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    Equal Opportunity Galaxies Are All About Diversity
    By News Staff | September 30th 2008 02:00 AM | Print | E-mail | Track Comments
    The detailed study, called the ACS Nearby Galaxy Survey Treasury (ANGST) program, explored a region called the Local Volume, where galaxy distances range from 6.5 million light-years to 13 million light-years from Earth.

    A typical galaxy contains billions of stars but looks smooth when viewed through a conventional telescope because the stars appear blurred together. In contrast, the galaxies observed in this new survey are close enough to Earth that the sharp view provided by Hubble's Advanced Camera for Surveys and Wide Field Planetary Camera 2 can resolve the brightness and colour of some individual stars. This allows scientists to determine the history of star formation within a galaxy and tease out subtle features in a galaxy's shape.

    These images taken with the NASA/ESA Hubble Space Telescope are close-up views of four galaxies from a large survey of nearby galaxies. The galaxies have very different masses and sizes and showcase the diversity found in the ANGST study. Although the galaxies are separated by many light-years, they are presented as if they are all at the same distance to show their relative sizes. The images, taken with Hubble's Advanced Camera for Surveys, reveal rich detail in the stellar populations and in the interstellar dust scattered between the stars. Hubble's sharp views reveal the colours and brightness of individual stars, which astronomers used to derive the history of star formation in each galaxy. In the composite image at the top, NGC 253 is ablaze with the light from thousands of young, blue stars. The spiral galaxy is undergoing intense star formation. The image demonstrates the sharp "eye" of the Advanced Camera, which resolved individual stars. The dark filaments are clouds of dust and gas. NGC 253 is the dominant galaxy in the Sculptor Group of galaxies and it resides about 13 million light-years from Earth. In the view of the spiral galaxy NGC 300, second from top, young, blue stars are concentrated in spiral arms that sweep diagonally through the image. The yellow blobs are glowing hot gas that has been heated by radiation from the nearest young, blue stars. NGC 300 is a member of the Sculptor Group of galaxies and it is located 7 million light-years away. The dark clumps of material scattered around the bright nucleus of NGC 3077, the small, dense galaxy at bottom, left, are pieces of wreckage from the galaxy's interactions with its larger neighbours. NGC 3077 is a member of the M81 group of galaxies and it resides 12.5 million light-years from Earth. The image at bottom, right, shows a swarm of young, blue stars in the diffuse dwarf irregular galaxy NGC 4163. NGC 4163 is a member of a group of dwarf galaxies near our Milky Way and is located roughly 10 million light-years away. These galaxies are part of a detailed survey called the ACS Nearby Galaxy Survey Treasury (ANGST) program. In the census, Hubble observed roughly 14 million stars in 69 galaxies. The survey explored a region called the "Local Volume", and the galaxy distances ranged from 6.5 million light-years to 13 million light-years from Earth. The Local Volume resides beyond the Local Group of galaxies, an even nearer collection of a few dozen galaxies within about 3 million light-years of our Milky Way Galaxy. The natural-colour images were constructed using observations taken in infrared, visible, and blue light. The observations of NGC 253 and NGC 300 were taken in September 2006; of NGC 3077 in November 2006; and of NGC 4163 in December 2006.


    "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", said Julianne Dalcanton of the University of Washington in Seattle (USA) 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."

    Many stars in nearby galaxies are the fossil equivalents of new stars forming in the far Universe. "When we look back in time at distant, young galaxies, we see lots of vigorous star formation. However, we can only guess as to what those galaxies might eventually turn into", Dalcanton explained. "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."

    Early results of the ANGST survey show the rich diversity of galaxies. Some are made up entirely of ancient stars, while others have been forming stars nearly continuously during their whole lives. 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", said team member Benjamin Williams of the University of Washington.

    The ANGST survey also includes maps of many large galaxies, including M81. "With these maps, we can track when the different parts of the galaxy formed", explained Evan Skillman of the University of Minnesota (USA), describing work by students Dan Weisz of the University of Minnesota and Stephanie Gogarten of the University of Washington.

    In a separate paper describing the star-formation history in M81, astronomers confirmed that massive spiral galaxies formed most of their stars in the early Universe. Analysing M81's outer disk, the astronomers found that most of the stars formed more than 7 billion years ago, when the Universe was half its present age. M81 and other mammoth galaxies also experienced rapid enrichment of chemical elements heavier than the hydrogen and helium produced during the Big Bang, 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", said Williams, the paper's lead author.

    "This rich survey will add to Hubble's legacy, providing a foundation for future studies", Dalcanton added. "With this information, we will be able to trace the complete cycle of star formation in detail."

    The survey's results were submitted to The Astrophysical Journal Supplement Series. Another paper that details the star-formation history in galaxy M81 has been submitted to The Astronomical Journal.