X-ray constraints on the population of high-redshift active galactic nuclei and prospects with NewAthena

Abstract

Supermassive Black Holes (SMBHs) play an important role within their host galaxies, however little is known about their initial formation and growth. X- ray luminosity functions (XLFs) of actively accreting SMBHs, known as Active Galactic Nuclei (AGN), trace the growth and evolution of SMBH populations across cosmic time. However, due to limitations in survey depth and sky area with current telescopes, current XLF models are poorly constrained at redshifts of 𝑧 > 4, until now. This thesis presents new measurements of the high-redshift XLF, using Swift-XRT, Chandra and looking to the future with NewAthena. I place limits on the bright-end of the XLF at 𝑧 = 5.7 − 6.4 using high-redshift AGN identified within the Extragalactic Serendipitous Swift Survey (ExSeSS), covering ∼ 2000deg2. Within ExSeSS I find one spectroscopically confirmed, serendipitously X-ray detected 𝑧 > 6 AGN, with an X-ray luminosity of 𝐿X =8.47+3.40−3.13 × 1044 erg s−1 and 𝑧 = 6.31 ± 0.03, and calculate an upper limit on the space density at higher luminosities where no additional sources are found, with which I place constraints on the shape of the XLF. These results are consistent with the rapid decline in the space densities of high-luminosity AGN. I also find these X-ray based measurements are consistent with estimates of the bolometric quasar luminosity function based on UV measurements at 𝑧 ≳ 6, although a large X-ray to bolometric correction factor is required at these high luminosities. I performed measurements of the 𝑧 = 4 − 10 XLF at intermediate luminosities, using deep Chandra observations of X-ray selected AGN within the COSMOS2020 galaxy survey. I first performed a cross-match to a deep Chandra source list, for which the X-ray sensitivity can be accurately quantified, before exploiting the available X-ray data further through direct extraction of X-ray counts at the positions of COSMOS2020 galaxies. With the resulting 𝑧 = 4 − 10 X-ray AGN sample, comprised of 21 blind detections and a further 11 directly extracted detections, I have measured the early space density of AGN at moderate-luminosities where the majority of early SMBH growth occurred.These measurements reveal higher space-densities than expected, based on the extrapolation of XLF models from lower redshifts. Whilst my measured space densities at 𝑧 = 4 − 5 are consistent with the model predictions, at 𝑧 = 5 − 7 I find space densities of the order of 10× the extrapolated model predictions which rises to ∼250× the model extrapolations at 𝑧 = 7 − 10. In addition I find evidence that a large fraction of the early AGN population is heavily obscured with an obscured fraction of 0.982+0.007−0.008; correcting for this obscuration further increases the measured space densities. Comparing to recent JWST results, these measurements begin to bridge the gap between the bright-end of the quasar luminosity function and the latest JWST observations of very early, low-luminosity AGN, indicating that a larger fraction of the first galaxies likely play host to a rapidly growing SMBH than previously thought. With these observational measurements of the XLF, I determine predictions for the source numbers detected with a survey performed by the Wide Field Instrument (WFI) on ESA’s next large X-ray mission, NewAthena. I perform full simulations of NewAthena WFI survey pointings for a small number of exposure times, using the SIXTE software. I then use these to calibrate an analytical description of the survey sensitivity of NewAthena. The resulting analytical sensitivity (which includes important source confusion effects) enables rapid predictions for a wider range of exposure times. Combining this model sensitivity and my XLF measurements I predict the number of high redshift X-ray AGN that will be visible to NewAthena, identifying a survey strategy that can be expected to find 1159 unabsorbed, 2748 absorbed and 1091 Compton thick AGN at 𝑧 = 4 − 10 within approximately one year of observing time. NewAthena can be expected to observe an area of sky two orders of magnitude larger than Chandra, to similar survey depths, and yield unprecedented samples of early AGN. The large number of observed X-ray AGN predicted indicates the power of NewAthena for investigating the high-redshift XLF.