Current Projects

The main goal of my thesis is to analyse disturbed, merging clusters that host radio relics. Radio relics are diffuse patches of emission which trace shock fronts induced by the major merging of clusters. In order to disentangle the physical processes at play, I combine radio observations ranging from the lowest to the highest frequencies (150 MHz up to 16 GHz), optical broad-band and narrow-band imaging in all filters (from B to z band) and spectroscopy. I use this multi-wavelength data to study the interplay between the merger, the resulting travelling shock waves, the intra-cluster medium and the galaxies.

What I am currently working on

I am currently leading or majorly contributing to a number of projects aimed at furthering our understanding of the `Sausage' cluster, a very interesting merging cluster which hosts double radio relics. All of these projects are currently being finalised, so keep posted for when they will appear on the ArXiv (hopefully by end of September).
  • Star forming properties of the galaxies within the `Sausage' cluster.
  • The nature of the star forming galaxies within the hot ICM of the cluster using Keck/WHT spectroscopy.
  • Weak lensing analysis of the cluster.
  • Redshift/dynamical analysis of the cluster.
  • Suzaku observations of the cluster environment.

Hα observations of merging clusters

We investigated the role of merger shocks in shaping the Hα luminosity function in galaxies withi disturbed clusters. We discovered extended Hα haloes in galaxies neighbouring the relics in Sausage cluster, which were possibly disrupted by the passage of the shock wave. Our results suggest that cluster mergers may play an important role in the evolution of cluster galaxies through shock-induced star formation. This effect depends on time since core passage for the subclusters and one would only see an enahcement in star formation in a recent merger. You can find the paper here.

Spectral modelling of radio relics

By using the BRATS spectral modelling software, we fitted spectral ageing models to some of the best radio datasets available for a radio relic -- the northern relic in the Sausage cluster. Our results indicate Mach numbers which are consistent with the X-ray derived Mach number (inconsistency which has been an issue in the field for a few years now). We could determin the shock advance speed and place constraints on the core passage of this merging cluster at 0.6 -- 0.8 Gyr ago. We find a systematic electron age across the relic, indicating the shock front in moving nortwards at constant rate.You can find the paper here.

Very high-frequency radio observations of merging clusters

With the aid of the AMI telescope, we have made a surprising, very high frequency detection of a cluster radio relic. The results indicates that our currently-accepted, simple view of radio relic formation needs improving. You can find the paper here.

Low-frequency radio observations of merging clusters

My first project dealt with a low radio frequency analysis of the cluster nicknamed the "Sausage", in which we pinpoint the electron injection and acceleration mechanisms operating in the diffuse radio emission. A high-resolution version of this paper can be found here.

Past Projects

Clusters with VISTA

My Master's thesis was concerned with deriving physical properties of two galaxy clusters discovered by the South Pole Telescope through the Sunyaev-Zel’dovich effect (SZE), by using coordinated optical and infra-red data. For cluster SPT-CL J0528-5259, at redshift 0.75, the positions of the members are yet unknown. Two sets of coordinates have been reported for this cluster: one location of the brightest cluster galaxy, derived from a spectroscopic analysis, and the second from SZE measurements. By investigating all the objects within 2 arcmin of the position of the brightest cluster galaxy, I found a clear overdensity of galaxies at the expected redshift and prove that this is the correct cluster location. I also obtain a rich red sequence of bright, massive elliptical galaxies in two colour - magnitude plots. By contrast, 21 members of SPT-CL J0546-5345 at z = 1.06 have been spectroscopically confirmed. For the 9 galaxies recovered in the infra-red data, I produced photometric redshifts matching spectroscopic measurements. I performed a test of various combinations of magnitude apertures, filters and galaxy models to establish that the best photometric redshifts are obtained when fitting both optical and infra-red fluxes, using Petrosian magnitudes, to templates for all galactic types. I derived the first stellar mass estimates for the members of this cluster and obtain consistent light to mass ratios. The investigation concluded that the best and most reasonable fits are obtained when the masses are derived from coordinated optical and infra-red filters and fixing all the galaxies as ellipticals.

Radial Velocities

During my summer studentship at the IoA in 2011, I worked with Prof Roger Griffin on a long-term observing programme of radial velocities of a set of ~80 seventh-magnitude late-type stars. Some of them proved to be spectroscopic binaries with exceptionally long periods, of the order of decades.

Optical Interferometry

This project started as my internship during the summer of 2009 within the Optical Interferometry group at MRAO. I first designed a virtual observatory to simulate observations that would be made with the upcoming Magdalena Ridge Observatory Interferometer. Upon its completion this will be the largest, most powerful optical interferometer in the world with up to 10 telescopes working in concert to obtain a resolution better than 1 mas. I used this programme for my Bachelor thesis in Astrophysics, where I investigated the capabilities to reconstruct the true morphology of a star + gas + dust ring system with an MROI-type of telescope.
The real "looks" of the star
Reconstruction with a ten-element interferometer

Magnetotail of the Earth

The topic of Physics Bachelor thesis, was also conceived during a summer internship on space plasma(2009, at MPE). My project was concerned with analyses on the processes occurring in the plasma sheet in the magnetosphere of the Earth, using Cluster mission data, a four-spacecraft mission. I used the three-point gradient estimation scheme to finding evidence of local magnetohydrostatic equilibria in the Earth’s magnetotail which are characterized by a balance of the pressure gradient and the Lorentz force. This particular method needs to be employed because, in the case of the pressure, the measurement instrument does not work on all Cluster spacecraft and it is impossible to discriminate between spatial and temporal variations.


During my PhD, I have given a number of talks at other institutes and at conferences. Below you can find video recordings of two such talks.

My seminar at the IAC in Tenerife, Spain, on November 10, 2013.
My talk at the Metrewavelength Sky Conference in Pune, India, on December 10, 2013.


Check out ADS for a list (albeit short) of my publications.


Huub Rottgering (Leiden)
David Sobral (Leiden/Lisbon)
Reinout van Weeren (CfA)
Jeremy Harwood (ASTRON)
Will Dawson (LLNL/Davis)
James Jee (Davis)
Dave Wittman (Davis)
Henk Hoekstra (Leiden)
Marcus Brüggen (Hamburg)
Matthias Hoeft (Tautenburg)
Richard Saunders (Cambridge)
Clare Rumsey (Cambridge)
Martin Hardcastle (Herts)
Huib Interma (NRAO)
Bjorn Emonts (CSIRO/Madrid)
George Miley (Leiden)