Post Doctoral Fellow @ The Indian Institute Of Astrophysics, Bengaluru
K. Aditya
Galactic Dynamics/Observational Astronomy
I am Aditya, I am presently a postdoctoral fellow at the Indian Institute of Astrophysics, Bengaluru.
I have completed my PhD dissertation at the Indian institute of Science Education and Research, Tirupati,
specializing in radio observation of superthin galaxies using the Giant Meterwave Radio Telescope (GMRT) and
galactic dynamics. Before that I obtained a Masters degree in Physics from Jamia Millia Islamia, New Delhi with
a specialization in quantum optics and optical spectroscopy and earned BSc with honors in Physics from University of Delhi.
What I Do
My primary focus is aimed at understanding how observed physical processes such as the star formation,
gravitational instabilities and metallicity are intertwined with galactic dynamics across morphologies and
redshifts. I study the dynamics of galaxies using HI 21 cm observation in conjunction with optical/NIR
photometry and IFU data. I work primarily using Giant Meter Wave Radio Telescope at Pune, India, to study
the distribution of neutral hydrogen in galaxies. I use Common Astronomy Software Applications (CASA) for
carrying out data analysis and study the kinematics and distribution of neutral hydrogen by fitting 3D
tilted ring models. I use Jeans equations, distribution function-based self-consistent models, and N-body
simulations to understand the role of dark matter in regulating the vertical structure, disc heating,
dynamics and stability of galaxies
Skills
Galactic dynamics
Modeling galaxies using axisymmetric and spherical Jeans equation.
Self-consistent equilibrium models of galaxies using AGAMA(Action Angle based Galaxy Modeling
Architecture).
N-body simulations using AREPO, Gadget-2, analysis using pynbody
HI observations and modeling
Radio interferometric data reduction (CASA)
Modeling the spectral line data cubes using 3D-tilted ring modeling; FAT, TiRiFiC, BBarolo
Statistical Methods
Data analysis and application of Bayesian statistics/ MCMC methods to multi-dimensional parameter
estimation problems, machine learning methods: Principal Component Analysis (PCA).
Programming
Proficient in Python, R, and associated libraries for studying galactic dynamics and data analysis
projects.
Science in Media
Different rates of star formation in spiral and irregular galaxies can help understand dark matter &
gravitational instabilities
Superthin galaxies are a class of bulgeless, low surface brightness galaxies with strikingly high
values of planar-to-vertical axes ratio ,indicating the
presence of an ultracold stellar disc. Using the multicomponent galactic
disc model of gravitationally coupled stars and gas in the force field of the dark matter halo as well
as the stellar dynamical code AGAMA (Action-based Galaxy Modelling
Architecture), we determine the
vertical velocity dispersion of
stars and gas as a function of galactocentric radius for five superthin galaxies (UGC 7321, IC 5249,
FGC 1540, IC2233, and UGC00711) using observed stellar and atomic hydrogen (H I) scale heights as
constraints, using a Markov chain Monte Carlo Method. We find that the central vertical velocity
dispersion for the stellar disc in the optical band varies between σ0s ∼ 10.2−18.4
and falls off with an exponential scale length of 2.6–3.2 Rd where Rd is the exponential stellar disc
scale length. Interestingly, in the 3.6 μm, the same, averaged over the two components of the stellar
disc, varies between 5.9 and 11.8
, both of which confirm the presence of ‘ultracold’ stellar discs in superthin galaxies.
Interestingly, the global median of the multicomponent disc dynamical stability parameter QN of our
sample superthins is found to be 5 ± 1.5, which higher than the global median value of 2.2 ± 0.6 for a
sample of spiral galaxies.
Dynamical modelling of disc vertical structure in superthin galaxy ‘UGC 7321’ in
braneworld gravity: an MCMC study
Low surface brightness (LSBs) superthins constitute classic examples of very late-type galaxies, with
their disc dynamics strongly regulated by their dark matter haloes. In this work, we consider a
gravitational
origin of dark matter
in the braneworld scenario, where the
higher dimensional Weyl stress term projected on to the three-brane acts as the source of dark matter.
In the context of the braneworld model, this dark matter is referred to as the ‘dark mass’. This model
has been successful in reproducing the rotation curves of several LSB and high surface brightness
galaxies. Therefore, it is interesting to study the prospect of this model in explaining the vertical
structure of galaxies which has not been explored in the literature so far. Using our two-component
model of gravitationally coupled stars and gas in the external force field of this dark mass, we fit
the observed scale heights of stellar and atomic hydrogen (H I) gas of superthin galaxy ‘UGC7321’
using the Markov Chain Monte Carlo approach. We find that the observed scale heights of ‘UGC7321’ can
be successfully modelled in the context of the braneworld scenario. In addition, the model predicted
rotation curve also matches the observed one. The implications on the model parameters are
discussed.
H I 21 cm observation and mass models of the extremely thin galaxy FGC 1440
We present observations and models of the kinematics and distribution of neutral hydrogen (H I) in the
superthin galaxy FGC 1440 with an optical axial ratio
a/b = 20.4. Using the Giant Meterwave Radio telescope (GMRT), we imaged
the galaxy with a spectral resolution of 1.7 km s−1 and a spatial resolution of 15
9 × 13
5. We find that FGC 1440 has an asymptotic rotational velocity of 141.8 km s−1. The structure of the
H I disc in FGC 1440 is that of a typical thin disc warped along the line of sight, but we cannot rule
out the presence of a central thick H I disc. We find that the dark matter halo in FGC 1440 could be
modelled by a pseudo-isothermal (PIS) profile with
, where Rc is the core radius of the PIS halo and Rd the exponential stellar disc scale length. We
note that in spite of the unusually large axial ratio of FGC 1440, the ratio of the rotational
velocity to stellar vertical velocity dispersion,
, which is comparable to other superthins. Interestingly, unlike previously studied superthin
galaxies which are outliers in the log10(j*) − log10(M*) relation for ordinary bulgeless disc
galaxies, FGC 1440 is found to comply with the same. The values of j for the stars, gas, and the
baryons in FGC 1440 are consistent with those of normal spiral galaxies with similar mass.
H I 21cm observations and dynamical modelling of the thinnest galaxy: FGC 2366
Superthin galaxies are bulgeless low-surface brightness galaxies with unusually high major-to-minor
axes ratio of the stellar disc, i.e. 10 < a/b < 20.
We present Giant Metrewave Radio Telescope (GMRT) H I 21cm radio-synthesis observations of FGC 2366,
the thinnest galaxy known with a/b = 21.6. Employing the 3D tilted-ring modelling using fully
automated TiRiFiC (FAT), we determine the structure and kinematics of the H I gas disc, obtaining an
asymptotic rotational velocity equal to 100 km s−1 and a total H I mass equal to 109M⊙. Using z-band
stellar photometry, we obtain a central surface brightness of 22.8 mag arcsec−2, a disc scale length
of 2.6 kpc, and a scale height of 260 pc. Next, we determine the dark matter density profile by
constructing a mass model and find that an Navarro–Frenk–White (NFW) dark matter halo best-fits the
steeply rising rotation curve. With the above mass inventory in place, we finally construct the
dynamical model of the stellar disc of FGC 2366 using the stellar dynamical code ‘AGAMA’. To identify
the key physical mechanisms responsible for the superthin vertical structure, we carry out a Principal
Component Analysis of the data corresponding to all the relevant dynamical parameters and a/b for a
sample of superthin and extremely thin galaxies studied so far. We note that the first two principal
components explain 80 per cent of the variation in the data, and the significant contribution is from
the compactness of the mass distribution, which is fundamentally responsible for the existence of
superthin stellar discs.
Stability of galaxies across morphological sequence
We investigate the stability of nearby disc galaxies and galaxies at redshift (z) equal to 4.5.We
explore the connection between the stability parameter (QRW) , star formation rate (SFR), gas fraction
(fGas), and the time-scale for growth of gravitational instabilities (τ).
We find that, despite differences in morphology 91 per cent of the nearby galaxies
have a minimum value of stability parameter (
) greater than 1 indicating stability against the growth of axisymmetric instabilities. The spirals
in our sample have higher median star formation rate, lower median QRW, a lower fGas and small time
scale for growth of gravitational instabilities than irregular galaxies. We find that the
gravitational instabilities in spirals convert a large fraction of gas into stars quickly, depleting
the gas reservoirs. On the other hand, star formation occurs more gradually over longer time-scales in
irregulars with a higher gas fraction. We then compare the stability of the nearby galaxies with
galaxies at
. We find that net stability levels in the nearby galaxies and the galaxies at
are primarily driven by the stellar disc suggesting the presence of an inherent mechanism that
self-regulates the stability. Finally, upon removing the contribution of the dark matter to the total
potential, the median QRW for the nearby galaxies and galaxies at
remains unchanged indicating that the baryons can self-regulate the stability levels, at least in a
statistical sense.
The study presents a theoretical framework for understanding the role of dark
matter on the stability of the galactic disc. We model the galaxy as a two-component
system consisting of stars and gas in equilibrium with an external dark matter halo.
We derive the equations governing the growth of perturbations
and obtain a stability criterion that connects the potential of the dark matter halo and the
gas fraction with the stability levels of the galaxy. We find that a two-component disc is more
susceptible to the growth of gravitational instabilities than individual components, particularly
as gas fractions increase. However, the external field, due to the dark matter halo, acts as a stabilizing
agent and increases the net stability levels even in the presence of a cold gas component. We apply the stability
criterion to models of the Milky Way, low surface brightness galaxies, and baryon-dominated cold rotating disc galaxies
observed in the early universe. Our results show that the potential due to the dark matter halo plays a significant role
in stabilizing nearby galaxies, such as the Milky Way, and low surface brightness galaxies, which would otherwise be prone
to local gravitational instabilities. However, we find that the baryon-dominated cold disc galaxies observed in the early
universe remain susceptible to the growth of local gravitational instabilities despite the stabilizing effect of the dark matter halo.
Challenges in modeling the dark matter halo of NGC 1052–DF2: Cored versus cuspy halo models
Aims. The discovery of NGC 1052−DF2 and subsequent modeling have shown that NGC 1052−DF2 is deficient
in dark matter and is in conflict with the standard stellar-to-halo mass ratio. In this work, we aim to
resolve the degeneracy between the dynamical models on the mass estimate of the NGC 1052−DF2.
Methods. We constructed mass models of NGC 1052−DF2 using an anisotropic distribution function with a radially varying anisotropy parameter and studied the effect of the various model parameters on the dark matter estimates. We used the observed stellar photometry as an input parameter to construct the distribution function and employed a Markov chain Monte Carlo (MCMC) method to estimate the dark matter model parameters.
Results. We find that mass models with a cuspy dark matter halo have comparable χ2 to models with zero dark matter. Moreover, the cuspy dark matter halo fails to consistently account for the observed velocity dispersion in the inner and outer regions of the galaxy. Consequently, we rule out the possibility of a cuspy dark matter halo for describing the mass models of NGC 1052−DF2. Our study shows that the cored dark matter halo model with a total mass of log(MDM/M⊙) = 10.5 explains the observed kinematics but requires an extraordinarily large scale length (20 kpc) and an outer cutoff radius (26 kpc). While the cored mass model provides a comparatively better fit, our findings emphasize that the mass models are largely unconstrained by the available kinematic data. Our results suggest that NGC 1052−DF2 may not only have an ultra-diffuse stellar distribution but that it can, within uncertainties in the available kinematic data, potentially host an ultra-diffuse dark matter distribution compatible with the standard stellar-to-halo mass relation (SHMR) predicted by galaxy formation and evolution models.
Contact
Get In Touch
Location
Indian Institute of Astrophysics
Bengaluru, 560 034
