Evolution of the AGN UV luminosity function from redshift 7.5

Determinations of the UV luminosity function of AGN at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of AGN data. We address this problem by constructing a sample of more than 80,000 colour-selected AGN from redshift z=0 to 7.5. While this sample is composed of multiple data sets with spectroscopic redshifts and completeness estimates, we homogenise these data sets to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z=0 to 7.5. The luminosity function has a double power law form at all redshifts. The break magnitude $M_*$ of the AGN luminosity function shows a steep brightening from $M_*\sim -24$ at z=0.7 to $M_*\sim -29$ at z=6. The faint-end slope $\beta$ significantly steepens from $-1.7$ at $z<2.2$ to $-2.4$ at $z\simeq 6$. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at $z\sim 6$ is subdominant (< 3%), although it can be non-negligible (~10%) if these luminosity functions hold down to $M_{1450}=-18$. Under reasonable assumptions, AGN can reionize HeII by redshift z=2.9. At low redshifts (z<0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of HI absorption lines in the IGM. Our global analysis of the luminosity function also reveals important systematic errors in the data, particularly at z=2.2--3.5, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, luminosity function analysis codes, and homogenised AGN data publicly available.Comment: 30 pages, 15 figures; accepted in MNRAS; code, data, and various fits at https://github.com/gkulkarni/QL

The transverse proximity effect in quasar spectra

The intergalactic medium is kept highly photoionised by the intergalactic UV background radiation field generated by the overall population of quasars and galaxies. In the vicinity of sources of UV photons, such as luminous high-redshift quasars, the UV radiation field is enhanced due to the local source contribution. The higher degree of ionisation is visible as a reduced line density or generally as a decreased level of absorption in the Lyman alpha forest of neutral hydrogen. This so-called proximity effect has been detected with high statistical significance towards luminous quasars. If quasars radiate rather isotropically, background quasar sightlines located near foreground quasars should show a region of decreased Lyman alpha absorption close to the foreground quasar. Despite considerable effort, such a transverse proximity effect has only been detected in a few cases...thesi

Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration

The HeII transverse proximity effect - enhanced HeII Ly{\alpha} transmission in a background sightline caused by the ionizing radiation of a foreground quasar - offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt+2017, we present a detailed model of the HeII transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We post-process outputs from a cosmological hydrodynamical simulation with a fluctuating HeII UV background model, plus the added effect of the radiation from a single bright foreground quasar. We vary the age $t_\mathrm{age}$ and obscured sky fractions $\Omega_\mathrm{obsc}$ of the foreground quasar, and explore the resulting effect on the HeII transverse proximity effect signal. Fluctuations in IGM density and the UV background, as well as the unknown orientation of the foreground quasar, result in a large variance of the HeII Ly{\alpha} transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far UV HeII Ly{\alpha} transmission spectra of the background quasars to our models, and extract joint constraints on $t_\mathrm{age}$ and $\Omega_\mathrm{obsc}$ for the six Schmidt+2017 foreground quasars with the highest implied HeII photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong HeII transmission spike, is relatively old $(22\,\mathrm{Myr})$ and unobscured $\Omega_\mathrm{obsc}<35\%$, whereas three others are either younger than $(10\,\mathrm{Myr})$ or highly obscured $(\Omega_\mathrm{obsc}>70\%)$.Comment: 19 pages, 6 figures, submitted to Ap

Lyman-alpha spectral properties of five newly discovered Lyman continuum emitters

We have recently reported the discovery of five low redshift Lyman continuum (LyC) emitters (LCEs, hereafter) with absolute escape fractions fesc(LyC) ranging from 6 to 13%, higher than previously found, and which more than doubles the number of low redshift LCEs.We use these observations to test theoretical predictions about a link between the characteristics of the Lyman-alpha (Lya) line from galaxies and the escape of ionising photons. We analyse the Lya spectra of eight LCEs of the local Universe observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope (our five leakers and three galaxies from the litterature), and compare their strengths and shapes to the theoretical criteria and comparison samples of local galaxies: the Lyman Alpha Reference Survey, Lyman Break Analogs, Green Peas, and the high-redshift strong LyC leaker Ion2. Our LCEs are found to be strong Lya emitters, with high equivalent widths, EW(Lya)> 70 {\AA}, and large Lya escape fractions, fesc(Lya) > 20%. The Lya profiles are all double-peaked with a small peak separation, in agreement with our theoretical expectations. They also have no underlying absorption at the Lya position. All these characteristics are very different from the Lya properties of typical star-forming galaxies of the local Universe. A subset of the comparison samples (2-3 Green Pea galaxies) share these extreme values, indicating that they could also be leaking. We also find a strong correlation between the star formation rate surface density and the escape fraction of ionising photons, indicating that the compactness of star-forming regions plays a role in shaping low column density paths in the interstellar medium of LCEs. The Lya properties of LCEs are peculiar: Lya can be used as a reliable tracer of LyC escape from galaxies, in complement to other indirect diagnostics proposed in the literature.Comment: 11 pages, 10 figures, accepted for publication in A&