The huge luminosities of AGN allow
us to see them when the Universe was only a fraction of its present age.
We can compare the number and luminosities of AGN in the distant past with
what we can see in the nearby Universe today. It is found that the Universe
was very different in the past, and AGN were much brighter - thus
they have evolved. The peak of AGN activity appears to have occurred when
the Universe was about a third of its present age.
This page very briefly describes recent work on the evolution of almost 200 AGN discovered in `RIXOS': the ROSAT International X-ray/Optical Survey. To examine the evolution of AGN, we use a mathematical tool known as the luminosity function. This is the number of AGN per unit volume per unit luminosity interval (or in English, how many AGN with some range of brightness are found in some fixed size blob of the Universe). As AGN evolve, the luminosity function changes. If the luminosity function is different at different redshifts then we know that AGN have evolved with time.
The different colours and symbols correspond to the luminosity function in different ranges of redshift (z). If AGN were not evolving then the luminosity functions from different redshifts should be the same. The dashed lines represent a model in which there are the same number of AGN at all redshifts, but the high redshift ones are more luminous.
Two possible models for AGN X-ray
luminosity evolution are shown above; both fit the data well. AGN appear
to be more luminous as redshift increases until about redshift 1.5. At
higher redshifts, AGN luminosities may start to decrease with redshift.
Full details are published in Page et al (1996), Monthly Notices of the
Royal Astronomical Society, volume 281, pages 579-590.
Other Extragalactic Astronomy pages:
MSSL home pages:
This page written by Mat Page (mjp@mssl.ucl.ac.uk).
Last modified 17th March 1998