As per this book, An Introduction to Active Galactic Nuclei by Bradley Peterson: "The nuclear emission must last more than 10^8 years, because Seyfert galaxies constitute about 1 in 100 spiral galaxies. This is a simple argument. One extreme scenario is that galaxies which are Seyferts are always Seyferts, in which case their lifetime is the age of the Universe (10^10 years). The opposite extreme is one where all spirals pass through a Seyfert phase (or phases) - since 1 spiral in 100 is currently in the Seyfert phase, it must last of order 1010 / 100 = 10^8 years."
The nuclear emission must last more than $10^8$ years, because Seyfert galaxies constitute about 1 in 100 spiral galaxies. This is a simple argument. One extreme scenario is that galaxies which are Seyferts are always Seyferts, in which case their lifetime is the age of the Universe ($10^{10}$ years). The opposite extreme is one where all spirals pass through a Seyfert phase (or phases) - since 1 spiral in 100 is currently in the Seyfert phase, it must last of order $10^{10} / 100 = 10^8$ years.
I can't seem to grasp this justification for the age of nuclear emissions. Could someone explain?
