Effect of a QSO on its host galaxy

Abstract

Various arguments and observations have led to a general belief that QSO phenomenon is a form of violent activity in the nuclei of galaxies. It is natural to expect that a powerful source of high energy radiation like a quasar would affect the properties of its host galaxy due to interaction of the radiation with the interstellar medium of the galaxy. A strong effect could perceptibly change the properties of the medium and lead to galactic winds, change the star formation rate and so on. The primary aim of the present thesis is to study such effects in detail. In order to study such interaction we need to know the properties of the quasar and the nature of its host galaxy. We assume standard range of values for quasar luminosity and spectrum and the mass of the black hole. There exist various conflicting arguments regading the morphology of the host galaxy. By considering observability of a galaxy at large distances we show that the galaxy associated with a quasar should be a spiral in general. We also show that the colour of these host galaxies is excessively blue and that it carmot be explained by standard population synthesis models. We consider Compton heating of the gas in the interstellar medium of the host galaxy due to X-ray radiation from quasars. Standard properties of the interstellar medium of a spiral galaxy are assumed. X-ray photoionization destroys important coolants and thus most o f the EUV and X-ray energy goes into heating the gas. In principle, the gas can be heated upto the temperature of ~ 10®i f at which point Compton heating and inverse Compton cooling balance. Such heating should drive the gas out from the galaxy setting up “galactic wind” . Balance between the mass loss from the ISM due to the wind and the mass input into the ISM due to the galaxy setting up “galactic wind” . Balance between the mass loss from the ISM due to the wind and the mass input into the ISM due to the stellar mass loss should determine the extent to which the ISM would be evacuated. We calculate this radius by considering host galaxy evolution taking into account proper initial mass function of the stars and time dependent star formation rate. We show that the evacuation radius is much smaller than that hitherto believed and that it is redshift dependent. The exact nature of the evolution of the ISM exposed to QSO radiation field is investigated by doing detailed hydrodynamic calculations. We show that a Lagrangean formulation of the problem is necessary. Results show that no ‘steady state’ wind is established and mass loss rate from the galaxy is time dependent. Results also show that the “hydrodynamic time scale” , i.e. the time scale over which expansion of gas takes place is more relevant to the problem compared to the “ Compton heating time scale” . This prevents the gas from reaching the inverse Compton temperature until it has evolved to large altitudes away from the galaxy. We predict star burst activity in the active galaxy owing to the increase in pressure in the hot intercloud medium. This can explain the blue colour of active galaxies. It is shown that the galaxy would be evacuated of almost 90% of its interstellar gas in about 10® yr. Hence, it was believed that the black hole would be starved of fuel and that would switch off the central engine-the quasar. We show that it need not be so if the gas to be accreted on the black hole is produced very near the black hole by disruption of stars.