About

Piero Madau is a Professor in the Astronomy & Astrophysics Department within the Physical & Biological Sciences Division. His research focuses on theoretical astrophysics and cosmology, with specific interests including the physics of the intergalactic medium, the first light and the dawn of galaxies, high-energy radiation backgrounds, the formation and evolution of massive black holes, and the cosmic history of star formation. He has also been involved in the 'Via Lactea Project,' which involves a suite of extremely high-resolution simulations of the assembly of the dark matter halo of the Milky Way. Madau holds a BS from the University of Florence obtained in 1983 and a PhD from the International School for Advanced Studies in Trieste completed in 1987.

Research topics

• Physics
• Astrophysics
• Astronomy
• Quantum mechanics
• Condensed matter physics

Selected publications

• Black holes in the shadows: The missing high-ionization lines in the earliest JWST active galactic nuclei

  Astronomy and Astrophysics · 2026-01-20

  preprintOpen access

  Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift, broad-line active galactic nuclei (AGNs), whose properties challenge standard models of black-hole growth and AGN emission. We analyzed a spectroscopic sample of 34 Type 1 AGNs from the JWST Advanced Deep Survey (JADES) survey, spanning redshifts 1.7 < z < 9, to constrain the physical nature of the accretion flows powering these sources with broad-line diagnostics statistically for the first time. At z > 5, we find a marked suppression of high-ionization emission lines (HeII, CIV, NV) relative to prominent broad H α and narrow [OIII] features. This contrast places strong constraints on the shape of the ionizing spectral energy distribution (SED) and on the physical conditions in the broad-line region (BLR). By comparing the observations to photoionization models based on SEDs of black holes accreting at sub-Eddington ratios, we show that standard AGN continua struggle to reproduce the observed broad-line ratios and equivalent widths (EWs) across a wide ionization parameter range. These results suggest the need for modified SEDs – either intrinsically softened due to super-Eddington accretion or radiative inefficiencies in the innermost disk, or externally filtered by intervening optically thick gas that absorbs or scatters the highest energy photons before they reach the BLR.

  PublisherDOI

• Dynamical Dark Energy Imprints in the Lyman-Alpha Forest

  arXiv (Cornell University) · 2026-01-02

  preprintOpen access

  The nature of dark energy (DE) remains elusive, even though it constitutes the dominant energy-density component of the Universe and drives the late-time acceleration of cosmic expansion. By combining measurements of the expansion history from baryon acoustic oscillations, supernova surveys, and cosmic microwave background data, the Dark Energy Spectroscopic Instrument (DESI) Collaboration has inferred that the DE equation of state may evolve over time. The profound implications of a time-variable, ``dynamical" DE (DDE) that departs from a cosmological constant motivate the need for independent observational tests. In this work, we use cosmological hydrodynamical simulations of structure formation to investigate how DDE affects the properties of the Lyman-Alpha ``forest'' of absorption features produced by neutral hydrogen in the cosmic web. We find that DDE models consistent with the DESI constraints induce a spectral tilt in the forest transmitted flux power spectrum, imprinting a scale- and redshift-dependent signature relative to standard Lambda-CDM cosmologies. These models also yield higher intergalactic medium temperatures and reduced Lyman-Alpha opacity compared to Lambda-CDM. We discuss the observational implications of these trends as potential avenues for independent confirmation of DDE.

  PublisherDOI

• Chasing the light: Shadowing, collimation, and the super-Eddington growth of infant black holes in JWST broad-line AGNs

  Astronomy and Astrophysics · 2026-03-12 · 7 citations

  preprintOpen access1st authorCorresponding

  Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift broad-line active galactic nuclei (BLAGNs) characterized by moderate luminosities, weak X-ray emissions, and faint high-ionization lines. We propose that a subset of these BLAGNs, the so-called “little blue dots” (LBDs) are accreting at super-Eddington rates and use geometrically thick, non-advective disk models to investigate photon scattering and shadowing within the polar funnel. Our models predict extremely blue optical-UV continuum slopes and highly collimated radiation fields where isotropic-equivalent luminosities exceed the Eddington limit in the polar direction, while shadowing suppresses emission at higher inclinations. This “searchlight” configuration naturally generates a stratified ionization structure: coronal and high-excitation narrow lines are produced along the symmetry axis, while the equatorial broad-line region (BLR) remains shielded from the hardest ionizing photons. We show that the anisotropic illumination of the BLR explains the observed faintness of high-ionization lines despite strong Balmer emission. For M BH = 10 7.5 –10 8 M ⊙ black holes accreting at Eddington ratios ∼10, standard BLR conditions predict He II λ 4686/H β in the range of 0.08–0.28. Notably, because inherently blue disk spectra provide a much higher ratio of ionizing to optical photons than standard quasar composites, the observed large Balmer equivalent widths are matched with typical BLR covering factors without invoking enshrouded geometries. Taken together, these findings support the view that super-Eddington accretion flows, shaped by thick disk geometries, may naturally account for the ionizing SED and emission line diagnostics of high- z LBDs, while offering a plausible pathway to rapid black hole growth at cosmic dawn.

  PublisherDOI

• Dynamical Dark Energy Imprints in the Lyman-Alpha Forest

  ArXiv.org · 2026-01-02

  articleOpen access

  The nature of dark energy (DE) remains elusive, even though it constitutes the dominant energy-density component of the Universe and drives the late-time acceleration of cosmic expansion. By combining measurements of the expansion history from baryon acoustic oscillations, supernova surveys, and cosmic microwave background data, the Dark Energy Spectroscopic Instrument (DESI) Collaboration has inferred that the DE equation of state may evolve over time. The profound implications of a time-variable, ``dynamical" DE (DDE) that departs from a cosmological constant motivate the need for independent observational tests. In this work, we use cosmological hydrodynamical simulations of structure formation to investigate how DDE affects the properties of the Lyman-Alpha ``forest'' of absorption features produced by neutral hydrogen in the cosmic web. We find that DDE models consistent with the DESI constraints induce a spectral tilt in the forest transmitted flux power spectrum, imprinting a scale- and redshift-dependent signature relative to standard Lambda-CDM cosmologies. These models also yield higher intergalactic medium temperatures and reduced Lyman-Alpha opacity compared to Lambda-CDM. We discuss the observational implications of these trends as potential avenues for independent confirmation of DDE.

  PublisherOA PDF

• Central Massive Black Holes Are Not Ubiquitous in Local Low-Mass Galaxies

  ArXiv.org · 2025-10-06

  preprintOpen access

  The black-hole occupation fraction ($f_\mathrm{occ}$) defines the fraction of galaxies that harbor central massive black holes (MBHs), irrespective of their accretion activity level. While it is widely accepted that $f_\mathrm{occ}$ is nearly 100% in local massive galaxies with stellar masses $M_\star \gtrsim 10^{10}~M_\odot$, it is not yet clear whether MBHs are ubiquitous in less-massive galaxies. In this work, we present new constraints on $f_\mathrm{occ}$ based on over 20 years of Chandra imaging data for 1606 galaxies within 50 Mpc. We employ a Bayesian model to simultaneously constrain $f_\mathrm{occ}$ and the specific accretion-rate distribution function, $p(λ)$, where the specific accretion rate is defined as $λ=L_\mathrm{X}/M_\star$, and $L_\mathrm{X}$ is the MBH accretion luminosity in the 2-10 keV range. Notably, we find that $p(λ)$ peaks around $10^{28}~\mathrm{erg~s^{-1}}~M_\odot^{-1}$; above this value, $p(λ)$ decreases with increasing $λ$, following a power-law that smoothly connects with the probability distribution of bona-fide active galactic nuclei. We also find that the occupation fraction decreases dramatically with decreasing $M_\star$: in high mass galaxies ($M_\star \approx 10^{11-12}M_\odot$), the occupation fraction is $>93\%$ (a $2σ$ lower limit), and then declines to $66_{-7}^{+8}\%$ ($1σ$ errors) between $M_\star\approx10^{9-10}M_\odot$, and to $33_{-9}^{+13}\%$ in the dwarf galaxy regime between $M_\star\approx10^{8-9}~M_\odot$. Our results have significant implications for the normalization of the MBH mass function over the mass range most relevant for tidal disruption events, extreme mass ratio inspirals, and MBH merger rates that upcoming facilities are poised to explore.

  PublisherOA PDFDOI

• Central Massive Black Holes Are Not Ubiquitous in Local Low-mass Galaxies

  The Astrophysical Journal · 2025-10-14 · 2 citations

  articleOpen access

  Abstract The black hole occupation fraction ( f occ ) defines the fraction of galaxies that harbor central massive black holes (MBHs), irrespective of their accretion activity level. While it is widely accepted that f occ is nearly 100% in local massive galaxies with stellar masses M ⋆ ≳ 10 10 M ⊙ , it is not yet clear whether MBHs are ubiquitous in less-massive galaxies. In this work, we present new constraints on f occ based on over 20 yr of Chandra imaging data for 1606 galaxies within 50 Mpc. We employ a Bayesian model to simultaneously constrain f occ and the specific accretion-rate distribution function, p ( λ ), where the specific accretion rate is defined as λ = L X / M ⋆ , where L X is the MBH accretion luminosity in the 2–10 keV range. Notably, we find that p ( λ ) peaks around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>28</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">erg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msubsup> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ; above this value, p ( λ ) decreases with increasing λ , following a power law that smoothly connects with the probability distribution of bona fide active galactic nuclei. We also find that the occupation fraction decreases dramatically with decreasing M ⋆ : in high-mass galaxies ( M ⋆ ≈ 10 11−12 M ⊙ ), the occupation fraction is &gt;93% (a 2 σ lower limit), and then declines to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>6</mml:mn> <mml:mn>6</mml:mn> <mml:msubsup> <mml:mo>%</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>7</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>8</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> (1 σ errors) between M ⋆ ≈ 10 9−10 M ⊙ , and to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>3</mml:mn> <mml:mn>3</mml:mn> <mml:msubsup> <mml:mo>%</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>9</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>13</mml:mn> <mml:mo>%</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> in the dwarf galaxy regime between M ⋆ ≈ 10 8−9 M ⊙ . Our results have significant implications for the normalization of the MBH mass function over the mass range most relevant for tidal disruption events, extreme mass ratio inspirals, and MBH merger rates that upcoming facilities are poised to explore.

  PublisherOA PDFDOI

• Exploring the fate of primordial discs in Milky Way-sized galaxies with the GigaEris simulation

  Monthly Notices of the Royal Astronomical Society · 2025-04-11 · 2 citations

  articleOpen accessSenior author

  ABSTRACT Recent observations with JWST and Atacama Large Millimeter/submillimeter Array (ALMA) have unveiled galaxies with regular discs at significantly higher redshifts than previously expected. This appears to be in contrast to our understanding of galaxy evolution at high redshift. Additionally, the stellar populations of the Milky Way suggest that the bulk of the Galactic thin disc formed after $z=1$, raising questions about the history, evolution, and survivability of primordial discs in Milky Way analogues. Here, we use GigaEris, a state-of-the-art N-body, hydrodynamical, cosmological ‘zoom-in’ simulation with a billion particles within the virial radius, to delve into the formation of the early kinematically cold discs (KCDs), defined by their ratio between the mean rotational velocity and the radial velocity dispersion, of a Milky Way-sized galaxy at redshifts $z\gtrsim 4$. Our analysis reveals a primarily inward migration pattern for disc stars formed at $z \gtrsim 6$, turning into a mix of inward and outward migration at later times. Stars migrating outwards undergo minimal kinematic heating, and might be identified as part of the thin disc formed at much later epochs. We find that approximately 76 per cent of all stars formed in the KCD at $z \sim 7$ become part of a pseudo-bulge by $z = 4.4$. This proportion decreases to below 10 per cent for KCD stars formed at $z \lesssim 5$. The inward migration of stars born in our KCDs at $z \gtrsim 4$ deviates from the expected inside-out formation scenario of thin discs at lower redshifts. Our results suggest a novel ‘two-phase’ disc formation process, whereby the early discs transform primarily into the pseudo-bulge within less than a billion years, and the present-day thin disc forms subsequently from higher angular momentum material accreted at later times.

  PublisherOA PDFDOI

• The origin of cold gas in the circumgalactic medium

  Astronomy and Astrophysics · 2024-02-19 · 16 citations

  articleOpen accessSenior author

  Context. The presence of cold gas ( T ≲ 10 4 K) in the circumgalactic medium (CGM) of galaxies has been confirmed in observations and in high-resolution simulations, but its origin is still a puzzle. Possible mechanisms are cold accretion from the intergalactic medium (IGM), clumps embedded in outflows and transported from the disk, and gas detaching from the hot CGM phase via thermal instabilities. Aims. In this work we characterize the history of cold CGM gas in order to identify the dominant origin channels at different evolutionary stages of the main galaxy. Methods. To this end, we tracked gas particles in different snapshots of the smoothed particle hydrodynamics (SPH) cosmological zoom-in simulation Eris2k. We performed a backward tracking of cold gas, starting from different redshifts until we could identify one of the followings origins for the particle: cold inflow, ejection from the disk, cooling down in situ, or stripping from a satellite. We also performed a forward tracking of gas in different components of the galaxy (such as the disk and outflows). Results. We find a clear transition between two epochs. For z &gt; 2, most cold gas (up to 80%) in the CGM comes from cold accretion streams as the galaxy is accreting in the cold mode from the IGM. At lower z , gas either cools down in situ after several recycles (with 10–20% of the gas cooling in outflow), or it is ejected directly from the disk (up to 30%). Outflows have a major contribution to the cold CGM gas budget at z &lt; 1, with almost 50% of the hot gas cooling in outflow. Finally, we discuss possible mechanisms for CGM cooling, showing that the thermally unstable gas with t cool / t ff &lt; 1 (precipitation-regulated feedback) is abundant up to r ∼ 100 kpc and cooling times are shorter than 50 Myr for densities n &gt; 10 −2 cm −3 .

  PublisherOA PDFDOI

• A new measurement of the mean transmitted flux in the Ly α and Ly β forest

  Monthly Notices of the Royal Astronomical Society · 2024-06-22 · 3 citations

  articleOpen access

  ABSTRACT We present new measurements of the mean transmitted flux in the hydrogen $\rm {Ly\,\alpha }$ and a relative transmitted flux measurement in $\rm {Ly\,\beta }$ using 27 008 quasar spectra from the Fourteenth Data Release (DR14) of the Extended Baryon Oscillation Spectroscopic Survey. Individual spectra are first combined into 16 composites with mean redshifts in the range of $2.8\lt z\lt 4.9$. We then apply Markov Chain Monte Carlo (MCMC) inference to produce a piecewise fit of the effective $\tau _{\rm {Ly\,\alpha }}$ (corrected for metal lines and optically thick absorption) assuming a spline point distribution. We also perform a relative $\Delta \tau _{\rm {Ly\,\beta }}$ measurement with the same data set, finding $\Delta \tau _{\rm {Ly\,\beta }}\lt 0.35$ at $z\lt 4.8$. The 6–8 per cent precision measurements in the rest frame 1075–1150 Å at ${\it z} \ \lt \rm {4.0}$ and 10–12 per cent precision measurements in the same region at ${\it z} \ \gt \rm {4.0}$ on $\tau _{\rm {Ly\,\alpha }}$, and our determinations of $\Delta \tau _{\rm {Ly\,\beta }}$, are dominated by systematic errors, likely arising from bias and uncertainties in estimates of the quasar continuum. Our $\tau _{\rm {Ly\,\alpha }}$ values show a smooth increase by a factor of 5 over the redshift range $z=2.4-4.4$.

  PublisherOA PDFDOI

• Searching for Intermediate-mass Black Holes in Globular Clusters through Tidal Disruption Events

  The Astrophysical Journal · 2024-03-01 · 9 citations

  articleOpen access

  Abstract Intermediate-mass black holes (IMBHs) may be the link between stellar mass holes and the supermassive variety in the nuclei of galaxies, and globular clusters (GCs) may be one of the most promising environments for their formation. Here, we carry out a pilot study of the observability of tidal disruption events (TDEs) from 10 3 M ⊙ &lt; M • &lt; 10 5 M ⊙ IMBHs embedded in stellar cusps at the center of GCs. We model the long super-Eddington accretion phase and ensuing optical flare, and derive the disruption rate of main-sequence stars as a function of black hole mass and GC properties with the help of a 1D Fokker–Planck approach. The photospheric emission of the adiabatically expanding outflow dominates the observable radiation and peaks in the near-ultraviolet/optical bands, outshining the brightness of the (old) stellar population of GCs in Virgo for a period of months to years. A search for TDE events in a sample of nearly 4000 GCs observed at multiple epochs by the Next Generation Virgo Cluster Survey yields null results. Given our model predictions, this sample is too small to set stringent constraints on the present-day occupation fraction of GCs hosting IMBHs. Naturally, better simulations of the properties of the cluster central stellar distribution, TDE light curves, and rates, together with larger surveys of GCs are all needed to gain deeper insights into the presence of IMBHs in GCs.

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: CDI- Type II: Towards Analyzing Complex Petascale Datasets: The Milky Way Laboratory

  NSF · $816k · 2011–2016

• The Imprint of Galaxy Formation on the Intergalactic Environment

  NSF · $552k · 2009–2013

• MRI: Acquisition of a High-Performance Parallel Computing Cluster for Astrophysics Research at the University of California, Santa Cruz

  NSF · $910k · 2012–2014

Frequent coauthors

• Lucio Mayer

  University of Zurich

  122 shared

• Francesco Haardt

  University of Insubria

  77 shared

• M. Kuhlen

  University of California, Berkeley

  69 shared

• James Wadsley

  McMaster University

  62 shared

• Thomas Quinn

  University of Washington

  54 shared

• Alessandro Lupi

  Istituto Nazionale di Fisica Nucleare, Sezione di Milano Bicocca

  51 shared

• Marta Volonteri

  Institut d'Astrophysique de Paris

  42 shared

• Stelios Kazantzidis

  National and Kapodistrian University of Athens

  39 shared

Education

• Ph.D., Astronomy

  University of California, Santa Cruz

  1996

• M.S., Astronomy

  University of California, Santa Cruz

  1993

• B.S., Physics

  University of California, Santa Cruz

  1991