The X-ray enhancements of radio-loud quasars at high redshift: New results at z=4--7
Open Access
- Author:
- Zuo, Zihao
- Area of Honors:
- Astronomy and Astrophysics
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- William Nielsen Brandt, Thesis Supervisor
Cindy Yuexing Gulis, Thesis Honors Advisor - Keywords:
- galaxies: high-redshift
quasars: general
X-rays: galaxies - Abstract:
- Highly radio-loud quasars (HRLQs; $\log R>2.5$) at $z\gtrsim 4$ show apparent enhanced \hbox{X-ray} emission compared to matched HRLQs at lower redshifts, perhaps due to a redshift-dependent fractional contribution to the \hbox{X-ray} luminosity from inverse-Compton scattering of cosmic microwave background photons (IC/CMB). Using new {\it Chandra\/} observations and archival \hbox{X-ray} data, we investigate this phenomenon with an optically flux-limited sample of 41 HRLQs at $z = 4$--5.5 all with sensitive \hbox{X-ray} coverage, the largest sample utilized to date by a wide margin. \hbox{X-ray} enhancements are assessed using \hbox{X-ray}-to-optical flux ratios and spectral energy distributions. We confirm the presence of \hbox{X-ray} enhancements at a \hbox{4.9--5.3$\sigma$} significance level, finding that the median factor of enhancement is $\approx 1.8$ at our sample median redshift of $z\approx 4.4$. Under a fractional IC/CMB model, the expected enhancement at lower redshifts is modest; e.g., $\approx 4$\% at $z\approx 1.5$. We also investigate a sample of seven radio-loud quasars (RLQs; $\log R>1$) at even higher redshifts of $z=5.6$--6.8, using new and archival \hbox{X-ray} data. These RLQs also show evidence for \hbox{X-ray} enhancements by a median factor of $\approx 2.7$ at a \hbox{3.7--4.9$\sigma$} significance level. The \hbox{X-ray} spectral and other properties of these $z=5.6$--6.8 RLQs, however, pose challenges for a straightforward fractional IC/CMB interpretation of their enhancements.