Hubble captured a rare ‘light echo’ from a star explosion

Light echoes (LEs) are produced when photons emitted by a transient source scatter off interstellar dust clouds. Often appearing as arcs or wisps of light, LEs were first detected in Galactic and extragalactic novae over a hundred years ago and successfully explained as scattering phenomena by Couderc.

A collaboration of astronomers from Dublin, Barcelona, Aarhus, New York, and Garching, using Hubble Space Telescope (HST) data, created a short gif video, showing first the supernova explosion at the very center, followed by light rings which appeared when light from the explosion hit various layers of dust in the vicinity.

Lead scientist Professor Maximillian Stritzinger of Aarhus University, Denmark, said: “The data set is remarkable and enabled us to produce very impressive colored images and animations that exhibit the evolution of the light echoes over five years. It is a rare phenomenon previously only documented in a handful of other supernovae.”

Dublin-based astrophysicist and co-author Dr. Morgan Fraser, UCD School of Physics, said: “While the James Webb Space Telescope has drawn much attention, its predecessor Hubble continues to provide incredible images of the universe. HST has been observing the sky for over three decades, so we can find things like this light echo that evolve slowly over many years.”

Co-author Dr. Lluis Galbany, Institute of Space Sciences, Barcelona, said: “The blast wave from this powerful supernova explosion is racing outwards at over 10,000 kilometers per second. Ahead of this blastwave is an intense flash of light emitted by the supernova, and this is what is causing the expanding rings we can see in the images. Supernovae are of interest as these cosmic explosions produce many heavy elements such as carbon, oxygen, and iron, which make up our galaxy, stars, and planet.”

Co-author Dr. Stephen Lawrence, Hofstra University, New York, 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 are 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 the smoke from a given shell, or how fast and in what direction was the wind blowing.”

The SN 2016adj supernova in question, located in the well-known odd galaxy Centaurus A and 10 to 16 million light years away from Earth, was first observed in 2016. The astronomers observed the region around the explosion as it slowly dissipated for five and a half years.

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.

Professor Stritzinger said: “Centaurus A is a huge elliptical galaxy. These are mostly quiet, dust free, and without younger stars prone to go off as supernovae, but Centaurus A is different. It is a strong radio astronomical source and contains prominent dust lanes with new stars forming within. This sign that it has “recently” gobbled up another smaller spiral galaxy, and matters have not yet settled down, as they might in a couple of hundreds of millions of years. Observing the development of these light echoes will help us gain more insight into these violent galaxy collisions.” 

Up to now, four distinct light echoes produced by four different dust sheets have been observed. The team, which includes Dr. Ferdinando Patat, European Southern Observatory, Garching, Germany, plans to follow up on the observations with the HST in the future, hoping that more light rings will emerge. Furthermore, it might be possible to obtain a spectrum of the light echoes, showing, in effect, the spectrum of the underlying supernova.

Journal Reference:

1. Maximilian D. Stritzinger et al. Hubble Space Telescope Reveals Spectacular Light Echoes Associated with the Stripped-envelope Supernova 2016adj in the Iconic Dust Lane of Centaurus A. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ac93f8