### Introduction

This document contains comments about the document Galaxy Rotation curve in Wikipedia
To order to read the document select:http://en.wikipedia.org/wiki/Galaxy_rotation_curve

At the right hand side there is a figure. This figure is misleading. The following picture shows the issues involved:
 ``` ^ V| BBBBBBBBBBBBBBBBBBBB e| *A l| * A C 0| * A C C c| * A C C C i| * A C t| * A C y| * A C | * A |* A ----------------------------------------------> Distance ```
• Curve A shows a typical calculated rotation curve of a Galaxy which only consists of a bulge. The first part of the curve represents the bulge. The velocity increases lineair. The second part follows Kepler's third law.
• Curve B shows a typical observed rotation curve of a spiral galaxy. Such a galaxy has a clear bulge and a disc. The shape of the curve is flat. The curve stops at the edge (spiral ends) of the galaxy i.e. of what is observed.
• Curve C shows the calculated galaxy curve of the mass calculated from the observed luminosities. The first part represents the bulge. The second part represents the observed disc and the third part represents the part outside the disc. This part follows Kepler's third law.
What the above figure clearly shows is a decrepency in between what is observed (Curve A) and what is calculated (Curve C)
What the above figure does not show is the amount of mass necessary to simulate a flat galaxy rotation curve.

In the Wikipedia document we read:
A general feature of the galaxy rotation curves that have been measured is that rotational velocity of stars and gas is constant as far from the galactic centre as can be measured (line B in the illustration): stars are observed to revolve around the centre of these galaxies at a constant speed over a large range of distances from the centre of any galaxy.
Generally speaking this sentence is correct, but a better sentence is:
• "The general appearance of the observed rotation curve of a spiral galaxy consisting of a central bulge and a disk is almost flat i.e. the stars of these galaxies are observed to revolve around the centre at a constant speed over a large range of distances. "
If disc galaxies had mass distributions similar to the observed distribution of stars and gas the rotation curves velocities should decline at large distances (dotted line A in illustration)
This sentence is misleading. A better sentence is:
• However if the galaxy rotation curves are calculated based on the mass distributions based on the observed distributions of the stars and gases than the velocity curves decline at large distances (dotted line A in illustration)
in the same way as do other systems with most of their mass in the centre, such as the Solar System or the moons of Jupiter, following the prediction of Kepler's Laws.laws of gravity to the observed matter.
This sentence can be removed. The issue is not Kepler's Law but Newton's Law.
The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the Newtonian-Keplerian prediction, assuming a centrally-dominated mass associated with the observed luminous material.
This sentence is highly misleading. When you are assuming a "centrally-dominated mass" than you are considering an elliptical galaxy. A spiral galaxy is different. Its mass is much more spread out.
When masses of galaxies are calculated solely from the luminosities and mass-to-light ratios in the disk, and if core portions of spiral galaxies are assumed to approximate to those of stars, the masses derived from the kinematics of the observed rotation and the law of gravity do not match.
This sentence is complex. IMO opinion a better sentence is:
• When masses of galaxies are calculated solely from the luminosities and mass-to-light ratios in the disk and in the bulge, and if based on those calculated masses the galaxy rotation curve is calculated then this calculated curve and the observed curve do not match.
In fact this sentence is almost the same as written before.
This discrepancy can be accounted for by a large amount of dark matter that permeates the galaxy and extends into the galaxy's halo.
This solution requires a critical investigation.
1. In general you should not propose one solution but many and critical investigate which one is the best.
2. Any calculated galaxy rotation curve based on mass derived solely from Luminosities and visible observations is suspicious because it is easy possible that a lot more mass exists in the bulge and disc in small objects which are invisible. With small it could be planet size objects or in the form of gases.
In fact with better observations more mass is detected, making the discrapency between what is observed versus what is calculated smaller.
3. The size of the galaxy (specific the disc) should also be considered larger as observed. It is wrong to assume that the galaxy all of a sudden stops at the end of the visible observed flat galaxy rotation curve.
4. Accordingly to Wikipedia dark matter is (mostly) nonbaryonic.
A solution based only on baryonic matter (elements of periodic table) is simpler. You have to demonstrate that such a solution is not feasible before assuming non-baryonic matter.

### Reflection

Accordingly to this Wikipedia document the solution of galaxy rotation problem (A better word is missing mass problem) is darkmatter. This so called dark matter should reside mostly outside the disc in the halo surrounding the disc (galaxy)
IMO the most obvious solution is first to try more cold baryonic matter inside the visible disc or outside but in the plane of the disc. If you perform such a simulation curve C will grow and become more flat. It is also amazing the little amount (density) of matter that is required to simulate almost flat rotation curves. For more detail see Visible matter and darkmatter

The document Evidence for cold baryonic matter fueling the Milky Way deduced from the first detection of dust emission in High-Velocity Clouds uses the word "cold baryonic matter" to indicate low metalicity gas. That usage is correct.
The same for the following document: Ultimate Energy Density of Observable Cold Baryonic Matter Which reads: "We demonstrate that the largest measured mass of a neutron star establishes an upper bound to the energy density of observable cold baryonic matter."

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Created: 11 July 2012
Updated: 12 August 2013