When an emission line from a galaxy is redshifted, it means that the light emitted by that galaxy is exhibiting a shift toward longer wavelengths. This is generally interpreted as a sign that the galaxy is moving away from the observer, according to the Doppler effect.
Based on the options you've provided:
-
The galaxy contains a small percent of your hydrogen - This is not relevant to redshift. The composition of the galaxy does not directly explain the redshift.
-
The galaxy contains a large percent of hydrogen - Similar to the first point, while the presence of hydrogen may affect specific spectral lines, it doesn't inherently explain redshift.
-
The galaxy is moving toward the viewer - This is incorrect in the context of redshift. A galaxy moving towards the observer would exhibit a blueshift, which is a shift toward shorter wavelengths.
-
The gas is moving away from the observer - This is the correct interpretation of a redshift. If the emission line is redshifted, it suggests that the gas in the galaxy is receding from the observer.
In summary, when an emission line from a galaxy is redshifted, it generally indicates that the galaxy (or the gas within it) is moving away from the observer.