Thursday, December 6, 2012

Supernovae Part II – Supernova Environments

In a previous post, we introduced readers to the exciting field of supernovae. Today we are going to delve a little deeper and discuss the environments where supernovae explode. These environments are interesting because they can provide information about the nature of the stellar systems that produce supernovae.  This can help us understand how and why some of the brightest events in the universe are produced.  

Type Ia supernovae are known as “standardizable candles”. Without going too deep into the science, a "standard candle" is an object whose observed brightness only depends on how far away it is. If you hold a flashlight 5 feet from your eye, it looks much brighter to you than if you hold it 100 feet away. By knowing the difference in brightness, you can estimate the distance to the flashlight. Now picture a flashlight with dying batteries. If you only see the flashlight far away, and have no idea how bright it would be up close, you can't estimate the distance. But if you somehow know how much power is left in the batteries, you can use that information to correct your distance estimate. In a nutshell, this is a "standardizable candle". The distance to a "standardizable candle" is not only based on it's observed brightness, but also on intrinsic properties of the object. For supernovae, what this means is that by measuring certain properties of the explosion through our observations, we can estimate how bright it should be if it were nearby, and therefore we can calculate how far away it is. 

In one of the most astounding tenets of astronomy, looking at objects that are farther away is analogous to looking back in time, because the speed of light is finite. It takes a long time for the light from these supernovae and their host galaxies to reach us. Finding supernovae that are farther away therefore allows us to trace the history of the expansion of the universe. The 2011 Nobel Prize in Physics was awarded for the role of type Ia supernovae in discovering dark energy, the mysterious force that is driving the acceleration of the expansion of the universe. Several CANDELS team members played key roles in the discovery, and team member Adam Riess was one of the prize recipients.

Recent research suggests that the brightness (and therefore our estimated distance) of a type Ia supernova depends in some way on the properties of it’s host galaxy. This suggests that the explosion mechanism for type Ia supernovae depends in some way on the surrounding environment. In the CANDELS Supernova Survey, we are searching for supernovae from a time when the universe was only 3 or 4 billion years old (we have measured the age of the universe to be around 14 billion years old). Galaxies from the early universe do not look like our own Milky Way galaxy; they are smaller, bluer (because young, hot stars are blue), and less polluted with heavier elements such as iron which are produced in the interior of stars and in supernovae. Studying the environments of these very far away supernovae is therefore important for tracing the expansion history, but also in our basic understanding of the systems that produce type Ia supernovae.

In the rest of this post, we will show some of the diverse galaxies that supernovae have been discovered in by the CANDELS supernova team.

CANDELS images of supernova host galaxies: images a), b), c), e) and f) are 0.004 degrees on each side. Image d) is 0.008 degrees on each side. For reference each image is approximately the size of a US quarter dollar viewed at 1/4 of a mile away.

a) This galaxy is one of the most nearby supernova hosts that we’ve discovered in CANDELS. The light that reached the Hubble Space Telescope to produce this image was emitted around 2 billion years ago. It is a fairly typical “edge-on” galaxy; we are viewing the galaxy right along the plane of the disk. If you look closely, this galaxy has a faintly visible dust lane; this material blocks the light emitted behind it and is therefore slightly darker.

b) This is a similar looking galaxy, but slightly farther away. The light in this image was emitted about 4 billion years ago.

c) Likely a similar shape galaxy to the previous two galaxies, however this one is rotated 90 degrees so that we’re viewing it from a very different angle. This galaxy is also quite a bit farther away; the light you are looking at was emitted 7 billion years ago! The blue clumps are likely bright, blue, massive, young stars, indicating significant recent star-formation. The supernova that went off in this galaxy may be a core-collapse supernova (core-collapse supernovae are much more likely to be found in blue regions like the ones in this image) instead of a type Ia supernova. A core-collapse supernova is the death of a massive star; when the star runs out of nuclear fuel, the outer regions of the star collapse onto a very dense, compact core, and the rebound of this material causes the explosion that we observe. The CANDELS supernova team is still working on the classification of all of our discovered supernovae.

d) Don’t be distracted by the galaxies to the left; the compact object in the center is the host galaxy of a supernova that exploded 6 billion years ago. This galaxy is observationally very different from the first 3 hosts. It is likely much less massive, and though it is difficult to draw conclusions about current star formation from just this image, it is not very blue, implying that it contains alot of fairly old stars.

e) Again not to be distracted by surrounding objects, the faint blue galaxy in the center of this image hosted a supernova 7 billion years ago. The size and shape of this galaxy is similar to d), but this galaxy is much more blue. It is highly likely that this galaxy has had more recent star formation.

f) These three galaxies all appear to be at the same distance from Earth, and so we believe they may be interacting, merging together at some point to form a larger galaxy. We are seeing them as they were about 8 billion years ago. A supernova that exploded in one of these galaxies is very likely a type Ia, and the environment appears different from some of the nearby type Ia supernovae. 

The CANDELS supernova team is hard at work classifying and analyzing the supernovae that we are discovering. You can see the diverse nature of galaxies that host supernovae; while star-forming (blue) galaxies are more likely to host a core-collapse supernova than old, red galaxies, this does not mean that type Ia supernovae cannot explode in blue galaxies. There are many questions that remain to be answered regarding the role of the host galaxy environment on type Ia supernova explosions. Understanding the environment of each supernova will be an important tool in constraining the nature of type Ia supernova progenitors. As the supernova environment may evolve as we discover them farther and farther away, understanding the role that the host galaxy plays in our distance estimate is a goal of the CANDELS supernova team.

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