Question

Imagine that you are studying an edge-on galaxy that is at a distance of ten mega-parsecs from you. The galaxy has an apparent radius of 4.4 arcminutes.

A) What is the actual radius of this galaxy, in kilo-parsecs?

B) You take a spectrum of the centre of the galaxy, and see the H-alpha absorption line of hydrogen at a wavelength of 657.8 nm. The laboratory wavelength of this line is 656.3nm. What is the redshift of the galaxy?

C) You now take spectra of both edges of the galaxy. At one edge, the absorption line is at 658.31 nm, while at the other edge of the galaxy, the absorption line is a 657.35 nm. How fast is the galaxy rotating?

D) Balance centrifugal force against gravity and hence calculate the mass of the galaxy (in solar masses)?

Answer 1