Galactic Interactions and Clusters

Hickson Compact Group 92
A portrait of Stephan's Quintet, also known as the Hickson Compact Group 92, revealing an assortment of stars across a wide colour range, from young, blue stars to aging, red stars. Credit: NASA, ESA and the Hubble SM4 ERO Team.

Galaxies

While the origin of magnetic fields in spiral galaxies is still speculative, a process known as the alpha-omega dynamo amplifies the weak magnetic fields to produce strong large-scale fields through the inductive effect of turbulence and differential rotation.

Nearby galaxies can help us achieve an outside perspective of galactic scale magnetic fields while retaining a decent spatial resolution.

Several observational tools are available to study cosmic magnetic fields with the most useful being radio synchrotron emission. The total synchrotron intensity can be used to estimate the total magnetic field strength. As synchrotron emission is intrinsically polarised, it can be used to determine the ordered component on the plane of the sky.  Such polarised emission is mainly seen in regions of low turbulence, specifically in the interarm regions of face-on galaxies.

With edge-on spiral galaxies, the large scale ordered fields in the disks lie plane-parallel along the disk of the galaxy and in the halo they have a X-shaped morphology. This is what is expected from observations of face-on galaxies and their magnetic field amplification by the action of a mean-field dynamo.

We use a variety of telescopes to observe nearby galaxies to help us learn more about the configuration, nature and amplification of galaxies’ magnetic fields and what contribution it has on the ISM. Interferometers such as the Jansky Very Large Array (JVLA) and LOw Frequency ARray (LOFAR) are commonly used to achieve these goals. Observations at low frequencies with LOFAR make it possible to learn more about the nature of the magnetic field and cosmic ray propagation at furthest reaches of galaxies with little contamination from thermal emission, while higher frequency observations with the JVLA allow us to observe ordered magnetic field through polarisation measurements.

 

Galaxy clusters

 Galaxy clusters are the largest gravitationally bound structures in the Universe. They form at the intersection of filaments in the cosmic web. Galaxy clusters were first discovered in the X-ray, as the hot intra-cluster medium (ICM) emits bright Bremsstrahlung radiation. Despite how bright galaxy clusters are in the X-ray, only about 5% of their mass is contained in the gas. The rest of the mass is contained in Dark matter.
 
We use radio observations to reveal information about galaxy clusters that is not revealed at other wavelengths. Radio observations look at synchrotron radiation, which trace relativistic electrons in micro-gauss magnetic fields. Large scale diffuse radio emission found in some clusters points to the fact that magnetic fields are present on cluster wide (sometimes Mpc) scales. These observations let us investigate how they might have formed, and how the magnetic fields can influence the evolution of galaxy clusters.
 

Research activities

  • Galaxies

    In the local Universe, large-scale double-lobed radio sources are almost always hosted by elliptical galaxies. This is consistent with leading galaxy formation models that suggest both elliptical galaxies and extended radio sources are the result of mergers. These models do not allow for spiral galaxies to host large-scale double-lobed radio sources. However, to-date, a handful of such sources have been observed to exist in the local Universe. We call these sources Spiral DRAGNs.

     

    M51

    The whirlpool galaxy and its surrounding: LOFAR radio map of the whirlpool galaxy M51 and its neighbourhood at a frequency of 150 MHz. The field covers 4 by 2.6 degrees, the observations were performed with the Dutch LOFAR high-band antennas. The map shows the distribution of hot electrons in M51 and also a large number of background galaxies.The inlay shows an enlarged view of M51 at 150 MHz (white contour lines) overlayed onto an optical image of M51 from the Digital Sky Survey (DSS).

     

    Radio-optical overlay image of galaxy J1649+2635. Yellow is visible-light image; Blue is the radio image, indicating the presence of jets. Credit: Mao et al., NRAO/AUI/NSF, Sloan Digital Sky Survey

  • Clusters

    Our research is mainly focused on understanding the production of large scale diffuse emission present in galaxy clusters. This emission, with typical scales of order 1Mpc, indicates the presence of large scale magnetic fields in galaxy clusters. Diffuse radio emission in galaxy clusters can be categorized as radio halos or radio relics or radio mini halos depending on their size, polarisation and position in the cluster.

    Radio Halos are enormous regions of diffuse emission found at the centre of galaxy clusters. Their formation is thought to be linked to the merger of galaxy clusters which are hugely energetic events (roughly equivalent to a trillion supernovae).  One formation scenario suggests that turbulence in the gas of the galaxy cluster accelerates particles to radio emitting energies leading to the production of radio halos. However not all merging galaxy clusters host radio halos. The reason for this is not yet clear, although it is potentially related to how much energy is released in a particular  merger event, where a weaker merger does not generate enough turbulence to form a radio halo.

     

    Diffuse radio emission in the galaxy cluster MACS J2243.3-0935 reveals a new radio halo. The background image shows an optical image of the cluster using the SDSS survey. Cantwell et al. 2016, MNRAS, 458, 1803.

     

    Radio Relics are typically found on the outskirts of galaxy clusters and are typically assumed to be generated by the passing of shocks from cluster mergers. Unlike radio halos, radio relics are often found to be highly polarised.

     

    Merging cluster from the MACS survey (Risley et al 2016). Purple is X-ray, blue is dark matter lensing reconstruction and red is radio at 325MHz from the GMRT.

     

    Radio relics and radio halos typically have very low surface brightness which poses challenges, both in their detection and study. In order to increase our signal to noise we make use of low frequency instruments such as LOFAR and the GMRT.

    The large amount of gas trapped in galaxy clusters also distorts the signal of the cosmic microwave background (CMB). This effect is know as the Sunyaev-Zel’dovich or SZ effect. Using high frequency instruments such as the AMI telescope in Cambridge we study the density profiles of galaxy clusters.

     

     

     

  • Interacting Galaxies in the Hercules Cluster.

    Star Clusters in Collision. This computer simulation shows the gravitational interaction of two young star clusters.

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