TTU Physics Department.

TTU Physics: Postdoctoral Research Fellow

Picture of Paul Sell.
Paul Sell, Ph.D.
Postdoctoral Research Fellow (Astrophysics)
Office Phone: (806) 742-3767
Office: 26 Science Building
Curriculum Vitae


I am an observational astronomer investigating extreme outflows in space driven by accretion, the infall of gas in deep gravity wells. I strive to better understand the processes that initiate and drive the outflows and the effect that they have on the space around them. The path my research has taken has forked in two directions: studies of unusual X-ray binaries and examinations of quickly-evolving galaxies. I describe this work in a bit more detail below, including links containing pictures of these systems.

First on the astronomically smallest scales, accretion can occur in X-ray binaries, mutually orbiting stellar systems with an evolving star and the one of two of the most extreme compact objects known: a black hole or a neutron star. When either a star puffs up or drives away a massive wind toward the end of its life, it dumps material onto the compact object, which begins to inspiral. The inspiraling motions whip matter around so strongly that some is thrown off and focused by strong magnetic fields. This collimated outflow of matter perpendicular to the disk is referred to as a jet, which can then send shockwaves out into space. I am interested in studying populations of X-ray binaries (the study I led of M81: journal article) and detailed investigations of individual sources. In case studies of two sources, Circinus X-1 (journal article) and Cygnus X-1 (journal article), I have probed outflowing shockwaves with the Chandra Space Telescope and ground-based observatories with the primary aim of measuring the amount of energy needed to drive them. Such measurements provide us insight into the source of the shockwaves and their capabilities to heat up and drive out gas around them. This work lead to the discovery of the supernova remnant of around Circinus X-1 (journal article), making it the youngest X-ray binary known (NASA press release).

Second, I have explored how the extreme feedback of energy through galaxy-wide outflows drives the evolution of galaxies over cosmic time. Using the Hubble and Chandra Space Telescopes and a wide range of other observatories, I have carefully inspected a small sample of galaxies (journal article) to deduce the source of powerful, wide, fast outflows. Through of set of detailed arguments, I deduced that the source of these outflows does not appear to be the supermassive black holes in the centers of these galaxies but appears to be the extreme bursts of star formation that have occurred (the radiation pressure, supernovae, and winds from the stars). The Hubble telescope observations revealed that these are the most compact galaxies for their mass yet found in the universe (NASA press release). Continuing radio telescope observations have imaged one of the outflows and was featured recently in Nature (journal article).