Cosmic Peaks
In the large-scale structure of our cosmos, matter is not distributed uniformly but arranged in a vast "cosmic web" of filaments, sheets, and nodes. The nodes of this cosmic web are extremely overdense regions where dark matter halos collapse earliest and grow most massive — representing the peaks of our universe. They act as the gravitational anchors for clusters of galaxies and are believed to be the birthplaces of the first supermassive black holes.
Distant quasars are expected to reside in these cosmic peaks. Through the JWST GTO EIGER program and my JWST Cycle 4 MOENCH program we study the environment around quasars in the early universe, mapping the galaxy overdensities around them. With a large program on the Chandra X-ray Observatory we hunt of accreting black holes hidden in these environments.
- Eilers et al. 2024: EIGER. VI. The Correlation Function, Host Halo Mass, and Duty Cycle of Luminous Quasars at z ≥ 6
- Yue et al. 2024: EIGER. V. Characterizing the Host Galaxies of Luminous Quasars at z ≥ 6
- Eilers et al. 2023: EIGER. III. JWST/NIRCam Observations of the Ultraluminous High-redshift Quasar J0100+2802
Quasar Light Echoes
Accreting black holes emit ultra-violet radiation that ionizes the intergalactic gas, carving out highly ionized bubbles around each quasar. Any changes in a quasar's luminosity produce outward-propagating ionization gradients, known as the quasars' "light echoes".
Flickering Quasars at Cosmic Dawn
In the nearby universe, quasars are known to vary in brightness over time, a signature that reveals the physics of accretion onto the supermassive black hole and provides a direct way to measure its mass. However, detecting this kind of variability at the earliest cosmic times has remained a major technical challenge, since time dilation stretches any flickering over decades.
Using a decade of infrared and X-ray monitoring, we detected variability in a quasar observed just 850 million years after the Big Bang — the earliest flickering quasar found to date. The variable spectrum reveals a geometrically thin, optically thick accretion disk, much like those seen around mature black holes today, suggesting that the most chaotic phase of early black hole growth happens even earlier.
The Lifetimes of Quasars
The lifetime of quasars denotes the timescale on which supermassive black holes grow and actively accrete material from their surrounding accretion disk. In our standard black hole growth picture, we expect this timescale to last about a billion years in order to grow a supermassive black hole from a small stellar remnant initial black hole seed.
Yet new techniques to constrain these black hole growth timescales, such as measuring the extent of the "proximity zones" observed in quasar spectra, suggest that quasar lifetimes only last about a million years — orders of magnitude shorter than expected. As PI of a JWST Cycle 2 proposal BEES: Black Hole Extended Emission Search to observe the extended nebular emission around quasars in order to estimate the quasar lifetimes from another angle.
- Ďurovčíková et al. 2026: BEES: Quasar lifetime measurements from extended rest-optical emission line nebulae at z∼6
- Morey et al. 2021: Estimating the Effective Lifetime of the z∼6 Quasar Population from the Composite Proximity Zone Profile
- Eilers et al. 2021: Detecting and Characterizing Young Quasars. II. Four Quasars at z∼6 with Lifetimes < 10,000 years
- Eilers et al. 2020: Detecting and Characterizing Young Quasars. I. Systemic Redshifts and Proximity Zone Measurements
- Eilers et al. 2017: Implications of z∼6 Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes
The Epoch of Reionization
Roughly a billion years after the Big Bang, the neutral hydrogen that filled the early universe was reionized by the first generations of galaxies and quasars, transforming the intergalactic medium into the ionized state we still observe today. Pinning down when and how this transition occurred is key to understanding the first sources of light in the cosmos.
Quasar spectra carry an imprint of this transition. Features like damping wings and Lyman-alpha forest opacity encode the amount and distribution of neutral hydrogen along the line of sight, allowing us to study the timing and morphology of reionization.
- Ďurovčíková et al. 2024: Chronicling the Reionization History at 6 ≤ z ≤ 7 with Emergent Quasar Damping Wings
- Eilers et al. 2018: The Opacity of the Intergalactic Medium Measured along Quasar Sightlines at z~6
Generative Models for Quasars
Measuring the masses of supermassive black holes in distant quasars is challenging, because the bright light from the accretion disk outshines the host galaxy by orders of magnitude. For most quasars, especially in the early universe, we rely on simple scaling relations between emission line widths and luminosities, which carry large uncertainties.
To overcome this, we are developing new machine-learning approaches of multi-modal data sets that learn the information on quasar properties encoded in their spectra. By modeling the spectra jointly with the quasars' physical properties in a low-dimensional space, we aim to predict accurate black hole masses and luminosities, even when parts of the spectra are missing or noisy.
- Lalchand & Eilers 2024: Shared Stochastic Gaussian Process Latent Variable Models: A Multi-modal Generative Model for Quasar Spectra
- Eilers et al. 2022: A Generative Model for Quasar Spectra