X-ray Isophotes in a Rapidly Rotating Elliptical Galaxy: Evidence of Inflowing Gas

TL;DR

This study models the X-ray emitting gas in the rotating elliptical galaxy NGC 4649, revealing inflow patterns, explaining temperature peaks, and addressing the cooling flow paradox through intermittent outflows, with implications for similar galaxies.

Contribution

It provides detailed gas dynamical simulations of NGC 4649, demonstrating inflow behavior, explaining observed temperature peaks, and proposing solutions to the cooling flow paradox involving intermittent outflows.

Findings

Inflow of about one solar mass per year at all radii.

Central temperature peak explained by compressive heating.

Cooling flow paradox addressed by intermittent outflows.

Abstract

We describe two-dimensional gasdynamical computations of the X-ray emitting gas in the rotating elliptical galaxy NGC 4649 that indicate an inflow of about one solar mass per year at every radius. Such a large instantaneous inflow cannot have persisted over a Hubble time. The central constant-entropy temperature peak recently observed in the innermost 150 parsecs is explained by compressive heating as gas flows toward the central massive black hole. Since the cooling time of this gas is only a few million years, NGC 4649 provides the most acutely concentrated known example of the cooling flow problem in which the time-integrated apparent mass that has flowed into the galactic core exceeds the total mass observed there. This paradox can be resolved by intermittent outflows of energy or mass driven by accretion energy released near the black hole. Inflowing gas is also required at intermediate kpc radii to explain the ellipticity of X-ray isophotes due to spin-up by mass ejected by stars that rotate with the galaxy and to explain local density and temperature profiles. We provide evidence that many luminous elliptical galaxies undergo similar inflow spin-up. A small turbulent viscosity is required in NGC 4649 to avoid forming large X-ray luminous disks that are not observed, but the turbulent pressure is small and does not interfere with mass determinations that assume hydrostatic equilibrium.