Scientific Collaborations



One of the ambitious theoretical astrophysics projects pursued at the BHI led by Natarajan & Narayan, Bridging Scales from the Event Horizon and beyond to cosmological scales involves the computational tracking of the inflow of gas across several orders of magnitude in galactic nuclei with GRMHD simulations. Over the past year and a half or so, we spent developing a novel multi-zone computational method that permits running GRMHD simulations spanning 8 orders of magnitude in spatial scale while ensuring two-way communication between partitioned zones. Our goal for this project is to bridge scales from pc to kpc to Mpc scale in understanding the black hole accretion problem in order to account self-consistently for the observed phenomenology like jets that are believed to originate on the smallest scales but are seen to transport energy to Mpc scales. To do this, our first step was to numerically test and demonstrate the validity of the classic Bondi-Hoyle-Lyttleton (BHL) analytic solution for gas accretion onto a compact mass.  Going beyond the analytic solution which assumes a value for gas density out to infinity as the boundary condition, with hydrodynamic simulations using the code KHARMA we have successfully shown convergence to the BHL accretion solution out to 10^7 R_g across numerical resolutions and adopted annular structure for the computation. We find this holds for a range of boundary conditions taken from cosmological simulations from the GIZMO code with and without external gravity. For the hydrodynamic case, we are the first group to explicitly demonstrate that the BHL solution holds out to the more realistic, cosmologically relevant boundary conditions. We are able to successfully map out the accretion rate from the Bondi radius to 10^7. We then did the simulation first using weak B fields and then strong B fields. Our key findings are as follows: we first demonstrated that our simulations reproduce the idealized classic Bondi solution for spherical gas accretion, and also agree with the general relativistic solution. Upon inclusion of magnetic fields, we find that once the accreting gas is magnetized, the magnetosphere of the supermassive black hole gets saturated with a strong magnetic field. This leads to the subsequent suppression of the accretion rate by over two orders of magnitude. This we find sets up continuous feedback across scales via turbulent magnetic convection. The accumulation of the field on horizon scales transforms the flow dynamics far from the supermassive black hole effectively coupling scales. Turbulent magnetic fields we found are pivotal to bridge scales by effectively coupling feeding and feedback.  This work represents a major breakthrough as it is the first time that accretion of gas feeding the black hole has been demonstrated to couple with feedback exerted by the black hole in the presence of magnetic fields. The first set of results have recently been published in ApJ Letters ( Cho et al. 2023, ApJ Letters, Vol. 959, Issue 2, L22-32)  and work on a longer, more detailed follow-up paper is underway.


Natarajan is an associate member of the NANOGrav collaboration. NANOGrav’s most recent dataset offers compelling evidence for gravitational waves with oscillations of years to decades. These waves are thought to arise from orbiting pairs of the most massive black holes throughout the Universe: billions of times more massive than the Sun, with sizes larger than the distance between the Earth and the Sun. We timed an array of ultra-precise millisecond pulsars with the world’s largest radio telescopes and used this network of cosmic clocks to measure invisible ripples in spacetime produced by merging extremely massive black holes. Our transformative experiment reveals unique insights into how galaxies have grown over cosmic time. Our peer reviewed collaboration papers from the 15 year data set can be found here.


Natarajan served as a member of the IPTA Detection Committee to develop the detection check-list for gravitational wave experiments from Pulsar Timing. The Committee was tasked with developing the scientific assessment framework to calibrate the results from the independent international teams working on probing the nanohertz gravitational wave background. The developed check-list that determined that all current PTAs have found evidence for the GWB and hence vetted for publication can be found here.


The Beyond Ultra-deep Frontier Fields and Legacy Observations (BUFFALO) is a 101 orbit + 101 parallel Cycle 25 Hubble Space Telescope (HST) Treasury program taking data from 2018 to 2020. BUFFALO will expand existing coverage of the Hubble Frontier Fields (HFF) in Wide Field Camera 3/IR F105W, F125W, and F160W and Advanced Camera for Surveys/WFC F606W and F814W around each of the six HFF clusters and flanking fields. This additional area has not been observed by HST but is already covered by deep multiwavelength data sets, including Spitzer and Chandra. As with the original HFF program, BUFFALO is designed to take advantage of gravitational lensing from massive clusters to simultaneously find high-redshift galaxies that would otherwise lie below HST detection limits and model foreground clusters to study the properties of dark matter and galaxy assembly. The expanded area will provide the first opportunity to study both cosmic variance at high redshift and galaxy assembly in the outskirts of the large HFF clusters. Five additional orbits are reserved for transient follow-up. BUFFALO data including mosaics, value-added catalogs, and cluster mass distribution models will be released via MAST on a regular basis as the observations and analysis are completed for the six individual clusters.


I am currently on NLST which was convened to assist the U.S. community in preparing for the 2020 astrophysics Decadal Survey and provide input to the Study Office and NASA HQ on the LISA mission. It consists of independent U.S. scientists with expertise in gravitational wave technologies, signal analysis, and astrophysics. I lead and participated in several white papers that were submitted for the Decadal Survey describing many open key questions in black hole physics that stand to be addressed by the LISA mission.


One of the principal architects and lead of the map-making project for the the Hubble Space Telescope Frontier Fields program, an initiative that produced images of six deep fields centered on strong lensing galaxy clusters drawn from  Abell catalog and the MACS survey, in parallel with six deep “blank fields” adjacent to these clusters. All the data and lensing mass models and maps produced by several independent groups are publicly available at Hubble Frontiers Fields Initiative website.  This project has generated some of the highest resolution maps of the dark matter distribution in cluster lenses.

Lenstool: Gravitational Lensing Modeling Software


PIs: Priyamvada Natarajan & Jean-Paul Kneib; Members: Mathilde Jauzac, Eric Jullo, Anson D’Aloisio, Johann Richard, Marceau Limousin, Jens Hjorth

Using nature’s telescopes – cluster-lenses – our goal is to study the detailed distribution of dark and luminous matter in clusters of galaxies as well as study the properties of the faint background galaxy populations that are brought into view by gravitational lensing. As part of a long term project to develop and implement modeling techniques, the publicly available package Lenstool: Gravitational Lensing Modeling Software is available here.


PIs: Priyamvada Natarajan, Christopher Reynolds & Pablo Laguna

Members: Bhaskar Agarwal,  Angelo Ricarte, Tamara Bogdanovic, John Wise, Aycin Aykutalp,
Laura Blecha, Qi Ge,  Matthew Kunz, Cole Miller, Brian Morsony, KwangHo Park, Massimo Ricotti, Gareth Roberg-Clark, Michael Marchand Swisdak III, Hsiang-Yi Karen Yang 

Multi-wavelength data now suggest that black holes play a significant role in galaxy formation from the earliest cosmic epochs. The discovery of a population of luminous quasars at z>6 powered by accreting supermassive black holes suggests that these behemoths were in place well before the universe was 10% of its current age. Therefore the seed black holes from which they form ought to have formed at z>10. Rapid mass growth of the seeds is required to produce these quasars. Simultaneously, it is also clear that during the process of assembly these black holes exert a profound impact on their galactic environments beyond the nuclear regions. The scientific goals of our TCAN project were to study massive black holes in their cosmological context with focus on understanding their formation, fueling and feedback.  We modeled their role employing a variety of techniques: analytic models derived from first principles, numerical models to delineate important physical processes and cutting-edge large simulations that combine codes tackle the diversity of scales in the problems. The intellectual challenge that we addressed arises from the complexity and multiple scales of the problem – the cascading physical scales, timescales relevant for processes as well as the energetics.