Postdoc Spotlight: Stephanie Tonnesen



Theorist Stephanie Tonnesen has held the Nashman Fellowship at the Observatories since 2014, and will be joining the Center for Computational Astrophysics at the Flatiron Institute in New York City, part of the Simons Foundation.  Driven by a lifelong quest to understand how our universe works, we sat down with her to talk about her research, her life, and the path that led to her field of research.



Q: What questions are you pursuing in your research?


My main question is to understand how galaxies go from healthy, star-forming spiral galaxies with plenty of cold gas to fuel their star formation, like our Milky Way, to “dead” galaxies that have no more cold gas and therefore cannot continue to grow and form stars.  


Specifically, I’m most interested how this evolution works in a galaxy cluster environment. Clusters are like the Washington DC’s of the universe, where there are hundreds to thousands of galaxies gravitationally bound to each other. In this galaxy city, you get all sorts of interactions due to gravity, like mergers and galaxy harassment--which are many fast galaxy fly-bys. However, galaxies can also interact directly with the intracluster medium, which is gas that has crashed into the gravitational potential of the cluster, and heated to the point that it cannot cool in the age of the universe.  


I study a mechanism that can cause galaxy “death” called ram pressure stripping. When a galaxy moves through a galaxy cluster, the intracluster medium can push out the the star forming fuel inside of a galaxy. The stars in the galaxy are unaffected by the diffuse intracluster medium, and leave their own gas behind as the galaxy plunges through the cluster.  This loss of fuel stops stars from forming.  Using detailed simulations, I have found that ram pressure stripping can remove a galaxy’s fuel even in a less dense suburb in addition to the cluster’s metro area.



This image only shows the gas that is in or has been stripped from the galaxy by ram pressure stripping.  The galaxy is moving toward the right, and the elliptical shape on the right is the gas still bound to the galaxy.  The stripped gas stretches to the left and shows much structure due to interactions and mixing with the surrounding gas.


Q: When did you realize you were interested in astronomy, and what path did you take to get here?


My inspiration for being a scientist was my mom who was a mathematician and electrical engineer. She built huge power stations and had her pink hard hat--a gag gift from her construction team. During summer breaks, she would make us kids do logic puzzles and build circuits. I don’t remember a time when I wasn’t interested in science.


Then in high school I needed some extra credit in chemistry, which we could get by going to special talks by Nobel laureates at the University of Toronto. I ended up at a talk about the multiverse, which I found fascinating. I started reading everything I could about astronomy, and I went to college knowing I wanted to major in astrophysics.  


I now have two kids and my 5 year old wants to be an astronaut. I hope I can inspire him the way my mother inspired me.


Q: What’s the most interesting place you’ve traveled for work?


The worst part of being a simulator is that you don’t get to travel to amazing telescopes! I love going observing, but I don’t love reducing the data. While I was at Carnegie I got to go to Soverato Italy for a “In the Footsteps of Galaxies” conference.   My collaborators in the MUSE team are based in Italy.


Q: What has the Nashman Fellowship meant to you?


My research is focused on comparing simulations to observations. Coming to Carnegie has been a huge boost to understanding observational data and working with observers. In addition, the freedom of being a Nashman Fellow means I get to pursue the research I find most interesting, and I was able to publish papers that I don’t think I would have done anywhere else.  These were answering harder questions that required more time than can usually be given to a project in a traditional research position.



From "The Ties that Bind" 


Q: What’s next for your research and your career?


This fall I’m moving back to the East Coast, as an associate research scientist at the Center for Computational Astrophysics at the Flatiron Institute which is part of the Simons Foundation. It will be exciting to be surrounded by computational astrophysicists, but also to be able to keep my Carnegie ties and develop some intersectional science between the institutes.


Scientifically, I will continue to study cluster galaxy evolution and  I’m also interested in whether the galaxy’s environment at larger scales will have an effect on galaxy growth and evolution. I’m also using simulations to interpret real observations. I’m currently running detailed simulations to compare with these incredibly detailed observations made with the MUSE instrument on ESO’s VLT.  I’d like to know what range of galaxy types that can evolve due to ram pressure stripping, and to more clearly define the environment in which ram pressure stripping can be effective.