Dr. Jeremy D. Murphy is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at Princeton University. The current cosmological paradigm includes a significant and unknown physical entity called "dark matter." While the presence of dark matter (DM) in galaxies is well established, the extent of galaxy DM halos and the role DM plays in galaxy formation and evolution is largely unknown. As the current cosmological paradigm governing the hierarchical clustering of DM halos makes specific predictions about their formation timescales and merger history, a clear understanding of the mass of DM halos on galaxy scales is needed. In this project the fellow will focus on directly constraining the mechanisms and timescales of elliptical galaxy formation.
The recent discovery of quiescent, red elliptical galaxies at redshift around z~2 raises many questions regarding the Lambda Cold Dark Matter hierarchical picture of galaxy formation. Fortunately, galaxies leave both chemical and kinematic clues to their formation histories. Advances in integral field spectroscopy and dynamical modeling have begun to open the window into understanding the DM distribution, DM central density, the stellar orbital anisotropy and chemical abundance gradients of elliptical galaxies. In this project the fellow will continue his study of the formation and evolution of massive elliptical galaxies. Specifically, this project has the following aims: (1) The fellow will use 2D stellar kinematics from VIRUS-P and orbit-based, axisymmetric dynamical modeling to constrain the DM halo mass, the DM central density, the stellar orbital anisotropy and stellar abundance gradients. From this he will be able to place direct constraints on the timescales and possible mechanisms of galaxy formation. (2) The fellow will compare mass estimates based on stellar kinematics to other methods of estimating mass in galaxies in order to quantify any systematics between these different methods. (3) The fellow will bring his experience with the commissioning of the VIRUS-P spectrograph, and his detailed study of the instrument's fiber optics, in order to successfully install a duplicate instrument on the DuPont telescope at Las Campanas Observatory. He will then use this instrument to expand his current galaxy sample into the Southern Hemisphere.
The broader impacts of this project include a significant educational component. The fellow will develop and teach a new course for the Prison Teaching Initiative (PTI). The PTI program teaches college-credit courses to inmates at two youth correctional facilities near Princeton University. The students can earn a two-year degree through the program and go onto four-year colleges upon release. The course will focus on physics and astronomy and aim to develop both a scientific approach to problem solving and an appreciation of the practical application of science in daily life. The large majority of evidence indicates that improving education in a prison population is the leading cause in reducing a return to crime upon release. The fellow will bring his previous teaching skills and enthusiasm for science to a deserving, underrepresented population.
My NSF-funded research was aimed at exploring both the extent and shape of the dark matter halos that surround the most massive galaxies in the local universe. The galaxies I study are truly massive, typically 10 to 100 times the mass of our own galaxy, the Milky Way. As approximately 90% of a galaxy's mass is in dark matter, it plays the dominant role in how a galaxy builds itself up over time, and how it interacts with other nearby galaxies. To carry out this research, I used the Mitchell Spectrograph1 at McDonald Observatory to measure how fast the stars are moving at a number of locations in the galaxy under study. Using spectroscopy, I was able to not only determine how fast the stars are moving (using the Doppler shift of the light, similar to the Doppler shift for sound) but also how the stars are orbiting the galaxy. Both of these pieces of information are critical for determining how much dark matter is present in the galaxy. Once this data is collected, I use the super-computers at the Texas Advanced Computing Center to reconstruct the stellar orbits and the dark matter distribution for the galaxy. This is accomplished by starting with a guess for how the dark matter is distributed in the galaxy, then comparing this guess to the data. This guess-and-compare process is continued until the best match is found. All of the galaxies I have studied to date contain significant amounts of dark matter, although the percentage of dark matter mass to total mass varies significantly between galaxies. One galaxy I studied is M49, the brightest galaxy in the Virgo galaxy cluster. The first figure shows the galaxy, overlaid with the 5 pointings with the Mitchell Spectrograph. The second figure shows how the mass of the stars and dark matter are distributed in M49 based on my measurements. At the full extent of my data, the dark matter outweighs the mass in stars by over a factor of 10. My NSF fellowship also included an outreach project which involved teaching for credit college-level math classes in several New Jersey state prisons. This project was through the Prison Teaching Initiative at Princeton University. I was the lead instructor for several classes and took a significant leadership and administrative role in the project. This work was tremendously rewarding and has proven to be some of the most meaningful experiences I have had in my life. To see someone working hard at something they are not sure they will ever conquer, and then do so, particularly in the harsh environment of a maximum security prison, was very inspiring. Footnote 1: The Mitchell Spectrograph uses fiber optics to channel the light from the telescope into the instrument. A significant component of my NSF research was directed towards understanding the optical aberrations that occur to the light as it passes through the optical fibers. This research has proven critical to both calibration of the Mitchell Spectrograph and the development of new instruments for astronomy. For example, the VIRUS Spectrograph, currently under construction to carry out the Hobby Eberly Telescope Dark Energy eXperiment (HETDEX), has used my studies of the fiber optics of the Mitchell Spectrograph to inform the design of its fiber optical system.
