MadSci Network: Astronomy
Query:

Re: Would radiation in a Seyfert or Quasar galaxy kill life on planets within?

Date: Thu Apr 29 12:46:04 1999
Posted By: Denise Kaisler, Grad student, Astronomy, UCLA, Division of Astronomy
Area of science: Astronomy
ID: 918081411.As
Message:

Let me focus on Seyfert galaxies, since these are spiral galaxies and
therefore somewhat akin to our own. 

A back-of the envelope calculation shows that a Seyfert galaxy doesn't
have much of an effect on how much radiation the planet recieves.

The average luminosity of a Seyfert Galaxy is 10e36 to 10e38 Watts. Most of
this luminosity originates from the Broad-Line Region, a small area near
the center of that galaxy which is light days to light weeks in diameter.

Let's say a star was located 8.5 kiloparsecs (1 kpc = 3.09e19 m) away from
the center of such a galaxy. This is how far the Sun is from the center of
the Milky Way. If the source of the Seyfert's emission is 10 light-days
across (a happy medium), then the flux received in the vicinity of the star
is given by

f2 = (r^2/d^2) f1

Where f1 = L/(4*pi*r^2)

r is the radius of the broad-line region 
(5 light-days or 1.296e14 m)
L is the luminosity of the galaxy (let's say 10e37 Watts assumed to be all
concentrated in this broad-line region or BLR)
f1 is the flux at the surface of the BLR
d is the distance of the star from the center of the galaxy

Plugging in the numbers gives us

f1 = 4.73e7 W/m^2

f2 = (1.30e14/8.5*3.09e19)^2 * 4.73e7 W/m^2
	
   = 23.45 W/m^2

Now this is only about 2% of the solar constant (the flux received at the
Earth's surface from the Sun). So even if the Earth was located in Seyfert
galaxy, the radiation from the central engine would be a small fraction of
what the Earth gets from the Sun. 

Of course, this would be a significant increase as far as life on Earth is
concerned. Such an increase or decrease would probably change the weather
patterns on the Earth, just as it did during the Maunder minimum, a period
in when very few sunspots were observed on the Sun. This coincided with the
"Little Ice Age" from 1640-1710, when the global temperature dropped by
about 2 degrees Farenheit - causing rivers to ice up and precipitating
major crop failures in Europe and other areas of the world.

But this would only be the case if the Earth were instantly transported to
a Seyfert galaxy. If the planet had been there all along, whatever life had
developed there would no doubt have adapted to the higher flux long ago. 

As far as the last part of your question goes, there is a theory that all
spiral galaxies pass through a stage where their nuclei are very active. It
is possible (though this is by no means a solid theory) that the Milky Way
was a Seyfert galaxy in the past. 

However, if the Milky Way had gone through such a phase, it would probably
have done early in its history, long before life arose on Earth. It is
astronomically improbable that mass extinctions on our planet were caused
by such a phenomenon.

Still, a very interesting question!







Current Queue | Current Queue for Astronomy | Astronomy archives

Try the links in the MadSci Library for more information on Astronomy.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-1999. All rights reserved.