of their stars in the early Universe. And for the case of M81, the astronomers found that most of the stars formed in the galaxy’s outer disc more than seven billion years ago, when the Universe was half its present age. M81, like other mammoth galaxies, have also experienced rapid enrichment of heavy elements such as carbon, through the deaths of massive stars in supernova explosions. "We were surprised by how quickly the elements formed and how the subsequent star-formation rate for the bulk of the stars in M81 changed after that," says Williams.

The ANGST survey has provided targets for future multi-wavelength surveys, which will enable scientists to combine the star-formation maps with the properties of gas and dust in the galaxies to trace the complete cycle of star formation in detail.

While this rich survey is certainly testament to the capabilities of a fully functioning Hubble, the telescope is currently awaiting its fate following a data storage and transmission malfunction over the weekend.