Kinematic constraints on structuring of the optical emission-line nebula in NGC 1275

Abstract

Bright optical nebulae are a relatively common feature of galaxy clusters with a central depression in their intracluster gas temperature suggestive of an X-ray cooling flow. In my thesis, I have measured, for the first time, the velocity field of nearly the entire optical nebula associated with NGC 1275, the central cD (giant elliptical) galaxy of the Perseus cluster. This nebula is the brightest example with the largest projected size in the sky. Our primary scientific objective is to address the physical processes that give rise to the complex morphology of this nebula, which comprises a multitude of mostly approximately radial but also a number of tangential emission-line _laments. This nebula spans up to ～140 kpc in the north-south direction, extending far beyond the half-light radius of NGC 1275. Popular models for the nature of the nebula invoke either an X-ray cooling flow, in which case the filaments should possess a velocity profile consistent with free fall, or gas drawn out from the central galaxy by buoyantly rising bubbles inflated by radio jets from the AGN, in which case the filaments should show a reversal in velocity along their lengths. We find that the velocity field of the nebula is incredibly complex, and in several important respects contradicts model predictions based on slit spectroscopy. We find that filaments previously thought to be very long integral structures actually comprise multiple shorter filaments that each have their own distinct kinematics. Furthermore, filaments that are apparently aligned and adjacent to each other often possess entirely different kinematics. We searched for filaments with velocity profiles consistent with free fall, but we could only find very few examples beyond the central bright nebular region where overlapping filaments complicate measurements of the individual filament kinematics. We also searched for filaments that show a reversal in velocity along their lengths, but again could find only several examples. Although the combination of a residual X-ray cooling flow and relatively cool gas drawn out by buoyant X-ray bubbles remains the most viable explanation for the nature of the nebula, both the morphology and kinematics of this nebula is complicated by the action of various X-ray bubbles that are currently expanding or rising through the nebula. As such, the velocity field of the filaments reflect the effects of the X-ray bubbles on the bulk flows within the intracluster medium, and trace streamlines that reveal motions in the surrounding X-ray gas. We point out evidences that support the idea that some filaments are being dragged by rising X-ray bubbles, others represent large-scale vortices behind bubbles, and yet others are draping the surface of and pushed outwards by expanding X-ray bubbles.