A Tale of Two Supernovae

Wednesday, November 15, 2017

The Perseus galaxy cluster, located about 240 million light-years away, is shown in this composite of visible light (green and r

The Perseus galaxy cluster, located about 240 million light-years away, is shown in this composite of visible light (green and red) and near-infrared images from the Sloan Digital Sky Survey. Unseen here is a thin, hot, X-ray-emitting gas that fills the cluster. Credit: Robert Lupton and the Sloan Digital Sky Survey Consortium.

A single measurement from the former Hitomi X-ray Observatory has revealed the origins of Type 1a supernovae while resolving a decades’ old debate about one of the most important sources of heavy metals in the universe.

Brian McNamaraIt was only one measurement, but it seems to be have settled the issue,” said Brian McNamara, a professor of Physics and Astronomy and a member of the Hitomi Collaboration. “It turns out that star formation in the Milky Way is not so very different from distant galaxies as we once thought.

The debate centres around two routes by which Type 1a supernovae may be occurring, each route producing different amounts of iron, chromium, magnesium, cobalt, and nickel, known as iron-peak elements.

The team used the Hitomi X-ray Observatory to measure the relative abundance of iron peak elements spread throughout the Perseus Galaxy Cluster.  The cluster is one of the most massive objects in the universe located 250 million light years away. They compared their observations against the latest atomic models to work their way back to likely formation scenarios, while reconciling their results with other previous observations.

What they found was the relative abundances of iron-peak elements in Perseus are the same as in own solar neighbourhood. Meaning that although the rate of start formation in the Milky Way and the Perseus Cluster are very different, the production levels of the heavy elements are the same. Star formation, on average, proceeded roughly the same in Perseus as it has in the Milky Way.

This result simplifies things,” said McNamara, also an affiliate with the Perimeter Institute for Theoretical Physics. “Previous studies suggested star formation histories were different in different types of galaxies. We see different rates of star formation today, but overall their histories are the same.

Their paper appears this week in Nature.

Although supernovae can happen under a variety of conditions, this particular type of supernova centres around a star known as a white dwarf. Type Ia supernovae play a key role in stellar cartography and they were critical in the discovery of dark energy.

The cataclysmic explosion is actually a runaway nuclear reaction set off when the white dwarf’s mass exceeds a critical limit of 1.44 solar masses. It happens in one of two ways: Under the first route, a white dwarf in binary orbit with another star syphons off matter from that star in a process known as accretion. In the second, two white dwarfs collide and merge with each other under their own gravitational forces.

By measuring the x-ray emission spectra coming from a star, galaxy or even galaxy cluster, astronomers can determine the relative abundances of elements comprising that body, including the iron peak elements. And knowing that the accretion route produces more iron peak elements than the collision route, it means astronomers in principle should be able determine which route contributed the most to the creation of iron-peak elements in the galaxy.

Unfortunately, early x-ray measurements were too crude to provide a definitive answer. X-ray spectra showed elliptical galaxies and giant galaxy clusters such as Perseus had an overabundance of iron peak elements, indicating Type Ia supernovae were dominated by accretion events. But if a long, drawn out accretion of matter on a white dwarf was taking place, astronomers should be seeing an increased brightness in that area - which they did not.

Then came the Hitomi X-ray Observatory. Launched in February 2016, Hitomi was the first satellite x-ray telescope to have the spectral resolution fine enough to measure each element individually from distant objects. It was in operation for only one month before it failed.  Nevertheless, it survived long enough to observe its key target, the Perseus cluster of galaxies.

Ultimately the iron peak elements are likely explained by a roughly equal contribution of colliding and accreting white dwarfs,” said McNamara. “Ideally, we would have more measurements to take an average, but for now this is as accurate as you’re going to get.

JAXA, the Japanese Space Agency, is considering building a new version of the Hitomi X-Observatory, with a launch goal of 2020.

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