Luminous starburst galaxies are where a lot of young stars are currently forming. They come in two
different varieties: starbursts where the star creation is spread evenly throughout the galaxy and those
where it is concentrated at its nucleus. Sometimes activity at the centre is so intense that fantastic
‘bubbles’ are created giving rise to streams of hot gas, or ‘superwinds’. XMM-Newton has recently
gained new insights into one such starburst galaxy, NGC 253.

In cosmic terms NGC 253 is very near, “only” some 8 million light years away. Its observation is also facilitated because the total amount of intervening matter between us and the galaxy is quite small and the X-rays it emits are not so absorbed. X-ray telescopes can therefore register lower energy X-rays. NGC 253 had already been examined by previous missions such as ROSAT.

XMM-Newton with its far greater collecting power, and ability to measure higher energy X-rays has identified new X-ray point sources in the galaxy, it has made an evaluation of the rate of star creation and a detailed analysis of the elemental composition of NGC 253’s superwind.

“Our study concentrated on the brighter stuff in NGC 253 stemming from its nucleus” explains Wolfgang Pietsch from the Max Planck Institute in Garching, and first author of a paper on this observation published in Astronomy and Astrophysics. “This is where we are likely to learn the most about star formation, and in addition about the creation of the wind that is flowing out from that region. We still don’t know whether there is an active galactic nucleus, for instance a mini-quasar powered by a black hole buried in the centre, although our new data makes this possibility look less likely.”

Basically, starbursts are where stars are born, but what is being observed is in fact the X-ray emitting supernova remnants, the remains of massive stars that have exploded and whose expelled matter will eventually contract under gravity to be recycled into new ones. “We have probed deeper into NGC 253’s nucleus than ever before” says Pietsch. “Its hot gas emission probably comes from a very high number of supernovae. The X-ray emission we have measured would need the explosion of one star in this small nuclear region about every five years. In contrast, for our entire Milky Way there is only about one supernova every 20-30 years. So this starburst galaxy is very prolific.”

A large amount of hot gas is created and expelled from the centre of such galaxies by multiple supernova explosions. It then exerts pressure on the surrounding interstellar medium as it ploughs into it with high velocity. “Imagine this galaxy as a flat pancake, with the hot gas like an expanding balloon in the middle” says co-author Martin Ward from Leicester University. “Eventually the radius of the balloon becomes equal to the thickness of the galaxy and it then breaks out at the top and bottom and one gets these fantastic superwinds or plumes that are clearly seen in X-rays.”

NGC 253’s plume is well known since the Einstein mission. It had also been studied by ROSAT. But XMM-Newton has now resolved it in different and higher energy bands and discovered something new.

“From the spectra obtained by the RGS instrument, we see that the cone-shaped plume is filled with low temperature material that may in part come from the nucleus” explains Pietsch. “But it is mainly material that has been heated by the wind that is flowing out and interacting with the surrounding environment. This is a different picture from what had been suggested by recent Chandra observations.”

“With XMM-Newton we can measure the elemental abundances within the superwind to high accuracy not possible before” says Martin Ward. “This is extremely interesting because it provides us with clues about the chemical enrichment of the inter-galactic medium.”

NGC 253 has not finished revealing fascinating details of the star formation processes and further XMM-Newton observations of this galaxy were carried out last December.

“XMM-Newton observations of NGC 253: resolving the emission components in the disk and nuclear area” is published in the January 2001 edition of Astronomy and Astrophysics, Our thanks to first author Wolfang Pietsch, and to Martin Ward.