On the Origin of the Extended Halpha Filaments in Cooling Flow Clusters

TL;DR

This study uses high-resolution Halpha imaging to explore the morphology and origins of filaments in cooling flow clusters, revealing their connection to cooling flows, magnetic fields, and conduction processes.

Contribution

It provides new high-resolution observations linking Halpha filaments to X-ray cooling flows and discusses the roles of magnetic fields and conduction in filament ionization and structure.

Findings

Halpha filaments are present in 35% of clusters.

Strong correlation between Halpha luminosity and X-ray cooling rate.

Filaments are likely influenced by magnetic fields and conduction.

Abstract

We present a high spatial resolution Halpha survey of 23 cooling flow clusters using the Maryland Magellan Tunable Filter (MMTF), covering 1-2 orders of magnitude in cooling rate, dM/dt, temperature and entropy. We find 8/23 (35%) of our clusters have complex, filamentary morphologies at Halpha, while an additional 7/23 (30%) have marginally extended or nuclear Halpha emission, in general agreement with previous studies of line emission in cooling flow cluster BCGs. A weak correlation between the integrated near-UV luminosity and the Halpha luminosity is also found for our complete sample, with a large amount of scatter about the expected relation for photoionization by young stars. We detect Halpha emission out to the X-ray cooling radius, but no further, in several clusters and find a strong correlation between the Halpha luminosity contained in filaments and the X-ray cooling flow rate of the cluster, suggesting that the warm ionized gas is linked to the cooling flow. Furthermore, we detect a strong enhancement in the cooling properties of the ICM coincident with the Halpha emission, compared to the surrounding ICM at the same radius. While the filaments in a few clusters may be entrained by buoyant radio bubbles, in general, the radially-infalling cooling flow model provides a better explanation for the observed trends. The correlation of the Halpha and X-ray properties suggests that conduction may be important in keeping the filaments ionized. The thinness of the filaments suggests that magnetic fields are an important part of channeling the gas and shielding it from the surrounding hot ICM.