Tuesday, April 23, 2013

CANDELS Finds the Most Distant Type Ia Supernova Yet Observed

Image credit: NASA
A couple weeks ago, the supernova science team from CANDELS was pleased to announce the discovery of the most distant Type Ia supernova ever seen at more than 10 billion light years away, a time when the universe was only about 3.5 billion years old. There wasn't ever much doubt that supernovae existed more than 10 billion years ago -- but it's still an exciting moment for us when we're able to photograph one with the Hubble Space Telescope. The image on the right shows the supernova's location in one of the five fields that CANDELS is searching. Almost every single dot in the upper panel is a distant galaxy, filled with billions of stars. Our team searches through these images every couple of months, hoping that one of these stars has exploded.

But ironically, the most exciting thing about this supernova for many of us is that we haven't seen more. In terms of gaining an understanding of how these objects are created, its really the lack of supernovae in the early universe that is the most telling observation. The Hubble has the power to observe one of these objects from more than 11 billion light years away, and we've been staring at the sky in search of them for nearly 3 years. So far, we've found only a single confirmed Type Ia supernova from 10-11 billion years ago -- a billion years of cosmic history.

In astronomy, our uncertainty about the universe is somewhat striking. 70% of the Universe is the mysterious "dark energy", completely unknown until about 15 years ago. Another 25% is dark matter -- also a mystery, since we only observe its gravitational effects. This leaves 5% of the universe that is composed of matter more familiar to us. In the context of this uncertainty, the supernova progenitor mystery might not seem so fundamental. And yet, every second, about 5-10 supernovae are going off somewhere in the observable universe with 10 billion times the brightness of our sun. These supernovae produce about half of the iron in the universe, some of the raw material for creating planets like the Earth. And the ways that these explosions happen -- both the stellar evolutionary steps that lead to the explosion and the physics of the detonation itself -- remain mysterious.

The composition of the universe.  Only 5% of the universe is normal matter, and only .03% of the universe consists of the heavier elements we're most familiar with on Earth.
Image source: http://www.lsst.org/lsst/public/dark_energy
This dearth of supernovae must be telling us something fundamental about the nature of these objects -- so what is it? Finding the earliest supernovae sheds light on this puzzle in a couple of different ways because fundamentally, the early universe was a very different place than the universe we live in today. There were fewer heavy elements, stars were younger, and galaxies were producing stars at a faster rate than they are today. The way in which supernovae form in this environment tells us something about their nature.

First of all, if supernovae are formed from younger stars, do they explode in the same way? Some theorists think that the answer is no -- it's predicted that only more massive stars have time to become supernovae within the first 3-4 billion years of the Universe, and nuclear fusion in more massive stars will result in a different blend of heavy elements in the core.  When the star's remnant (called a white dwarf) gains matter from a binary companion and explodes, its brightness is powered by the radioactive decay of elements fused in the explosion; a different chemical composition results in a different brightness. We're looking for such a change in brightness -- it can tell us a bit more about how supernovae are created.

Secondly, if a burst of star formation occurs in a galaxy, how long will it be before the Type Ia supernovae start exploding? How quickly stars evolve and form supernovae can tell us a lot. If most Type Ia supernovae occur when two white dwarfs form in a binary system and then slowly merge together, the time it takes them to explode will be based on the distribution of initial separations that binary stars are born with. If, on the other hand, they tend to occur when a normal or giant star is slowly pouring mass onto a white dwarf at a given speed, one might expect something closer to a single characteristic time from formation to explosion. By knowing when most of the stars formed in the universe, and observing how quickly supernovae are exploding at different ages of the universe, we can determine which model for supernovae is correct. This knowledge may also make it easier to understand the physics of a white dwarf's detonation.

All of this work can be tied back to the fact that Type Ia supernovae are not just interesting in and of themselves; they are cosmological tools with a characteristic brightness that can be used to set a distance scale for the Universe. Understanding better how supernovae explode can be used to answer questions like: why are supernovae dimmer in smaller galaxies? What process creates unpredictable and unusual supernovae like Type Iax? How can we better calibrate these tools to learn more about dark energy?

These are some of the questions that our team hopes to answer in the coming years. In the meantime, we're searching the night sky for exploding stars -- and each time we find one, it tells us a bit more about the universe we live in.

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