Two roads diverged in a wood, and I took the one less traveled by, and that has made all the difference.

~Robert Frost

I am a visiting scientist at the Center for Astrophysics (CfA), Harvard & Smithsonian, MA, USA and an Adjunct Postdoctoral Fellow at the University of New South Wales (UNSW), Sydney, Australia. I am working as a part of the Astro3D Galaxy Evolution with Lenses (AGEL) collaboration. I work on gravitational lens modeling and use the modeling results to understand galaxy evolution theories and cosmology. I joined UNSW as an ASTRO3D Postdoctoral Fellow in September 2022. I completed my PhD at the Centre for Astrophysics and Supercomputing (CAS), Swinburne University of Technology (SUT), Melbourne, Australia, in 2021 which was followed by a short-term postdoctoral researcher position between September 2021 to May 2022. My PhD thesis "Morphology-dependent Black Hole Mass Scaling Relations" presents a detailed investigation of the correlation between supermassive black hole mass and various host galaxy properties obtained using state-of-the-art two-dimensional modeling and multi-component decomposition technique. My PhD research advocates that the black hole--galaxy correlations are dependent on the host galaxy morphology, where the morphology is shaped by the formation and evolutionary tracks followed by a galaxy. This work adds a major step towards understanding how the supermassive black holes co-evolve with their host galaxy.

Research Highlights!

Gravitational Lens Modeling

Model of the gravitational lens AGEL0142 obtained using GLEE. Panels from top left to right represent the observed lens configuration, reconstructed lens model, and normalized residual. Panels from bottom left to right are convergence map of the deflector’s density profile, magnification model, and the reconstructed background source in the source plane. (Ref: Sahu et al. 2024)

Galaxy Evolution with Strong Gravitational Lensing

This project aims to study the evolution of total mass profile properties of galaxies with cosmic time and test the two-phase model of galaxy formation and evolution. The gravitational lens modeling can help obtain the total (baryonic plus dark matter) mass density profile of the foreground lens (aka deflector) galaxy. This can be used to understand the dark matter profile in the lens galaxies. Further, based on the correlations of density, mass, size, and dark matter fraction with the cosmic time (redshift) for a statistical sample of a particular class of galaxies, one can investigate the mass assembly history of that type of galaxy. In Sahu et al. (2024) , I modeled a pilot sample of seven lenses from the Astro3d Galaxy Evolution with Lensing (AGEL) survey using state-of-the-art lens modeling software GLEE (Suyu and Halkola et al. 2010,2012). We updated the high redshift (z>0.5) end of the total mass density profile slope (γ) versus redshift (z) relation (AGEL data marked with blue stars). This paper compares γ—z relation from advanced simulations, dynamical observations, and all lensing observations and investigates possible reasons behind conflicts between studies and how to resolve them.

Galaxy Modeling and Multi-Component Decomposition

In order to measure various galaxy properties for my PhD research, I performed 2D modeling and multi-component decomposition of a sample of local early-type galaxies using their Spitzer space telescope images. One example is NGC 4371 which is a multi-component early-type galaxy with a bulge, barlens, bar, ansae, and disk. The top image set shows the galaxy image (3.6mu), the 2D isophotal model, and the residual. The bottom panels show the isophotal averaged surface brightness profiles along the galaxy's major-axis and equivalent-axis (geometric mean of major and minor axis), respectively. In addition, the surface brightness profile is followed by residual (data-fit), ellipticity profile, position angle profile, and fourth Fourier harmonic (cosine) coefficient profile. Galaxy components are identified by visual inspections at various contrast levels, features in the ellipticity, position angle, higher Fourier harmonic coefficient profiles, and kinematic evidence. (Ref: Sahu et. al. (2019a))

Morphology dependent Black Hole Mass versus Spheroid Mass relation

Black hole mass versus bulge mass relation is dependent on galaxy morphology. Where early-type galaxies with a disk (ES, S0), early-type galaxies without a stellar disk (E), and late-type galaxies (LTGs, S) define different relations between black hole mass and bulge (spheroid) mass. See Sahu et. al. (2019a) and Graham & Sahu (2023a,b)

More Highlights ... Click on the panels to expand

As expected from the morphology dependence in the BH mass versus spheroid mass diagram. BH mass versus spheroid size relation is also morphology dependent, where early-type galaxies with a disk (ES, S0), early-type galaxies without a stellar disk (E), and late-type galaxies (LTGs) follow different relations. See Sahu et. al. (2021) and Graham & Sahu (2023a,b)

Black hole mass also correlates with galaxy stellar mass. However, this correlation is also different for early-type galaxies with a disk (ES, S0), early-type galaxies without a disk (E) and late-type galaxies (ETGs). With this relation, the black hole mass in other galaxies can be estimated using the galaxy stellar mass without going through the complex multi-component decomposition process to obtain bulge mass. See Sahu et. al. (2019a) and Graham & Sahu (2023a,b)

Massive core-Sérsic (merger driven) and Sérsic (gas abundant accretion or wet merger driven) galaxies define two different relations between black hole mass and the central stellar velocity dispersion. The distinct relations for the two sub-samples may be linked with their evolutionary tracks. See Sahu et. al. (2019b)

Massive core-Sérsic (merger driven) and Sérsic (gas abundant accretion or wet merger driven) galaxies define two different relations between galaxy luminosity (absolute magnitude) and the central stellar velocity dispersion. This plot is based on the V-band data from Lauer et al.(2007). We found the same break when using our ETG sample with 3.6 mu imaging data. See Sahu et. al. (2019b)

There are many interesting correlations between black hole mass and the spheroid density at various radii in addition to the BH mass versus density at/within half-light radius being morphology-dependent just as the Mbh--Msph and Mbh--Re diagrams. We found a correlation between BH mass and spheroid density at black hole's sphere of influence (soi). This relationship is particularly interesting because of its interesting applications e.g., predicting gravitational waves, tidal disruption events etc. There is more to it, see Sahu et al. (2022).

The latest morphology-dependent black hole scaling relations and morphology-aware galaxy mass function can be used to improve the strain model for gravitation wave background specifically in the low frequency (micro-nano Hertz) regime. This model will then be used to improve the constraints (amplitude, event rate) for the detection of long-wavelength gravitational waves by pulsar timing arrays and laser interferometer space antenna (LISA). More details can be found in my paper Sahu et al. (2022, ApJ, 927 (1), 67). If you are interested in working on this project, get in touch.

Image credits: Moore et al. (2015), Liu & Eatough (2017), and LIGO

Current Research

Image Credit: Nandini Sahu

Cosmography with Double Source Plane Lenses

I am currently working on cosmography using independent measurement of distance ratios obtained by lens modeling of a double source plane lens (DSPL). For lens modeling, I am using python-based software LENSTRONOMY. This case study shows that 1) multi-source source plane lenses can be used to constrain cosmological parameters independently, and 2) they complement other independent probes, such as cosmic microwave background (CMB), by providing orthogonal constraints. (Ref: Sahu et al. 2025, in preparation)

A Brief Summary My PhD Thesis

Black holes are one of the most mysterious objects in our Universe. Observations suggest that they exist in a continuum of mass starting from stellar-mass black holes to super-massive black holes (SMBH); though, undisputed detections of majority of intermediate-mass black holes is yet to happen. A galaxy may have millions of stellar-mass black holes but only one SMBH at its centre with mass in between about a million to billion times the solar mass. For decades astronomers are trying to study how the central black hole may govern various properties of the host galaxy and vice-versa. Our work adds another step to this study.

We have performed careful, multi-component, photometric-decompositions of the largest-to-date sample of galaxies with dynamically measured (central) SMBH masses. These decompositions enabled us to measure the bulge masses and reliably identify the galaxy morphologies. We explored the black hole mass scaling relations for various sub-morphological classes of the galaxies, including galaxies with and without a rotating stellar disk, early-type (E, ES, S0) versus late-type galaxies (all spirals), barred versus non-barred galaxies, and Sérsic versus core-Sérsic galaxies. Consequently, we have discovered significantly modified correlations of black hole mass with galaxy properties, i.e., the spheroid/bulge stellar mass, the total galaxy stellar mass, the central stellar velocity dispersion, central luminosity/mass concentration (Sérsic index), effective half-light radius, and the internal/spatial stellar mass density.

The final scaling relations are dependent on galaxy morphology, which is fundamentally linked with the formation and evolutionary paths of galaxies. These modified scaling relations more accurately predict the black hole masses in other galaxies, pose ramifications for the virial-mass f-factor, and offer insights into simulations and theories of black hole-galaxy co-evolution. Additionally, these scaling relations will improve the predictions for the ground-based and space-based detection of long-wavelength gravitational waves by the pulsar timing arrays and the upcoming Laser Interferometer Space Antenna (LISA), respectively.

Publications

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Curriculum Vitae available on request

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