ESA Herschel: Revealing Milky Way's warm heart

What heats gas near supermassive black holes at the centre of galaxies? 

Astronomers have looked at the centre of our Galaxy, the Milky Way, with ESA's Herschel Space Observatory and discovered a rich variety of molecules at surprisingly high temperatures - up to 1000 K. 

The new data suggest that the molecular gas is heated up by shocks, in addition to ultraviolet radiation from massive stars close to the Galactic Centre. 

Shocks develop in the gas as the material surges towards Sagittarius A*, the region harbouring the supermassive black hole at the heart of the Milky Way.

About 26 000 light-years away, the central region of the Galaxy, known as Sagittarius A*, or Sgr A*, hosts the closest super-massive black hole to Earth.

With an estimated mass equivalent to about four million times that of our Sun, this black hole currently accretes matter from its surroundings at a very gentle pace, like the majority of super-massive black holes in massive galaxies across the Universe.

A few hundred times closer to Earth than the nearest galaxy hosting an actively accreting black hole, Sgr A* provides a unique chance to study the environment of super-massive black holes in great detail.

However, we observe the Galactic Centre through the dense disc of the Milky Way, where gas and dust in the spiral arms of the Galaxy absorb visible light.

The best way to study the interstellar material around Sgr A* is thus via infrared and radio observations.

New research based on spectroscopic data from ESA's Herschel Space Observatory has resolved the innermost portion of the Milky Way – a few light-years around Sgr A* – for the first time at far-infrared wavelengths.

The team of astronomers, led by Javier Goicoechea from the Centro de Astrobiología in Madrid, Spain, was able to isolate the far-infrared emission from all the interstellar components that surround Sgr A* – neutral atomic, molecular and ionised gas, as well as dust.

Javier Goicoechea

In particular, they exploited the characteristic signature from several molecules to trace the temperature, density and other properties of the material that orbits the central black hole and is possibly falling onto it.

"We detected a surprisingly rich variety of molecules in the environment of Sgr A* that really exceeded our expectations," comments Goicoechea.

"The molecules range from highly excited carbon monoxide and water vapour, to hydrogen cyanide and many light molecules that play a critical role in the chemistry of the interstellar medium. Some of them had not been detected before Herschel," he adds.

More information on ESA Herschel site here