SARAO Fellow at the University of Pretoria
and the University of Manchester

## Research Interests

My research is focused on high resolution radio studies of deep fields. The aim is to use wide-field VLBI to identify faint radio-selected uJy AGN across cosmic time. When used in combination with lower resolution data from the eMERGE project, we are able to study the physical interplay between AGN and star formation. In addition, this research hopes to shed light on the role of `radio-quiet' AGN in galaxy formation and evolution. Some of my research is focused radio studies of nearby HII regions in dwarf galaxies as part of the Little THINGS survey. I have extensive experience with VLBI calibration techniques (see Radcliffe+16) and am one of the founding developers of the e-MERLIN CASA pipeline (see my github). I am also involved in the Development in Africa with Radio Astronomy (DARA) project which helps to teach the next generation of radio astronomers in preparation for the African VLBI Network and SKA.

## Publications

From Radcliffe+16: Compact radio source J123659+621833, which illustrates the effect MSSC has on the structure, fluxes, and noise levels achieved. Left panel:Source when calibrated with only standard phase referencing. This has a peak brightness of $824\,\mathrm{\mu Jy\,beam^{-1}}$ with a maximum S/N of 56.1. Right panel:The source with MSSC applied. The peak brightness is now $1.28\,\mathrm{mJy}$ with greatly improved S/N of 115.8.

### Nowhere to Hide: Radio-faint AGN in GOODS-N field. I. Initial catalogue and radio properties

#### Abstract

Context. The occurrence of active galactic nuclei (AGN) is critical to our understanding of galaxy evolution and formation. Radio observations provide a crucial, dust-independent tool to study the role of AGN. However, conventional radio surveys of deep fields ordinarily have arc-second scale resolutions often insufficient to reliably separate radio emission in distant galaxies originating from star-formation and AGN-related activity. Very long baseline interferometry (VLBI) can offer a solution by identifying only the most compact radio emitting regions in galaxies at cosmological distances where the high brightness temperatures (in excess of 105 K) can only be reliably attributed to AGN activity. Aims: We present the first in a series of papers exploring the faint compact radio population using a new wide-field VLBI survey of the GOODS-N field. This will expand upon previous surveys, permitting the characterisation of the faint, compact radio source population in the GOODS-N field. The unparalleled sensitivity of the European VLBI Network (EVN) will probe a luminosity range rarely seen in deep wide-field VLBI observations, thus providing insights into the role of AGN to radio luminosities of the order $10^{22}\,\mathrm{W\,Hz^{-1}}$ across cosmic time. Methods: The newest VLBI techniques are used to completely cover an entire $7.5^\prime$ radius area to milliarcsecond resolutions, while bright radio sources (S > 0.1 mJy) are targeted up to $25^\prime$ from the pointing centre. Multi-source self-calibration, and a primary beam model for the EVN array are used to correct for residual phase errors and primary beam attenuation respectively. Results: This paper presents the largest catalogue of VLBI detected sources in GOODS-N comprising of 31 compact radio sources across a redshift range of 0.11-3.44, almost three times more than previous VLBI surveys in this field. We provide a machine-readable catalogue and introduce the radio properties of the detected sources using complementary data from the e-MERLIN Galaxy Evolution survey (eMERGE). The catalogue is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/vizbin/qcat?J/A+A/619/A48

### Measuring size evolution of distant, faint galaxies in the radio regime

#### Abstract

We measure the evolution of sizes for star-forming galaxies as seen in 1.4 GHz continuum radio for z=0-3. The measurements are based on combined VLA+MERLIN data of the Hubble Deep Field, and using a uv-stacking algorithm combined with model fitting to estimate the average sizes of galaxies. A sample of ~1000 star-forming galaxies is selected from optical and near-infrared catalogues, with stellar masses M ~ 1010-1011 M and photometric redshifts 0-3. The median sizes are parametrized for stellar mass M* = 5 x 1010 M as Re=A x (H(z)/H(1.5))αz. We find that the median radio sizes evolve towards larger sizes at later times with αz = -1.1∓0.6, and A (the median size at z ~ 1.5) is found to be 0.26∓0.07asec or 2.3∓0.6 kpc. The measured radio sizes are typically a factor of 2 smaller than those measure in the optical, and are also smaller than the typical Hα sizes in the literature. This indicates that star formation, as traced by the radio continuum, is typically concentrated towards the centre of galaxies, for the sampled redshift range. Furthermore, the discrepancy of measured sizes from different tracers of star formation, indicates the need for models of size evolution to adopt a multi-wavelength approach in the measurement of the sizes star-forming regions.

### Multi-source self-calibration: Unveiling the microJy population of compact radio sources

#### Abstract

Very long baseline interferometry (VLBI) data are extremely sensitive to the phase stability of the VLBI array. This is especially important when we reach uJy rms sensitivities. Calibration using standard phase-referencing techniques is often used to improve the phase stability of VLBI data, but the results are often not optimal. This is evident in blank fields that do not have in-beam calibrators. We present a calibration algorithm termed multi-source self-calibration (MSSC) which can be used after standard phase referencing on wide-field VLBI observations. This is tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field North and the Hubble Flanking Fields. MSSC uses multiple target sources that are detected in the field via standard phase referencing techniques and modifies the visibilities so that each data set approximates to a point source. These are combined to increase the signal to noise and permit self-calibration. In principle, this should allow residual phase changes caused by the troposphere and ionosphere to be corrected. By means of faceting, the technique can also be used for direction-dependent calibration. Phase corrections, derived using MSSC, were applied to a wide-field VLBI data set of the HDF-N, which comprises of 699 phase centres. MSSC was found to perform considerably better than standard phase referencing and single source self-calibration. All detected sources exhibited dramatic improvements in dynamic range. Using MSSC, one source reached the detection threshold, taking the total detected sources to twenty. This means 60% of these sources can now be imaged with uniform weighting, compared to just 45% with standard phase referencing. In principle, this technique can be applied to any future VLBI observations.

Note: if you use this code please remember to cite Radcliffe et al. (2016) A&A 587 A85

### Nowhere to Hide - Wide-field VLBI of the Hubble Deep Field-North

#### Abstract

We present a new ultra-deep, wide-field VLBI survey targeting the central part of the Hubble Deep Field-North region using the European VLBI Network (EVN). The survey covers a large, 180 arcmin2, area to milliarcsecond resolutions and microJy sensitivities. Results are presented from the first of three epochs of observations, concentrating on a new calibration technique termed multi-source self-calibration (MSSC). MSSC uses the combined response of multiplefaint sources to improve the phase stability of wide-field VLBI data sets. On average, MSSC improved the signal to noise of detected target sources by 27% in naturally weighted images and 63% in uniformly weighted images when compared to phase referenced calibration. We conclude with the first science results from this survey which concentrate on the redshift ~2 sub-mm source, J123642+621331 (Waddington et al. 1999). Previous studies have indicated the presence of star-formation, together with a dusty, embedded AGN (Muxlow et al. 2005). This was confirmed by previous VLBI observations (Garrett et al. 2001, Chi et al. 2013), and makes the object a prime candidate to study the interplay between AGN and starburst activities at high redshifts. Combination imaging using the eMERGE (P.I. Muxlow) survey data (a combination of JVLA+VLA+MERLIN+eMERLIN) together with these new EVN data allows us to obtain radio images at a range of differing angular resolutions. This allows us to separate contributions from star-formation and embedded AGN.

### Star formation: The role of high resolution radio surveys

#### Abstract

The role of star-formation is critical for galaxy evolution. It is a ubiquitous process in galaxies that is both influenced by, and drives their evolution. In this article we will review the current status of high resolution studies of star-formation processes within galaxies from the local to the distant Universe. In particular this review will focus on the state-of-the-art radio surveys that are now underway which provide some of the highest angular resolution and unobscured views of star-formation in galaxies. These surveys significantly add to our understanding of the processes of star-formation and provide the first glimpse of what will be achievable with the SKA in the coming decades.

## Teaching & Outreach

### Development in Africa with Radio Astronomy (DARA)

Left: University of Zambia students and lecturers. Right: Fourier transform tool used to teach about the spatial filtering effect of interferometers

Manchester is playing a leading role in the future long-term development of the SKA by training the next generation of African students who will run telescopes of the SKA2 in 8 countries. The DARA (Development in Africa with Radio Astronomy) initiative is within the UK Newton project and is coordinated with South Africa. DARA is headed by Professor Melvin Hoare (Leeds), Professor Peter Wilkinson, Dr. Robert Beswick (Manchester), Professor Michael Jones, Professor Matthew Jarvis (Oxford), Professor Martin Hardcastle, Dr. Mark Thompson (Hertfordshire) together with Dr. Roelf Botha (HartRAO; South Africa). Other members are taking roles in the training and have recently returned from teaching in Kenya and Zambia.

Personally, I am involved in the teaching of radio interferometry data reduction techniques using CASA and AIPS. The aim is to get the students from raw correlated interferometry data to beautiful images at the end (with some computer training included). This project has been active since 2015, and I have taught for the past 2 years.

To see more about the project you can have a look at the main website

#### Materials

There are materials that are useful for anyone who wants to get into radio interferometry or need help with their observations. There are both lectures and some hands-on workshops where you can reduce some real data! They were created by Anita Richards, Luke Hindson, Hayden Rampardarath and myself.
You can find the materials here

### Observational Astronomy

Color image of starburst galaxy M82 using combined BVR filters on the Gratama telescope. Credit: M. Verheijn

I was a teaching assistant (TA) for the observational astronomy course taught by prof. Scott Trager (Groningen). Students gain real experience of using an optical telescope as they have to conduct an observing project using the Gratma telescope in the Blauuw Observatory. An example is shown in the image above.

## By email:

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