An international team of astronomers using ESA’s Herschel Space Observatory has shown that the number of stars that form during the early lives of galaxies may be influenced by the massive black holes at their hearts.
This artistically modified image of the local galaxy Arp 220, captured by the Hubble Space Telescope, helps illustrate the Herschel results. The bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, indicate that the central black hole's activity is intensifying (NASA / JPL-Caltech / R. Hurt)
All large galaxies have a massive black hole at their center, each millions of times the mass of a single star. For over a decade scientists have been puzzled as to why 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 the answer lies 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.
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 this super-heated material can be trillions of times as bright as the Sun, with brighter emissions 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 millimeter. This sub-millimeter 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.
“Herschel provides a new perspective and is conducting a number of surveys of galaxies near and far, in order to unravel the mysteries of the formation and evolution of galaxies across cosmic time,” explained Dr. Göran Pilb’ratt, the ESA Herschel project scientist.
The latest study, published in Nature, uses 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.
“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,” explained lead author Dr. Mat Page of the University College London. “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.”
Galaxies with massive black holes were found to have high rates of star formation, with some forming stars at a thousand times the rate of our own Milky Way galaxy 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.
