Talk:Galaxy effective radius

Circular Symmetry
Is "circularly symmetric" really correct? Or should it read "spherically symmetric" instead? -- LemiNW 17:28, 25 November 2006 (UTC)
 * Definitely ! Doctore (talk) 16:54, 22 May 2013 (UTC)
 * lol

Out of date
The information in the page is so out of date, words just fail me.

These papers on this were published around 1969. Whoever introduced this text here has absolutely zero idea of how or why effective radius means. The graphic regarding galaxies shown here is irrelevant.

The ideas behind this date back to the idea of the theoretical estimating of mass distribution. It relies on finding a radius-magnitude relationship. The concept is to describe the mass density / density profile out to the edges of the galaxy, to the theoretical dark matter halo. Reff is represented in arcsec, and sometimes relies, in modern times, on tracers using globular or planetary nebulae. I.e NGC 5128 tracers is 300 arcsec / 6 arcmin, for example. The galaxy is 25.7×20.0 arcmin, or roughly five times larger. Hence, describing galaxies by there half-light diameters is just nuts. Worse is that you have to also know the surface brightness, calculated from the magnitude, then assume the distribution of the light, follows some general law (which has differing values based on some average, that is different between astronomical types), and somehow out pops some diameter.

Please if you know what this about say so, else stop mucking around with things you don't understand. IMO. it calls either deletion or starting completely from scratch.


 * Q. Do spiral galaxies have effective radii? (Why doesn't the article say so?)


 * Q. What is the distribution form of a density profile of an elliptical galaxy including the dark-matter halo.

In the end, why so complicated. Foe most uses, the diameter is based on the RC3, UGC or PGC galaxy catalogues gives diameters based on photographic emulsions. Known the magnitude limit, sets the galaxy size, which is based on the exposure time. This determines the apparent size of the galaxy. Knowing that, means you determine, from the distance, the true scale of the galaxy. Please forget this half-radius stuff on elliptical galaxies. Arianewiki1 (talk) 06:02, 22 January 2015 (UTC)