Department of Astronomy

Compact Objects

Stellar mass black holes and neutron stars litter the Milky Way galaxy are the compact remnants of massive star evolution. Some of these so-called compact objects find themselves in orbit with other, regular stars, from which they can siphon off matter. This “growth through cannibalization” process is called accretion and it turns an otherwise stealthy black hole into one of the brightest objects in the universe since the big bang, most of it coming it at X-ray energies. These object are thus aptly named X-ray binaries.

Professor Heinz’s group studies how these “small” black holes grow and how their growth (a) affects their environment and (b) how we can use observations of X-ray binaries to study matter under the most extreme conditions and hopefully learn more about the nature of spacetime itself around these most extreme of objects.

On much large scales, black holes are also found in the center of every massive galaxy. Compared to the black holes in X-ray binaries, they weigh in at about a million to a billion times more than stellar mass black holes. Nonetheless, they act in ways very similar to stellar mass black holes. Professor Heinz’s group is studying the large scale impact of these objects on the galaxies they live in, and beyond.

Recent studies show that these supermassive black holes can affect cosmic structure on scales that are a hundred times bigger than a typical galaxy by releasing tightly bundled streams of relativistic plasma called jets.

Using numerical simulations, they are studying the impact of black holes on their environments and the role growing blackholes play in the growth of cosmic structure. These models are complemented by a program of X-ray observations of compact objects of all scales, from X-ray binaries to supermassive black holes in galaxy clusters.

Run by Sebastian Heinz.

AGN Feedback

Black holes have a significant impact on their environment. Over the past 10 years, a consensus has emerged that galaxy clusters are heated by the action of relativistic jets, blasting into the intergalactic medium from the supermassive black hole of the central massive elliptical galaxy of the cluster. This heating might be enough to counter-balance the radiative losses the cluster gas is experiencing constantly, as it emits X-rays that can be observed from space based X-ray telescopes.

How this heating occurs and whether the heat can, in fact, be distributed to just the right locations in the cluster is still an issue of contentious debate. We are running state-of-the-art hydro simulations of galaxy clusters under the influence of energy injection from a central jet. We use fully cosmologically evolved clusters and inject powerful, collimated jets into them from the cluster center. A movie of this process can be accessed from Fig. 2. The key difference to earlier studies is that we include, for the first time, the dynamic state of the cluster, rather than assuming that it is a spherically symmetric, hydro-static atmosphere. This has important consequences for the dynamical evolution of radio sources (jets and the large scale bubbles of hot gas they blow into the intergalactic medium).

One of the important early results is that internal cluster dynamics, excited both by the action of the jets themselves and simply due to residual motions from previous dynamical encounters, as well as turbulence induced by cluster substructure (e.g., moving galaxies) can redistribute the gas in the cluster center in such a way that it erases any channels that were previously carved by the jet. This means that subsequent powerful jet outbursts can efficiently couple with the dense gas of the inner cluster, making it possible for the jet to heat the gas that needs it most (the coldest gas near the center of the cluster).

Publications

Visualization

XIM is a virtual X-ray observatory of numerical simulations of galaxy clusters and other sources of diffuse thermal emission. It can be used to generate data cubes for Chandra and can be customized for other telescopes as well. For example, it can be used to evaluate the capabilities of future missions, like Athena and Astro-H.

XIM is a set of IDL tools designed to turn standard gridded output from a hydrodynamical simulation into a virtual X-ray observation. The output from XIM is a spectral data cube.

This package contains the modules necessary to perform X-ray spectral simulations:

  • Athena, IXO and Chandra on-axis response matrices
  • an interface to the APEC/APED thermal emission code
  • an interface to MARX for sophisticated Chandra ray tracing simulations
  • an interface for spectral models other than thermal
  • output as IDL data files and fits events files

Publications

Microquasar Feedback

Black holes and neutron stars can accrete matter from their binary companion stars. In the process, they can not only release large amounts of radiation (mostly in the X-ray band, which gives them their designation as “X-ray binaries”), but they can, under certain circumstances, launch relativistic jets – tightly funneled streams of relativistic, magnetized gas. In this case, we also call the object a “microquasar”.

As this gas streams away from the compact object, it must eventually encounter the interstellar medium. Like jets from supermassive black holes, these microquasar jets can shock gas, push it aside into shells, and inflate diffuse “radio lobes” of synchrotron emitting plasma.

Professor Heinz’s group is studying the dynamics and observable characteristics of this interaction, both theoretically and through a program of broad-band observations of a number of objects.

For example, we have recently discovered a large-scale, jet-driven X-ray shock around the neutron star Circinus X-1. No other neutron star or black hole within our own galaxy shows anything like this. A careful analysis shows that the shockwaves are only about 1600 years old, a relatively recent phenomenon in astronomical terms, and that this neutron star is incredibly powerful.

While only a few dozen X-ray binaries suitable for such detailed study are known within the Milky Way, this observation clearly shows the power of deep observations of individual objects (a fact often forgotten in the era of survey science).

Publications

Multi-Phase Fluids

Evidence is mounting that a lot of the energy released by AGN jets is funneled into sound waves that traverse the cluster and, without some form of dissipation, will take their energy with them to the outskirts of the cluster – not ideal for heating the centers of galaxy clusters.The presence of relativistic gas itself makes the the intergalactic medium (IGM) a multi-phase gas. These bubbles and filaments of hot, relativistic gas react very differently to the passage of sound and shocks waves: Their high internal sound speed makes them act like hydraulic pistons that broadcast the arrival of the wave immediately over their entire surface. They also present much lower inertia to the arriving wave. The result is that that the wave transforms a stationary bubble into a rotating vortex ring – not unlike a smoke ring.

This process of vorticity creation through the interaction of waves with multi-phase gases is known in the fluid dynamics community as the Richtmyer-Meshkov instability. We are studying the efficiency of the Richtmyer-Meshkov instability in the context of galaxy clusters using hydrodynamic and magneto-hydrodynamic simulations.

In order to calculate the energy in the vortex field, we developed an algorithm to split the velocity field into a purely potential and a purely rotational component. The former has zero vorticity, the latter has zero divergence. We can then use the vorticity equation ω=curl(curl v) to calculate the vector potential that gives the rotational velocity component, from which we calculate vrot.

The implication is that the IGM, which is filled with filaments and bubbles of relativistic plasma, is very good at converting energy carried by sound waves and shock waves into kinetic energy in a vortex field that stays behind. Dissipative processes then have much more time to act on this energy and heat the cluster gas. The amount of energy that can be extracted is directly proportional to the volume filled by relativistic plasma. We estimate that for a typical cluster, a volume filling fraction of a few percent should be sufficient to extract enough energy to heat the cluster.

Publications