Accretion And Jet Physics In Low Luminosity Active Galactic Nuclei

In this dissertation, the origin of the emission from low luminosity activegalactic nuclei(LLAGNs) in multi-wavelength bands(e. g., radio, optical and X-ray) is explored based on the black hole accretion-jet model calculations. Wefurther investigate the relation between the black hole accretion and the circum-stances near the black hole.In chapter 1, we briefly introduce the conception of active galactic nu-clei(AGN) and its taxonomy, and the recent progresses of the researches on thecentral engines of LLAGNs. We also describe the radiatively inefficient accretionflows(RIAFs) that developed in last decade and its applications in astrophysics.In chapter 2, we use the observational data in radio, optical and X-ray wave-bands, for a sample of active galactic nuclei (AGNs) with measured black holemasses, to explore the origin of radio emission from nearby low-luminosity activegalactic nuclei (LLAGNs). The maximal luminosity of an advection dominatedaccretion flow (ADAF) can be calculated for a given black hole mass, as there is acritical accretion rate m_crit, above which the ADAF is no longer present. We findthat the radio luminosities are higher than the maximal luminosities expectedfrom the ADAF model for most sources in this sample, which implies that theradio emission is dominantly from the jets in these sources. The radio emissionfrom a small fraction of the sources (15/60, hereafter referred to as radio-weaksources) in this sample can be explained by the ADAF model. However, com-paring the observed multi-band emission data with the spectra, calculated forthe ADAF or ADIOS (adiabatic inflow-outflow solution) cases, we find that nei-ther ADAF nor ADIOS models can reproduce the observed multi-band emissionsimultaneously, with reasonable magnetic field strengths, for these radio-weaksources. A variety of other possibilities are discussed, and we suggest that theradio emission is probably dominated by jet emission even in these radio-weakLLAGNs.In chapter 3, we try to check the possible relation between the black hole growth and matter on large scale. It is argued that there is a linear correlationbetween star formation rate (SFR) and accretion rate for normal bright activegalactic nuclei (AGNs). However, it is still unclear whether this correlation holdsfor LINERs, of which the accretion rates are relatively lower than those of normalbright AGNs. The radiatively inefficient accretion flows (RIAFs) are believed tobe present in these LINERs. In this work, we derive accretion rates for a sampleof LINERs from their hard X-ray luminosities based on spectral calculations forRIAFs. We find that LINERs follow the same correlation between star forma-tion rate and accretion rate defined by normal bright AGNs, when reasonableparameters are adopted for RIAFs. It means that the gases feed the black holeand star formation in these low-luminosity LINERs may follow the same way asthat in normal bright AGNs, which is roughly consistent with recent numericalsimulations on quasar evolution.In chapter 4, we investigate the X-ray origin in FR Is using the radio,submillimetre, optical, and Chandra X-ray data of a small sample consistingof eight FR I sources. These sources are very dim, with X-ray luminositiesL_X/L_Edd～10^-4—10^-8 (L_X is the X-ray luminosity between 2-10 keV). We tryto fit the multiwaveband spectrum using a coupled accretion-jet model. In thismodel, the accretion flow is described by an advection-dominated accretion flow(ADAF) while in the innermost region of ADAF a fraction of accretion flow istransferred into the vertical direction and forms a jet. We find that X-ray emis-sion in the source with the highest L_X(～1.8×10^-4 L_Edd) is from the ADAF. Theresults for the four sources with moderate L_X(～several×10^-6 L_Edd) are com-plicated. Two are mainly from the ADAFs, one from the jet, and the other fromthe sum of the jet and ADAF. The X-ray emission in the three least luminoussources (L_X(?)1.0×10^-6 L_Edd) is dominated by the jet although for one sourceit can also be interpreted by the ADAF due to the low-quality X-ray data. Wealso investigate the fuel supply in these sources. We find that the accretion ratesin four sources among the five in which we can have good constraints to theiraccretion rates must be higher than the Bondi rates. This implies that other fuelsupply, such as the gas released by the stellar population inside the Bondi radius,should be important.