Astronomy and Astrophysics Programme (AAP)

Analytical Formalism to Study the 21 cm Signal from Cosmic Dawn and Epoch of Reionization

2020-12-14T11:43:50Z

Analytical Formalism to Study the 21 cm Signal from Cosmic Dawn and Epoch of Reionization Raste, Janakee The epoch of Cosmic Dawn and Reionization is one of the most important time periods of the universe when first sources of radiation like stars and galaxies were formed, and they changed the properties of their surrounding medium. 21 cm radiation emitted due to the hyperfine splitting of ground state of neutral hydrogen is one of the most important probes to study the state and dynamics of neutral medium during this epoch. I present an analytical formalism to compute the fluctuating component of the 21 cm signal from CD/EoR. I have used excursion set formalism to calculate the size distribution of randomly distributed selfionized regions, and calculate the isotropic spin temperature profiles around them, taking into account the effect of X‐rays and Ly‐alpha photons. I present an analytical formalism to compute the two‐point correlation function for this topology, where small ionization bubbles are surrounded by large and shallow spin temperature profiles, which can overlap with one another and merge with a background value far away from any source. Using geometric and probabilistic arguments, I compute the global 21 cm signal, its auto‐correlation and power spectrum in the redshift range 10 < z < 30. Our results from this analytical formalism agree reasonably well with existing results in the literature from N‐body simulations

Confronting realistic MHD simulations of solar eruptions with observed space based data

2025-06-06T07:58:44Z

Confronting realistic MHD simulations of solar eruptions with observed space based data Maity, Samriddhi Sankar Solar flares and Coronal Mass Ejections (CMEs) are among the most violent and energetic phenomena observed in the solar atmosphere, resulting from the sudden release of immense amounts of energy. A typical solar flare is characterized by a rapid increase in light emission across a wide range of the electromagnetic spectrum, whereas a CME is defined as the expulsion of vast quantities of plasma and high-energy particles from the Sun into space. Both solar flares and CMEs play crucial roles in shaping space weather, which can have profound effects on Earth. The intense radiation from solar flares can disrupt satellite communications, navigation systems, and power grids, while CMEs can cause geomagnetic storms that impact Earth's magnetosphere, leading to auroras and potential damage to satellites and other space-based technologies. Understanding these phenomena is vital for both scientific research and technological advancements. Solar flares and CMEs both provide insights into the fundamental processes of energy release and particle acceleration in the Sun's atmosphere and offer a unique perspective on the Sun's magnetic field dynamics. Studying the mechanisms behind these events helps scientists develop models to predict solar activity and mitigate its impacts on space weather. This knowledge is essential for improving space weather forecasting and developing strategies to protect Earth's technological infrastructure from solar disturbances. In addition to their scientific importance, solar flares and CMEs are also of great interest for space exploration. Understanding the behavior of these solar phenomena is critical for ensuring the safety of astronauts and spacecraft. As human space exploration extends beyond Earth's orbit, predicting and preparing for the effects of solar flares and CMEs will be essential for the success of long-duration missions. Thus continued research into these dynamic solar events will enhance our ability to predict and mitigate their impacts, contributing to the advancement of space science and the protection of Earth's technological systems. In the thesis, we provide an overview of solar eruptions, alongside a discussion of the numerical equations that govern the magneto-hydrodynamic (MHD) simulations used in our study. We also describe the observational instruments utilized to gather data, allowing us to compare our simulation results with observational insights. Next we provide our study regarding the photospheric magnetic imprints of solar flares associated with coronal mass ejections (CMEs). Solar flares often leave distinct imprints on the magnetic field at the photosphere, typically observed as abrupt and permanent changes in the downward-directed Lorentz force within localized regions of the active region. Our study aims to differentiate eruptive and confined solar flares by analyzing the variations in the vertical Lorentz force. We focus on 26 eruptive and 11 confined major solar flares, all stronger than the GOES M5 class, observed between 2011 and 2017. For this analysis, we utilize SHARP vector-magnetograms obtained from NASA's Helioseismic and Magnetic Imager (HMI). In addition to observational data, we incorporate data from two synthetic flares derived from a $\delta$--sunspot simulation as reported by \cite{chatterjee2016repeatedflare}. Our methodology involves estimating changes in the horizontal magnetic field and the total Lorentz force integrated over areas around the polarity inversion line (PIL), which encompasses the flare locations. To achieve this, we developed a semi-automatic contouring algorithm that delineates the region near the Polarity Inversion Line (PIL) where the most significant magnetic changes occur. Our findings indicate a rapid increase in the horizontal magnetic field along the flaring PIL, coinciding with significant changes in the downward-directed Lorentz force in the same vicinity. A crucial aspect of our results is the identification of a threshold in Lorentz force changes. All confined flares in our study exhibit total Lorentz force changes of less than $1.8 \times 10^{20} dyne$. This threshold proves to be a significant factor in effectively distinguishing between eruptive and confined flares. Moreover, for eruptive events where the change in Lorentz force is below the threshold, we noticed a significantly higher ribbon distance between the parallel flare ribbons, typically exceeding 15 Mm at the onset time of the flare. This indicates a potential implication between the ribbon separation and the magnitude of the Lorentz force change during eruptive events. Therefore, ribbon separation could serve as an additional factor to consider when studying the magnetic imprints associated with the solar flares. We applied the similar procedure to the B \& C class synthetic flare events and noticed a remarkable resemblance in the temporal evolution with the observational data. Our observation indicates that the Lorentz force propagates from the reconnection site towards the photosphere. This provides valuable insights into understanding the mechanisms of flare-related upward impulse transmission, which is crucial for the associated coronal mass ejection (CME) dynamics. Our study not only enhances the understanding of the magnetic and dynamic characteristics of solar flares but also has significant implications for predicting the potential impact of these solar events on space weather. The ability to distinguish between eruptive and confined flares based on Lorentz force changes could lead to better understanding of the relation between the sunspot topology and the ejective flaring. Finally, we shed light on the changes in reconnection flux throughout the evolution of CMEs, from their onset to eruption. Additionally, we correlate these reconnection flux changes with the velocity of the ejected material. Coronal mass ejections (CMEs) are among the most powerful drivers of space weather, with magnetic flux ropes (MFRs) widely considered their primary precursors. However, the three-dimensional variation in reconnection flux during the evolution of MFRs throughout CME eruptions remains insufficiently understood. Here, we present a detailed study utilizing a realistic three-dimensional magneto-hydrodynamic (3D MHD) model to explore the temporal evolution of reconnection flux during MFR evolution. Our approach integrates both numerical simulations and observational data to provide a comprehensive analysis. We begin our investigation with an initial coronal configuration characterized by an isothermal atmosphere and a potential arcade magnetic field, beneath which an MFR emerges at the lower boundary. Our model incorporates radiative cooling and a coronal heating function. Additionally, we have included field-aligned Spitzer thermal conduction. However, our model does not account for solar wind. As the MFR rises, we observe significant stretching and compression of the overlying magnetic field. This dynamic process leads to the formation of a current sheet, initiating magnetic reconnection. The reconnection process gradually intensifies, eventually resulting in the impulsive expulsion of the flux rope. Our simulation generates two homologous CME eruptions, each characterized by an impulsive increase in kinetic energy and a corresponding release of magnetic energy. The peak velocities of the CMEs in our simulation are approximately 224 $\rm km \, s^{-1}$ and 213 $\rm km \, s^{-1}$. In the first eruption, the magnetic flux rope exhibits only torus instability, while in the second eruption, it demonstrates both torus and kink instabilities. Our analysis focuses on the temporal evolution of reconnection fluxes during these two successive MFR eruptions, with the twisted flux continuously emerging through the lower boundary. To complement our simulations, we perform a parallel analysis using observational data from NASA's Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) along with Solar TErrestrial RElations Observatory (STEREO-A) spacecraft for a specific eruptive event. Our findings indicate that changes in reconnection flux play a crucial role in determining CME dynamics. Specifically, the acceleration of CMEs are linearly correlated (CC = 0.58 and 0.81) to the amount of the reconnection flux, highlighting the importance of reconnection dynamics in the overall process of CME initiation and propagation. This nearly realistic simulation of a solar eruption offers significant insights into the complex dynamics of CME initiation and progression. The ability to model and understand the temporal evolution of reconnection flux in three dimensions provides a more accurate and detailed picture of the mechanisms driving CMEs. Consequently, our study enhances the understanding of how the reconnection flux changes over time during the solar eruption processes and demonstrates the vital role of reconnection flux and velocity of CMEs from the onset to the eruption. Finally, we provide a brief overview of our future goals. In this thesis, we have presented realistic magneto-hydrodynamic (MHD) simulations of solar eruptions alongside observed space-based data. By comparing the simulation results with observational data from instruments, we aim to enhance our understanding to better represent the solar eruptions and their impacts. Our research not only closes the gap between theoretical models and observational data but also underscores the crucial role of concurrent observation and simulation in understanding solar eruptions. By leveraging both observational data and modeling efforts, this thesis lays the groundwork for further improvements of such lower corona models and techniques to a border range of solar eruptions study and their implications for solar physics.

Effect of AGN on the ISM of their hosts: A multi-wavelength perspective

2025-06-06T09:08:49Z

Effect of AGN on the ISM of their hosts: A multi-wavelength perspective Nandi, Payel Active Galactic Nuclei (AGN) are among the high luminosity (1011 − 1014 L⊙ ) sources in the observable Universe, believed to be powered by the accretion of matter onto supermassive black holes (SMBHs; 105 − 1010 M⊙ ) at the centres of galaxies. The energy released from AGN could affect their host galaxies and their large-scale environment through a process called feedback. AGN feedback is generally invoked in galaxy formation models and simulations to explain the observed correlation between the mass of SMBHs and various host galaxy properties. A viable feedback mechanism in AGN is outflows. The driving force behind such outflows, though debated, can have an impact on the interstellar medium (ISM) of their hosts by inhibiting (negative feedback) or enhancing (positive feedback) star formation (SF). In massive galaxies, there are evidences that AGN regulate SF in their hosts via the jets, injecting energy into the gaseous halos and regulating the cooling of gas onto the galaxies. Recent observations available from a limited number of sources point to AGN having both positive and negative impacts on the SF characteristics of their hosts. However, the nature and details of the impact AGN have on the evolution of their host galaxies remain controversial and uncertain. Also, AGN affecting SF was not known in dwarf galaxies before a couple of years ago. But, only very recently, there are observational evidences of dwarf galaxies hosting AGN, thereby challenging theoretical models that generally invoke supernovae feedback in dwarf galaxies. In this thesis, we carried out a systematic investigation on a sample of AGN to find clues to (a) what triggers outflows in AGN, (b) the impact of AGN on the SF of their hosts and (c) the nature of AGN feedback in a dwarf AGN hosted by an intermediate-mass black hole (IMBH; 104 − 105 M⊙ ). Towards this, we utilized imaging data from the Chandra X-ray Observatory, the Ultra-Violet Imaging Telescope (UVIT) aboard AstroSat, the Hubble Space Telescope (HST), the Himalayan Chandra Telescope (HCT), and the Very Large Array (VLA). Additionally, we incorporated spatially resolved spectroscopic data in the optical and infrared bands from Gemini and SDSS (MaNGA), as well as millimetre data from ALMA. The findings of this thesis are summarized in four chapters, as described below. First, we aimed to understand the driver of ionized outflows in AGN. For this, we carried out a systematic investigation of the [O III]λ 5007 emission line on a sample of AGN, consisting of radio-detected and radio-undetected sources using MaNGA and Faint Images of the Radio Sky at Twenty cm (FIRST) survey data. We found radio-detected sources to show an increased outflow detection rate compared to radio-undetected sources. We noticed a strong correlation between outflow characteristics and bolometric luminosity in both samples, except that the correlation is steeper for the radio-detected sample. Our findings suggest (a) ionized gas outflows are prevalent in all types of AGN, (b) radiation from AGN is the primary driver of ionized gas outflows, (c) radio jets play a secondary role in enhancing the gas kinematics over and above that caused by radiation and (d) SF is quenched in the very central regions of the galaxies in the sample studied due to AGN activity. Then, we investigated the impact of AGN on the SF characteristics of their hosts. This was done by mapping the star-forming regions in galaxies hosting AGN and looking for any correlations between the deduced SF and AGN properties. It is natural to expect that the influence of the central AGN on their hosts could decrease from the centre to the outskirts of the galaxies. In the nearby Universe, Seyfert and LINER type AGN are ideal targets to investigate this connection as the resolution offered by ground-based imaging observations will enable one to probe SF on scales from a few hundred parsecs to a few tens of kpc. Most studies of SF in Seyfert galaxies have been conducted in the optical, IR, or radio wavelengths using ground and space-based observatories. While there have been a few studies in the UV band using Galaxy Evolution Explorer (GALEX), the resolution provided by GALEX is often insufficient to resolve SF on parsec scales. A limited number of studies have used the HST, which offers the capability to resolve parsec-scale structures, but it has a restricted field of view, making it observationally expensive to study a large number of sources comprehensively. To address this gap, we undertook a systematic investigation of the SF properties of Seyfert galaxies using UVIT. From an investigation of the SF characteristics on a sample of eight AGN, using UVIT observations, we found a positive correlation between the total surface density of SF and extinction. For five sources, we found a gradual decline of both the surface density of SF and extinction from the centre to the outer regions. We found the ratio of the star formation rate (SFR) in the nuclear region to the total SFR to be positively correlated with the Eddington ratio. This points to the influence of AGN in enhancing the SF characteristics of the hosts. This impact is found to be dominant only in the central few kpc regions with lesser effect on the larger scales probed in this thesis. Afterthat, we investigated the nature of AGN feedback on a dwarf AGN, NGC 4395. This is important, as dwarf AGN are believed to be powered by the lower mass end of SMBHs. In the literature, theoretical studies on the regulation of SF in dwarf galaxies have been attributed to radiation from young stars and supernova explosions. However, recent theoretical studies do indicate that AGN could play a significant role in regulating SF in dwarf galaxies. Observationally, there is evidence of AGN feedback operating in dwarf galaxies covering angular sizes smaller than about an arcmin. Therefore, detailed studies on SF characteristics of dwarf galaxies hosting AGN are needed, firstly, to characterize their SF properties and secondly, to find evidence of the feedback process, if any, in them. Using data from UVIT, we identified a total of 284 star-forming regions extending up to a distance of 9 kpc. Of those, 120 regions were also identified in the Hα continuum subtracted image. The detection of fewer star-forming regions in Hα is attributed to the lower spatial resolution as well as the shallowness of the Hα image relative to UV. On inspection of the spatial distribution of the surface density of SFR in the UV, we found three star-forming regions near the AGN that have a high surface density of SFR. One out of the three star-forming regions in the UV is also found to have a high surface density of SFR in Hα and younger age. This could possibly hint at positive feedback from the AGN. At 1.4 GHz, we found a few complexes having enhanced radio emission. These complexes contain a larger number of star-forming regions, with the majority of them having higher SFR. These complexes are known to host supernova (SN) remnants. The star-forming regions in these complexes have higher SFR in Hα and 24 µm, compared to other star-forming regions, arguing for SN-induced SF. Then, we aimed to understand the effect of jets on the ISM. There is hardly any observational evidence of jet−ISM interaction and its impact on the host galaxies of AGN on parsec scales. This was investigated on the dwarf AGN, NGC 4395, powered by an IMBH. Using high-resolution observations at 15 GHz from the VLA and the HST, we found evidence of radio jet−ISM interaction on the scale of an asymmetric triple radio structure of ∼ 10 pc size. The high-resolution radio image and the extended [O III]λ 5007 emission, indicative of an outflow, are spatially coincident and are consistent with the interpretation of a low-power radio jet interacting with the ISM. The spatial coincidence of molecular H2 λ 2.4085 along the jet direction, the morphology of ionized [O III]λ 5007, and displacement of the CO(2−1) emission argues for conditions less favourable for SF in the central ∼ 10 pc region . Finally, we provide a summary of the thesis work, highlighting its key findings and significance. Additionally, we discuss the unique contributions of this research and outline potential future directions that could further enhance our understanding of the topic.

The Formation and Evolution of Bars, and Its Impact on Galaxy Dynamics

2020-12-14T11:44:02Z

The Formation and Evolution of Bars, and Its Impact on Galaxy Dynamics Kataria, Sandeep Kumar In this thesis we have studied bar formation and evolution using N-body simulations. We have focused on the effect of bulge mass and concentration on bar formation and evolution, and derived a new bar formation criterion. We also tested the criterion with observations collected from literature. We then investigated the effect of bulge mass on bar pattern speed since observations do not give a clear idea of the nature and origin of slow and fast of bars in galaxies. Bars also a potential candidate for changing the dark matter profile in the central regions of galaxies as they transports angular momentum from disk to the dark halo. We have also studied this effect using N-body simulations