[Fwd: Re: Quasar classification (in Re: WD-Ontology)]

Mon Mar 5 15:17:34 PST 2007

-------- Original Message --------
Subject: Re: Quasar classification (in Re: WD-Ontology)
Date: Mon, 5 Mar 2007 18:08:09 +0000 (GMT)
From: Anita M. S. Richards <a.m.s.richards at manchester.ac.uk>
To: Ed Shaya <eshaya at umd.edu>
References: <20070221162208.3kfmmx1swk4kowgk at webmail.sic.rm.cnr.it> 
<200703012258.35784.kjetil at kjernsmo.net> <45E8AD76.4000902 at umd.edu> 
<200703032048.24671.kjetil at kjernsmo.net> 
<Pine.LNX.4.62.0703032123240.16359 at hecate.jb.man.ac.uk> 
<45EC5197.2030400 at umd.edu>

Hi Ed,

Thanks for an expert view. Most of waht you say illustrates well the need
not to make definitions rigid - that way they cannot be contradictory -
and there has already been plenty of development of 'ranking' algorithms
so that it should be easy to avoid ending up with everything as both a
sub- and a super-set of everything else...

cheers
a

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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).

On Mon, 5 Mar 2007, Ed Shaya wrote:

>
>> 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.
>
> "define it as an AGN"? I think you need to differentiate between making a 
> tentative identification and defining.  Sometimes you can make a good guess 
> at the type if you know only the luminosity in one band and have a 
> statistical reason for arguing most sources in the field are at z>1 or so.  A 
> fleshed out ontology would tell you what other classifications are reasonable 
> and under what conditions the identification would be correct (eg, "as long 
> as z>1.6 and B<15, and variability is < .2 mag, then this is certainly a 
> quasar").
>> 
>> 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?
>
> I have.  Primordial black holes have been hypothesized by several illustrious 
> astronomers and a supermassive one feeding on the IGM would probably act like 
> any other QSO.  It just would not have a galaxy. Arguments for such objects 
> are reasonable since it does seem quite hard to accrete so much matter in 
> time.  Recently, there have been several papers (just Google on "primordial 
> supermassive blackholes") contemplating the idea that old elliptical galaxies 
> began life as these primordial blackholes that later gathered a galaxy around 
> them.  I guess that this possibility, though highly speculative, has been the 
> underlying reason for keeping the term QSO disjoint from AGN.
>
>> 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...
>
> Again, this is probabilistic identification.  This time because there are 
> only upper limits on the sizes and no distances.  So, yes ontology alone is 
> not useful for identification when you have insufficient data to positively 
> identify.  I don't get the point of coming up with use cases that are too 
> hard for the human mind let alone a simple algorithm.  On the other hand, if 
> there is data that provide an upper limit of 2e8 km diameter, and a M_B = 
> 4.8+/-.3 and  B-V = 0.7+/-.2, then ontology could tell this is a G2V star.
>> 
>> 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.
>> 
> Every astroObject has a position (set of coordinate values) in the 
> N-dimensional observation space.  We carve up this space into a reasonable 
> set of subspaces which we call the object classes.  Over most of this space 
> the clustering of the properties demarcate for themselves acceptable ranges 
> for each class.  But it is true that in certain projections (ie, you have 
> limited wavelength coverage, resolution etc) things can get messy.
>
>> 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).
>> 
> I agree except for the last bit.  We must take away contradictory definitions 
> because the system will crash.  We can deal with contradictions only in that 
> individuals can have their private ontology versions that are somehow amended 
> to their way of thinking.  Our private versions might contradict each others.
>> 
>> 
>> 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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