The evolutionary connection between unobscured and obscured quasars

• Main Authors: Hsu, Li-Ting, 徐麗婷
• Other Authors: Chen, Lin-wen
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/57474878186937572217

碩士 === 國立臺灣師範大學 === 地球科學系 === 99 === Type 2 quasars (QSO2) are intrinsically as luminous as type 1 quasars (QSO1), but obscured by a large amount of dust. Furthermore, the evolutionary connection between QSO1 and QSO2, if any, remains unclear. One key clue to understand the nature of type 2 quasars is to investigate the scale and origin of the dust that causes obscuration, whether it is attributed to circumnuclear dust or to a dust torus in the galactic nucleus. In this thesis,we study the evolutionary connection between QSO1 and QSO2 by statistical analyses of several multiwavelength properties including the environment,dust property, radio luminosity, and Eddington ratio. We find that the average galaxy counts surrounding QSO2 within R = 1.5 Mpc are greater than those of QSO1 by a factor of 1.4, implying that these two types of quasars have different origins. In addition, the dust amount in QSO2 is related to the central power; the more dusty QSO2 prefer to have higher radio luminosities and lower accretion rates, whereas QSO1 have a weak correlation between dust amount and central power. This different dust-radio luminosity relation between QSO1 and QSO2 implies that the dust extinction is not only due to the orientation effect of torus, but also attributed to a larger-scale dust. Finally, based on the evolutions of dust amount and Eddington ratio, we demonstrate that the amount of dust in QSO2 diminishes while their accretion rates increase with time, involving that QSO2 may turn into clear and bright QSO1. Our study suggests that QSO2 are at an early evolutionary stage of QSO1.