Re: Galaxies without dark matter halos?

Joseph Lazio <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> >>>>> "g" == greywolf42  <[EMAIL PROTECTED]> writes:
>
> g> Craig Markwardt <[EMAIL PROTECTED]> wrote in message
> g> news:[EMAIL PROTECTED]
>
> >> (2) 2D projection effects are a fact of our natural world.  They
> >> must be corrected for, in order to infer physical accelerations.
>
> g> The observable universe is 3D.  2D 'projection' normally is the
> g> result of theory or of limited representation.  In this specific
> g> case, we have 'statistically corrections' to the 'observed
> g> accelerations' for 'theoretical projection effects.'  A 2D-3D
> g> correction would not be statistical.  A theory of some kind has
> g> been imposed onto the data.  This makes the result questionable.
>
> I think we will all agree that the Universe is 3-D.  At the risk of
> stating the obvious, the sky is not 3-D.  If one watches an object in
> orbit and sees that it describes an elliptical path, does this mean
> that the orbit is intrinsically elliptical or that the orbit is
> circular and inclined to our line of sight?

I thought all orbits were elliptical, since Kepler.  ;)  (Sorry, couldn't
resist.)

The inclination of the orbit is one of those things that it is very
difficult to determine in astronomy.  We can't tell -- just by looking --
whether we're looking nearly edge-on or straight down onto the plane of the
orbit.

> (It's a similar argument to why don't all spiral galaxies look
> circular?  Because their disks are inclined to our line of sight.)

However, we expect all disk galaxies to be roughly circular.  Whereas
stellar orbits are expected to be elliptical -- sometimes highly so.

> g> This merely repeats the logical fallacy that if it is 'old' it must
> g> be wrong.  Scientific observations are NEVER 'outdated'.  An
> g> observation may later be found to be erroneous -- due to a specific
> g> finding of a flaw used in the instruments or methods.  At which
> g> point the study or experiment is superseded.  But it is never
> g> superseded simply by having somebody else use different methods
> g> that come to different conclusions.
>
> >> Their study used an imager with ~1 arcsec seeing, and did not
> >> resolve any stars within 1-2 arcsec of Sgr A.
>
> g> And this statement is relevant, how?
>
> Because of a reason you don't cite above.  Scientific observations can
> be superseded by improvements in technology.

But I *did* discuss this earlier in the thread (you snipped it).  And I
pointed out that such a view was incorrect and unscientific.

> That doesn't mean the
> old observations were "wrong," they are just not as useful.  If you
> are trying to study the motions of stars in the central region of the
> Galaxy, angular resolution is vital, the more the better.  At 1 arcsec
> resolution one has to worry about confusion (two stars being so close
> together that they appear as one).  Moreover, 1 arcsec at the distance
> of the Galactic center corresponds to about 0.04 pc.  Current
> observations can probe well within this region while older ones,
> because of their limited angular resolution, could not.

Later publication still does not invalidate prior observation.  Only if you
can explain -- from fundamental causation -- why earlier observations were
'wrong' is the prior work invalidated.

greywolf42
ubi dubium ibi libertas