Measure the "days of light", consider the black hole
By Avishai Gal-Yam
A research group from Tel Aviv University has been investigating for several years the properties of black holes located at the center of so-called "active" galaxies. These are galaxies from which a lot of radiation is emitted, which is commonly assumed to originate from matter falling into the black hole that resides in their center.
Dr. Shay Caspi, Prof. Hagai Netzer and Prof. Dan Maoz from Tel Aviv University have been observing active galaxies in recent years using the telescope of the Weiss Observatory at Mitzpe Ramon. In an article published these days in "The Astrophysical Journal" and is part of the doctoral thesis of Caspi, a student of Prof. Netzer, the members of the group present the analysis of the results from seven years of observations. The members of the group, using a method known as "reverberation mapping", were able to measure the mass of 17 extremely large black holes located at the center of active galaxies.
The research method is based on tracking two types of radiation coming from the black hole area. The first type is called "continuum radiation": it is radiation consisting of a sequence of frequencies - starting with infrared radiation and ending with X-ray radiation. Matter that reaches the vicinity of the black hole swirls around it and falls into it in a structure called an "adsorption disk". It is common to assume that most of the continuum radiation is emitted from this disk.
The second type of radiation is called "line radiation". This radiation includes only light of certain colors - radiation at frequencies characteristic of different elements, such as hydrogen and oxygen. The accepted assumption is that the radiation from the absorption disk hits gas clouds located at a certain distance from the black hole and stimulates them to emit the line radiation.
From monitoring active galaxies we know that the intensity of radiation from them varies greatly from time to time. When the intensity of the continuum radiation from the disc changes, this also causes a change in the intensity of the line radiation, which is excited by the continuum radiation. However, this change is not immediate: the light emitted from the disk takes several days to travel the distance between the absorption disk and the clouds emitting the lines - these clouds are several "light days" from the center (a light day is the distance traveled by light, which moves at a speed of 300 km per second, for one day).
And so, only after some time it is possible to notice that the radiation of the lines has changed according to the change in the radiation of the continuum. This is similar to returning sound waves - an echo - from the walls of a distant building, or from the hills on the other side of a wadi, after a certain period of time depending on the distance. Prolonged monitoring of the intensity of the continuum radiation and the radiation of the lines allows the researchers to measure the time gap between changes in the continuum radiation and the resulting changes in the line radiation, and to deduce from it how long it takes for the light to reach the region of the black hole to the clouds emitting the lines. Since the speed of light is known, measuring this time is equivalent to measuring the distance between the line-emitting clouds and the black hole. In addition to this measurement, an accurate measurement of the lines allows determining the speed at which the gas clouds from which the lines are emitted surround the black hole.
By combining the data collected by the researchers on the distance between the gas clouds and the black hole and the speed of their movement around it, the researchers deduce the mass of the black hole using Kepler's laws - the laws developed by the astronomer Johannes Kepler in the 16th century to describe the movement of the planets around the sun.
The work recently published by researchers from Tel Aviv University (which also includes two American researchers) includes the largest and most comprehensive sample studied so far. With the help of the data analysis, the researchers were able to point out a connection between the mass of the black hole and the energy emitted from the center of the galaxy - an important link in their understanding of the processes of creating black holes in galaxies and the universe.
The results of the research constitute significant progress in describing the connections between the properties of giant black holes and the properties of the radiation emitted from their surroundings, which can be discovered in observations. These results bring the scientists another step closer to understanding the physical conditions in this unusual environment.
{Appeared in Haaretz newspaper, 22/5/2000}
