School of Astrophysics
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Item Radio and Submm Studies of Active Galactic NucleiChatterjee, SuchetanaThe radio dichotomy in the observed quasar population (only 10%-15% of all quasars are radio-loud) in the Universe is still an unsolved problem in extragalactic astronomy. Using the spectroscopic quasar catalog from the Sloan Digital Sky Survey, we show in this study, that the radio-loud fraction (RLF) of quasars increases with the full-width-half-maxima (FWHM) of the Hβ and MgII emission lines. We then construct a sample of high broad line (HBL; FWHM greater than 15, 000 km s−1) quasars and study their properties in a comprehensive manner. Our study show that the bolometric and optical continuum luminosity of the radio loud (RL) quasars are higher while there is no significant difference in other fundamental properties between the RL and radio-quiet (RQ) populations in our sample. By comparing them with the non-HBL Hβ broad emission line quasars, we find that the HBL sources have the lowest Eddington ratios in addition to having a very high RLF. This is consistent with the theories of jet formation, in which jets are launched from low Eddington ratio systems. We then investigate the host galaxy properties of our HBL quasars by modeling their spectral energy distribution using the X-CIGALE module. Our preliminary studies show that among the host galaxy properties, stellar mass, the luminosity absorbed by the dust, star-formation rate, and stellar population age are higher for our radio loud quasars while e-folding time is higher for our radio quiet quasars. We note that our sample size is small to robustly predict the physical implications of this result. To probe the energetic outflows from quasars we model the thermal Sunyaev-Zeldovich (SZ) effect from quasar feedback. We use the cosmological simulation SIMBA which includes different prescriptions for quasar feedback. From these theoretical simulations, we perform mock observations of the Atacama Large Millimeter Array to characterize the feasibility of direct detection of the quasar SZ signal. Our results show that for all the systems we get an enhancement of the SZ signal, when there is radiative feedback, while the signal gets suppressed when the jet mode of feedback is introduced in the simulations.Item Unveiling the cosmic dawn and epoch of reionization using cosmic 21 cm signalSamui, Saumyadip; Datta, Kannan KumarThe cosmic dawn (CD) and epoch of reionization (EoR) is one of the least understood episodes of universe. The cosmological 21-cm signal from neutral hydrogen, which is considered as a promising tool, is being used to observe and study the epoch. A signif icant part of this thesis focuses on the semi-analytical modeling of the global HI 21-cm signal from CD considering several physical processes. Further, it investigates the nature of galaxies that dominate during CD and EoR using current available observations. Understanding di!erent physical processes through which the thermal and ioniza tion states of the intergalactic medium (IGM) during CD and EoR evolved is the key to unlocking the mysteries of the early universe. The study of the 21-cm signal is a power ful tool that can be used to investigate di!erent physical processes in the early universe. It provides us with a window into the time before the formation of the "rst galaxies. Our study is partly motivated by the "rst-ever detection of the global 21-cm signal reported by the EDGES low-band antenna and ongoing observations by several global 21-cm ex periments. One of the promising avenues to interpret the EDGES signal is to consider a ‘colder IGM’ background. In our "rst work, we study the redshift evolution of the primordial magnetic "eld (PMF) during the dark ages and cosmic dawn and prospects of constraining it in light of EDGES 21-cm signal in the ‘colder IGM’ background. Our analysis has been carried out by considering the dark matter-baryon interactions for the excess cooling mechanism. We "nd that the colder IGM suppresses both the residual free electron fraction and the coupling coe#cient between the ionized and neutral components. The Compton heating also gets a!ected in colder IGM backgrounds. Consequently, the IGM heating rate due to the PMF enhances compared to the standard scenario. Thus, a signi"cant fraction of the magnetic energy, for B0 . 0.5 nG, get transferred to the IGM, and the magnetic "eld decays at a much faster rate compared to the simple (1 + z)2 scaling during the dark ages and cosmic dawn. We also "nd that the upper limit on the PMF depends on the underlying DM-baryon interaction. Higher PMF can be allowed when the interaction xv cross-section is higher and/or the DM particle mass is lower. Our study shows that the PMF with B0 up to ⇠ 0.4 nG, which is ruled out in the standard model, can be allowed if DM-baryon interaction with suitable cross-section and DM mass is considered. However, this low PMF is an unlikely candidate for explaining the rise of the EDGES absorption signal at lower redshift. We further consider, in detail, the heating of the IGM owing to cosmic ray protons generated by the supernovae from both early Pop III and Pop II stars. The low-energy cosmic ray protons from Pop III supernovae can escape from minihalos and heat the IGM via collision and ionization of hydrogen. Moreover, the high-energy protons generated in Pop II supernovae can escape the hosting halos and heat the IGM via magnetosonic Alfvén waves. We show that the heating due to these cosmic ray particles can signi"- cantly impact the IGM temperature and hence the global 21-cm signal at z ⇠ 14