Hubble saw Multiple Light Echoes Reflecting off Rings of Dust From a Supernova Explosion

When stars reach the end of their life cycle, they experience gravitational collapse at their centers and explode in a fiery burst (a supernova). This causes them to shed their outer layers and sends an intense burst of light and high-energy short-wavelength radiation (like X-rays and gamma-rays) out in all directions. This process also creates cosmic rays, which consist of protons and atomic nuclei that are accelerated to close to the speed of light. And on rare occasions, supernovae can also create “light echoes,” rings of light that spread out from the site of the original explosion.

These echoes will appear months to years after the supernova occurs as light from the explosion interacts with the layers of dust in the vicinity. Using the Hubble Space Telescope (HST), an international team of astronomers was able to document the emergence and evolution of multiple light echoes (LEs). The team traced these echoes to a stripped-envelope supernova (SN 2016adj) located in the central dust lane of Centaurus A, a galaxy located 10 to 16 million light-years away in the constellation of Centaurus.

The research was led by Professor Maximilian Stritzinger and his colleagues from the Department of Physics and Astronomy at Aarhus University. They were joined by researchers from the European Southern Observatory (ESO), O’Brien Centre for Science North at University College Dublin (UCD), the Institute of Space Sciences (ICE, CSIC), the Institut d’Estudis Espacials de Catalunya (IEEC), Hofstra University, and New Mexico State University. The paper that describes their findings recently appeared in the Astrophysical Journal Letters.

The core and subsequent layers of a dying star. Each layer has been left over from millions of years of fusing each subsequent element into the next one. Credit: Wikimedia

The SN 2016adj supernova was first seen in 2016 and took place in the well-studied galaxy of Centaurus A. This type of supernova (stripped-envelop) refers to the fact that the star undergoing collapse has already blown away its outer hydrogen envelope and was surrounded by an outer helium layer before exploding. Despite its relative proximity, there is still considerable debate about this galaxy’s fundamental properties and proper distance. Astronomers are still unsure if it is a lenticular or giant elliptical galaxy and estimate its distance at 10 to 16 billion light-years from Earth.

In the five and a half years since SN2016adj was discovered, astronomers have watched the area around the supernova as it slowly faded. Using images from Hubble, Stritzinger and his colleagues created a short video that shows the supernova fading and the evolution of the light echoes that followed. The gif video (shown below) shows the progression throughout a one-thousand, nine-hundred, and eleven-day period. This includes the light from the original explosion visibly fading, followed by the appearance of an illuminated ring that expands slowly outwards.

The data set is remarkable and enabled us to produce impressive colored images and animations that exhibit the evolution of the light echoes over the five years that followed. It is a rare phenomenon previously documented with only a handful of other supernovae. As Prof. Stritzinger said in a recent UCD Research News release:

“Centaurus A is full of dust lanes, and when the sideways spreading light from the supernova hit these dusty areas over time, they lit up further and further away from the original supernova position, creating a series of expanding rings of emission called light echoes. The variations in these rings during the years of observation enable researchers to probe the layout of the dust lanes in the galaxy near the explosion. The data suggests that they consist of columns of dust with large holes in between, resembling a chunk of Swiss cheese.”

Gif-video of the HST images acquired of SN 2016adj. Credit: HST/NASA/Caltech-JPL/Stritzinger et al. (2022)

Based on their observations, astronomers estimate that the blast wave created by the explosion moved outwards at over 10,000 km per second (36 million km/h; 22.37 million mph). Ahead of this wave is the intense flash of light released from the supernova, which is absorbed by the clouds of dust and gas thrown off by the supernova explosion, which caused the expanding rings of light captured in these images. These events are particularly interesting to astronomers since they are how the Universe was seeded with heavy elements like carbon, oxygen, and iron.

The abundance of these elements led to the formation of the first planet roughly thirteen billion years ago and the eventual emergence of life. As co-author Dr. Stephen Lawrence of Hofstra University said:

“A good everyday analogy is to imagine the finale of a fireworks show – the bright burst of light from a shell at the end of the show will light up the smoke from earlier shells that is still lingering in the area. By comparing a series of photographs taken over several minutes, you could measure all sorts of information that is not directly related to the most recent explosion that is lighting up the scene, things like how many shells had previously exploded, how opaque is the smoke from a given shell, or how fast and in what direction was the wind blowing.”

These observations were immensely significant since only four distinct light echoes from four different dust sheets have been observed so far. Looking ahead, the team hopes to conduct follow-up observations with Hubble, hoping that more light rings will emerge. They also hope to obtain spectra from the light echoes, which will reveal the dust clouds’ composition and the supernova’s chemical makeup. In the meantime, these results show how the venerable Hubble can still make major discoveries over thirty years later. As co-author Dr. Morgan Fraser of the UCD School of Physics:

“While the James Webb Space Telescope has drawn much attention, its predecessor Hubble continues to provide incredible images of the Universe. HST has now been observing the sky for over three decades, so we can find things like this light echo that evolve slowly over many years.”

Further Reading: UCD, Astrophysical Journal Letters