Probing Environment of AGNs based on their feedback processes

Abstract

It is broadly accepted that the presence of a supermassive black hole (SMBH) at the center of an active galaxy is not merely triggering the mass accretion onto it but may play a crucial role in predicting the final evolution of its host galaxy. The generation of energy via accretion of surrounding matter onto the SMBH, known as Active Galactic Nucleus (AGN) feedback, in the form of winds/outflows or jets, can interact with the stellar environment of its host and may set o the heating, compression or expulsion of the gases residing in the host galaxy. The two main modes of the AGN feedbacks are the radiative mode and kinetic mode. The radiative mode occurs in highly luminous sources, delivering high radiation pressure and hence resulting in the ejection of gases in the form of strong outflows. On the other hand, the kinetic mode scenario is perhaps due to the radio jets mainly occurring in low luminous AGNs. In this thesis, we study these modes of the AGN feedback to understand the dynamics of the gases in the central region of the AGN as well as the metal enrichment of the IGM. We have performed a detailed photometric and spectroscopic analysis for the continuum variability and absorption/emission line study of the AGNs to address some serious questions in this field. It is widely believed that the cool gas clouds traced by Mg ii absorption, within a velocity o set of 5000 kmsð€€€1 relative to the background quasar are mostly associated with the quasar itself, whereas the absorbers seen at larger velocity o sets towards us are intervening absorber systems and hence their existence is completely independent of the background quasar. This paradigm has been recently questioned based on an analysis of 45 blazars, showing that the number density of intervening Mg ii absorbers (dN=dz) towards them is nearly 2 times higher in comparison to the normal quasar sightlines. Given its serious implications, it becomes important to revisit this finding, by enlarging the blazar sample and subjecting it to an independent analysis. In this thesis, we have assembled a 3 times larger sample of blazars (191 blazars), albeit with moderately sensitive optical spectra. Our analysis with this enlarged sample shows that the dN=dz of the Mg ii absorbers statistically matches with results known for normal quasars sightlines. However, our analysis revealed that associated Mg ii absorbers remain a significant contributor to dN/dz up to a velocity o set of 0.2c measured relative to the blazars in contrast to the commonly used upper limit of 5000kmsð€€€1. We extend our study of associated systems using the high ionization absorption lines, such as C iv. In this study, we mainly focus on the appearance and disappearance of C iv absorption troughs in broad absorption line (BAL) quasars to explore for the leading cause responsible for their extreme variation. We have derived a new set of 94 BAL quasars ( 1.7 < zem < 4.4) exhibiting an appearance of C iv BAL troughs over 0.3ð€€€4.8 rest-frame years by comparing the Sloan Digital Sky Survey Data Release (SDSSDR)-7, SDSSDR-12, and SDSSDR-14 quasar catalogs. We have compared the quasar’s intrinsic, BAL trough, and continuum parameters of this new appearing BAL quasar sample with a disappearing BAL quasar sample from the literature. We have found that the appearing BAL quasars are brighter compared to the disappearing BAL quasars. The appearing BAL sample is also found to have shallower BAL troughs compared to the disappearing BAL sample. The distribution of quasar continuum variability parameters between the two samples is clearly separated with the appearance of the BAL troughs being accompanied by the dimming of the continuum and vice versa. Spectral index variations in the two samples also point to the anti-correlation between the BAL trough and continuum variations consistent with the "bluer when brighter" trend in quasars. We found that the intrinsic dust model is less likely to be a favorable scenario in explaining BAL appearance/disappearance. Our analysis suggests that the extreme variations of BAL troughs like BAL appearance/disappearance are mainly driven by changes in the ionization conditions of the absorbing gas. In this thesis, we have also accomplished a first systematic search for blazar-like BAL quasars to inquire if a blazar subset exists even among the BAL quasars. We have carried out intra-night optical variability (INOV) study of a well-defined sample of 10 BALð€€€blazar candidates selected based on a flat spectrum and high linear polarization at radio wavelengths. A small population of BALð€€€blazars can be expected in the ‘polar model’ of BAL quasars. However, no such case was found, since none of our 30 monitoring sessions devoted to the 10 BALð€€€blazar candidates yielded a positive detection of INOV, which is highly abnormal in blazars. This suggests that the physical conditions in the relativistic jets of BALð€€€blazars are less conducive for strong INOV. An evidence in support of this scenario has also been inferred in this thesis based on the INOV DC of 11% for a comparison sample of 9 blazars (non-BAL) matching in magnitude and redshift with that of BALð€€€blazars. In summary, our study rule out the puzzling excess of the dN/dz of intervening Mg ii galaxies towards the blazars in contrast to the normal quasars. Our analysis suggests that fluctuating ionizing continuum mainly drive BAL trough’s appearance and disappearance. At last, our INOV study of BALð€€€blazar candidates hints that the physical conditions in the relativistic jets of the BAL quasars di er from the blazars.