Kinematic relationship between multiphase gas, excitation, and star-forming sites in cool-core nebulae

Abstract

The giant elliptical galaxies at the centers of galaxy groups and clusters sometimes host luminous emission-line nebulae comprising ionized, atomic, and molecular gas. When well resolved, these nebulae exhibit filamentary structures that are immersed in the surrounding hot X-ray-emitting intracluster gas. Appearing when the entropy of the intracluster medium at the cores of groups or clusters is relatively low, these nebulae are linked to cooling of their surrounding hot gas. Despite decades of study, however, the nature of such cool-core nebulae remains poorly understood; even their basic physical properties such as density and temperature, along with the physical conditions that lead to their occasional star formation.

Identifying the mechanism(s) that energizes cool-core nebulae lies at the heart of a better understanding of these nebulae. To this end, I study the kinematic relationship between the atomic (perhaps partially ionized) and molecular phases of the cool-core nebula in NGC5044, the giant elliptical galaxy at the center of the NGC5044 group. This galaxy exhibits only a weak AGN and no recent star formation, leaving conduction of heat or penetration of energetic particles from the surrounding intracluster gas as the most likely mechanisms for energizing its nebula. I find that the gas components emitting in H$ \alpha $, H$ _2 $ 1-0 S(1) at $ 2.12 \si{\um} $, and CO share essentially identical kinematics. If confined to spatially distinct volumes, the different gas components should quickly decouple upon infall owing to ram pressure with the intracluster medium. Instead, the different gas components likely co-exist within the same volume, not possible if energized purely through thermal collisions but possible if powered by energetic particles.

I find the ratios in H$ \alpha $, H$ _2 $, and CO lines to be quite uniform throughout NGC5044, with an H$ _2 $ to H$ \alpha $ ratio remarkably similar to that in NGC1275, the giant elliptical galaxy at the center of the Perseus cluster. In addition, I measure the [OI] and [OIII] line intensity throughout NGC5044, and demonstrate that the line ratios rule out thermally-energized gas but provide further support for gas powered by energetic particles. These results, together with the need for the different gas components in NGC5044 to be kinematically coupled, promise stronger constraints on theoretical models based on energetic particles for powering cool-core nebulae once predictions for the emission-line spectrum of NGC5044, like that published for NGC1275, become available.

Finally, I present an [OIII] image of the nebula in NGC1275. I confirm the discovery of a second giant HII region in its inner nebula, and demonstrate the ability to detect giant HII regions at the outskirts of the nebula where the vast majority of star clusters in NGC1275 have recently formed. I also show that some of the young star clusters host bright HII regions, in contradiction with the expectation that their natal gas should be quickly dispersed during the formation of very massive star clusters. Finally, I demonstrate that, like NGC5044, the entire nebula of NGC1275 likely glows weakly in [OIII], further emphasizing the need for energetic particles to explain its peculiar line ratios.