Quasar Demographics

The quasar luminosity function (QLF) is the fundamental statistic to constrain the redshift evolution of the quasar population. It is an important demographic probe that enables comparisons with cosmological simulations and semi-analytic models to disentangle different supermassive black hole seeding and growth scenarios.

During my PhD I have constrained the QLF at z=3-5 and determined that the bright-end slope does not flatten with redshift as previously thought, but remains steep. More recently, I was an integral part of the Pan-STARRS1 distant quasar survey targeting quasars at z>6. Following the discovery of 55 additional quasars, I was able to determine the most precise estimate of the z∼6 quasar luminosity function to date. The image shows the evolution of the QLF from z∼3 to z∼6, highlighting the steep bright-end slope and the rapid decline in quasar number density with increasing redshift.
My postdoc Francesco Guarneri is now working on extending these studies to z≈6.5-8 using the new quasar discoveries in the Euclid Wide Survey in the Data Release 1 footprint.

Quasar-galaxy clustering is a powerful tool to constrain quasar host dark matter halo masses and thus connect SMBH growth to structure formation in the early Universe. I have published the first constraints of the quasar-galaxy cross-correlation function at z≈7.3, based on the discovery of eight galaxies in two quasar fields. Our results indicate that the host dark matter halo masses of z≈7.3 quasars are still massive, but less massive than those of z∼6 quasars, providing tentative evidence for a non-monotonic redshift evolution of quasar clustering properties.

Relevant publications

• The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey Footprint. II. The North Galactic Cap Sample (Schindler et al. 2018)
• The Extremely Luminous Quasar Survey in the Sloan Digital Sky Survey Footprint. III. The South Galactic Cap Sample and the Quasar Luminosity Function at Cosmic Noon (Schindler et al. 2019a)
• The Pan-STARRS1 z > 5.6 Quasar Survey. III. The z ≈ 6 Quasar Luminosity Function (Schindler et al. 2023)
• A first look at quasar-galaxy clustering at z ≃ 7.3 (Schindler et al. 2026)

SMBH galaxy co-evolution and quasar feedback

It is well known that the masses of supermassive black holes correlate with the mass of the central stellar component of their host galaxies, indicating a co-evolution. Quasar feedback is thought to be the mechanism that regulates this co-evolution. At z>6, however, SMBHs appear to be over-massive compared to the local relation, suggesting that SMBH growth precedes galaxy growth in the early Universe. Ideally one would like to trace the evolution of the SMBH-galaxy relation across cosmic time.

In a pilot study I used molecular CO transitions to trace the cold gas kinematics in two z∼3.5 hyper-luminous quasars. Using NOEMA interferometric observations, my PhD student Radha Gharapurkar is now working on a larger sample at these redshifts to fill the gap between the reionization-era quasars and the local Universe.

Quasar spectra also offer insight into the chemical profile and gas kinematics of the accreting material. In the X-SHOOTER/ALMA sample we find that z>5.7 quasars are already enriched in iron, indicating a fast chemical evolution at the centers of these early massive galaxies. In addition, the spectra more commonly show large velocity shifts of high-ionization lines, indicative of vigorous (outflowing) gas motion close to the accretion disk. Similarly, based on the XQR-30 quasar sample at redshifts z=5.8-6.6, we find evidence for a higher incidence of fast outflowing dense gas , possibly indicating more vigorous feedback effects on the surrounding galactic gas.

Using the z=3.5 hyper-luminous quasar sample for a comparative study, my PhD student Radha Gharapurkar is now investigating whether quasar feedback at z>6 is indeed more common or a general result of the vigorous accretion in these sources.

The images on the left shows Centaurus A revealing the lobes and jets emanating from the active galaxy’s central black hole, an example of quasar feedback in the local Universe.
Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)

Relevant publications

• The X-SHOOTER/ALMA Sample of Quasars in the Epoch of Reionization. I. NIR Spectral Modeling, Iron Enrichment, and Broad Emission Line Properties (Schindler et al. 2020)
• A Closer Look at Two of the Most Luminous Quasars in the Universe (Schindler et al. 2021)