Multiple radial cool molecular filaments in NGC 1275

Abstract

We have extended our previous observation of NGC 1275 (Perseus A (Per A), the central giant elliptical galaxy in the Perseus Cluster) covering a central radius of 10 kpc to the entire main body of cool molecular gas spanning 14 kpc east and west of the center. We find no new features beyond the region previously mapped, and show that all six spatially resolved features on both the eastern and western sides (three on each side) comprise radially aligned filaments. Such radial filaments can be most naturally explained by a model in which gas deposited "upstream" in localized regions experiencing a X-ray cooling flow subsequently free falls along the gravitational potential of Per A, as we previously showed can explain the observed kinematics of the two longest filaments. All the detected filaments coincide with locally bright Hα features, and have a ratio of CO(2-1) to Hα luminosity of 10 -3; we show that these filaments have lower star formation efficiencies than the nearly constant value found for molecular gas in nearby normal spiral galaxies. On the other hand, some at least equally luminous Hα features, including a previously identified giant HII region, show no detectable cool molecular gas with a corresponding ratio at least a factor of 5 lower; in the giant HII region, essentially all the pre-existing molecular gas may have been converted to stars. We demonstrate that all the cool molecular filaments are gravitationally bound, and without any means of support beyond thermal pressure should collapse on timescales ≲106 yr. By comparison, as we showed previously, the two longest filaments have much longer dynamical ages of 107 yr. Tidal shear may help delay their collapse, but more likely turbulent velocities of at least a few tens of km s-1 or magnetic fields with strengths of at least several 10 μG are required to support these filaments. © 2009. The American Astronomical Society.