## Theses and Dissertations

#### DOI

10.17077/etd.m5j7epqy

Dissertation

Spring 2017

#### Degree Name

PhD (Doctor of Philosophy)

Physics

Philip Kaaret

Hai Fu

Kenneth Gayley

Andrea Prestwich

Steven Spangler

#### 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.

#### Public Abstract

Approximately 400,000 years after the Big Bang, the Universe had expanded and cooled enough such that protons and electrons – which had been disassociated up to that point – joined to form neutral atoms. From this neutral material, the first stars and galaxies formed. These stars’ intense ultraviolet radiation began to undo the neutralization process, ripping electrons from atoms (ionization) within the remaining neutral material. This continued until nearly all the neutral material throughout the Universe was re-ionized. However, ultraviolet photons were not the sole ionizing radiation at this time. X-rays from extremely hot material falling into black holes could have significantly affected the timing and structure of this re- ionization process. My thesis focuses on addressing the question: what role did X-rays play during this time?

Currently, X-ray telescopes cannot see these extremely distant black holes. Instead, nearby galaxies with the right properties are used as proxies. Compared to the first galaxies, most galaxies we see today have a higher fraction of elements heavier than helium. These elements are produced through the life and death of generations of stars. Therefore, the proxy galaxies must have very low fractions of heavy elements and should have just begun producing stars.

In measuring the X-ray emission from 41 local proxies to the first galaxies, I found a significant increase in X-ray producing objects compared to normal galaxies seen today. By incorporating this information into simulations, we can better understand the role X-rays played in shaping the early Universe.

xiii, 106 pages

#### Bibliography

Includes bibliographical references (pages 91-106).