Astronomers at UCL, using ESA’s Herschel Space Observatory, have shown that the number of stars that form during the early lives of galaxies may be controlled by the massive black holes at their hearts.
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 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.
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.
The latest study, led by 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.