Quasar classification (in Re: WD-Ontology)

[Anita M. S. Richards]

On Sat, 3 Mar 2007, Kjetil Kjernsmo wrote:

> 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.
>
Yes, it is - and I don't think that the IVOA should attempt to create 
rigid definitions when there are many existing practices which make sense 
but only in limited contexts.  For example, I would say (following 
Padovani et al. 2004 and refs therein) that anything with an X-ray 
luminosity > 10^35 W and a very hard x-ray photon index is an AGN *in the 
GOODS fields* and anything with an X-ray luminosity > 10^37 W is a QSO. 
If an object is only detected in the X-ray, then there is no distance 
information to allow the luminosity to be calculated, but the hardness of 
the X-ray photon index may mean that you can define it as an AGN even 
though no optical host has *yet* been detected.  After all, a 
non-detection is only relative to some arbitrary set of observational 
sensitivities.

Hence I would say that it is probably uncontroversial to make QSO a 
sub-set of AGN?  I have never come across the use of QSO to describe 
something which could not also be described as AGN although the converse 
is not the case?

Another example which only makes sensse in a limited context is "stars are 
points, galaxies are extended" which is fairly accurate (pace QSOs!) for 
some optical data but falls down for e.g. X-ray surveys of distant objects 
(or even some optical high-sensitivity, but not-so-high-resolution 
surveys) where everything except nearby galaxies is point-like - and for 
interferometry which can resolve some stars...

So either we have to add wavelength and resolution to any definition, or 
(probably more practically) allow multiple definitions and build up a tree 
of supersets and subsets based on accumulated application to real 
catalogues etc., not the prejudices of those of us on this mailing list, 
if that's possible.

Regarding individual objects, the IVOA's reasoning could _add_ 
classifications, e.g. QSO + extended optical galaxy at same position is 
also AGN (although I would say that anyway as a case of inheritance) - but 
that should somehow be distinguished from definitions taken straight from 
the literature.  But we should not _take classifications away_ even if 
they seem to be contradictory (maybe within some time limit e.g. 20 yr).



Incidentally, microquasars are always referred to as such and are indeed a 
separate phenomenon and not a subset of any of the above...

best wishes


Anita


- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AstroGrid Astronomer
MERLIN/VLBI National Facility, University of Manchester, 
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K. 
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).