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Register a new userConvective cores in galactic cooling flows
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We use hydrodynamic simulations with adaptive grid refinement to study the dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the efficiency of heat supply to the gas. We consider a number of potential heating mechanisms including Type Ia supernovae and sporadic nuclear activity of a central supermassive black hole. As a starting point for this research we use an equilibrium hydrostatic recycling model (Kritsuk 1996). We show that a compact cooling inflow develops, if the heating is slightly insufficient to counterbalance radiative cooling of the hot gas in the central few kiloparsecs. An excessive heating in the centre, instead, drives a convectively unstable outflow. We model the onset of the instability and a quasi-steady convective regime in the core of the galaxy in two-dimensions assuming axial symmetry. Provided the power of net energy supply in the core is not too high, the convection remains subsonic. The convective pattern is dominated by buoyancy driven large-scale mushroom-like structures. Unlike in the case of a cooling inflow, the X-ray surface brightness of an (on average) isentropic convective core does not display a sharp maximum at the centre. A hybrid model, which combines a subsonic peripheral cooling inflow with an inner convective core, appears to be stable. We also discuss observational implications of these results.
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A currently active radio galaxy sits at the center of almost every strong cooling core. What effect does it have on the cooling core? Could its effect be strong enough to offset the radiative cooling which should be occuring in these cores? In order to answer these questions we need to know how much energy the radio jet carries to the cooling core; but we have no way to measure the jet power directly. We therefore need to understand how the radio source evolves with time, and how it radiates, in order to use the data to determine the jet power. When some simple models are compared to the data, we learn that cluster-center radio galaxies probably are energetically important -- but not necessarily dominant -- in cooling cores.
Almost every strong cooling core contains an active radio galaxy. Combined radio and X-ray images reveal the dramatic interaction which is taking place between the radio jet and the central cluster plasma. At least two important questions can in principle be answered by comparing the new data to theoretical models. The first is how the radio jet propagates, and disrupts, in the cooling core environment: why are these cluster-center radio sources unusual? The second is the effect the radio jet has on the cooling core: is it energetically important to the core? Thanks to the new data we are beginning to be able to answer these questions.
We present an analytical study of the effect of small convective cores on the oscillations of solar-like pulsators. Based on an asymptotic analysis of the wave equation near the center of the star, we derive an expression for the perturbations to the frequencies of radial modes generated by a convective core and discuss how these perturbations depend on the properties of the core. Moreover, we propose a diagnostic tool to isolate the predicted signature of the core, constructed from a particular combination of the oscillation frequencies, and we validate this tool with simulated data. We also show that the proposed tool can be applied to the pulsation data soon expected from satellite missions such as CoRoT and Kepler to constrain the amplitude of the discontinuity in the sound speed at the edge of the convective core, the ratio between the sound speed and the radius at this same location, and the stellar age.
Central cluster galaxies (cDs) in cooling flows are growing rapidly through gas accretion and star formation. At the same time, AGN outbursts fueled by accretion onto supermassive black holes are generating X-ray cavity systems and driving outflows that exceed those in powerful quasars. We show that the resulting bulge and black hole growth follows a trend that is roughly consistent with the slope of the local (Magorrian) relation between bulge and black hole mass for nearby quiescent ellipticals. However, a large scatter suggests that cD bulges and black holes do not always grow in lock-step. New measurements made with XMM, Chandra, and FUSE of the condensation rates in cooling flows are now approaching or are comparable to the star formation rates, alleviating the need for an invisible sink of cold matter. We show that the remaining radiation losses can be offset by AGN outbursts in more than half of the systems in our sample, indicating that the level of cooling and star formation is regulated by AGN feedback.
Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power law cooling function. The characteristic length and time scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently rapidly the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one and three dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period.
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