The title of this section on the ALFALFA website is more officially: Galaxy Evolution and Dynamics within Local Large Scale Structure. However, that seemed like a mouthful for what is a rather straightforward idea. We know that there is large scale structure within the universe; there are clusters and filaments of galaxies and voids that lack galaxies. The picture below is from the Millenium Simulation and shows this structure; the brighter colors represent regions that have a higher density of matter (more galaxies).
One of the interesting questions in extragalactic astronomy is understanding how these different types of environments affect the galaxies that reside there. What are the differences between a galaxy in a cluster and one in a void? For example, we know that galaxies in the interiors of clusters tend to have less neutral hydrogen than similar galaxies in a less dense region of space. This is because clusters have hot gas that strips the neutral hydrogen out of galaxies as they fall into the cluster center.
The survey area of ALFALFA was designed to help address these questions about the role of environment. The “spring sky” of ALFALFA includes the Virgo Cluster, a nearby galaxy cluster. Since the Virgo Cluster is relatively close, distances can be estimated for galaxies using what are termed “secondary methods”. These methods make use of some quantity that we can measure (such as rotational velocity or period) which is related in a known manner to the absolute brightness of an object. Having distance estimates for these galaxies and recessional velocities allows us to place the galaxies relative to the cluster center in position space and velocity. Then, we can study what happens to galaxies as they approach the cluster center and try to figure out exactly what changes occur, especially in terms of their gas content.
If you want to study a cluster environment, you need a reference to compare against. For this reason, the “fall sky” of ALFALFA includes part of a nearby void. This gives us the opportunity to study galaxies that are isolated and have presumably developed unaffected by neighbors and interactions. By comparing galaxies from high and low density environments, we can start to understand how the environment affects these different galaxies. How are the galaxies similar? How are they different? In answering those questions, we learn in what ways environment matters.
In addition, ALFALFA includes several galaxy groups in the survey area. Galaxy groups are a medium density environment; there aren’t nearly as many galaxies as in a cluster but there are enough galaxies that interactions do occur. Studying galaxy groups allows us to determine when environment becomes important. Does we see effects as soon as the environment is slightly dense? Or is there some critical level at which environment becomes important?