The Link Between Extended Line Emission and AGN Feedback in Brightest Cluster Galaxies

Abstract

This thesis presents a study of the optical emission-line regions of low-redshift brightest cluster galaxies (BCGs) and how these are connected to energy feedback in cluster cores. The cores of massive galaxy clusters appear to undergo cycles of self-regulating energy feedback, in which the output of nuclear activity in BCGs offsets much of the hot cluster gas cooling. Many of the BCGs in these `cool-core' clusters are surrounded by extended filamentary nebulae that produce characteristic low-ionisation emission spectra. The origin and sources of excitation of these structures are not well understood.

A sample of BCGs was observed with a new wide-field, optical integral-field spectrograph. These galaxies produce strong line emission with a range of integrated luminosities. Maps of the morphology and velocities of the line-emitting gas are constructed from these data.

A deep observation of the nearby galaxy NGC4696, provides a detailed view of the complex emission nebula. In the inner nebula, a bright spiral filament coincides closely with a prominent dust lane. The data reveal velocities in this structure consistent with infalling, orbit-like trajectories encircling the nucleus. A model is presented in which some of the gas in the filaments originated in the accretion of a gas-rich dwarf galaxy. It is suggested that the filaments are excited by ~200km/s ram-pressure-driven shocks as the gas clouds fall through the halo. The shock models described reproduce the optical spectrum of the nebula.

Three BCGs with intermediate line luminosities are studied to investigate how the filament structures are related to the processes of feedback in the cluster cores. Two systems, Abell 3581 and Sersic 159-03, appear to be experiencing phases of feedback dominated by AGN heating. The structure and kinematics of the emitting gas in Abell 3581 show evidence for significant interaction between the radio outflows and the optical nebula, and possible accretion flows into the nucleus. X-ray and radio data indicate that Sersic 159-03 is dominated by AGN feedback, but the kinematics of the optical nebula are consistent with infall or outflow along the filaments. The third system, 2A0335+096, is dominated by a cooling and accumulation phase of feedback. The outer nebula forms a disk-like structure, ~14kpc in radius, with a velocity field of rotation about the BCG. Overall, our data are consistent with ongoing AGN-driven feedback in these systems.

A comparative study of the line-luminous galaxies in the sample is also presented. The data for several systems show evidence for a correlation between the velocity dispersions and line flux ratios in the nebulae, suggesting a strong contribution from shocks to the excitation. The large-scale excitation properties of the nebulae are assessed using optical diagnostic diagrams and discussed in the context of mixing between shock and stellar photoionisation excitation models. It is suggested that both shock and photoionisation excitation contribute significantly to the properties of the emission in these BCGs, to a degree that varies among the galaxies.