Galaxy halos generally show a decrease in rotational velocity with increasing height above the miplane. This can be explained by the Galactic Fountain model, in which gas is ejected from the disk by supernovae. As the gas rises, its rotational velocity decreases, and it moves radially outward. This results in what is called a lagging halo -- rotational velocities are lower for halo gas than for disk gas. This was originally accepted as the entire reason for a lagging halo. However, simulations of halos show that the galactic fountain alone cannot account for the magnitude of the lag observed in several galaxies. An external source of low angular momentum gas, such as accretion, is needed. Computer models in which ~10% of halo gas is from accretion and ~90% of halo gas is from outflow best match observed velocity gradients.

Recent observations have shown that evidence of accretion (eg, from the IGM, molecular clouds, or companion galaxies) is more common than previously thought. This supports the notion that infalling gas plays a non-trivial role in galaxy halo formation and evolution. Additionally, current star formation rates indicate that most galaxies should have used up all of their gas by now if there were no source to replenish the gas. Since galaxies are still forming stars, there must be an accretion mechanism that is providing new material to form stars.

My PhD project involves measuring velocities of H-alpha-emitting halo gas as a function of height above the midplane in edge-on galaxies in order to measure the magnitude of the lagging halo. I aided in the implementation of a multi-slit spectroscopic setup for this project at the 3.5m telescope at Apache Point Observatory in New Mexico. A slit-mask with a series of uniform, parallel slits allows for simultaneous observations of H-alpha emission at several different radii in a target galaxy. A narrow band filter prevents overlapping of spectra from neighboring slits. This is an ongoing project, and optical observations will be augmented by radio and UV data to characterize galaxy halos. The goal of this project is to determine to what extent halo rotation is coupled to disk rotation by measuring rotatational velocity gradients for a sample of nearby, edge-on galaxies. This has implications for the contributions of infalling gas and outflowing gas to the origin and evolution of galaxy halos. I am collaborating with the HALOGAS team (PI: G. Heald, ASTRON), which is studying cold gas accretion in the local universe. The lags that I measure will be incorporated into their galaxy models, which will further constrain the kinematic characteristics of those galaxies. (Advisor: R. Walterbos, New Mexico State University, 2007-present)

For more information, visit my department research page.

For travel pictures from summer internships I've had in grad school, check out my pictures page!