Re: Galaxies without dark matter halos?

Joseph Lazio <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> >>>>> "g" == greywolf42  <[EMAIL PROTECTED]> writes:
>
> g> John Chandler <[EMAIL PROTECTED]> wrote in message
> g> news:[EMAIL PROTECTED]

> [Regarding the measurements of angular motions:]

Actually, I wasn't discussing angular motions.  We were discussing the
physical orbits of stars.  John then made a comment about astrometry being
able to determine the 3D physical orbit of a star without any other input --
which I am attempting to clarify.

> >> If not, then your attempt to distinguish between what is "observed"
> >> and what is "merely calculated" is pointless quibbling.
>
> g> Differentiating between observations and theoretical calculation is
> g> never "quibbling."  In this case, I was alluding to the fact that
> g> we do not get orbital speeds from astrometry (observation) until we
> g> include a theoretical calculation of distance (calculation).
>
> As is often the case, I get the impression that people are talking
> past each other, in part because they are using the same words to mean
> different things.  My sense is that John Chandler is using terminology
> with which most astronomers would be comfortable.  I'm at a bit of a
> loss to understand some of greywolf's distinctions.
>
> greywolf, perhaps you could explain what you mean by a "theoretical
> calculation of distance."  I don't understand how one would start from
> some first-principle theory and calculate the distance to a star.  The
> most obvious difficulty with this language would be in the case of a
> binary system with a measured parallax.  How could a measured parallax
> be termed a "theoretical calculation"?

Parallax measurements are fairly direct measurements of distance.  I would
not call them 'theoretical.'  However, once we depart from the directness of
parallax, our measurements of distance become more and more theoretical.  As
they depend upon the explicit use of some theory of categorization and
addition of varying numbers of assumptions and results of other
calculations.  In the 'central galactic mass' case at hand, the
identification of the distance to the 'orbiting star' requires some sort of
determination that the star is indeed at the center of the galaxy (and not
on a line between us and the center) and what the distance to the center of
the galaxy is.  Both of these assumptions or calculations are subject to
some reasonable probability of systematic error.

The observation of movement of an image of a star on a photo plate (or
equivalent) is the observation.  Determination of the speed of the star in
orbit is a calculation -- not an observation.

> More generally, I can see that
> some kinds of distance estimates might involve input from, say, models
> of stellar atmopheres.  That still strikes me as not what most people
> mean by a "theoretical calculation," though.

Then I would disagree with 'most people.'  However, the models of stellar
atmospheres are undeniably theoretical.  Regardless of what 'most people'
call them.

> >> The astrometric positions themselves are of course calculated in
> >> turn from the filar micrometer readings, plate measurements, and
> >> other "actual" observations.
>
> g> Your statement, to which I was replying was: "What's observed
> g> astrometrically is the projected angular velocity."  We actually
> g> observe an angular velocity (not projected) on the celestial
> g> sphere.
>
> Again, this seems to be at odds with common astronomical terminology.
> What we desire is the space, or true 3D, velocity of the star.

Correct.

> What we observe is the motion on the celestial sphere.

Correct, as I noted to John.

> Common astronomical
> terminology is that what is observed is the "velocity projected on the
> celestial sphere."

The terminology immediately above is quite clear, if -- strictly speaking --
dimensionally impossible.  Velocity is always vector distance per unit time.
Motion on the celestial sphere can only be in units of radians or degrees
per unit time.

However, the words "on the celestial sphere" are absolutely critical in the
above terminology.  And these words were not attached to the term 'projected
velocity' when John used them. (I then attempted to clarify the situation --
apparently failing at this attempt).  John made a specific claim about the
3D physical orbit of stars.

Regardless of the terminology about movement of points of light on a plate,
the subject under discussion is still the motion of stars in the galaxy.

greywolf42
ubi dubium ibi libertas