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  In addition to black-hole binary stars in our Galaxy, we have learned that there are black holes of a completely different type in different places in the universe. On this page is a negative black-and-white photograph of a galaxy called NGC4151. Underexposed photographs make the center of the galaxy look like a single bright point, but they don't reveal the rest of the picture. More exposed photographs make the center look completely saturated, but enable us to see a spiral galaxy around it. As you can see in this example, a particular type of galaxies, also known as active galaxies, have an extremely bright, central object which emit light, radio waves, and X-rays. This, we have come to think, is because at the center of active galaxies are gigantic black holes whose mass is 1 to 100 million times larger than that of the sun, and which absorb gas and stars around them. I use the word "gigantic," but to put that in perspective, their event horizon is only 1/100 millionth of the whole galaxy.We can understand the formation of a black hole like Cygnus X-1 as the end of an ordinary star. But then, how did these gigantic black holes, with a mass 1 to 100 million times that of the sun, come into being? That is a question that had remained unsolved for more than 20 years, but recently we have gained a clue: the existence of intermediate-mass black holes.  This is the spiral galaxy called M83, located in the southern sky. On the left is a visible-light photograph; on the right is an image taken with the new X-ray astronomical satellite Chandra. As you can see, X-rays emit diffuse light along the spiral arms of the galaxy. This indicates high-temperature gases, such as solar coronas, exist widespread in the galaxy. The center of the galaxy is very bright, and probably a gigantic black hole is hidden there. In addition to this light, we can see bright point sources in the x-ray photo. Plotted in red, the visible-light photo makes it clear that these bright dots are distributed almost along the arm of the galaxy. The vast majority are thought to be places where gases are absorbed by black holes and neutron stars.Among these X-ray point sources, about the ten brightest ones emit several dozen or hundred times as much X-ray energy as Cygnus X-1. We have called them Ultra Luminous X-ray Sources (ULX). There are some galaxies, such as ours, that have no ULXs, while others, such as M83, have around ten. It would seem that galaxies with more star-formation activity have more ULXs. The ASCA satellite has allowed us to observe some of the ULXs, and we have found that their X-ray spectrum and certain other characteristics are similar to those of Cygnus X-1 and other similar stars. So we can regard them as black-hole binary stars that emit X-rays the same way as Cygnus X-1, though they are far brighter. If that's the case, would Cygnus X-1 become a ULX if it accumulated more gas? The answer is no. There is a limit to the brightness of a black hole, since the X-rays it emits push back against the gasses attracted to it, and prevent their downward movement and accumulation. The smaller the black hole, the less gas it is capable of accumulating. So Cygnus X-1, whose mass is only ten times that of the sun, cannot become a ULX by any means. It would take a black hole that is several dozen or hundred times as heavy as the sun to become a ULX. That's why we have come to regard ULXs as "intermediate-mass black holes," somewhere between small black holes like Cygnus and gigantic ones found at the center of galaxies. We think that by absorbing gas and stars, small black holes become intermediate-mass black holes and sink towards the center of a galaxy, where they merge with each other to form one gigantic black hole. |
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