Determining the Velocity of the NGC 300 Galaxy

The spiral galaxy, NGC 300, is one of the closest galaxies, at approximately 7 mly, to our own Milky Way galaxy. It is gravitationally bound to the NGC 55 galaxy, which is estimated to be about 1 mly away from NGC 300. We want to figure how quickly the galaxy is moving in relation to our perspective; to do that you must be aware of a phenomenon known as redshifting.

Redshifting occurs when the light source moves away from the observer, causing the wavelengths to be elongated into the “redder” part of the electromagnetic spectrum. By filtering redshifted photons received from a position near the nucleus of the galaxy, in the form of wavelengths manifested as emission lines, through an elemental gas, we see how light can be absorbed by the electrons in the atoms of this gas. By using these absorption lines, which are upside-down spikes in the spectral graph, we can compare the redshifted lines to the lines from the same type of elemental light source on Earth. The origin of redshifting comes from an effect called the Doppler shift, which is illustrated in the equation below:

where

• $\lambda ’$ is the shifted wavelength from the galactic object;
• $\lambda_0$ is the absolute wavelength observed in a lab by absorption lines;
• $v$ is the velocity of the galaxy;
• $c_0$ is the constant 299,792 km/s, the speed of light.

Below is the infrared spectrum of wavelengths, observed from the NGC 300 galaxy, which was captured by the ISO telescope. Use the absorption lines to determine the shifted wavelength, and then use this to solve for the galaxies radial velocity.

The observed wavelength is $\lambda_0= 158.543$ .

What is NGC 300’s radial velocity?