Research

The evolution of galaxies is significantly impacted by the way the galaxy interacts with its surroundings. Gas falling into the galaxy can enrich the material used to make stars while gas outflowing from the galaxy can remove fuel for star formation and heat the surrounding gas, preventing gas from falling back in to form stars. The complicated cycle is driven by stellar winds and supernovae which push gas from the interior of the galaxy outward above the galactic plane. As the gas travels outward, it interacts with cold clouds of gas which are the seeds for star formation. These winds can push the clouds out of the galaxy or tear them apart. Observations have shown it must be a mix of the two. The two phases, the hot wind and cold clouds, appear to coexist in the area surrounding the galaxy. By understanding the interaction between hot gas and cold clouds, we can have a better idea how these outflows regulate the evolution of the galaxy as a whole.  

An excellent example of a galaxy with outflows is M82, the Cigar Galaxy – shown below in an image from NASA, ESA and the Hubble Heritage Team. The light emitted by sulfur is shown in red highlighting the material flowing outwards within these winds. 

In order for the hot wind and cold clouds to coexist, there must be something keeping the cold from being torn apart. One potential solution is that magnetic fields embedded through the wind and cloud would wrap around the cloud, protecting the cold material from hydrodynamic instabilities. This idea, called magnetic draping, has been a proposed solution to keep the wind from destroying the cold by adding extra pressure at the boundary between the cloud and wind.  In this project (Cottle et al. 2020) I performed computer simulations of the wind-cloud interaction with the added effects of magnetic fields to investigate the viability of this solution.

For magnetic fields parallel (aligned) to the wind, there wasn’t any increase in the cloud lifetime or stability over simulations of wind-cloud interactions without magnetic fields. 

For magnetic fields perpendicular (transverse) to the wind, rather than the expected outcome of clouds surviving for longer times before being destroyed, I found that the magnetic fields caused the cloud to lose mass more rapidly than in the case without magnetic fields. As the wind bends the field lines around the cloud it squishes the cloud material in the orthogonal direction accelerating the rate material is torn away from the cloud.

Below are volume renderings of these simulations showing the symmetry of the cloud in the the aligned case and the lack thereof in the transverse case. 

The primary method of observing these outflows is the absorption of light by gas that is between us and a bright source, such as another galaxy or the outflow’s host galaxy. These absorption profiles are dependent on the amount of gas that is along the sightline, or the column density, and can tell us how much of a particular ion can be found in the gas. 

Absorption profiles are a very useful way of detecting the multiple phases within the outflows, as the types of ions you can detect depend on the temperature of the gas that did the absorbing. Low ionization energy ions like Mg II indicate the presence of cold material, while high ionization energy ions such as Ne VIII trace the hotter material in the wind. Intermediate energy ions such as C IV and O VI are useful in detecting material between the two phases which are thought to be produced on the boundary between the wind and cloud. Being able to estimate where these ions are found in simulations is a key step to connecting numerical simulations of outflows to observations. 

I modeled the column density profiles of various ions within hydrodynamic simulations of outflows (Cottle et al. 2018) with a post-processing tool called Trident. Trident estimates the number density of an ion given characteristic information of the simulation cell, such as metallicity and temperature with the assumption that the gas is in ionization equilibrium. With these number densities, I estimated the column densities and average velocities of commonly observed ions to model absorption profiles. These profiles can then be related back to observations to help characterize the dominant physics in these outflow interactions.

For the online data products from this project, go  here.

The ionization equilibrium approach failed to recreate observed absorption profiles. Low energy ions were over estimated while high energy ions were only found within the wind and much high velocities than they have been observed. This lead me to conclude that the ionization reactions need to be accounted for within the simulations rather than relying on equilibrium estimations. 

I am currently working on simulations including a non-equilibrium chemistry solver called MAIHEM to track the ionization through the simulation and achieve better estimates on the distribution of various ions within the wind-cloud interaction. These simulations predict more production of the intermediate and high energy ions along the boundary of the cloud, indicating a better fit to observations. 

My work at Western Washington University focused on constructing a catalog of candidate young stellar objects within Orion.These sources were selected uniformly through the constellation based on excess flux in the infrared as well as variability. A large subset of this catalog was used as targets for the APOGEE-2 survey; a study focused on understanding the characteristics of young stellar objects and comparing results across surveys. 

Another project of mine involved understanding the applicability of measuring an alias of a high frequency signal in lieu of the original signal. This is primarily useful for radio astronomy where signals are interpreted as Fourier transforms. When converted from analog to digital, signals are aliased at complementary frequencies removing the need to use specialized technology. Most of this work has focused on proof of concept.