Unseen "stuff" controls the universe.

"NV" == Nicolaas Vroom writes:

NV "Joseph Lazio" schreef in bericht
NV ...
"JR" == Jeff Root writes:

My recollection of rotation curves is such as those shown by Kent
(Figure 1, URL:
http://adsbit.harvard.edu/cgi-bin/np...J.....93..816K
).

NV This document at page 5 = page 821 shows the following sentence:
NV "In principle, the bulge and the disk must be treated differently
NV since they contribute in different ways to the rotation curve" Why
NV differently ? What is meant with: "different ways" In principle
NV the rotation curve at each distance r from the center of the
NV galaxy depents on the mass dm at each position x,y,z

I don't have my copy of Binney & Tremaine in front of me, but I have a
dim recollection that the gravitational potential of a disk is
different than that of a spherical distribution. That has an impact
on the rotation curve. See
URL:http://www.astro.su.se/%7Epawel/gradprobl.2.html for examples.

NV Next they write: "Also they are likely to have different M/L
NV ratios" Why ?

NV IMO it is not so strange that M/L ratios are different but if that
NV is the case and when there is a disk it becomes more difficult to
NV calculate the mass of the disk accurately.

The stars are different. Bulges are dominated by low-mass stars,
comparable to the Sun. Most of the light from a disk comes from the
high-mass stars (which are absent in a bulge), but the high-mass stars
contribute very little to the overall mass of the disk. (A 10 solar
mass star might have a luminosity of 100 solar luminosities.)

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