This project addresses the issue of how gas flows into spiral galaxies, a process essential to support the long histories of star formation in these systems. The PI and co-PI will use the adaptive-mesh hydrodynamics code Enzo to carry out two series of simulations of gas flow into disk galaxies. The first series will consist of very high resolution cosmological simulations of galaxy halos similar to that of the Milky Way; the goal is to resolve the interface between the cold and hot phases of the gas and determine whether cold clouds condensing from the hot phase make their way to the disk. The second series will simulate a vertical column in and above the galactic disk, with the goal of determining whether and how thermal instabilities serve to channel the cooling medium into the disk. The code will also be used to construct simulated observations of emission and absorption lines arising in the cooling medium, for comparison with available spectroscopic data. The work will in part be performed by, and will include the mentoring of, a postdoctoral fellow. The postdoc, PI, and co-PI will participate in a program to train graduate students to visit middle schools to conduct interactive outreach programs, and visualizations constructed from the simulations will be made available for planetarium shows or other public venues.
Galaxies are the basic building blocks of the universe and large, disk galaxies such as our own Milky Way consist of hundreds of billions of stars as well as clouds of gas and dust. The Milky Way is forming stars out of the gas and dust at the rate of about one star per year and if it keeps doing so at this rate without replenishment, it will run out of material in a few hundred million years. Since the Milky Way is much older than this, it seems likely that it has a source of new gas for star formation. While some of that comes from small galaxies falling into the Milky Way, another possible source is the accretion of diffuse gas from the intergalactic medium as suggested by large-scale cosmological simulations. However, how this gas accretion occurs and what observational signatures can be used to look for it, are unclear. In this project, we carried out a systematic study of this gas accretion, focusing in particular on the interface between the galactic disk and the much larger but much lower-density gas "halo" that has been suggested as a repository for gas inflowing toward the disk. Using super-computer simulations which followed the formation and evolution of a disk galaxy over cosmological time-periods, we showed that most of the gas flowing toward the disk through the galactic halo does so in a hot, ionized form. This gas, with temperatures of a few million degrees is too diffuse to be easily seen and yet ends up depositing enough mass into the disk to sustain the current star formation rate. The gas does not come in evenly, but consists of filaments, or streams, within the spherical halo gas. This picture is consistent with current observations and suggests new ways that future instruments could detect the gas, both in our Milky Way and external systems. One question galaxy simulations had difficulty answering in detail is why the gas stayed hot and did not cool. Previous simulation work had suggested that the hot halo gas, which is predicted to radiate energy in the form of X-rays, might cool and condense into small, dense clouds. However, using a set of very high-resolution simulations that focused on a small region of the halo, we demonstrated that the gas was unlikely to cool, unless it was strongly perturbed. In summary, the results from this project made great strides towards clarifying how fresh fuel flows from intergalactic space, through the hot halo, and finally into the galactic disk, where it can be formed into stars.
