Topic: Early March Astronomy Bulletin

[Clive]

SUPERNOVA FROM THE YEAR 185
NOIRLab

In the year 185 CE, Chinese astronomers recorded a 'guest star' in the night sky, one that shone brightly for about eight months before it faded away. The 'guest star' is now understood to be a core-collapse supernova explosion which astronomers refer to as SN 185. The explosion occurred about 8,000 light years away from us towards the direction of Alpha Centauri and left behind a supernova remnant known as RCW 86.

The link between a supernova explosion and the remnant structure hasn't always been well understood. For a long time, astronomers thought that for a remnant to reach the size of RCW 86 would require at least around 10,000 years. This is what evidence from core-collapse supernovae indicates. However, if this indeed was the case, it would mean the remnant is far older than the supernova explosion that occurred in 185 CE.

But a recent study (2006) found that the size of the remnant meant that it was expanding unusually fast. Using that to estimate the age of the remnant put it at around 2,000 years old, which strongly agrees with the theory that the remnant is indeed the result of the explosion in 185. One may wonder, though, what really drives such a rapid expansion velocity?

To answer that, astronomers used X-ray observations of the remnant. These observations revealed the presence of the element iron. Iron points to a different type of supernova explosion, known as a Type Ia supernova. This type is seen in a binary system: when one star has become a white dwarf, it eats away material from its companion star. Eventually the white dwarf cannot accrete any more mass and it explodes violently.

This is what is thought to have happened in the case of SN 185. The white dwarf stripped the material off its companion star, its stellar winds clearing the local area of dust and gas and creating a cavity around the binary. Then when the white dwarf exploded the cleared space allowed for the stellar remnants to expand quickly, unhindered by gas or dust, quickly reaching the size we see today.

A VERY EARLY BLACK HOLE

RAS

Astronomers have discovered what appears to be one of the first supermassive black holes to have formed in the early Universe. The black hole sits at the heart of a galaxy named COS-87259. This galaxy is considered to be very extreme as it's undergoing star formation at a rate that is 1000 times that of the Milky Way. COS-87259 also contains more than a billion solar masses of interstellar dust.

The black hole at the centre of this galaxy is surrounded by cosmic dust. Dust absorbs visible and UV wavelengths and, in turn, emits strongly in the infrared region of the electromagnetic spectrum, in particular the mid-infrared part. For this reason, this black hole is bright almost exclusively in the mid-infrared region. The black hole also generates a jet, which propels material out of the host galaxy. The discovery was made looking at a very small patch of sky, leading to the astonishing conclusion that these objects may be more abundant in the early Universe than previously thought so.

So far, astronomers knew about one other type of supermassive black hole, within quasars. Quasars, however, tend to be unobscured by cosmic dust, unlike this new black hole. Quasars are also typically less abundant in the early Universe, with only a few tens of them found over the whole sky. The discovery of COS-87259's supermassive black hole raises many questions concerning early Universe black holes, their formation and the conditions of their host galaxies.