Quasar classification (in Re: WD-Ontology)

[Richard McMahon]

Just to say that, as an astronomer I agree with Anita. The situation is
also complicated by the fact that distances are not known for any AGN, so
a luminosity is always uncertain and depends on for instance the
cosmological framework one assumes. One measures with an error
estimate(maybe) a redshift or radial velocity and then make assumptions
and about the current rate of expansion of the Universe, matter density
and Lamda and the k-correction to get the luminosity at a fixed rest
wavelength.

AGN( or Active Galactic Nucleus) is the top level class; QSO, Quasar,
Seyfert I, Seyfert II are all members of this AGN class. An object can be
simultaneously an AGN, a QSO, a Quasar and a Seyfert I or Seyfert II. Some
objects have been known to change from a Seyfert I to a Seyfert II.

For info, Schmidt and Green(1983) define a quasar as an object
with a B band absolute magnitude brighter than m(B)=-23.



On Sat, 3 Mar 2007, Anita M. S. Richards wrote:

> Date: Sat, 3 Mar 2007 21:59:10 +0000 (GMT)
> From: Anita M. S. Richards <a.m.s.richards at manchester.ac.uk>
> To: Kjetil Kjernsmo <kjetil at kjernsmo.net>
> Cc: IVOA semantics <semantics at ivoa.net>
> Subject: Re: Quasar classification (in Re: WD-Ontology)
>
>
>
>
> 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).
>

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