## Theses and Dissertations

Dissertation

Spring 2017

#### Degree Name

PhD (Doctor of Philosophy)

Physics

Philip Kaaret

#### Abstract

The focus of this dissertation is to investigate the effect of metallicity on high-mass X-ray binary (HMXB) formation and evolution as a means to understand the evolution of the early Universe~$(z>6)$.

Understanding the population and X-ray output of HMXBs are vital to modelling the heating and ionization morphology of the intergalactic medium during the epoch of reionization. Current X-ray instruments are unable to directly detect very high redshift HMXBs, making it impossible to constrain population sizes in this way. Instead certain local galaxies may be used as analogs to infer the properties of galaxies in the early Universe. These local analogs should have properties consistent with those expected for the first galaxies, such as low-metallicity, compact morphology, and intense recent star formation.

I present an X-ray population study of 25 blue compact dwarf galaxies (BCD), using multiwavelength data and Bayesian analysis techniques. We find a significant enhancement of the HMXB population in low-metallicity environments and suggest the same may be true in the early Universe.

I continue the investigation of HMXB populations in a sample of 10 moderate metallicity $(Z \geq 0.3\, Z_\odot)$, local star-forming galaxies known as Lyman Break Analogs (LBAs).

I find evidence of a $L_X$--SFR--metallicity plane in the combined sample of BCDs, LBAs, and regular star-forming galaxies.

Then I study a third type of local analog to early Universe galaxies, the Green Pea galaxies. These are a subclass of luminous compact galaxies (LCGs) which show strong [OIII]$\lambda5007$\AA\ emission indicative of extreme, recent star-formation. This pilot study was carried out to look, for the first time in X-rays, at this recently established class of galaxies and use them to test the $L_X$--SFR--metallicity plane.

Determining the spectral properties of bright HMXBs in low-metallicity environments also has important implications for models of X-ray heating leading up to the Epoch of Reionization.

I examined the X-ray spectra of VII~Zwicky~403, one of the nearby BCD galaxies from the first study and contrast this with the only other low-metallicity BCD with high-quality spectra, I~Zw~18. In the high flux state, the spectrum of VII~Zw~403 is hard but drops off exponentially at higher energies $(E>5\,\text{keV})$. This lies in contrast with the softer blackbody accretion disk spectrum seen from I~Zw~18 in its high flux state.

I conclude with a brief summary of the thesis and discuss recent relevant theory and simulation work done by other groups.

xiii, 106 pages

#### Bibliography

Includes bibliographical references (pages 91-106).

COinS