Question

Explain how X-ray observations are used to determine the temperature, T (r), and den- sity, ρgas, of the hot gas in an elliptical galaxy. What quantities are observed directly and how are they used to ultimately determine T and ρgas?

Answer 1

SOLUTION

An elliptical galaxy, is a set of stars of ellipsoidal geometry. In the literature, the observations of the x-ray emissions are the spectral character (luminescence), the quality of the energy emission networks, the generation of the signals, the signals of the emission of x-rays. The spectral observations show the gas temperature around .

Indeed, the heating of the gas is due to the fact that within the elliptical system the stars that contain it move at great speeds, with respect to other nearby stars. The gas is ejected by each star within the galaxy at very high velocity, and is determined, by the speed of star dispersion of the galaxy. This accomplishes collisions between the gas distributions and therefore thermalizes the kinetic energy of the gas, generating the amount of heat proportional to the amount of gas injected. Therefore we have:

where

: is the stellar velocity dispersion

: is the mean mass per particle in the gas in units of the proton mass mp

: is the proton mass

: is the Boltzmann constant.

Another aspect of observation are the profiles of brightness on the surface of the galaxy, which indicate that the density of the hot gas is reduced for large radii of the galaxy, by means of the following relationship

In conclusion, the spectral observation of the x-rays emitted by the elliptical galaxies, essentially the measurements of gas heating, and the brightness profiles on the surface of the gas, allows us to obtain information on the temperature and density of the galaxy in an elliptical way .