Radio galaxies & the evolving Universe

Radio galaxies

Radio galaxies are powered by the accretion onto supermassive black holes, and one of their striking features, from existence of which they actually owe the name, are radio jets and radio lobes. The jets are launched at the vicinity of the central supermassive black hole, and then drill their way through the intergalactic, and intracluster medium. Eventually they terminate and excess energy that they transfer is deposited in the radio lobes. Depending on both the power of the jets and the density of the medium in which they propagate, they may reach distances of up to Mpc from the central black hole. This makes some of them one of the largest single objects in the Universe.

The origin of radio emission in these sources in exactly the same as in the galactic X-ray binaries that exhibit relativistic jets (see end of this page). In fact, X-ray binaries are often considered as reminiscent of the radio galaxies and quasars, but on a much smaller scale (hence also their name `microquasars').

Various flavours of a radio galaxy

The looks - morphologically

Radio galaxies have been classed into two main types, FRI and FRII, following seminal work of Bernie Fanaroff and Julia Riley back in 1974. This classification has proven to be extremely powerful, yet its physical origins are still not fully understood. Then, in 2000 Gopal-Krishna and Paul Wiita highlighted existence of peculiar, hybrid type: the hybrid morphology radio sources (HyMoRS). These hybrids would exhibit characteristics if both FRI and FRII classes at the same time (that is, they would have a jet looking like an FRI one on the one side, and like an FRII on the other side from the host). HyMoRS are rather fascinating objects, and we still know very little about them. To change this we recently carried out a Radio Galaxy Zoo project focussing on these particular objects. Our first findings have been now published (Kapinska et al. 2017).

Left: The giant relic FR I radio galaxy B1545-321 (radio lobes)with recently restarted activity (bright green/red jet emission close to the galaxy core). 2.4 GHz ATCA image. Credits: Saripalli, Subrahmanyan, & Shankar.

Galaxy groups and clusters

Galaxy groups and clusters are the largest gravitationally bound objects known that have formed though the cosmic structure formation. These structures may contain tens (groups) to thousands (clusters) of galaxies. Although they are gravitationally bound by their mutual attraction, the velocities especially of galaxies in clusters suggest that additional mass component (dark matter) or additional attractive force (apart from gravity) is necessary to explain the observations.

Galaxy clusters are often studied in X-rays, which have revealed the presence of intercluster gas referred to as intercluster medium (ICM). The ICM consists of hot plasma (10^7 - 10^9 K) that is composed mostly of ionised hydrogen and helium (most of the cluster baryonic material). The X-ray observations are, however, limited; due to the sensitivity of the instruments clusters only up to z ~ 1can be studied with this method. Luckily, the galaxy clusters may be found and investigated through other techniques such as gravitational lensing or radio observations.

Radio galaxies being usually hosted by massive elliptical galaxies, are often found in centres of galaxy clusters and groups. Since often they are harboured by the most massive central cluster galaxy and are one of the most powerful objects, radio galaxies have significant impact on the ICM, and hence the evolution of galaxy clusters (via so-called kinetic AGN feedback).

Below: A composite image of galaxy cluster MS0735.6+7421. Credits: B. McNamara, NASA, ESA, CXC, STScI.

Relativistic jets of stellar black holes

During my master's studies I was working on observational radio astronomy with Dr. James Miller-Jones at the University of Amsterdam in the Netherlands. My master's thesis work focused on low frequency radio observations (~300 MHz) of three well known X-ray binaries, SS 433, Cygnus X-3 and GRS 1915+105.

X-ray binary star systems

X-ray binaries are binary star systems that contain a compact object (i.e. black hole or a neutron star) which accretes matter from a companion (donor) star. The latter is usually a main system star, and based on its mass there are distinguished high and low mass binary X-ray systems (HMXBs and LMXBs respectively). The companion star orbits the compact object, and under the presence of strong gravity of the compact object a transfer of matter from the donor occurs via accretion flow, and further formed accretion disc.

Below: An artist impression of an X-ray binary. Credits: Prof. R. Hynes, Lousiana State University.
An artist impression of an X-ray binary. Credits: Prof. R. Hynes, Lousiana State University.

As suggested by their name the systems emit predominantly in X-rays; their X-ray luminosities reach >10^35 erg/s. However, these objects are detected at wavelengths ranging from radio, through optical to high energy radiation. As the emission originates from the underlying physical processes, observations of the objects at various wavelengths give an insight into particular parts of the binary system: optical and infrared light is emitted by the companion star and the accretion disc, while the X-ray radiation is produced close to the compact object. Sometimes, in exceptionally violent or powerful objects, twin and perpendicular to the accretion disc relativistic outflows of matter are created, the so-called jets. They are often visible in radio waves, and hence radio observations of X-ray binaries may provide a signature for interplay between the binary and its environment as well as current state of their activity. The X-ray binaries that emit radio jets are often called microquasars due to their reminiscence of the larger and more powerful quasars in the centres of galaxies. Attempts have been made to provide a unification theory for black holes of all scales, which would also link compact objects of X-ray binary systems and supermassive black holes in the centre of galaxies.

Radio observations

What exactly is seen in radio waves? The radio emission in X-ray binaries has an origin in relativistic electrons spiralling in magnetic fields (synchrotron emission). An accreting object sheds its excess angular momentum and energy by forming jets. These outflows propage through the medium surrounding the X-ray binary. If a matter condensation is reached on the outflow way, shocks are created - these may be visible as radio knots at some distance from the binary. When the jet terminates, the excess energy transported by the outflows are stored in the surrounding medium creating a cocoon around the object. The radio knots in the jet, the outflow termination point, and the cocoon may be observable in radio waves, especially in lower frequencies (MHz).

A few peculiar, and beautiful, examples of X-ray binaries (especially as seen in radio waves) include SS 433 and Cygnus X-1.

SS 433:

Cyg X-1:

Cyg X-1 (position marked with a cross) and its jet blown ring observed in 1.4 GHz with WSRT.

Credits: Gallo et al., 2005, Nature 436, 819.

The ring of Cyg X-1 (1.4 GHz, WSRT) showing the sketch of the underlying model and jet activity.

Credits: Gallo et al., 2005, Nature 436, 819.