# Confusing?

Alberto Micol Alberto.Micol at eso.org
Wed Nov 24 03:00:57 PST 2004

```Robert Hanisch wrote:

>On p.7, the paragraph beginning "The order in which words are arranged..."
>seems like circular reasoning to me.  It says the order matters only if the
>order matters.  I think the standard should be clear on this -- does the
>order matter or not?  The text elsewhere suggests yes.
>
>I find Section 3.4 confusing...does anyone else?  phot.color is a difference
>of two magnitudes, but does NOT have the associated word arith.diff.  A
>temperature ratio is not a temperature, so phys.temperature;arith.ratio
>seems backwards to me.  I can't use arith.ratio for M/L, so instead I see
>that this is defined as phys.mass.light.  The UCD
>phys.mass;phys.luminosity;arith.ratio would seem to make more sense.  If
>phot.color;em.opt.B;em.opt.V means B-V and phot.color;em.opt.V;em.opt.B
>means V-B, then order is clearly important.  The concern this all raises for
>me is that the construction rules for new UCDs are not very clear, and that
>the existing UCD1+s have a certain (large) amount of arbitrariness.
>
>

I found myself exactly in the same position as Bob. It IS confusing.
Nevertheless...

The fundamental requirement of the UCDs is (quote):

the primary word carries most of the meaning as to "what the quantity is".

Hence, Bob's suggestion for
*phys.mass;phys.luminosity;arith.ratio*
would make the UCDinterpreter think that the described quantity is a mass.
It is not.

The fact is that, despite being a ratio, it is a quantity that carries some
physical meaning (for a galaxy it could give indications on the type of
stars and
the amount of dark matter), hence the existence of a ucd.

The same is true for example for other quantities, for which nobody objects
the availability of a proper UCD. One for all: velocity, it is a ratio
between a distance
and a time ...

(After all, the dimensional analysis of a quantity will always reveal a
ratio between
mass length and time, some times even for adimensional quantities)

Other more complex examples of the same problem are those UCDs
which simply describe a different mathematical expression of the same
thing,
eg phot.mag and phot.flux.

phot.mag it is the 2.5 log10 of a flux ratio
One could say that the correct ucd is:  *phot.flux;em.opt.V;arith.mag*
expliciting the algorithm applied to the flux using a special arith.mag

The question is: Why to complicate things?

In fact, not to complicate things, * phot.color;em.opt.V;em.opt.B* complies
to that fundamental requirement (it is a color) and expresses an
implicit arith.diff.

So, it is confusing, but it is difficult to keep it simple.
What is the way out?
Can the UCDs be built in a more more mathematical way, so to avoid confusion
or would that complicate too much the problem?

For the time being I'm afraid we have to live with such fuzzy definition.
Let's hopw that UCD3 will address all this.

Alberto

```