About

Ramesh Narayan is the Thomas Dudley Cabot Professor of the Natural Sciences at Harvard University. He is affiliated with the Harvard-Smithsonian Center for Astrophysics, located at 60 Garden Street, MS-51, Cambridge, MA. His contact information includes an office phone number (617) 496-9393, fax (617) 495-7093, and email rnarayan@cfa.harvard.edu. The available information references his curriculum vitae, recent publications, and links to the Harvard-Smithsonian Center for Astrophysics and Harvard University Astronomy Department. No additional details about his research focus, background, or key contributions are provided in the text.

Research topics

• Astrophysics
• Physics
• Astronomy
• Computational physics
• Optics
• Computer Science
• Geology
• Nuclear physics
• Quantum mechanics
• Remote sensing

Selected publications

• Bridging Scales: How Much Do Supermassive Black Holes Grow in the Suppressed Bondi Regime?

  The Astrophysical Journal Letters · 2026-02-05 · 1 citations

  articleOpen access

  Abstract The coevolution of supermassive black holes (SMBHs) and their host galaxies remains one of the central open questions in cosmology, rooted in the coupling between accretion, feedback, and the multiscale physics that links the event horizon to the circumgalactic medium. Here we bridge these scales by embedding a first-principles, GRMHD-informed prescription for black hole accretion and feedback—derived from multizone simulations that self-consistently connect inflows and outflows from the horizon to the Bondi radius—within cosmological magnetohydrodynamic zoom-in simulations of ∼10 14 M ⊙ halos. These GRMHD results predict a “suppressed Bondi” regime in which magnetic stresses and relativistic winds strongly reduce effective accretion rates in a spin-dependent manner. We find that black holes cannot grow efficiently by accretion until they exceed ∼10 7 M ⊙ , regardless of the feedback strength. Beyond this threshold, systems bifurcate: low-spin ( η ∼ 0.02) black holes continue to accrete without quenching star formation, while high-spin ( η ≳ 0.3) black holes quench effectively but become starved of further growth. Early, massive seeding partially alleviates this tension through merger-driven assembly, yet an additional cold or super-Eddington accretion mode appears essential to reproduce the observed SMBH population and the empirical black hole–galaxy scaling relations. Our results demonstrate that GRMHD-informed feedback models can account for the maintenance-mode behavior of low-luminosity active galactic nuclei like M87*, but cannot by themselves explain the full buildup of SMBH mass across cosmic time. A unified, multiregime framework is required to capture the evolving interplay between spin-dependent feedback, cold inflows, and mergers in driving coevolution.

  PublisherDOI

• Bridging Scales in Black Hole Accretion and Feedback: Subgrid Prescription from First Principles

  arXiv (Cornell University) · 2026-02-17

  preprintOpen access

  Understanding how supermassive black holes (BHs) couple to their host galaxies across a vast spatial and temporal dynamic range remains a central challenge in galaxy evolution. Using the multizone framework -- designed to capture bidirectional inflow--outflow from the event horizon to the Bondi scale -- we present a suite of long-duration GRMHD simulations spanning BH spins $|a_\ast|=0$--0.9 and Bondi radii $R_B/r_g=4\times10^2$--$2\times10^6$. From these simulations we derive spin-dependent subgrid prescriptions from first principles, applicable to hot accretion flows with low-Eddington ratios ($f_{\rm Edd}\lesssim10^{-3}$), for adoption in cosmological simulations and semi-analytic models. We provide compact analytic fits for the time-averaged accretion rate $\dot M(R_B,a_\ast)$ and feedback power $\dot E_{\rm fb}(R_B,a_\ast)$ with respect to the Bondi rate $\dot{M}_B$, which are largely insensitive to the initial gas configuration and magnetic field strength. To capture intrinsic time-variability, we also quantify the full distributions of $\dot M$ and feedback efficiency $η$, both well described by lognormal statistics, with widths that increase toward larger $R_B$. We further measure self-consistent spin evolution in the hot accretion mode, finding that the spin-up parameter varies as $s(a_\ast)\simeq -3.7\,a_\ast$, which implies a very long spindown timescale $t_s\simeq 12(10^{-3}/f_{\rm Edd})\,{\rm Gyr}$. Thus, BH spins are effectively frozen during phases of quiescent accretion. Compared to conventional small-domain GRMHD calculations, our simulations, which reach dynamical equilibrium across horizon-to-galaxy scales, yield systematically different long-term accretion, feedback, and spin properties, cautioning against direct extrapolation from small-scale GRMHD simulations when constructing galactic-scale subgrid models.

  PublisherDOI

• Bridging Scales in Black Hole Accretion and Feedback: Subgrid Prescription from First Principles

  The Astrophysical Journal Letters · 2026-05-07 · 1 citations

  articleOpen access

  Abstract Understanding how supermassive black holes (BHs) couple to their host galaxies across a vast spatial and temporal dynamic range remains a central challenge in galaxy evolution. Using the multizone framework—designed to capture a bidirectional inflow–outflow from the event horizon to the Bondi scale—we present a suite of long-duration GRMHD simulations spanning BH spins ∣ a * ∣ = 0–0.9 and Bondi radii R B / r g = 4 × 10 2 –2 × 10 6 . From these simulations we derive spin-dependent subgrid prescriptions from first principles, applicable to hot accretion flows with low Eddington ratios ( f Edd ≲ 10 −3 ), for adoption in cosmological simulations and semianalytic models. We provide compact analytic fits for the time-averaged accretion rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mi>M</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>a</mml:mi> <mml:mo>*</mml:mo> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> and feedback power <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mover accent="true"> <mml:mi>E</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mi>fb</mml:mi> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:msub> <mml:mo>,</mml:mo> <mml:msub> <mml:mi>a</mml:mi> <mml:mo>*</mml:mo> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> with respect to the Bondi rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mover accent="true"> <mml:mi>M</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mi mathvariant="normal">B</mml:mi> </mml:msub> </mml:math> , which are largely insensitive to the initial gas configuration and magnetic field strength. To capture intrinsic time variability, we also quantify the full distributions of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> and feedback efficiency η , both well described by lognormal statistics, with widths that increase toward larger R B . We further measure self-consistent spin evolution in the hot accretion mode, finding that the spin-up parameter varies as s ( a * ) ≃ −3.7 a * , which implies a very long spin-down timescale t s ≃ 12(10 −3 / f Edd ) Gyr. Thus, BH spins are effectively frozen during phases of quiescent accretion. Compared to conventional small-domain GRMHD calculations, our simulations, which reach dynamical equilibrium across horizon to galaxy scales, yield systematically different long-term accretion, feedback, and spin properties, cautioning against direct extrapolation from small-scale GRMHD simulations when constructing galactic-scale subgrid models.

  PublisherDOI

• Bridging Scales: Modeling Suppressed Bondi Accretion on Black Holes and Its Impact on Galaxy Growth

  The Astrophysical Journal · 2026-03-23 · 1 citations

  articleOpen access

  Abstract The accretion and feedback processes governing supermassive black hole (SMBH) growth span an enormous range of spatial scales, from the Event Horizon to the circumgalactic medium. Recent general relativistic magnetohydrodynamic (GRMHD) simulations demonstrate that strong magnetic fields can substantially suppress gas accretion onto black holes. These simulations show that magnetic fields create magnetically arrested disk states, reducing inflow rates by up to 2 orders of magnitude relative to classical predictions. We incorporate this magnetic suppression prescription from recent GRMHD studies into D ark S age , a semianalytic model that tracks SMBH and galaxy coevolution over cosmic time. Implementing the suppression across different accretion rate regimes, we explore its impact on the distribution of black hole masses, stellar masses in galaxies, and active galactic nucleus (AGN) luminosities. We find that restricting suppression to sub-Eddington accretors ( f Edd &lt; 3 × 10 −3 ) and rescaling AGN feedback efficiencies gives simultaneous agreement with the observed local distributions of both galaxy and black hole masses. At early cosmic times ( z &gt; 6), super-Eddington growth episodes dominate in our model, reproducing the high number densities of luminous AGN recently discovered by the James Webb Space Telescope. Our results highlight the critical sensitivity of galaxy assembly to the coupling between small-scale accretion physics and large-scale feedback regulation. Magnetic suppression of hot gas accretion can reconcile low-redshift constraints while preserving the rapid black hole growth required at early cosmic epochs, thereby providing a physically motivated bridge between horizon-scale GRMHD simulations and cosmological galaxy-formation models.

  PublisherDOI

• Slow-light Effect in the Jet-launching Region of M87

  The Astrophysical Journal · 2026-03-11

  articleOpen accessSenior author

  Abstract We explore the impact of “slow-light” radiative transfer—i.e., general relativistic radiative transfer calculations in which the simulated fluid evolves while light rays are propagating through it—in general relativistic magnetohydrodynamic models of the M87 jet. Because the plasma in the jet-launching region is accelerated to relativistic velocities, and because the jet in M87 is nearly aligned with the line of sight (offset by ∼17°), a slow-light treatment is important for accurate modeling of the observable structure. While fast-light images exhibit prominent helical or loop-shaped features in the jet—which we associate with narrow bundles of magnetic field lines—these features become stretched and smoothed-out in slow-light images. Our slow-light images instead exhibit a double-edged, cone-like morphology that is more consistent with observations of M87 than conventional fast-light images. We find that the radius at which the plasma transitions from subrelativistic to relativistic velocities is imprinted on slow-light images via a transition from loop-dominated at small distances from the black hole (BH) to edge-dominated at a larger distance, with the loop-edge transition occurring at larger distances for lower BH spins. The jet image dynamics also vary with BH spin, with low-spin models producing jets that exhibit substantial “wobbling,” while high-spin models produce jets that are straighter and more stable in time. The spin-dependent jet morphology and variability are revealed by slow-light imaging both because slow-light effects are enhanced as the plasma velocity becomes more relativistic and because the plasma acceleration is itself a strong function of the spin.

  PublisherDOI

• Bridging Scales in Black Hole Accretion and Feedback: Subgrid Prescription from First Principles

  ArXiv.org · 2026-02-17

  articleOpen access

  Understanding how supermassive black holes (BHs) couple to their host galaxies across a vast spatial and temporal dynamic range remains a central challenge in galaxy evolution. Using the multizone framework -- designed to capture bidirectional inflow--outflow from the event horizon to the Bondi scale -- we present a suite of long-duration GRMHD simulations spanning BH spins $|a_\ast|=0$--0.9 and Bondi radii $R_B/r_g=4\times10^2$--$2\times10^6$. From these simulations we derive spin-dependent subgrid prescriptions from first principles, applicable to hot accretion flows with low-Eddington ratios ($f_{\rm Edd}\lesssim10^{-3}$), for adoption in cosmological simulations and semi-analytic models. We provide compact analytic fits for the time-averaged accretion rate $\dot M(R_B,a_\ast)$ and feedback power $\dot E_{\rm fb}(R_B,a_\ast)$ with respect to the Bondi rate $\dot{M}_B$, which are largely insensitive to the initial gas configuration and magnetic field strength. To capture intrinsic time-variability, we also quantify the full distributions of $\dot M$ and feedback efficiency $η$, both well described by lognormal statistics, with widths that increase toward larger $R_B$. We further measure self-consistent spin evolution in the hot accretion mode, finding that the spin-up parameter varies as $s(a_\ast)\simeq -3.7\,a_\ast$, which implies a very long spindown timescale $t_s\simeq 12(10^{-3}/f_{\rm Edd})\,{\rm Gyr}$. Thus, BH spins are effectively frozen during phases of quiescent accretion. Compared to conventional small-domain GRMHD calculations, our simulations, which reach dynamical equilibrium across horizon-to-galaxy scales, yield systematically different long-term accretion, feedback, and spin properties, cautioning against direct extrapolation from small-scale GRMHD simulations when constructing galactic-scale subgrid models.

  PublisherOA PDF

• Density jump as a function of the field for parallel relativistic collisionless shocks

  Journal of Plasma Physics · 2025-07-21

  articleOpen accessSenior author

  Collisionless shocks are frequently analysed using the magnetohydrodynamic (MHD) formalism, even though the required collisionality hypothesis is not fulfilled. In a previous work (Bret &amp; Narayan, 2018 J. Plasma Phys. vol. 84 , p. 905840604), we presented a model of collisionless shock displaying an important departure from the expected MHD behaviour, in the case of a strong flow aligned magnetic field. This model was non-relativistic. Here, it is extended to the relativistic regime, considering zero upstream pressure and upstream Lorentz factor $\gg 1$ . The result agrees satisfactorily with Particle-in-Cell simulations and shows a similar, and important, departure from the MHD prediction. In the strong field regime, the density jump $r$ , seen in the downstream frame, behaves like $r \sim 2 + 1/\gamma _{\mathrm{up}}$ , while MHD predicts 4 ( $\gamma _{\mathrm{up}}$ is the Lorentz factor of the upstream measured in the downstream frame). Only pair plasmas are considered.

  PublisherOA PDFDOI

• Extreme Magnetic Fields Around Black Holes

  Astrophysics and space science proceedings · 2025-01-01 · 1 citations

  book-chapterOpen accessSenior author
  PublisherOA PDFDOI

• Galaxy formation with wave/fuzzy dark matter: The core-halo structure and the solitonic imprint

  Astronomy and Astrophysics · 2025-06-12 · 1 citations

  articleOpen access

  Dark matter-dominated cores have long been claimed for the well-studied local group dwarf galaxies. More recently, extended stellar halos have been uncovered around several of these dwarfs through deeper imaging and spectroscopy. Such core-halo structures (inner flat core and a characteristic r −3 asymptotic outer halo profile) are not a feature of conventional cold dark matter (CDM). In contrast, smooth and prominent dark matter cores are predicted for wave/fuzzy dark matter ( ψ DM). The question arises as to what extent the visible stellar profiles should reflect this dark matter core structure. Here we compare cosmological hydrodynamical simulations of CDM, “WDM” (model used as a proxy for ψ DM) &amp; ψ DM, aiming to predict the stellar profiles for these three DM scenarios. We show that cores surrounded by extended halos are distinguishable for ψ DM, where the stellar density is enhanced in the core due to the presence of the relatively dense soliton. Our analysis demonstrates that, in our simulations, a distinctive core-halo structure does not appear in the case of CDM in the DM, gas, or stars. Whereas we do find a core-halo transition for DM, gas, and stars for ψ DM, and the scale of this transition is in line with the predicted core radius set by the soliton scale anticipated for the adopted boson mass of 2.5×10 −22 eV. The presence of a core-halo structure in the stellar profile for Galaxy 1 for ψ DM is visible for the most massive and the first galaxy to form in the simulation. Clearly, further simulations are needed to establish how strict this possible relationship is between the DM and stellar core-halo profile as a potential observational discriminator. Furthermore, we observe the anticipated asymmetry for ψ DM due to the soliton's motion (jumping and random walk), a distinctive characteristic not found in the symmetric distributions of stars in the warm and CDM models.

  PublisherDOI

• Bridging Scales: Coupling the Galactic Nucleus to the Larger Cosmic Environment

  The Astrophysical Journal Letters · 2025-03-10 · 6 citations

  articleOpen access

  Abstract Coupling black hole (BH) feeding and feedback involves interactions across vast spatial and temporal scales that are computationally challenging to model. Tracking gas inflows and outflows from kiloparsec scales to the event horizon for non-spinning BHs in the presence of strong magnetic fields, H. Cho et al. report strong suppression of accretion on horizon scales and low (2%) feedback efficiency. In this letter, we explore the impact of these findings for the supermassive BHs M87* and Sgr A*, using high-resolution, non-cosmological, magnetohydrodynamic simulations with the FIRE-2 model. Without feedback, we find rapid BH growth due to “cooling flows,” with 2% feedback efficiency, while accretion is suppressed, the rates still remain higher than constraints from Event Horizon Telescope (EHT) data for M87* and Sgr A*. To match the EHT observations of M87*, an efficiency greater than 15% is required, suggesting the need to include enhanced feedback from BH spin. Similarly, a feedback efficiency of &gt;15% is needed for Sgr A* to match the observationally estimated star formation rate of ≲2 M ⊙ yr −1 . Even with 100% feedback efficiency, the simulation-predicted Sgr A* accretion rate remains higher than EHT-inferred levels on average, while only episodically matching it, suggesting that Sgr A* is currently in a temporary quiescent phase. Bridging accretion and feedback across scales, we conclude that higher feedback efficiencies, possibly due to nonzero BH spin, are necessary to suppress “cooling flows” and match both the observed accretion and star formation rates in M87* and Sgr A*.

  PublisherDOI

Recent grants

• Three Dimensional Radiation GRMHD Simulations of Accretion Flows Around Black Holes

  NSF · $698k · 2013–2019

• Measuring Black Hole Spin: Physics of the Inner Region of an Accretion Disk

  NSF · $131k · 2008–2012

• Studies of Accretion onto Black Holes

  NSF · $331k · 2018–2023

Frequent coauthors

• G. Desvignes

  596 shared

• Jonathan Weintroub

  Center for Astrophysics Harvard & Smithsonian

  546 shared

• Kazunori Akiyama

  516 shared

• Ue‐Li Pen

  476 shared

• Shiro Ikeda

  The University of Tokyo

  465 shared

• Sheperd S. Doeleman

  423 shared

• Lindy Blackburn

  Center for Astrophysics Harvard & Smithsonian

  412 shared

• R. P. J. Tilanus

  Dutch Research Council

  405 shared

Education

• Ph.D., Astronomy

  Harvard University

  1991

• M.S., Astronomy

  Harvard University

  1986

• B.S., Physics

  Harvard University

  1983