Quasar classification (in Re: WD-Ontology)

[Kjetil Kjernsmo]

On Saturday 03 March 2007 00:04, Ed Shaya wrote:
> This is not quite so black and white here.  There may have been the
> intention on someone's part to ensure that something is classified as
> a QSO or an AGN but not both.  It is customary, to call it a QSO when
> there is no observed sign of an underlying galaxy and then to change
> its classification when (and if) deeper imaging reveals a galaxy
> there. Some QSOs have defied extremely deep and intensive searches
> for an underlying galaxy.  This type of problem needs some more
> examination. 

Yes, indeed. I have never worked with actually classifying objects, but 
Julian Krolik writes in his AGN book (on page 19) that "In practice, 
the only distinction [between a Seyfert 1 and a "radio-quiet quasar"] 
is whether a host galaxy is visible. When it is, the AGN is called a 
Seyfert galaxy, whereas when none is visible, it is called a quasar."

This makes the whole definition look somewhat arbitrary, and I feel that 
in many cases it is. 

However, in Krolik's description, there is no doubt that both classes 
are AGNs. Are you suggesting that there are cases where you would not 
classify an extra-galactic (as in beyond the Milky Way) object an AGN 
unless you see evidence of a host galaxy? I suppose that makes sense, 
but I haven't encountered that practice myself. I thought the formation 
of highly energetic objects such as quasars relied too much on galaxy 
formation to be classified separately. But then, I haven't stayed 
uptodate for the last five years. 

OTOH, there are microquasars, that share many characteristics of 
quasars, but are galactic objects themselves.

I could take on the task of writing Julian Krolik, since I have been 
outside of the community for a number of years, it would be fun to do 
something astronomy-related again.

> Another related problem comes with Seyfert I and Seyfert 
> II which are now thought to be the same class but observed from a
> different viewing angle.

Yes, indeed, and you have subclasses Seyfert 1.5, 1.2, 1.8, etc. Also, 
there has been cases where one object has evolved back and forth over 
time. In fact, I'm not sure it is meaningful at this point to have 
those refinements of Seyfert galaxies in the ontology, unless it is 
intended to make it very detailed. 

Best,

Kjetil
-- 
Kjetil Kjernsmo
Programmer / Astrophysicist / Ski-orienteer / Orienteer / Mountaineer
kjetil at kjernsmo.net
Homepage: http://www.kjetil.kjernsmo.net/     OpenPGP KeyID: 6A6A0BBC