Ghost Cavities Research Articles

GAS SLOSHING AND BUBBLES IN THE GALAXY GROUP NGC 5098

We present results from Chandra observations of the galaxy pair and associated galaxy group NGC 5098, and find evidence for both gas sloshing and AGN heating. The X-ray brightness images show diffuse emission with a spiral structure, centered on NGC 5098a, and a sharp edge in the diffuse emission surrounding much of the galaxy at about 30 kpc. The spiral structure in the X-ray surface brightness and temperature maps, the offset between the peak of the cool gas and the central AGN, and the structure of the cold front edges all suggest gas sloshing in the core. The most likely perturber is the nearby galaxy NGC 5098b, which has been stripped of its gaseous atmosphere. Detailed images of the core reveal several X-ray cavities, two of which, just north and southeast of the central AGN, correlate with radio emission and have bright X-ray rims, similar to buoyant bubbles seen in the ICM of other systems. We estimate the pressures in the bubbles and rims and show that they are roughly equal, consistent with these being young features, as suggested by their close proximity to the central AGN. We assume that the other X-ray cavities in the core, which show no correlation with existing radio observations, are ghost cavities from previous AGN outbursts. An estimate of the mechanical energy required to inflate the cavities indicates that it is sufficient to offset radiative cooling of the gas for 15 Myr. Therefore, for a typical cycle time of 10^7 yrs, the central AGN energy output is enough to balance cooling over long timescales.

Long-Term Evolution of Magnetized Bubbles in Galaxy Clusters

We have performed nonlinear ideal magnetohydrodynamic simulations of the long-term evolution of a magnetized low-density plasma formed by a radio galaxy in a stratified cluster medium. It is found that about 3.5% of the initial magnetic energy remains in the bubble after ~8 × 109 yr, and the initial magnetic bubble expansion is adiabatic. The bubble can survive for at least 8 × 109 yr due to the stabilizing effect of the bubble magnetic field on Rayleigh-Taylor and Kelvin-Helmholtz instabilities, possibly accounting for ghost cavities as observed in Perseus A. A filament structure spanning about 500 kpc is formed along the path of bubble motion. The mean value of the magnetic field inside this structure is ~0.88 μG at ~8 × 109 yr. Finally, the initial bubble momentum and rotation have limited influence on the long-term evolution of the bubble.

Captures of Red Giant Stars by Black Holes in Elliptical Galaxies: Feedback to the Hot Gas

The highly disturbed hot gas in elliptical galaxies, as revealed in many {\em Chandra} X-ray images, implies a source of energy in the galactic nucleus. In some elliptical galaxies faint X-ray ``ghost'' cavities appear without corresponding radio lobes. It has been suggested that ghost cavities are caused by short-lived activity with a timescale of $\sim 10^3-10^4$ years, but this is difficult to understand within the popular paradigm of active galactic nuclei. We suggest an episode model for ghost cavities, invoking captures of red giant stars by the black hole located at the center of the elliptical galaxies at a typical rate of $10^{-5}$yr$^{-1}$ per galaxy. The accretion of tidally disrupted red giant stars onto the black hole powers activity in a timescale of a few years. The total energy channeled into the jet/outflow during the cooling time of the hot gas is $\sim 10^{56}$ erg, which is the typical work required to form the observed cavities. In this scenario, the faint cavities are produced by the feedback following accretion of the debris of the captured red giant stars onto the black holes. We apply the present model to several elliptical galaxies and find that it can explain the formation of the ghost cavities. This model can be tested in the future by comparisons between radio and X-ray observations.

Buoyant radio lobes in a viscous intracluster medium

Ideal hydrodynamic models of the intracluster medium (ICM) in the core regions of galaxy clusters fail to explain both the observed temperature structure of this gas, and the observed morphology of radio-galaxy/ICM interactions. It has recently been suggested that, even in the presence of reasonable magnetic fields, thermal conduction in the ICM may be crucial for reproducing the temperature floor seen in many systems. If this is indeed correct, it raises the possibility that other transport processes may be important. With this motivation, we present a numerical investigation of the buoyant evolution of AGN-blown cavities in ICM that has a nonnegligible shear viscosity. We use the ZEUS-MP code to follow the three-dimensional evolution of an initially static, hot bubble in a β-model ICM atmosphere with varying degrees of shear viscosity. With no explicit viscosity, it is found that the combined action of Rayleigh‐Taylor and Kelvin‐Helmholtz instabilities rapidly shred the ICM cavity and one does not reproduce the intact and detached ‘ghost cavities’ observed in systems such as Perseus-A. On the other hand, even a modest level of shear viscosity (corresponding to approximately 25 per cent of the Spitzer value) can be important in quenching the fluid instabilities and maintaining the integrity of the bubble. In particular, we show that the morphology of the north-west ghost cavity found in Perseus-A can be reproduced, as can the flow pattern inferred from the morphology of Hα filaments. Finally, we discuss the possible relevance of ICM viscosity to the fact that many of the active ICM cavities (i.e. those currently associated with active radio lobes) are not bounded by strong shocks, the so-called ‘shock problem’. Ke yw ords: hydrodynamics ‐ cooling flows ‐ galaxies: jets ‐ X-rays: galaxies: clusters.

Discovery of Ghost Cavities in the X-Ray Atmosphere of Abell 2597

A Chandra image of the central 100 kpc of the Abell 2597 cluster of galaxies shows bright irregular X-ray emission within the central dominant cluster galaxy (CDG) and two low surface brightness cavities located 30 kpc from the nucleus of the CDG. Unlike the cavities commonly seen in other clusters, the cavities in Abell 2597 are not coincident with the bright central radio source. Instead, they appear to be associated with faint extended radio emission seen in a deep Very Large Array radio map. We interpret the ghost cavities as buoyantly rising relics of a radio outburst that occurred between 50 and 100 Myr ago. The demography of cavities in the few clusters studied thus far shows that galactic radio sources experience recurrent outbursts on an ~100 Myr timescale. Over the lifetime of a cluster, ghost cavities emerging from CDGs deposit 1059-1061 ergs of energy into the intracluster medium. If a significant fraction of this energy is deposited as magnetic field, it would account for the high field strengths in the cooling flow regions of clusters. The similarity between the central cooling time of the keV gas and the radio cycling timescale suggests that feedback between cooling gas and the radio source may be retarding or quenching the cooling flow.