Investigating the Nature and Structure of Broad Line Regions in Active Galactic Nuclei

Abstract

Active galactic nuclei (AGN) are the innermost regions of the active galaxies, which outshine their hosts. Since these objects are located at far distances, even with large telescope apertures and improvements in detector technology, resolving the inner regions of AGN remains a formidable challenge. Nevertheless, understanding the innermost regions of these galaxies is critical in understanding the galaxy evolution and the dynamics of matter in the vicinity of a Supermassive Black Hole (SMBH). In the absence of direct methods to resolve the inner regions of AGN, indirect methods are used. In this thesis, I explored a few important indirect methods to understand the innermost (sub-parsec) regions. Firstly, we used the reverberation mapping technique to estimate the accretion disk sizes for a sample of AGN with previously known SMBH mass estimates to look for a correlation between the estimated disk size, known SMBH mass, and luminosities. The results show that the accretion disk sizes computed using this method are, on average, 3.9 times larger than the predictions of the Shakura Sunyev (SS) standard disk model. Additionally, we find a weak correlation between the obtained accretion disk sizes and the SMBH mass. Next, we present initial results from a new accretion disk monitoring program titled: ‘Investigating the central parsec regions around supermassive black holes’(INTERVAL) to probe the accretion disk structure of Super Eddington Accreting AGN.We report the results for IRAS 04416+121 and find the disk sizes to be about 4 times larger than the SS disk model. In the next step, we calibrated the narrow-band photometric reverberation mapping (PRM) technique to develop tools for a large systematic narrow-band PRM project which can enable the measurement of SMBH mass and BLR sizes for a large number of quasars at a fraction of the telescope’s time. To develop an optimum strategy for executing a successful PRM project, we used simulations to test the effect of cadence, variability of the light curves, and the length of light curves in recovering the reverberation lags. Secondly, we used the method of microvariability observed in the accretion disk continuum to study the dichotomy between AGNs with and without detected jets. Through a monitoring program spanning 53 sessions with a minimum duration of 3 hours, we demonstrated that the jets can induce significant microvariability in the optical continuum in these AGNs, and the AGNs with confirmed jets are about 3 times more variable on short time scales than the AGNs without a confirmed jet. We also find evidence that Narrow Line Seyfert 1 (NLSy1) galaxies with γ−ray detection have blazar-like behavior in terms of microvariability. Finally, we performed statistical analysis on a large sample of low luminosity AGNs using optical spectra to investigate the dynamics of matter in BLR and infer the properties of the NLSy1 galaxies. Based on a sample of 144 NLSy1 and 117 BLSy1 galaxies, we find that the NLSy1 galaxies are more likely to have outflow signatures than their broad-line counterparts hinting toward the disk wind origin of the material in BLR. Further, through a principal component analysis of this data, we find out the principal components for NLSy1 galaxies differ from the BLSy1 galaxies, which points towards the fact that the NLSy1 galaxies are not just a subclass of BLSy1 galaxies but could be occupying their own parameter space. In summary, we have measured accretion disk sizes for a handful of AGN, which adds to the growing number of AGN with accretion disk size measurement. We have calibrated the PRM technique and developed simulations to execute a successful narrow-band PRM campaign to get the BLR sizes and SMBH mass for a large sample of AGN. We have found evidence that relativistic jets induce microvariability in the AGN optical continuum from the accretion disk. Finally, we have also found that the emission line region in the NLSy1 galaxies is likelier to show outflows through the Hβ emission line as compared to the BLSy1 galaxies.