Lars Hernquist
· ProfessorHarvard University · Astronomy
Active 1982–2024
About
Lars Hernquist is the Mallinckrodt Professor of Astrophysics at the Center for Astrophysics | Harvard & Smithsonian. His research interests include the theoretical study of dynamical processes in cosmology and galaxy formation and evolution. He conducts numerical simulations of stellar dynamical and hydrodynamical systems, contributing to the understanding of complex astrophysical phenomena. Additionally, Hernquist investigates the physics of compact objects, with a particular focus on neutron stars and the interplay between thermal and magnetic processes in strongly magnetized neutron stars.
Research topics
- Astrophysics
- Physics
- Astronomy
- Quantum mechanics
- Optics
- Computational physics
- Nuclear physics
- Mathematics
- Geometry
Selected publications
Probing Plasma Composition with the Next Generation Event Horizon Telescope (ngEHT)
Galaxies · 2023 · 9 citations
- Physics
- Astrophysics
- Computational physics
We explore the plasma matter content in the innermost accretion disk/jet in M87* as relevant for an enthusiastic search for the signatures of anti-matter in the next generation of the Event Horizon Telescope (ngEHT). We model the impact of non-zero positron-to-electron ratio using different emission models, including a constant electron to magnetic pressure (constant βe model) with a population of non-thermal electrons as well as an R-beta model populated with thermal electrons. In the former case, we pick a semi-analytic fit to the force-free region of a general relativistic magnetohydrodynamic (GRMHD) simulation, while in the latter case, we analyze the GRMHD simulations directly. In both cases, positrons are being added at the post-processing level. We generate polarized images and spectra for some of these models and find out that at the radio frequencies, both of the linear and the circular polarizations are enhanced with every pair added. On the contrary, we show that, at higher frequencies, a substantial positron fraction washes out the circular polarization. We report strong degeneracies between different emission models and the positron fraction, though our non-thermal models show more sensitivities to the pair fraction than the thermal models. We conclude that a large theoretical image library is indeed required to fully understand the trends probed in this study, and to place them in the context of a large set of parameters which also affect polarimetric images, such as magnetic field strength, black hole spin, and detailed aspects of the electron temperature and the distribution function.
Fast, Slow, Early, Late: Quenching Massive Galaxies at z ∼ 0.8
The Astrophysical Journal · 2022 · 159 citations
- Physics
- Astrophysics
- Astronomy
Abstract We investigate the stellar populations for a sample of 161 massive, mainly quiescent galaxies at 〈 z obs 〉 = 0.8 with deep Keck/DEIMOS rest-frame optical spectroscopy (HALO7D survey). With the fully Bayesian framework Prospector , we simultaneously fit the spectroscopic and photometric data with an advanced physical model (including nonparametric star formation histories, emission lines, variable dust attenuation law, and dust and active galactic nucleus emission), together with an uncertainty and outlier model. We show that both spectroscopy and photometry are needed to break the dust–age–metallicity degeneracy. We find a large diversity of star formation histories: although the most massive ( M ⋆ > 2 × 10 11 M ⊙ ) galaxies formed the earliest (formation redshift of z f ≈ 5–10 with a short star formation timescale of τ SF ≲ 1 Gyr), lower-mass galaxies have a wide range of formation redshifts, leading to only a weak trend of z f with M ⋆ . Interestingly, several low-mass galaxies have formation redshifts of z f ≈ 5–8. Star-forming galaxies evolve about the star-forming main sequence, crossing the ridgeline several times in their past. Quiescent galaxies show a wide range and continuous distribution of quenching timescales ( τ quench ≈ 0–5 Gyr) with a median of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">〈</mml:mo> <mml:msub> <mml:mrow> <mml:mi>τ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>quench</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">〉</mml:mo> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.50em"/> <mml:mi>Gyr</mml:mi> </mml:math> and of quenching epochs of z quench ≈ 0.8–5.0 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">〈</mml:mo> <mml:msub> <mml:mrow> <mml:mi>z</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>quench</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">〉</mml:mo> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>1.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ). This large diversity of quenching timescales and epochs points toward a combination of internal and external quenching mechanisms. In our sample, rejuvenation and “late bloomers” are uncommon. In summary, our analysis supports the “grow-and-quench” framework and is consistent with a wide and continuously populated diversity of quenching timescales.
Monthly Notices of the Royal Astronomical Society · 2021 · 216 citations
Senior authorCorresponding- Physics
- Astrophysics
- Astronomy
ABSTRACT We introduce the thesan project, a suite of large volume ($L_\mathrm{box} = 95.5 \, \mathrm{cMpc}$) radiation-magnetohydrodynamic simulations that simultaneously model the large-scale statistical properties of the intergalactic medium during reionization and the resolved characteristics of the galaxies responsible for it. The flagship simulation has dark matter and baryonic mass resolutions of $3.1 \times 10^6\, {\rm M_\odot }$ and $5.8 \times 10^5\, {\rm M_\odot }$, respectively. The gravitational forces are softened on scales of 2.2 ckpc with the smallest cell sizes reaching 10 pc at z = 5.5, enabling predictions down to the atomic cooling limit. The simulations use an efficient radiation hydrodynamics solver (arepo-rt) that precisely captures the interaction between ionizing photons and gas, coupled to well-tested galaxy formation (IllustrisTNG) and dust models to accurately predict the properties of galaxies. Through a complementary set of medium resolution simulations we investigate the changes to reionization introduced by different assumptions for ionizing escape fractions, varying dark matter models, and numerical convergence. The fiducial simulation and model variations are calibrated to produce realistic reionization histories that match the observed evolution of the global neutral hydrogen fraction and electron scattering optical depth to reionization. They also match a wealth of high-redshift observationally inferred data, including the stellar-to-halo-mass relation, galaxy stellar mass function, star formation rate density, and the mass–metallicity relation, despite the galaxy formation model being mainly calibrated at z = 0. We demonstrate that different reionization models give rise to varied bubble size distributions that imprint unique signatures on the 21 cm emission, especially on the slope of the power spectrum at large spatial scales, enabling current and upcoming 21 cm experiments to accurately characterize the sources that dominate the ionizing photon budget.
The Astrophysical Journal · 2021 · 4 citations
- Physics
- Astrophysics
- Astronomy
Abstract We present a comprehensive analysis of the shape of dark matter (DM) halos in a sample of 25 Milky Way-like galaxies in TNG50 simulation. Using an enclosed volume iterative method, we infer an oblate-to-triaxial shape for the DM halo with median T ≃ 0.24. We group DM halos into three different categories. Simple halos (32% of the population) establish principal axes whose ordering in magnitude does not change with radius and whose orientations are almost fixed throughout the halo. Twisted halos (32%) experience levels of gradual rotations throughout their radial profiles. Finally, stretched halos (36%) demonstrate a stretching in the lengths of their principal axes where the ordering of different eigenvalues changes with radius. Subsequently, the halo experiences a “rotation” of ∼90° where the stretching occurs. Visualizing the 3D ellipsoid of each halo, for the first time, we report signs of a reorienting ellipsoid in twisted and stretched halos. We examine the impact of baryonic physics on DM halo shape through a comparison to dark matter only (DMO) simulations. This suggests a triaxial (prolate) halo. We analyze the impacts of substructure on DM halo shape in both hydrodynamical and DMO simulations and confirm that they are subdominant. We study the distribution of satellites in our sample. In simple and twisted halos, the angle between satellites’ angular momentum and the galaxy’s angular momentum grows with radius. However, stretched halos show a flat distribution of angles. Overlaying our theoretical outcome on the observational results presented in the literature establishes a fair agreement.
Monthly Notices of the Royal Astronomical Society · 2020 · 182 citations
- Physics
- Astrophysics
- Quantum mechanics
ABSTRACT Supermassive black holes (SMBHs) that reside at the centres of galaxies can inject vast amounts of energy into the surrounding gas and are thought to be a viable mechanism to quench star formation in massive galaxies. Here, we study the $10^{9-12.5}\, \mathrm{M_\odot }$ stellar mass central galaxy population of the IllustrisTNG simulation, specifically the TNG100 and TNG300 volumes at z = 0, and show how the three components – SMBH, galaxy, and circumgalactic medium (CGM) – are interconnected in their evolution. We find that gas entropy is a sensitive diagnostic of feedback injection. In particular, we demonstrate how the onset of the low-accretion black hole (BH) feedback mode, realized in the IllustrisTNG model as a kinetic, BH-driven wind, leads not only to star formation quenching at stellar masses $\gtrsim 10^{10.5}\, \mathrm{M_\odot }$ but also to a change in thermodynamic properties of the (non-star-forming) gas, both within the galaxy and beyond. The IllustrisTNG kinetic feedback from SMBHs increases the average gas entropy, within the galaxy and in the CGM, lengthening typical gas cooling times from $10\!-\!100\, \mathrm{Myr}$ to $1\!-\!10\, \mathrm{Gyr}$, effectively ceasing ongoing star formation and inhibiting radiative cooling and future gas accretion. In practice, the same active galactic nucleus (AGN) feedback channel is simultaneously ‘ejective’ and ‘preventative’ and leaves an imprint on the temperature, density, entropy, and cooling times also in the outer reaches of the gas halo, up to distances of several hundred kiloparsecs. In the IllustrisTNG model, a long-lasting quenching state can occur for a heterogeneous CGM, whereby the hot and dilute CGM gas of quiescent galaxies contains regions of low-entropy gas with short cooling times.
Resolving small-scale cold circumgalactic gas in TNG50
Monthly Notices of the Royal Astronomical Society · 2020 · 180 citations
Senior authorCorresponding- Physics
- Astrophysics
- Astronomy
ABSTRACT We use the high-resolution TNG50 cosmological magnetohydrodynamical simulation to explore the properties and origin of cold circumgalactic medium (CGM) gas around massive galaxies (M⋆ &gt; 1011 M⊙ ) at intermediate redshift ($z \sim 0.5$). We discover a significant abundance of small-scale, cold gas structure in the CGM of ‘red and dead’ elliptical systems, as traced by neutral H i and Mg ii. Halos can host tens of thousands of discrete absorbing cloudlets, with sizes of order a kpc or smaller. With a Lagrangian tracer analysis, we show that cold clouds form due to strong $\delta \rho / \bar{\rho } \gg 1$ gas density perturbations that stimulate thermal instability. These local overdensities trigger rapid cooling from the hot virialized background medium at ∼107 K to radiatively inefficient ∼104 K clouds, which act as cosmologically long-lived, ‘stimulated cooling’ seeds in a regime where the global halo does not satisfy the classic tcool/tff &lt; 10 criterion. Furthermore, these small clouds are dominated by magnetic rather than thermal pressure, with plasma β ≪ 1, suggesting that magnetic fields may play an important role. The number and total mass of cold clouds both increase with resolution, and the mgas ≃ 8 × 104 M⊙ cell mass of TNG50 enables the ∼ few hundred pc, small-scale CGM structure we observe to form. Finally, we make a preliminary comparison against observations from the COS-LRG, LRG-RDR, COS-Halos, and SDSS LRG surveys. We broadly find that our recent, high-resolution cosmological simulations produce sufficiently high covering fractions of extended, cold gas as observed to surround massive galaxies.
Monthly Notices of the Royal Astronomical Society · 2020 · 209 citations
- Physics
- Astrophysics
ABSTRACT Supermassive black hole feedback is thought to be responsible for the lack of star formation, or quiescence, in a significant fraction of galaxies. We explore how observable correlations between the specific star formation rate (sSFR), stellar mass (Mstar), and black hole mass (MBH) are sensitive to the physics of black hole feedback in a galaxy formation model. We use the IllustrisTNG simulation suite, specifically the TNG100 simulation and 10 model variations that alter the parameters of the black hole model. Focusing on central galaxies at z = 0 with Mstar &gt; 1010 M⊙, we find that the sSFR of galaxies in IllustrisTNG decreases once the energy from black hole kinetic winds at low accretion rates becomes larger than the gravitational binding energy of gas within the galaxy stellar radius. This occurs at a particular MBH threshold above which galaxies are found to sharply transition from being mostly star forming to mostly quiescent. As a result of this behaviour, the fraction of quiescent galaxies as a function of Mstar is sensitive to both the normalization of the MBH–Mstar relation and the MBH threshold for quiescence in IllustrisTNG. Finally, we compare these model results to observations of 91 central galaxies with dynamical MBH measurements with the caveat that this sample is not representative of the whole galaxy population. While IllustrisTNG reproduces the observed trend that quiescent galaxies host more massive black holes, the observations exhibit a broader scatter in MBH at a given Mstar and show a smoother decline in sSFR with MBH.
The diversity and variability of star formation histories in models of galaxy evolution
Monthly Notices of the Royal Astronomical Society · 2020 · 105 citations
- Physics
- Astrophysics
- Astronomy
ABSTRACT Understanding the variability of galaxy star formation histories (SFHs) across a range of time-scales provides insight into the underlying physical processes that regulate star formation within galaxies. We compile the SFHs of galaxies at z = 0 from an extensive set of models, ranging from cosmological hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE), zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models (Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the variability of these SFHs on different time-scales using the power spectral density (PSD) formalism. We find that the PSDs are well described by broken power laws, and variability on long time-scales (≳1 Gyr) accounts for most of the power in galaxy SFHs. Most hydrodynamical models show increased variability on shorter time-scales (≲300 Myr) with decreasing stellar mass. Quenching can induce ∼0.4−1 dex of additional power on time-scales &gt;1 Gyr. The dark matter accretion histories of galaxies have remarkably self-similar PSDs and are coherent with the in situ star formation on time-scales &gt;3 Gyr. There is considerable diversity among the different models in their (i) power due to star formation rate variability at a given time-scale, (ii) amount of correlation with adjacent time-scales (PSD slope), (iii) evolution of median PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs. The PSD framework is a useful space to study the SFHs of galaxies since model predictions vary widely. Observational constraints in this space will help constrain the relative strengths of the physical processes responsible for this variability.
Recent grants
NSF · $597k · 2003–2007
Towards a Predictive Theory of Galaxy Formation: Cosmological Gas Accretion and Galactic Outflows
NSF · $746k · 2013–2017
NSF · $425k · 2018–2022
Frequent coauthors
- 426 shared
Volker Springel
- 341 shared
Mark Vogelsberger
Massachusetts Institute of Technology
- 265 shared
Shy Genel
Columbia University
- 208 shared
Dylan Nelson
Heidelberg University
- 202 shared
Paul Torrey
- 187 shared
Annalisa Pillepich
- 174 shared
Federico Marinacci
University of Bologna
- 149 shared
Rüdiger Pakmor
Education
- 1994
Ph.D., Astronomy
Harvard University
- 1989
B.A., Physics
Stanford University
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