Date of Degree

2-2017

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor(s)

Timothy Paglione

Committee Members

Saavik Ford

Barry McKernan

Reshmi Mukhurjee

Matthew O'Dowd

Timothy Paglione

Subject Categories

Other Astrophysics and Astronomy | Stars, Interstellar Medium and the Galaxy

Keywords

High energy astrophysics, gamma-ray astronomy, Fermi Gamma-ray Space Telescope, molecular clouds, cosmic rays

Abstract

We analyze gamma-ray emission from nearby, interstellar molecular clouds in order to calibrate current tracers of the interstellar medium and to probe local cosmic ray gradients. Gamma-rays detected by the Fermi Gamma-ray Space Telescope are created when cosmic rays collide with atomic nuclei in the interstellar medium, and thus provide a unique, unbiased view of the distribution of gas. The gamma-ray flux per proton in the interstellar medium, also known as the gamma-ray emissivity, contains information about the density of high energy cosmic rays. These cosmic rays are born in supernovae shock waves and diffuse throughout the Galaxy. The cosmic ray density therefore traces star formation. Previous models of cosmic ray propagation predict small density gradients in the Solar neighborhood. We analyze the gamma-ray emission from 93 nearby molecular clouds. We find no evidence for a local cosmic ray density gradient, and that the variance in the emissivity is larger than previously reported by the Fermi science team. Nor do we find a gradient in Xco = N(H2)/WCO, which is predicted to increase with Galactocentric radius. Finally, we suggest future work which may finally detect a local cosmic ray density gradient. A significant result would constrain the population of cosmic ray sources, thus revealing the distribution of star formation close to the Solar System.

 
 

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