Globular Cluster Colour

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				*Analysis of Globular Clusters Colour*

This analysis is part of a sequence of logical steps that are described in Investigation.

This analysis is based on the same data as identified in Analysis of Globular Clusters. Data on Globular Cluster B-V colour is added from SIMBAD. A description B-V colour can be found for example here. A lower B-V colour value indicates a bluer object. A higher B-V colour value indicates a redder object.

Note that the colour variation being investigated is typically a reddening of the globular cluster which is either due to dust which absorbs higher energy photons, or due to the age of the stars in the cluster. This is not related to “red-shift” which is caused by an object’s recession velocity or by the expansion of the universe.

A graph of B-V colour against globular cluster radius is shown on the right. No strong relationship is apparent. In the analysis below the B-V colour data is corrected for the effect of dust in the intervening space and a pattern emerges.

Colour Correction MW1 – Dust in the plane of the Milky Way

On the right globular cluster B-V colour is plotted against Galactic Latitude of the globular cluster. This is the visual angle of the globular cluster above the plane of the galaxy. It can be seen that globular clusters which are visually closer to the plane are redder. It can also be seen that this effect is slightly asymmetrical. It is assumed that this reddening is due to dust in the in the plane of the Milky Way, and that the asymmetry is because the earth is not exactly on the plane of the Milky Way.

A method for correcting for this effect is described with reference to the diagram on the right. The dust is assumed to have a constant density within a 3-dimensional disc that is aligned with and centred on the plane of the galaxy. The disc has a thickness 2T. T is assumed to be much less than the actual distance of any globular cluster from the plane.

The earth is offset from the plane by a fraction f of the half-thickness of the disc. Therefore the earth’s offset from the plane is fT.

The length of the path that light from each globular cluster has to travel through the dust disc to reach the earth is calculated as a factor of T.

A value is subtracted from the B-V value of the globular clusters that is proportional to the length of this light path. The variables in this correction are then manually adjusted to get the minimum standard deviation of the resulting corrected B-V data. This corrected B-V data is referred to below as MW1 B-V data.

The graph on the right shows the corrected MW1 B-V Colour against Galactic Latitude.

On the right 2 graphs are used to explore further reddening effects. Perhaps surprisingly there is no obvious reddening of the globular clusters that are visually close to the centre of the Milky Way and no obvious reddening with distance. This suggests that the reddening corrected for above is caused by dust that is close to the earth (relative to the scale of the galaxy).

The graph on the right shows MW1 B-V data plotted against globular cluster radius. Compare this with the graph at the top of the page. It can be seen that the globular clusters appear to be arranged in families that have a linear relationship between MW1 B-V colour and log(radius).

The optimum MW1 correction to maximise this effect is given by:-

MW1 B-V Colour = B-V Colour – 0.068 × (length of light path through dust disc)/T

Where the offset f = -0.11 (below the galaxy plane)

According to the AGM Theory

The reason for the appearance of the linear relationships is unknown and assumed not to be caused by anti-gravity matter. However this observation is referenced later in the Analysis of Elliptical Galaxies.