A galaxy's mass. Erge-on spiral galaxies are suitable for the determination of their masses. Prof. I.D.Karachentsev and his colleagues complied a cataloge of such galaxies and obtained their mass spectroscopically. The spectrum of one galaxy, FGC 1908 in Dragon, is given below. As the figure shows, the spectrograph slit was aligned with the major axis of the galaxy.
Vertical lines crossing the spectrum are emission from the night sky.
Other emissions belong to the galaxy. Their laboratory wavelengths are indicated. You are required to repeat the estimate of the galaxy's mass. Explain why a two-dimensional spectrum of the galaxy looks exactly like that. Estimate the mass of FGC 1908 and compare it to the mass of our galaxy.