ALFALFA will detect more than 25,000 galaxies by their neutral hydrogen content. A common question that I run into is: “What do you need so many galaxies for? What are you going to do with all of them?” A lot of people think that someone is going to sit down and study all those galaxies individually and wonders why they need so many. No one is going to study every galaxy in the survey in detail; after all, the graduate students would like to graduate at some point in time. Now, some projects might use all the galaxies but will only look at their basic properties. Other science involves looking at a small subsample of the survey in minute detail.
One of the goals of surveys is to create a large sample of galaxies so that their distribution in the universe can be studied. This includes the distribution of mass of the galaxies, quantified as the HI (neutral hydrogen) mass function. The interesting questions are: How many galaxies are low mass? How many are high mass? How are the masses distributed? Also of interest is how the galaxies are distributed spatially in the universe, including how far apart they tend to be on average. This question is studied through the use of a two-point correlation function which quantifies how galaxies are correlated on the sky. Both of these questions will use all the galaxies found in ALFALFA but only require very general, relatively easy to acquire properties of position and mass.
The other approach to surveys is to use them to help identify the few galaxies you are interested in studying in a lot of detail. Perhaps you want to know information about a sample of galaxies such as metallicity (chemical enrichment), star formation history, kinematics, interactions, etc. All of this information requires time-consuming follow-up work to the survey, so it’s not reasonable (at least initially) to obtain it for all the galaxies in the survey. Instead, you might pick a small subsample on which to focus your attention. This could be a specific interesting type of galaxy, such as the galaxies with the lowest or highest hydrogen masses. It could also be a specific area of the sky where galaxies are gravitationally bound to each other, such as the Virgo Cluster or Leo Group. The survey allows you to identify lots of the interesting galaxies that are rare and covers many areas of the sky that are interesting.
That’s a brief explanation of why we like surveys and all the galaxies they offer. We can use all the galaxies as test particles to examine the structure of the universe, or we can study a few galaxies of interest in much more detail.