Ionized nebulae surrounding brightest cluster galaxies

Abstract

We present integral field spectroscopic observations of six emission-line nebulae that surround the central galaxy of cool core clusters. Qualitatively similar nebulae are observed in cool core clusters even when the dynamics and possibly formation and excitation source are different. Evidence for a nearby secondary galaxy disturbing a nebula, as well as active galactic nucleus- and starburst-driven outflows are presented as possible formation mechanisms. One nebula has a rotational velocity of the same amplitude as the underlying molecular reservoir, which implies that the excitation or formation of a nebula does not require any disturbance of the molecular reservoir within the central galaxy. Bulk flows and velocity shears of a few hundred km s^-1 are seen across all nebulae. The majority lack any ordered rotation, their configurations are not stable so the nebulae must be constantly reshaping, dispersing and reforming. The dimmer nebulae are cospatial with dust features whilst the more luminous are not. Significant variation in the ionization state of the gas is seen in all nebulae through the non-uniform [NII]/Hα ratio. There is no correlation between the line ratio and Hα surface brightness, but regions with excess blue or ultraviolet (UV) light have lower line ratios. This implies that UV from massive, young stars act in combination with an underlying heating source that produces the observed low-ionization spectra.