This project consists of three complementary observational efforts, intended to test the conceptual picture that galaxies may gain mass from their surroundings not by accretion of hot plasma that cools as it settles into the galaxy, but rather by accretion of gas that has already cooled in the intergalactic environment and remains cold as it flows into the galaxy, possibly on orderly streams with preferred directions. This idea has gained theoretical support in recent years through hydrodynamical simulations of galaxy formation; observational tests to be performed by the PI consist of radio observations, primarily in the 21 cm neutral hydrogen line, of (1) the outer parts of the nearby Virgo cluster of galaxies, (2) large volumes surrounding two distinctly different clusters at redshift z=0.2, and (3) galaxies located in extremely under-dense void regions. This work will exploit the newly enhanced capabilities of the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands and the Expanded Very Large Array (EVLA) in the US. The new data will be combined with archived data to compare the morphologies, star forming properties, environments, and (where appropriate) locations within the clusters of galaxies with, and without, spatially extended cold gas. The observations will be compared against predictions made by high-resolution hydrodynamical simulations performed by a collaborator on the project; and the Virgo observations, in particular, will be made available to the research community through a website. The work will involve the participation of undergraduate and graduate students from under-represented groups, and will be highlighted in public presentations in schools and other venues.
