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 an ASTRO3D Postdoctoral Fellow at the University of New South Wales, Sydney, Australia. I am working as a part of the Astro3D Galaxy Evolution with Lenses (AGEL) collaboration. I am currently modeling high redshift gravitational lenses and using lens modelling results to understand the galaxy evolution models. Before joining UNSW, I completed my PhD at the Centre for Astrophysics and Supercomputing (CAS), Swinburne University of Technology (SUT), Melbourne, Australia, in 2021. This was followed by a short-term postdoctoral researcher position between September 2021 to May 2022. My PhD thesis titled "Morphology-dependent Black Hole Mass Scaling Relations" presents the investigation of correlation between black hole mass and various host galaxy properties obtained using state-of-the-art two-dimensional modelling and multi-component decomposition technique. These results are published in a series of four papers in the Astrophysical Journal. My work 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. Thus, this work adds another major steps towards understanding the how the supermassive black holes co-evolve with the host galaxy, and provides ramifications of black hole-galaxy coevolution theories.

Brief Summary

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.

Research Highlights!

Galaxy Modeling and Multi-Component Decomposition

We performed a 2-dimensional (2D) isophotal analysis, first extracting a 2D luminosity model using Isofit and Cmodel (Ciambur 2015) to capture the radial gradients in the ellipticity, position angle, and Fourier harmonic coefficients describing the isophote's deviations from a pure ellipse, and then performing a multi-component decomposition using the isophotal-averaged 1D surface brightness profile along the major and geometric-mean axis of the galaxies. For this purpose, we used the software Profiler (Ciambur 2016), which is inbuilt with many functions for specific galaxy components (see Section 3 in Sahu et al. 2019a, for more details).

NGC 7457

NGC 7457 is an early type galaxy with a bulge and a disk. Image set shows, galaxy (3.6mu) image, 2D isophotal model, and residual obtained by subtracting the model from the galaxy image. The galaxy model is generated using in-house softwares Isofit and Cmodel.

More Highlights ... Click on the panels to expand

NGC 4371 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), 2D isophotal model, and the residual. Bottom panels show the isophotal averaged surface brightness profiles along the galaxy 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, 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 evidences (when available in literature).

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) define different relations between black hole mass and bulge (spheroid) mass. See Sahu et. al. (2019a) and Graham & Sahu (2023a,b)

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)

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).

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)

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

Gravitational Lensing

I am using Hubble Space Telescope observations of strong gravitational lenses to understand more about the evolutionary tracks followed by galaxies. The gravitational lens modelling can help obtain the total (baryonic plus dark matter) mass density profile of the foreground lens. This can further be used to undertstand the dark matter fraction profile for the lens galaxies. Based upon the correlations of mass density slope, mass, size, dark matter fraction with cosmic time (redshift) for an ensemble of lenses one can understand the mass assembly history of the lens galaxy. Sahu et al. (2023, in preparation)

Publications

Talks/Presentations/Posters

Curriculum Vitae available on request

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