BLACK HOLE ASTROPHYSICS
In black hole physics, Natarajan has worked on detailed technical calculations of relevant astrophysical processes and their implications (small-scale physics) as well as on modeling populations of black holes, their global properties, and their impact on their surroundings. The challenge in black hole physics arises from the coupling of physical and temporal scales in modeling the formation, fueling and feedback from black holes – Natarajan’s work has focused on integrating phenomena across scales in order to provide a deeper understanding of how black holes grow and evolve.
Currently she is working on trying to understand the formation and evolution of inter-mediate black holes; the abundance and fate of wandering black holes and on computing the multi-messenger observational signatures of active and dormant black holes in the universe.
Natarajan has made many significant contributions to our understanding of the growth history of black holes. In an early influential paper, she laid the methodological groundwork for incorporating black hole growth into the larger framework of the assembly of galaxies in the standard cold dark matter model in two papers: High-redshift galaxies, their active nuclei and central black holes & First Stars, Very Massive Black Holes, and Metals. The framework and subsequent refinements she developed, have enabled new science – the demographic modeling of black hole populations as a function of cosmic epoch that once again permit direct comparison with multi-wavelength observations of black holes.
She has also been instrumental in developing a new channel for the formation of BH seeds – direct collapse black holes – from the collapse of massive, pristine, pre-galactic disks in the early universe Supermassive black hole formation during the assembly of pre-galactic disks and once again incorporating the physics of this mechanism into the larger framework of the assembly of structure in the universe, The mass function of high redshift seed black holes .
Recently, she has worked on discriminating the various seed formation mechanisms with multi-wavelength data from current and upcoming observational facilities: The observational signatures of supermassive black hole seeds and Unveiling the First Black Holes With JWST: Multi-wavelength Spectral Predictions. The detection of the source UHZ1 by JWST and Chandra, and its properties match predictions made in Natarajan et. al (2017) lending support for the mechanism of direct collapse of gas in the early Universe to make heavy seeds. Specifically, the properties of the host galaxy and that of the black hole in UHz1 uniquely suggest that black hole seeds form via multiple pathways in the Universe.
She has worked on several problems in black hole physics that explore astrophysical processes on a range of scales, like questions of the timescale on which alignment of the spins of the black hole and the disk occur The Alignment of Disk and Black Hole Spins in Active Galactic Nuclei the role of gas in effecting binary black hole mergers (Accretion during the merger of supermassive black holes Accretion during the Merger of Supermassive Black Holes) and evolution of the eccentricity of supermassive black hole binaries Eccentricity of Supermassive Black Hole Binaries Coalescing from Gas-rich Mergers. These calculations, enabled by her work are central to the emerging new field of multi-messenger astrophysics wherein electromagnetic counter-parts to SMBBH mergers are expected to play a critical role.
Notably a prediction that she made in 1999 of the existence and detectability of outflows driven by quasars in Sunyaev–Zeldovich decrements with no clusters? was confirmed with the observational discovery of an S-Z decrement generated by the outflowing gas in first decrement generated by the outflowing gas from a quasar.
She had also predicted in 1999 that gravitational instability driven fragmentation of the quasar activity driven outflowing shells of gas would lead to the formation of dwarf galaxies that would be either highly dark matter deficient or entirely lacking in dark matter in a paper titled Quasar outflows and the formation of dwarf galaxies well prior to the reported detection in 2018 of A galaxy lacking dark matter.