in dwarf galaxies
by Mehmet Alpaslan
for ASTRONOMY NOW
Posted: 28 April 2011
Astronomers at the California Institute of Technology in Pasadena using the Galaxy Evolution Explorer (GALEX) have observed some of the most powerful stellar explosions taking place in some of the smallest and faintest galaxies in the nearby Universe.
The most massive stars end their lives spectacularly in cataclysmic explosions known as supernovae – explosions so energetic that they can easily outshine their host galaxies. But astronomers analysing data from the Palomar Transient Factory have recently noted that a significant number of very bright stellar explosions are occurring in dwarf galaxies up to 1,000 times smaller than our own Milky Way Galaxy.
Through the work of Don Neill and the rest of his team, this puzzle is now one step closer to being solved. Using data from GALEX, a space telescope designed to observe the ultraviolet light emitted from newly forming stars, the team were able to confirm that the dwarf galaxies hosting these powerful explosions had very low masses and star formation rates.
Can you spot the difference? Each of these images, known to galactic astronomers as ‘postage stamps,’ show a galaxy that has hosted a supernova. The only difference is that the bottom row are the dwarf galaxies where the most powerful explosions took place. Image: NASA/JPL-Caltech.
“The most powerful explosions of massive stars are happening in extremely low-mass galaxies,” said Neill. “New data are revealing that the stars that start out massive in these little galaxies stay massive until they explore, while in larger galaxies they are whittled away as they age, and are less massive when they explode.”
In galaxies where there are no new stars forming, there is no mechanism to create heavy elements. This is because stars generate energy through fusion, where they stick together lighter elements like hydrogen and helium to create heavier carbon and oxygen -- which they eventually send back into the interstellar medium in the later stages of their lives. With no new stars forming, the dwarf galaxies observed by GALEX have no stellar chemical factories to make heavy elements.
Co-author Michael Rich adds: “dwarf galaxies are especially interesting to astronomers, because they are quite similar to the kinds of galaxies that may have been present in our young Universe, shortly after the big bang.” In these primordial galaxies, where there is an abundance of light elements like hydrogen and helium, the most massive stars are able to grow to be much larger than their cousins in newer, bigger galaxies. The heavier elements found in these galaxies are more easily blown away by stars, so they do not stay as massive as their ancient counterparts and consequently create weaker blasts when they finally go supernova.
Neill is keen to continue investigating this phenomenon. “I am actually working on much deeper data with GALEX taken as part of my guest investigator program. These deeper data will provide better measurements of the star formation in the tiny hosts of these ultra-bright supernovae. We will compare these star formation rates with models of how stars form in different environments, in particular environments with very few heavy metals, to see if these models can accommodate the large progenitors required to power the bright supernovae.”
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