X-rays emitted from black holes sterilise galaxies

All large galaxies have a massive black hole in the middle, each millions of times the mass of a single star. A puzzle that has remained unsolved for over a decade is that the masses of the black holes are linked to the size of the round central bulges at the hearts of galaxies.

The suspicion has long been that this is due to the events in the early lives of the galaxies, when the stars in the bulge were forming. To study this phase, astronomers need to look at very distant galaxies, so far away that we see them as they were billions of years ago.

"Space telescopes like Herschel let us look back in time, and that's just what we need to do to find out how today's galaxies were built" said Dr Mat Page (UCL Mullard Space Science Laboratory).

Although the black holes themselves cannot be seen, the material closest to them can get incredibly hot, emitting large amounts of light over a very wide range of wavelengths, from radio waves to x-rays. The light from the hot central material can be trillions of times as bright as the Sun, with brighter emission indicating a more massive black hole. There are also strong flows of material (winds and jets) expelled from the region around the black hole.

The hot material near the black hole outshines almost all the light from rest of the host galaxy, except for the light with wavelengths just less than a millimetre. This sub-millimetre light is invisible to normal telescopes but is seen by the Herschel Space Observatory and indicates the rate at which stars are being formed in the galaxy.

The latest study, led by Dr. Mat Page, used images from the SPIRE camera on board Herschel to calculate the amount of star formation in distant galaxies. This can be compared with the X-rays detected by NASA's Chandra X-ray satellite, which indicates the growth-rate of the black hole.

"Galaxies were forming stars like crazy when the Universe was young, but trying to see the light from star formation against the glare from the hot stuff around the black hole has been almost impossible until now. That's all changed with the new wavelengths opened up by Herschel's SPIRE camera," said Dr Page.

Galaxies with massive black holes were found to have high rates of star formation, with some forming stars at a thousand times the rate our own Milky Way galaxy does today. But intriguingly, the Herschel results show that the fastest-growing black holes are in galaxies with very little star formation - once the radiation coming from close to the black hole exceeds a certain power, it tends to "switch off" star formation in its galaxy.

Prof. Seb Oliver (who co-leads the HerMES project) said "This fantastic result provides an amazing link between black hoes and star formation in the early Universe. It is a huge clue to this decade old riddle and could mean that once a black hole is big enough and producing enough radiation it somehow shuts-down the formation of stars in the surrounding galaxy"

The most likely explanation is that the incredibly strong winds from around these very powerful black holes are preventing the gas and dust in the rest of the galaxy from forming stars.

"This allows the black holes to regulate the formation of stars, limiting the total number of stars that form." said Dr Myrto Symeonidis also from UCL's Mullard Space Science Laboratory, a co-author of the study.

Prof. Matt Griffin of Cardiff University, who is the Principal Investigator of the international team which built the Herschel-SPIRE instrument said: "This important discovery shows how the great sensitivity of SPIRE is allowing us to look back in time and understand the early history and development of the galaxies that populate today's universe. Only a small fraction of the instrument's observations have been fully analysed so far, and we're looking forward to many more exciting results."

Image: Artists impression of a star forming galaxy with an outflow from its active galactic nucleus. Credit: ESA//Herschel/HerMES ; NASA/CSX

Links:

UCL Mullard Space Science Laboratory
Herschel Space Observatory
Research in Nature