Sersic Profiles for Galaxies

[Norris Adams]

I am working my way through a Masters textbook on Galaxy Formation. I am an engineer (Bachelor level) , not a physicist, but I have reached page 31 without (completely) losing the thread! My problem is getting my head around the Sersic profile. This equation relates the surface brightness at any radial distance of a given galaxy to the so-called Effective (or Half Light) Radius at a given wavelength.

The shape of the curve (i.e. the Sersic profile) defining the drop off in luminosity as you move away from the centre is described by this equation (for varying values of integer n).

I just wondered where this equation came from? Is it empirically derived or based on some physical law? Alternatively, is it just a mathematical device to help draw a suite of brightness curves? If so, why does it work in practice? Why should galaxies be constrained by this law to have such fixed and predicable light curves (from centre to rim)?

I am hoping there are some clever astronomers or astrophysicists who may be able to help! 

Many thanks!!

 

Sersic Profiles.docx

[robin_astro]

Yes is it an empirical relationship (but does not work for all galaxies) and so must have some physical significance but the physical basis for it is unknown (or was as 2014) according to this paper for example which suggests a connection to early time galaxy formation models

https://iopscience.iop.org/article/10.1088/2041-8205/790/2/L24/pdf

[Norris Adams]

Many thanks Robin, for taking the time to reply.

The paper you supplied is certainly very helpful in throwing some more light on this question, even if it seems to show that the jury is still out on exactly why it works as it does. (I haven't reached the end yet)!

NB - I looked at some of your spectroscopy links too. Fascinating. (Yet another astronomical interest is the last thing I need right now!!!)

Much obliged for all your help!