Research

I am drawn to problems where theoretical models can be put to a real test. That is where the physics lives, and where we learn something new about the Universe. I develop models using tools that range from analytical calculations to large cosmological simulations, and use inference to ask how well their predictions survive contact with observations.

The central thread of my research is the formation and growth of supermassive black holes across cosmic time, and the quasars they power. I combine large-volume cosmological simulations with empirical models for black holes and quasars, and confront them with observations: quasar demographics, clustering measurements from JWST and wide-field surveys, and the imprint quasars leave on the intergalactic medium. The questions I keep returning to are: Where does quasar activity take place? How do quasars relate to their host halos and galaxies? How long does a quasar shine, and how does that set the growth of its black hole?

Within this theme, I have built semi-empirical models for black hole growth, developed population models for quasar clustering, interpreted JWST measurements of quasar environments, used quasar proximity zones as a clock for black hole growth, explored what gravitational waves tell us about black hole mergers, studied massive black holes in hydrodynamical simulations, and looked into the puzzling JWST population of “little red dots”.

I am also interested in the broader physics of galaxy formation and cosmology, and I have worked on several other topics, including galactic outflows and extended [CII] halos, the morphology of protoplanetary discs, and parameter inference for gravitational-wave signals.

Main research topics

Semi-empirical models for black hole growth

Growth histories of individual black holes in the black hole mass versus halo mass plane, coloured by redshift
How do supermassive black holes assemble their mass, and what does that history look like for an individual object rather than for the population as a whole? Semi-empirical models offer a way into this question: they place a black hole in every dark matter halo of a cosmological simulation, grow it through a few physically motivated rules, and let the data decide what those rules should be. Because the halos come from a simulation, the models inherit a realistic cosmic environment, and because the rules are few, they can be fitted to observations rather than tuned by hand. I am particularly interested in the stochastic nature of accretion, where a single timescale sets both how fast a black hole grows and how long it shines as a quasar, tying the growth of the population to the lightcurves of individual objects. This is the idea behind BAQARO, a framework for black hole growth and quasar activity across cosmic time. Alongside it, I am interested in analytical descriptions of black hole growth, such as the continuity equation for the black hole mass function, and in how they relate to the stochastic growth histories seen in the simulations.

Relevant publications

  • Pizzati et al. 2026 (submitted), BAQARO: Tracing Stochastic Black Hole Growth Histories and Quasar Lightcurves in a Cosmological Context

Quasar clustering and population models

Schematic of the quasar population model
Quasar clustering measures how strongly quasars trace the underlying matter, and thus how massive the dark matter halos that host them are. Together with the luminosity function, it tells us what fraction of halos host an active quasar at any given time, and hence how long quasars shine. Wide-field surveys have shown that quasars are remarkably strongly clustered at high redshift, so much so that reproducing the measurements at \(z\approx4\) has challenged models for over a decade. I am interested in population models that connect quasars to dark matter halos in large cosmological simulations, and reproduce their clustering and demographics at the same time. The key ingredient is the stochastic relation between quasar luminosity and halo mass: once it is constrained, the models predict where quasars live, how long they are active, and how their environments evolve across cosmic time.

Relevant publications

  • Pizzati et al. 2024a, Revisiting the extreme clustering of \(z\approx4\) quasars with large volume cosmological simulations
  • Pizzati et al. 2024b, A unified model for the clustering of quasars and galaxies at \(z\approx6\)

JWST measurements of quasar environments

Quasar host halo mass function at z~6
JWST has made it possible to measure the clustering of quasars and galaxies deep into the Epoch of Reionization. Slitless spectroscopy maps the [OIII]-emitting galaxies around bright \(z\gtrsim6\) quasars, revealing a wide range of environments, from sparse fields to rich overdensities, and delivering the first quasar–galaxy cross-correlation measurements at these redshifts. I work with the EIGER and ASPIRE teams on interpreting these measurements: turning galaxy maps into host halo masses and duty cycles, understanding the role of cosmic variance and satellite galaxies in the small volumes that JWST probes, and connecting the environments of the first quasars to how their black holes grew. The same techniques are now being pushed to \(z>7\).

Relevant publications

Quasar proximity zones and lifetimes

Lyman-alpha transmission around a quasar for a range of quasar lifetimes, showing how the proximity zone grows with lifetime
A luminous quasar ionizes the intergalactic medium around it, and the region of enhanced transmission in its spectrum, the proximity zone, records how long and how steadily the quasar has been shining. Proximity zones are one of the few clocks for quasar activity and black hole growth, and at \(z>7\) they also carry information on the neutral fraction of the surrounding gas. I am interested in what these zones tell us once we move beyond the picture of a quasar that turns on at constant luminosity. My work combines stochastic quasar lightcurves from semi-empirical growth models with radiative transfer through the intergalactic medium, to ask what proximity zones look like when quasars flicker, what they can and cannot constrain about the growth history of the black hole, and how this affects what we infer about the state of the early Universe.

Relevant publications

Black hole mergers and gravitational waves

Fraction of black hole mass growth due to mergers, as a function of redshift and black hole mass
Supermassive black holes grow in two ways: by accreting gas, and by merging with one another when their host galaxies and halos merge. Accretion is what makes quasars shine and is well charted by their demographics; the merger channel is much harder to observe, and its contribution to black hole growth is still uncertain. Gravitational waves offer the cleanest way to see it: pulsar timing arrays have found evidence for a nanohertz background from the population of supermassive black hole binaries, and LISA will detect individual mergers out to the highest redshifts. I am interested in whether these signals can constrain the merger channel of black hole growth, and in whether they are consistent with what quasars tell us. Quasar observables and binary observables such as the pulsar-timing background probe the same black hole population through different channels, and a model that follows black holes through both accretion and mergers in a cosmological context can be confronted with both at once, testing whether one picture of black hole growth can account for everything we see.

Massive black holes in cosmological hydrodynamical simulations

Large-volume cosmological hydrodynamical simulation Image: Schaye et al. 2023
Cosmological hydrodynamical simulations follow the growth of cosmic structure while modeling the physics of gas, stars, and black holes, and now reproduce many properties of galaxies. The population of supermassive black holes they produce, and especially the rare, luminous quasars that can only be captured in very large volumes, is much less tested. I use large-volume simulations such as FLAMINGO to compare their quasar populations with observations, from the luminosity function to the clustering of bright quasars and the halos that host them. This is a direct test of the black hole growth and feedback prescriptions these simulations rely on, and a complementary view to the empirical models above, where the physics is inferred from the data rather than imposed.

Relevant publications

The JWST little red dots

Mock little red dots analysis
JWST has revealed an abundant population of compact, red, broad-line sources, the “little red dots”, whose nature is still debated. If they are accreting black holes, they far outnumber the unobscured quasars of similar luminosity found in wide-field surveys, which would call for a revision of how supermassive black holes grow in the early Universe. I am interested in how these objects fit into the picture of early black hole growth, and in particular in what their demographics and clustering tell us about the halos they live in and their relation to unobscured quasars: whether the two populations can share the same halos, or whether little red dots represent a different phase in the life of early black holes.

Relevant publications

  • Pizzati et al. 2025, “Little Red Dots” cannot reside in the same dark matter halos as comparably luminous unobscured quasars

Other topics

Galactic outflows and [CII] halos at high redshift

Galactic outflow (M82) Image: NASA
ALMA has revealed that many galaxies at \(z>4\) are surrounded by extended halos of [CII] emission, reaching 10–15 kpc, which even detailed zoom-in simulations struggle to reproduce. During my Master's work I studied whether these halos are the signature of galactic outflows: gas expelled by star formation that cools rapidly in the inner halo and lights up in [CII]. A semi-analytical outflow model reproduces the observed emission profiles, including those of the ALMA ALPINE survey, suggesting that star-formation-driven feedback was already shaping galaxies well into the Epoch of Reionization.

Relevant publications

Turbulence and morphology of protoplanetary discs

Protoplanetary disc model
The strength of turbulence in protoplanetary discs controls how gas accretes and how planets form, but it is notoriously hard to measure. One handle is the vertical thickness of the dust layer, which turbulence stirs up. With ALMA resolving rings and gaps in disc images, the thickness can be read off from projection effects: a gap looks shallower along the disc's minor axis than along its major axis, by an amount that depends on the disc's vertical structure. Applying this method to high-resolution DSHARP observations with radiative transfer models, we found that the dust layers are thin, implying low levels of turbulence.

Relevant publications

  • Pizzati et al. 2023, Constraining turbulence in protoplanetary discs using the gap contrast: an application to the DSHARP sample

Overlapping gravitational-wave signals in next-generation detectors

Overlapping gravitational-wave signals
Next-generation gravitational-wave detectors such as Cosmic Explorer and the Einstein Telescope will observe hundreds of thousands of compact binary mergers per year. Signals will be so frequent that they overlap in time, a regime in which standard parameter-estimation pipelines are not guaranteed to work. We quantified the biases that arise when existing pipelines are applied to overlapping signals, and showed that they can be handled with current infrastructure as long as the mergers are more than a second or two apart, while closer pairs require new analysis strategies.

Relevant publications