2021

Abstract: Magnetars are slowly rotating, young, and isolated neutron stars exhibiting highly energetic behaviors, as in the case of soft-gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs). They are believed to be powered by ultrastrong magnetic fields, B ~ 10^14 - 10^15 G, which exceed the quantum critical field for electrons. Recently, they have been studied with paramount interest by almost every modern X-ray telescope. Despite the success, the traditional picture of magnetars has been challenged by the discovery of a low-field (~ 6.1 × 10^12 G) magnetar, SGR 0418+5729. It remains mysterious over the decades to interpret the evolutionary stage (or age) of such a puzzling source within the magnetar paradigm. We provide a novel approach to estimate a magnetar's age by considering the self-consistent time evolution of a plasma-filled oblique pulsar with the state-of-the-art magnetospheric particle acceleration gaps and magnetic field decay mechanisms. The rotational period of magnetars increases over time due to the extraction of angular momentum by gravitational-wave radiations, magnetic dipole radiations, and particle winds. These torques also change the obliquity angle between the magnetic and rotation axes. In the peculiar case of SGR 0418+5729, we find a dipolar magnetic field of ~ 1.0 × 10^14 G and a realistic age of ~ 18 kyr; both are consistent within the magnetar paradigm.

Abstract: The field of cosmology has made significant progress in both the theoretical and observational areas of research. However, questions about the actual physical observables in cosmology, and the magnitude of the different corrections therein, are still open for analysis and discussion. My work aims to explore and emphasise on the significant relativistic contributions to the observable galaxy number counts. We start by mathematically proving the frame-invariance of galaxy number counts in Einstein and Jordan frames, and then move on to computing the full-sky correlation function and power spectrum including all relativistic effects. We eventually narrow down our interest to specific effects, namely, redshift space distortions and weak gravitational lensing. We study the modelling of the angular power spectra in the redshift space, accounting for the intermediately nonlinear regime. Then we establish the importance of including lensing corrections to the main signal of probes such as galaxy-galaxy lensing, and for measurements in case of tests of gravity like E_g statistics. Finally, we also discuss the shape and size correlations, and gravitational flexions of galaxies occurring as a part of weak lensing data, arising out of the phenomenon of intrinsic alignment. During my talk, I will briefly introduce all these aspects, and provide more details about nonlinear contributions to angular power spectra in the redshift space, and the significance of the weak lensing corrections in density-shear correlations and E_g statistics.

Abstract: We attempt to address the old problem of accretion physics: the origin of turbulence and hence transport of angular momentum in accretion discs. We undertake the problem by introducing an extra force in the flow, i.e., in the governing Navier-Stokes equation describing the local accretion flow in the Keplerian accretion disc.  The extra force, which is expected to be stochastic in nature hence behaving as noise, could naturally result from grain-fluid interactions, feedback from outflows in astrophysical discs, etc. In our exploration, first we assume the forcing does not change the background flow due to its smallness. The local flow without the force is stable under linear perturbations. We attempt to check the stability of the local flow with the said extra force.  To do so, we establish the evolution of nonlinear perturbation, which is the modified Landau equation. We show that even in the linear regime, under suitable forcing and Reynolds number, the otherwise least stable perturbation evolves to a very large amplitude, leading to nonlinearity and plausible turbulence. Hence, forcing essentially leads a linear stable mode to unstable. We further show that nonlinear perturbation diverges at a shorter timescale in the presence of force, leading to a fast transition to turbulence. We subsequently explore the effect of forcing on the same background flow. We show that depending on the strength of forcing and boundary conditions suitable for the systems under consideration, the background plane shear flow and, hence, local accretion disc velocity profile modifies to parabolic flow, which is plane Poiseuille flow or Couette-Poiseuille flow, depending on the frame of reference. In the presence of rotation, both the flows become unstable at a smaller Reynolds number, compared to their nonrotating counterparts, under pure vertical as well as three-dimensional perturbations. Interestingly, while the rotational effect stabilizes pure plane share flow, it destabilizes parabolic shear flow. Hence, the local accretion disc in the presence of an extra force becomes unstable even for the Keplerian rotation, and hence plausibly, turbulence will pop in there. This dynamics is expected to be repeated throughout the disc, explaining the transport of angular momentum and matter along outward and inward directions, respectively. 

Abstract: The Tibet ASgamma experiment is located at 4,300m above sea level, in Tibet, China. The experiment is composed of a 65,700 m2 surface air shower array and 3,400 m2 underground water Cherenkov muon detectors. The surface air shower array is used for reconstructing the primary particle energy and direction, while the underground muon detectors are used for discriminating gamma-ray induced muon-poor air showers from cosmic-ray (proton, helium,...) induced muon-rich air showers. Recently, the Tibet ASgamma experiment successfully observed gamma rays in the 100 TeV region from some point/extended sources as well as sub-PeV diffuse gamma rays along the Galactic disk. In this talk, The observational results as well as their interpretations will be presented, followed by some future prospect.

Abstract: Gas, dust and stars make a galaxy.  In the disk-shaped spiral galaxies, gas constantly feeds to star formation. Massive stars die and stir the gas, which results in turbulent motions. Later, along with gravitational instabilities, creates fragments of isolated gas clouds, some of which overcome the pressure from thermal motion and turbulence and collapse to form a new generation of stars. We observe 21-cm spectral line emission from the neutral hydrogen gas in galaxies to probe the statistical characteristics of the structures in the column density and velocity in the interstellar medium (ISM).  Our investigations show that the energy input in the interstellar medium originates at scales comparable to the size of the galaxy and cascades down to smaller scales, where it thereby influences the star formation. The methodology used to  access the statistics of the column density and velocity structure, interpretation of the measurements and their importance in ISM physics will be discussed.

Abstract: One of the main drivers of galaxy evolution is interaction/merger between galaxies. Theoretically, such interactions are expected in all galaxy mass ranges. Their effect on massive galaxies in the local universe has been studied extensively and shown that they create tidal features, trigger star formation activity and affect the structure and dynamics of galaxies. However, similar studies of low mass galaxies are rare and hence, are essential to understand their assembly process. In this context, I will present some of the recent results from our studies of interacting low mass galaxies in the local universe. In the first part I will discuss the structural and kinematic studies of the Magellanic Clouds, which are the nearest interacting system of low mass galaxies. In the second part, I will discuss the results from our study on the star formation properties of a sample of nearby interacting low mass galaxies.

Abstract: In the standard cosmology, it is believed that there are two weak and distinct band-limited absorption features, near 20 MHz (z~70) and 90 MHz (z~15) in the global cosmological 21 cm signal which are signatures of collisional gas dynamics in the cosmic dark ages and Lyman-alpha photons from the first stars at cosmic dawn, respectively. A similar prediction of two distinct band-limited, but stronger, absorption features is expected in models with excess gas cooling, which have been invoked to explain the anomalous EDGES signal. In this work, we explore a novel mechanism, where dark matter spin-flip interactions with electrons through a light axial-vector mediator could directly induce a 21 cm signal which is characteristically different from either of these. We find generically, that our model predicts a strong, broadband absorption signal extending from frequencies as low as 1.4 MHz (z~1000), from early in the cosmic dark ages where no conventional signal is expected, all the way up to 90 MHz, depending upon the epoch of star formation and X-ray heating. We will discuss a rich set of spectral features that could be probed in current and future experiments looking for the global 21 cm signal as well as some complementary laboratory tests of short range spin-dependent forces between electrons.

Abstract: The long-standing 'missing satellite' problem is one of the few hard challenges that pose significant difficulties in understanding galaxy formation and evolution under Lambda-CDM cosmology. The number of low mass galaxies around large galaxies, predicted by Lambda-CDM, is far more in numbers than observed. Despite several theoretical and observational efforts, the discrepancy persists. Several recent studies indicate that a population of the satellite galaxies is camouflaged as the Compact/Ultra-Compact High-Velocity Clouds (CHVC/UCHVC) around the Galaxy. We developed a formalism based on hydrostatic equilibrium to identify these satellite galaxies from the pool of CHVC/UCHVCs. In this talk, I will describe the formalism and apply it to a recently discovered satellite galaxy Leo-T, which was thought to be a UCHVC.

Abstract: Massive stars end their lives when their core collapses under the influence of gravity. In some cases, the collapse results in a bright and spectacular event where they form a core-collapse supernova (CCSN). In some other cases, the star fails to explode and eventually forms a black hole (BH). Despite many efforts, we have yet to answer the question of which massive stars will end their lives as a CCSNe and which ones will collapse into a BH. Here, we investigate the impact of the equation of state (EOS) of the dense nuclear matter on the outcome of core-collapse and subsequent nucleosynthesis. We model the simulation using the parametrized spherically symmetric explosion method PUSH which includes general-relativistic hydrodynamics and neutrino transport. We use different supernova EOS and study the variation in explosion properties and nucleosynthesis yields for stars with different metallicity and ZAMS mass. We will discuss how the nuclear EOS influences the outcome of the core collapse, and how it impacts the explosion properties and nucleosynthesis yields. We will also discuss the remnant mass distribution from the different nuclear EOS.

Abstract: Magnetic field (B-field) is one of the important constituents of the interstellar medium. By characterizing with direction and strength, B-field couples with gas and dust grains through ions and governs the formation of molecular clouds into filamentary morphologies and eventually regulates the formation of baby stars in the dense cores. Multi-wavelength polarization technique is a promising tool to probe the plane-of-the-sky component of B-field across various orders of magnitude in gas densities and spatial scales. The B-field, turbulence, gravity, and stellar feedback interact with each other and dictate the structures of the clouds and star formation efficiency. However, the interplay among these parameters remain poorly constrained. This is because of the fact that in comparison to other key agents it is hard to probe the B-field and hence to measure the field strength. Thanks to the recently available wide-field optical and near-infrared polarimeters as well as sensitive far-infrared and sub-millimeter polarimeters, through which it is now possible to overcome this problem. In a recent study, we have found that despite of having ordered B-field morphology (based on optical and near-infrared polarimery) and quiescent physical conditions at low-density, large-scale of B213/Taurus region, we evidenced a complex B-field morphology at ~0.01 to ~0.1 pc scales of B213 cores (Eswaraiah et al. 2021, ApJL, 912, 27). This study was conducted based on the observations acquired from JCMT SCUBA2-POL2 as a part of the BISTRO project. These results imply that the B-field may become complex in the dense cores due to gas inflows into the filament, even in the presence of a substantial magnetic flux. In another study, we have witnessed a compressed B-field draped around the dense, massive clumps of Sh 201 (Eswaraiah et al. 2020, ApJ, 897, 90). This we attribute to the feedback effect of the H II region on the surface of the massive clumps. We hypothesize that the interplay of the thermal pressure imparted by the H II region, the B-field morphology, and the various internal pressures of the clumps (such as magnetic, turbulent, and gas thermal pressures) has various implications on guiding the expanding ionization fronts to form bipolar bubbles, and shielding and stabilizing the clumps against HII region feedback. Finally, I will briefly discuss the results from other ongoing works and shed some light on future directions.

Abstract: Over the previous two decades, general relativistic magneto-hydrodynamic (GRMHD) simulations have contributed immensely towards understanding the evolution of black hole accretion disks and relativistic jet launching. Advanced numerical algorithms that fully utilise the boom in computational resources over recent years have played a vital role in enabling simulations to resolve disk turbulence and jet dynamics. I will be presenting my work on two such important advancements: the warping of jets by a spinning black hole, and the formation of small magnetised blobs, or plasmoids, crucial for understanding the jet morphology of M87 as well as the flaring state of our own supermassive black hole, Sagittarius A* or Sgr A*, both of which are Event Horizon Telescope (EHT) targets. Using our group’s in-house developed state of the art GPU-accelerated GRMHD code H-AMR, I will show that the misalignment of black hole spin and disk rotational axes, which is naturally expected, substantially affects the jet’s morphology and, thus, needs to be accounted for when interpreting the horizon-scale images of M87*. Next, with one of the highest resolution GRMHD simulations ever produced, I will show how capturing the small scale physics of plasmoids could potentially revolutionise our understanding of nIR and X-ray variability in Sgr A*. Further, we are able to resolve jet boundary instabilities with such simulations that could provide an explanation as to the origin of the spine-sheath configuration of jets, and hold important implications for multi-wavelength observations of AGN jets.

Abstract: The discovery of astrophysical gravitational waves has opened a new avenue to explore the cosmos using transients. I will discuss a few new frontiers in the field of physical cosmology and fundamental physics that can be explored using gravitational wave signal detectable from the currently ongoing network of gravitational wave detectors such as LIGO/Virgo, and in the future from gravitational wave detectors such as KAGRA, LIGO-India, LISA, Einstein Telescope, and Cosmic Explorer. I will elucidate the existence of synergies between electromagnetic probes and gravitational wave probes and their importance in understanding the standard model of cosmology and the fundamental laws of physics that govern it.

Abstract: Observations of Faraday rotation measure (RM) of polarized radio sources located either inside or behind galaxy clusters suggest that the intracluster medium (ICM) is magnetized. The observed fields are of micro Gauss strength and correlated on several kilo-parsec scales. ‘Fluctuation’ dynamos appear to be ideally suited for amplifying dynamically insignificant seed magnetic fields to observable strengths. While Faraday RM provides information about the line-of-sight (LOS) component of the field, synchrotron emission and its polarization are the other two complimentary observables that furnish information about the magnetic field in the plane of sky. Aided by numerical simulations of fluctuation dynamos, I will discuss certain key results on the properties of polarized synchrotron emission and the role that Faraday rotation plays in inferring the polarized structures in the ICM. In particular, some of the prime issues that I intend to address concerns the Faraday depth (FD), how can one relate the power spectrum of FD to that of the magnetic field, the statistical nature of the total and polarized synchrotron emission and how these are affected by frequency dependent Faraday depolarization and the effects of different turbulent driving scales. The results from our study underlines the need for high frequency observations (≥ 5 GHz) to effectively probe the properties of polarized emission in the ICM.

Abstract: The epoch of Cosmic Dawn, when the first stars and galaxies were born, is widely considered the final frontier of observational cosmology today.  The technique of intensity mapping (IM) has emerged as the powerful tool to explore this phase of the Universe  by measuring the integrated emission from sources over a broad range of frequencies. A particular advantage of IM is that it provides a tomographic, or three-dimensional picture of the Universe, unlocking several thousand times more independent modes of information than one can obtain from conventional probes.  In addition to hydrogen (the most abundant element), there are exciting prospects for using intensity mapping in the submillimetre wavelengths, from the carbon monoxide (CO), ionized carbon and oxygen ([CII] and [OIII]) lines, as tracers of large-scale structure. I will illustrate how the description of dark matter haloes can be extended to describe the abundances and clustering of  molecular and ionic species in the early universe. This innovative approach allows us to fully utilize the latest available observations to constrain cosmological parameters from future observations. Combined with the information content of multi-messenger probes, this will also elucidate the properties of the first supermassive black holes at Cosmic Dawn. I will present a host of fascinating implications for constraining physics beyond the LCDM model, including tests of the theories of inflation,  the nature of dark matter and dark energy.

Abstract: General relativistic simulations of astrophysical phenomena is a computationally challenging task as the relevant equations are often nonlinear partial differential equations that couple vastly different spatial and temporal-scales. Our algorithms for tackling these problems have essentially remained unchanged for the past several decades. In this talk, I will discuss the current state of art in numerical relativity, and introduce a novel relativistic astrophysics code, SpECTRE, that combines the quasi-locality of discontinuous Galerkin (DG) methods with a task-based model for parallelizing computation. The robustness of the DG method allows for the use of high-resolution shock capturing methods in regions where (relativistic) shocks are found, while exploiting high-order accuracy in smooth regions. A task-based parallelism model allows efficient use of the largest supercomputers for problems with a heterogeneous workload over disparate scales. SpECTRE's goal is to achieve more accurate solutions for challenging relativistic astrophysics problems such as compact binary mergers and core-collapse supernovae.

Abstract: We have generated many advective accretion disk structures around the black holes with changing nature of the inflow gases at the outer accretion boundary. These studies have been carried out in the relativistic/semi-relativistic hydrodynamics regime by both time- independent and time-dependent numerical studies. Interestingly, we found that some boundary conditions can produce shocks with jet-like features in the inner part of the disk. However, other boundary conditions are not allowed shocks in the disks, which can have winds or no winds. Based on those disk structures and the nature of accretion solutions, we have predicted many physical processes in the accretion flow and their applications in the black hole X-ray binaries (BXBs), active galactic nuclei (AGNs), and tidal disruptive events (TDEs). So, I shall discuss those results in my talk with their possible future studies.

Abstract: The Sunyaev-Zeldovich (SZ) effect is an independent and powerful cosmological probe. Current inferences are however limited by a number of astrophysical uncertainties. The measurement of SZ effect in Planck data has yielded a value of $\sigma_8$ that is lower and in mild tension (~2 \sigma) with the value inferred from measurement of the primary CMB anisotropies. In the first part of the talk I will discuss the role of relativistic correction in the SZ effect, how this effects the analysis and finally argue how these mitigate part of the $\sigma_8$ tension. In the second part of my talk I will introduce a new modeling framework which involves masking most of the high SNR clusters in the Planck data. I will argue that so doing, counter intuitively,  increases the constraining power of the SZ measurements. Applying this novel analysis method to Planck data we deduce three interesting results: (i) $\sigma_8$  values derived from the revised analysis are consistent with primary CMB anisotropies (ii) bias in the value of $\sigma_8$ is driven by the CIB-tSZ correlation (iii) we see first signs of detection of the two halo correlation in the Planck y-map. I will conclude by discussing some of the ongoing work and future directions.

Abstract: Massive stars are primary sources of chemical yields and mechanical & ionizing luminosities. Therefore, understanding massive stars' evolution is crucial for comprehensive studies of the chemical and ionization evolution of galaxies. In this talk, I will focus on the chemical evolution part where I will discuss the origin of nitrogen at low metallicities because most observational calibrations of metallicities (O/H) implicitly depend upon the intrinsic N/O ratio especially at low metallicities, and therefore, understanding the origin and evolution of the N/O ratio in the interstellar medium (ISM) of galaxies is essential if we are to complete our picture of the chemical evolution of galaxies at high redshift. However, the observed N/O ratio (log(N/O)∼ −1.5) is nearly independent of O/H, albeit with a very large scatter (~1.5 dex) at low metallicities (12+log(O/H) <= 7.5). This plateau and scatter in N/O at low metallicities puzzled astronomers for decades and the origin of them had remained unexplained to date. In this talk, I will describe how these several heretofore unexplained features of the N/O distribution at low O/H can be explained by the N seen in metal-poor galaxies being mostly primary nitrogen that is returned to the ISM via pre-supernova winds from rapidly rotating massive stars (M >= 10 MSun, v/vcrit >= 0.4). This mechanism naturally produces the observed N/O plateau at low O/H. I will further explain the origin of large scatter in N/O at low O/H that it arises naturally from variations in star-formation efficiency. By contrast, models in which the N and O come primarily from supernovae provide a very poor fit to the observed abundance distribution. These peculiar abundance patterns observed at low O/H are a signature that dwarf galaxies retain little of their SN ejecta, leaving them with abundance patterns typical of winds. I will present the structure and origin of WNL stars, and their implications on the ionizing luminosity budgets in my talk at RRI on 23rd March 2021 at 3:15 pm IST.

Abstract: The inflationary paradigm of the early Universe has been extraordinarily consistent with the observations of the Cosmic Microwave Background (CMB) radiation, however, the microphysics governing it is not well understood and tested. In the standard cold inflation, the inflaton couplings to the other fields are neglected, and thus, due to a nearly exponential expansion, the Universe attains a supercooled state during the inflationary phase. On the other hand, there is another well-motivated description, known as warm inflation, where due to the dissipative effects in a coupled inflaton-radiation system, the Universe has a non-zero temperature during the inflationary phase.

In this talk, I will focus on the warm inflation scenario and investigate its imprints on the large and the small scale observations. I will discuss the primordial power spectrum generated during warm inflation models and estimate the microphysical parameter space consistent with the CMB observations. Further, I will also discuss the role of inflaton dissipation in the formation of primordial black holes (PBH) in warm inflation. Furthermore, I will also talk about the bounds on the abundance of the generated PBHs and the possibility of PBH remnants to constitute the dark matter.

Abstract: Clusters of galaxies are the Universes' most massive gravitational potential wells that hold large reservoirs of baryons in the form of diffuse gas called the intra-cluster medium (ICM). The ICM is mainly thermal gas of temperature up to ten million Kelvin that is weakly magnetised with field strengths of 0.1 - a few micro Gauss. The cosmic rays and magnetic fields, referred to as the non-thermal components, elude detection in most spectral bands and thus have remained the least understood components of the ICM. The relativistic electrons in the ICM manifest the non-thermal components at low radio frequencies (<= GHz) via synchrotron radiation providing a direct probe of their life cycles. From the radio surveys with the GMRT in the past few years, it has emerged that cluster mass and re-acceleration of seed relativistic electrons by shocks and turbulence are the factors important in the generation of the megaparsec-scale diffuse radio sources termed as radio halos, mini-halos and relics. Cluster mergers are responsible for driving shocks and turbulence in the ICM and the hadronic collisions and radio galaxies are the likely sources of the seed relativistic electrons. With sensitive radio observations now enabled by the SKA precursors and pathfinders such as the Upgraded Giant Metrewave Radio Telescope and LOFAR, a large sample of clusters with presence of such diffuse sources has been uncovered. I will describe our work on studying the highest redshift radio halo cluster El Gordo and towards the supercluster Saraswati with the uGMRT.

Abstract: When the fuel of a very massive star is spent, it collapses due to its own gravitational pull and eventually becomes a very small region of arbitrarily high matter density, that is a 'Singularity', where the usual laws of physics may breakdown. If this singularity is hidden within an event horizon, which is an invisible closed surface from which nothing, not even light, can escape, then we call this object a black hole (BH). But if the event horizon does not form, we are left with the tantalizing option of observing a naked singularity (NS). An important question then is, how to observationally distinguish a NS from a BH. I shall show that the precession frequency of a gyroscope orbiting a BH or a NS is sensitive to the presence of an event horizon. A gyroscope circling and approaching a BH from any direction behaves increasingly 'wildly,' that is, it precesses increasingly faster, without a bound. But, in the case of a NS, the precession frequency changes by a small amount, in a regular well-behaved manner. I shall also show that the precession of orbits of matter falling into a rotating BH or a NS can be used to distinguish these exotic objects. This finding could be used to distinguish a NS from a BH in reality, because the precession frequencies could be measured in X-ray wavelengths, as the infalling matter radiates X-rays. In fact, we have recently reported the first observational indication of gravitomagnetic monopole (gravitational analogue of Dirac's magnetic monopole) using the X-ray data from a star, collapsed into the most extreme object in the universe, a Singularity.

Abstract: In the Standard Model of cosmology (known as ΛCDM model), the dark matter (DM) is assumed to be a cold, collisionless and ideal fluid, which explains the large-scale structure data very well. However, this model fails to explain certain observations on the small scales (such as core-cusp, missing satellites), anomalous 21-cm signal, interpretation of the cosmological constant, etc. These provide a hint for physics beyond the standard model of cosmology.

In this talk, I will focus on the viscous dark matter and argue that it may solve some of the problems faced by the ΛCDM model. Viscous dark matter has rich physics and interesting consequences in contrast to the perfect DM fluid. In our study, we consider the viscous Self-Interacting Dark Matter (SIDM) and argue that its viscosity is sufficient to account for the present accelerated expansion of the Universe and can explain the low redshift observational data. Further, I also investigate the SIDM microphysics using the Kovtun-Son-Starinets (KSS) bound. Furthermore, I also show that the viscous DM fluid dissipates energy and can generate photons in the low-frequency tail of Cosmic Microwave Background (CMB) radiation which may explain the anomaly in the 21-cm signal reported by EDGES collaboration.

Abstract: Constructing dynamical models for interacting galaxies constrained by their observed structure and kinematics crucially depends on the correct choice of the values of their relative inclination (i) and viewing angle (θ) (the angle between the line of sight and the normal to the plane of their orbital motion). We construct Deep Convolutional Neural Network (DCNN) models to determine the i and θ of interacting galaxy pairs, using N-body + Smoothed Particle Hydrodynamics (SPH) simulation data from the GALMER database for training. GalMer simulates only a discrete set of i values (0◦, 45◦, 75◦ and 90◦) and almost all possible values of θ values in the range, [−90◦,90◦]. Therefore, we have used classification for i parameter and regression for θ. In order to classify galaxy pairs based on their i values only, we first construct DCNN models for (i) 2-class (i = 0 ◦, 45◦) (ii) 3-class (i = 0◦, 45◦, 90◦) classification, obtaining F1 scores of 99% and 98% respectively. Further, for a classification based on both i and θ values, we develop a DCNN model for a 9-class classification using different possible combinations of i and θ, and the F1 score was 97%. To estimate θ alone, we have used regression, and obtained a mean squared error value of 0.12. Finally, we also tested our DCNN model on real data from Sloan Digital Sky Survey. Our DCNN models could be extended to determine additional dynamical parameters, currently determined by trial and error method.

Abstract: There are many mechanisms by which primordial magnetic fields can be generated in the early universe. Though small, these fields can be subsequently amplified and sustained by a dynamo mechanism in the early universe. In this talk, I would like to present two new methods in which primordial magnetic fields can be generated in the early universe from topological defects. Topological defects are relics of symmetry breaking phase transitions in the early universe. Recently, we have shown that collapsing Z(3) domain walls and the wakes behind a moving Abelian Higgs cosmic string will result in the generation of a primordial magnetic field in the early universe.

Abstract: Collisional ring galaxies are a relatively small fraction of all galaxies that have undergone a recent interaction and are believed to be formed by head-on collisions of two galaxies. In these galaxies, an intruder galaxy passes through the centre or close the centre of a rotating disc of a larger galaxy, creating an outwardly propagating density wave in the larger galaxy and thus triggers star formation in a circular ring, which eventually stops once the wave moves forward and leaves behind an ageing stellar population in its wake. The Cartwheel galaxy is a famous example of this class. Large scale cosmological simulations can produce collisional ring galaxies whose star formation properties are found to be consistent with observations. However, these studies often ignore the central region or the host of collisional ring galaxies. In this talk, I report the discovery of a bar, a pseudo-bulge, and an unresolved point source in the archetype collisional ring galaxy Cartwheel.  The newly discovered bar is not recognizable as such in the higher spatial resolution of the Hubble Space Telescope images.  I will discuss the importance of our findings in the context of bar survival in a drop-through collision.

Abstract: Astrophotonics is the application of versatile photonic technologies to channel, manipulate, and disperse guided light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. The photonic platform of guided light in fibers and waveguides has opened the doors to next-generation instrumentation for both ground- and space-based telescopes in optical and near/mid-IR bands, particularly for the large and extremely large telescopes (ELTs). Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturize the next generation of spectrometers for large telescopes and space-based telescopes. In this talk, I will discuss some of the recent results from our efforts to design and fabricate high-throughput on-chip spectrometers based on Arrayed Waveguide Gratings (AWG). These devices are ideally suited for capturing the AO-corrected light and enabling new and exciting science such as large-scale near-IR galaxy surveys to map the cosmic filaments or characterizing exoplanet atmospheres. I will also discuss specific approaches to make this technology science-ready for the ELT era.