I talked in an earlier post about what the 21 cm line is but you might be wondering why we are interested in observing this line. Why spend so much time on a survey focused on detecting this transition line?
One of the main reasons is that it allows a different method of selecting galaxies. To date, most large surveys have been optical (or near infrared) surveys. This means that galaxies are identified based on the stars contained within them that shine at optical wavelengths. With ALFALFA, we are instead looking for galaxies based on their 21 cm line emission, which is a transition line of neutral hydrogen. So we are identifying galaxies based on their gas, rather than stellar, content. As a result, ALFALFA is able to easily find galaxies that don’t have a lot of stars (low surface brightness galaxies) but that have lots of gas; these are the types of galaxies that are often overlooked by optical surveys. Of course, ALFALFA misses galaxies that have no gas and lots of stars (such as ellipticals) that are detected in optical surveys, so you want surveys at all different wavelengths.
Observing the 21 cm line offers a few other advantages that are worth mentioning. If you know the total flux of radiation from the 21 cm line at your telescope and the distance to a galaxy, you can compute exactly the mass of neutral hydrogen in that galaxy. This gas mass can then be used to compare properties across different galaxies and look for correlations. One of the other major advantages is that due to how radio telescopes receive radiation, when observing a source you obtain spectral information naturally. By spectral information, I am referring to the fact that you record the flux as a function of frequency (or velocity from the Doppler Effect). You can see this below with data from an example galaxy – a classic double-horned HI (neutral hydrogen) line profile. The total flux of the galaxy comes from calculating the area underneath the peak. There’s also other information available here, though. The center of the profile gives the redshift, or recessional velocity, of the galaxy which can be used to estimate a distance. The width of the profile corresponds to the rotational velocity of the galaxy (with an inclination factor). The beauty of 21 cm observations is that we record all this data at once. If you wanted to do the same optically, you would first need to find a galaxy in a photometric survey (essentially, take a picture of it with a telescope) and then target it a second time for spectroscopic follow-up to find a redshift for the galaxy. Of course, optical surveys have their advantages too.